interesting article i found comparing manifold vacume to ported vacune
02-13-2004, 01:05 PM
#1
Member
Thread Starter
Join Date: Aug 2003
Location: SB, IN
Posts: 193
Likes: 0
Received 0 Likes
on
0 Posts
interesting article i found comparing manifold vacume to ported vacune
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
AGAIN I DID NOT WRITE THIS, JUST TRANSPORTED IT HERE. bUT I DO BELIEVE IT FULLY.
i found this on camaros.net in the performance section.
i think chickenman35 would probably agree with this, hope it clears up some of the debate
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
AGAIN I DID NOT WRITE THIS, JUST TRANSPORTED IT HERE. bUT I DO BELIEVE IT FULLY.
i found this on camaros.net in the performance section.
i think chickenman35 would probably agree with this, hope it clears up some of the debate
02-15-2004, 02:21 AM
#2
02-15-2004, 11:10 AM
#4
Supreme Member
Join Date: Jul 2001
Location: Pueblo Co
Posts: 3,974
Likes: 0
Received 0 Likes
on
0 Posts
Car: 1989 C4
Engine: L98
Transmission: 700r4
Axle/Gears: 307
Very good information especilly the #VC-1810 number. Ive been looking for a can like that for awhile but not seriolsy digging since I dont do that much HYW driving now days. But when I do vacuum is only in the 12/13 range holding a steady speed, high altitude sucks!
02-15-2004, 09:30 PM
#5
Senior Member
Join Date: Dec 2001
Location: Coquitlam, BC
Posts: 896
Likes: 0
Received 1 Like
on
1 Post
Car: 86\92 Mutant
Engine: 355CI 430HP
Transmission: T-5 with mods
Axle/Gears: 7.625", Eaton Posi, 3.73
Re: interesting article i found comparing manifold vacume to ported vacune
[i]
i think chickenman35 would probably agree with this, hope it clears up some of the debate [/B]
i think chickenman35 would probably agree with this, hope it clears up some of the debate [/B]
Yes, very few people realise that full manifold vacuum was the norm before the mid-seventies. Ported vacuum was entirely emissions driven.
The other thing that people sometimes misunderstand, is where the ported vacuum source is located. It is NOT taken from the Venturi area at all. Ported vacuum is NOT generated by airflow speed through the venturi.
The ported vacuum source is taken just above the throttle butterfly. Thus it see's MANIFOLD vacuum just as a full Manifold vacuum port does. Manifold ported vacuum has the source located just below the throttle butterfly.
Thus, Ported vacuum values will be exactly the same as Manifold vacuum values....with the exception that Manifold vacuum is active at idle and ported is not.
As explined previously, cars with modified engines, particularily bigger cams will benifit from running Manifold vacuum vs Ported vacuum. Generally, the idle will be more stable and the "tip in" throttle response will be better. Of course the timiing curve should be tailored to fit the application. Replacing the stock GM vacuum canister with a Crane adjustable unit is a good idea. The stock HEI generally has about 20 degrees built in . This is too much...10 to 15 degrees with some more static timing is a better way to go.
02-16-2004, 05:27 PM
#6
Supreme Member
I'll agree with a lot of what you said, and I definitely appreciate somebody putting togehter such a well thought-out and detailed post on the subject.
I'll throw in a few observations........
1. Like was said above, ported advance shows within 1"Hg the same vacuum as full manifold vacuum anywhere above light throttle in most (all?) cases in my expereince. I've actually driven around with 2 vacuum gagues duck-taped to the windshield- one from each type of port. So on most carbs they show the same vacuum above about 5-10% throttle, differeing only near idle.
2. Vacuum advance isn't just to compensate for leanness. At part throttle the vacuum in the manifold means that the cylinder isn't seeing a dense charge of A/F. It's real "thin" and that, as much as anything, causes the slow burn rate, requiring some additional advance for best economy, etc. Hence, vacuum advance (AKA load-compensated advance).
3. I've worked on a CRAPLOAD of 70s emissions-era distributors over the years. Mainly, the GM HEI. Typical curve for them still only has about 20-22* centrifugal advance (very similar to a pre-emissions distributor that would usually have about 24*) and stock timing spec is for about 4-6* initial. So, they're only getting a true 24-28* total timing (not including vacuum advance) at WOT. However, the amount of vacuum advance they have is A LOT. Usually around 22-26*. In short, they run a WEAK initial and centrifugal but then prop up the part throttle advance by giving it a TON of vacuum advance. I don't claim super-special knowelge why they do this- but that's how they're almost always set up. For a performance application simply bumping the initial up to 12*, installing slightly quicker centrifugal advance springs and limiting vacuum advance down to 12-16* can make these slugs gain a WHOLE LOT of new life, with no other changes. Also, the vast vast majority of emissions-era vacuum advance cans bring it "all in" well below 15"Hg, in my expereince. 9-11"Hg being typical. I strongly suspect is the super-tall "exonomy" rear end gears they used (2.19 - 2.42 being typical for a 70s-era V8 car) always kept the engine under a fair bit of a load and never allowed much manifold vacuum to be generated. Hence the need to bring in the vacuum advance at fairly low vacuum levels.
Last one, and this only applies to non-stock performance applications. Running the vacuum advance to a ported source IS a valid thing to do in some circumstances. A motor with relatively high compression and a small cam will sometimes "diesel" or "run on" if you hook up the vacuum advance up to full manifold vacuum. It keeps exhaust temps cool, but it increases combustion chamber temps. That can cause stuff to start to get hot inside the combustion chamber at idle and when you turn the key off it "glow plugs" and diesels on for a bit. With a big cam (little vacuum at idle) running full manifold vacuum to the distributor I definitely agree with you- it can cause a problem like you describe where the vacuum advance kinda "floats" in the middle of it's range and the idle gets unstable and often "rolls" up and down. Switching to ported vacuum can often cure this, although it's may or may not be the best way to fix the problem.
I'll throw in a few observations........
1. Like was said above, ported advance shows within 1"Hg the same vacuum as full manifold vacuum anywhere above light throttle in most (all?) cases in my expereince. I've actually driven around with 2 vacuum gagues duck-taped to the windshield- one from each type of port. So on most carbs they show the same vacuum above about 5-10% throttle, differeing only near idle.
2. Vacuum advance isn't just to compensate for leanness. At part throttle the vacuum in the manifold means that the cylinder isn't seeing a dense charge of A/F. It's real "thin" and that, as much as anything, causes the slow burn rate, requiring some additional advance for best economy, etc. Hence, vacuum advance (AKA load-compensated advance).
3. I've worked on a CRAPLOAD of 70s emissions-era distributors over the years. Mainly, the GM HEI. Typical curve for them still only has about 20-22* centrifugal advance (very similar to a pre-emissions distributor that would usually have about 24*) and stock timing spec is for about 4-6* initial. So, they're only getting a true 24-28* total timing (not including vacuum advance) at WOT. However, the amount of vacuum advance they have is A LOT. Usually around 22-26*. In short, they run a WEAK initial and centrifugal but then prop up the part throttle advance by giving it a TON of vacuum advance. I don't claim super-special knowelge why they do this- but that's how they're almost always set up. For a performance application simply bumping the initial up to 12*, installing slightly quicker centrifugal advance springs and limiting vacuum advance down to 12-16* can make these slugs gain a WHOLE LOT of new life, with no other changes. Also, the vast vast majority of emissions-era vacuum advance cans bring it "all in" well below 15"Hg, in my expereince. 9-11"Hg being typical. I strongly suspect is the super-tall "exonomy" rear end gears they used (2.19 - 2.42 being typical for a 70s-era V8 car) always kept the engine under a fair bit of a load and never allowed much manifold vacuum to be generated. Hence the need to bring in the vacuum advance at fairly low vacuum levels.
Last one, and this only applies to non-stock performance applications. Running the vacuum advance to a ported source IS a valid thing to do in some circumstances. A motor with relatively high compression and a small cam will sometimes "diesel" or "run on" if you hook up the vacuum advance up to full manifold vacuum. It keeps exhaust temps cool, but it increases combustion chamber temps. That can cause stuff to start to get hot inside the combustion chamber at idle and when you turn the key off it "glow plugs" and diesels on for a bit. With a big cam (little vacuum at idle) running full manifold vacuum to the distributor I definitely agree with you- it can cause a problem like you describe where the vacuum advance kinda "floats" in the middle of it's range and the idle gets unstable and often "rolls" up and down. Switching to ported vacuum can often cure this, although it's may or may not be the best way to fix the problem.
02-16-2004, 05:58 PM
#7
Supreme Member
Join Date: Sep 2001
Location: Decatur, Illinois
Posts: 2,906
Likes: 0
Received 0 Likes
on
0 Posts
I have read that with a manual transmission that you can get detonation when you let of slightly and get back in on a shift when running wot? Is there any truth to that? I know that according to the manual that cam w/ my demon dp, it said to connect to ported vacuum.
Ben
Ben
Trending Topics
02-16-2004, 10:59 PM
#8
Supreme Member
I just did some investigating over at camaros.net and discovered that the man that wrote this article is a mechanical engineer and has worked for GM and Chrysler for 37 years. The last 6 years was the Viper plant manager. I plan to email him on this subject. I want to dig deeper in this subject. This is very controversial.
02-17-2004, 12:39 AM
#9
Senior Member
Join Date: Dec 2001
Location: Coquitlam, BC
Posts: 896
Likes: 0
Received 1 Like
on
1 Post
Car: 86\92 Mutant
Engine: 355CI 430HP
Transmission: T-5 with mods
Axle/Gears: 7.625", Eaton Posi, 3.73
Originally posted by 94-6spd
I just did some investigating over at camaros.net and discovered that the man that wrote this article is a mechanical engineer and has worked for GM and Chrysler for 37 years. The last 6 years was the Viper plant manager. I plan to email him on this subject. I want to dig deeper in this subject. This is very controversial.
I just did some investigating over at camaros.net and discovered that the man that wrote this article is a mechanical engineer and has worked for GM and Chrysler for 37 years. The last 6 years was the Viper plant manager. I plan to email him on this subject. I want to dig deeper in this subject. This is very controversial.
Pre 1972-73 it was very common for some manufacturers to run manifold vacuum advacnce. I worked for Datsun at the time and they were all set up this way from the Factory. So were Chryslers and Honda's pre- 1974. It all matters how you, or the mfg, tune the curve.
I have read that with a manual transmission that you can get detonation when you let of slightly and get back in on a shift when running wot? Is there any truth to that? I know that according to the manual that cam w/ my demon dp, it said to connect to ported vacuum.
Ben
Ben
I debated some of these beliefs in this post:
https://www.thirdgen.org/techbb2/sho...hreadid=202171
Debate on Ported vs Manifold vaccuum starts about half way down.
Last edited by Chickenman35; 02-17-2004 at 03:22 AM.
02-19-2004, 09:00 AM
#10
Member
Thread Starter
Join Date: Aug 2003
Location: SB, IN
Posts: 193
Likes: 0
Received 0 Likes
on
0 Posts
Yes, very few people realise that full manifold vacuum was the norm before the mid-seventies. Ported vacuum was entirely emissions driven
Hey chickenman35 i was born 12/70. HAHA ...that's why i can't seem to remember this. Seriously the only way i've seen it hooked up was to ported vac, till i started reading your threads and doing comparison testing.
Hey chickenman35 i was born 12/70. HAHA ...that's why i can't seem to remember this. Seriously the only way i've seen it hooked up was to ported vac, till i started reading your threads and doing comparison testing.
02-19-2004, 10:27 AM
#11
Supreme Member
The vacuum advance canisters used in GM cars through the emissions era were not "restricted" in any way that caused them to move or react slowly. I'm sure it takes some time for the vacuum to change through the length of vacuum line from the carb to the distributor but the canisters and the ports on the carb were free-flowing. i.e. the vacuum canister could react close to instantaneously. I'm sure it's possible that SOME setup existed somewhere on something to move it slowly, but the vast majority of setups (everything I have ever worked on) was not set up that way.
Put a piece of hose on a vacuum canister, if you have one laying around. Draw vacuum on it using your mouth and then quickly release the vacuum. You will actually hear the vacuum advance canister SLAMMING back to it's fully-retarded position inside the distributor. If you've got it out of the distributor you can actually see it do this.
Put a piece of hose on a vacuum canister, if you have one laying around. Draw vacuum on it using your mouth and then quickly release the vacuum. You will actually hear the vacuum advance canister SLAMMING back to it's fully-retarded position inside the distributor. If you've got it out of the distributor you can actually see it do this.
02-23-2004, 10:15 AM
#12
Supreme Member
Join Date: Jan 2001
Location: Rock Hill, SC
Posts: 1,411
Likes: 0
Received 3 Likes
on
3 Posts
Car: 1999 Pontiac T/A Firehawk
Engine: ***'s Engine
Transmission: T56
It's certainly nice to see a few more people on here that'll advocate at least trying manifold advance. Every non-CC setup I've ever used has preferred it for best performance and best driveability (emissions may be another matter).
02-24-2004, 10:21 PM
#13
Senior Member
Join Date: May 2002
Location: Tucson AZ where the 3k ft of elevation kills your time
Posts: 783
Likes: 0
Received 0 Likes
on
0 Posts
Car: 89 camaro rs
Engine: 383 .06 over
Transmission: th350
Axle/Gears: 3.73 10 bolt
Last one, and this only applies to non-stock performance applications. Running the vacuum advance to a ported source IS a valid thing to do in some circumstances. A motor with relatively high compression and a small cam will sometimes "diesel" or "run on" if you hook up the vacuum advance up to full manifold vacuum. It keeps exhaust temps cool, but it increases combustion chamber temps. That can cause stuff to start to get hot inside the combustion chamber at idle and when you turn the key off it "glow plugs" and diesels on for a bit. With a big cam (little vacuum at idle) running full manifold vacuum to the distributor I definitely agree with you- it can cause a problem like you describe where the vacuum advance kinda "floats" in the middle of it's range and the idle gets unstable and often "rolls" up and down. Switching to ported vacuum can often cure this, although it's may or may not be the best way to fix the problem.
but i have one questions perhaps i am not the only one who doesn't know this but i feel like it would help clear up some or the misunderstandings i have had.
i am not sure what you guys mean buy
1. manifold vacume
2. ported vacume
3. centrifugal advance
02-25-2004, 04:15 PM
#14
Supreme Member
Join Date: Jan 2003
Location: Woodbury, NJ
Posts: 1,059
Likes: 0
Received 0 Likes
on
0 Posts
Car: 87' Iroc
Engine: 350
Transmission: 700R4
That post was GREAT....lots of good knowledge in here.
I'm running full manifold vacuum at idle now, I still can't get my car to run right but I'm definately getting that "slight" idle RPM fluctuation. Im have to pick up a vacuum adanve can.
I've got almost 11:1 CR with the XE268H, which has some overlap. I was told I need to run like 16* intial timing, around 32* total @ WOT. My problem is part throttle. I can' t seem to get more then 8MPG no matter how much I mess with the timing.
The 71000 Series Accel Distributor I have said it had 10* vacuum advance @ 10" WC....and it is adjustable....but I don't really know what the adjustment does, how much total vacuum advance it gives, and how much I should try and run to get better gas milage.
I'm running full manifold vacuum at idle now, I still can't get my car to run right but I'm definately getting that "slight" idle RPM fluctuation. Im have to pick up a vacuum adanve can.
I've got almost 11:1 CR with the XE268H, which has some overlap. I was told I need to run like 16* intial timing, around 32* total @ WOT. My problem is part throttle. I can' t seem to get more then 8MPG no matter how much I mess with the timing.
The 71000 Series Accel Distributor I have said it had 10* vacuum advance @ 10" WC....and it is adjustable....but I don't really know what the adjustment does, how much total vacuum advance it gives, and how much I should try and run to get better gas milage.
02-25-2004, 05:18 PM
#15
Supreme Member
Join Date: Sep 2001
Location: Decatur, Illinois
Posts: 2,906
Likes: 0
Received 0 Likes
on
0 Posts
What is your exact setup? I had the xe268 in a 9.5ish to one 350. I had hooker long tubes, duals, performer rpm ect... I got about 15 mpg w/ the q jet and the t56 w/ 3.45's. When I had the auto and qjet it was only about 13. When I put the demon 750 dp on it dropped back down to around 13 w/ the t56, but it was completely untuned other than idle mixture screws. I pulled about 15" of vacuum at idle.
I had the total mechanical and base set to 36. I had about 14 initial and 22 mechanical. I had a stock vacuum can so I just had whatever stock vac advance was. I was running ported vacuum(just because that is what the book that came w/ my demon said to do). There is a large amount of difference between our compression though.
Dont know if any of that will help.
Ben
I had the total mechanical and base set to 36. I had about 14 initial and 22 mechanical. I had a stock vacuum can so I just had whatever stock vac advance was. I was running ported vacuum(just because that is what the book that came w/ my demon said to do). There is a large amount of difference between our compression though.
Dont know if any of that will help.
Ben
02-25-2004, 07:32 PM
#16
Supreme Member
Join Date: Jan 2003
Location: Woodbury, NJ
Posts: 1,059
Likes: 0
Received 0 Likes
on
0 Posts
Car: 87' Iroc
Engine: 350
Transmission: 700R4
Yea, the compression is definately changing things for us at part throttle.
My buddy built his 350 same times as me, same cam, pretty much the same build (diff heads/intake) but with 9.7:1 CR. He gets like 13MPG with minimal tuning and 373 on a 200r4.
I just filled my tank up, I drove 86 miles and the tank took 10.2 gallons. I'm a little confused.
I was getting 7.7MPG and pinging, so i retarded the engine like 4-6 degrees. I then got 9.1MPG with no pinging, so I advanced the engine 1 degree, my MPG went down to about 8.6 (tonights fill up verfied this). The thing is, I advanced the engine back up 1 degree, and lost MPG....but the engine still wasn't pinging. I guess I'll retard it 2-3 degrees and see if it goes back up. I just though I wanted as much advance as I could get w/o pinging, but even though I'm not pinging, advacning the engine more dropped my MPG. It still seems like I'm trying way too hard to get 10MPG, which isn't that impressive....I mean...does compression affect MPG that mucH? 10.7:1 isn't that ridiculous....
My buddy built his 350 same times as me, same cam, pretty much the same build (diff heads/intake) but with 9.7:1 CR. He gets like 13MPG with minimal tuning and 373 on a 200r4.
I just filled my tank up, I drove 86 miles and the tank took 10.2 gallons. I'm a little confused.
I was getting 7.7MPG and pinging, so i retarded the engine like 4-6 degrees. I then got 9.1MPG with no pinging, so I advanced the engine 1 degree, my MPG went down to about 8.6 (tonights fill up verfied this). The thing is, I advanced the engine back up 1 degree, and lost MPG....but the engine still wasn't pinging. I guess I'll retard it 2-3 degrees and see if it goes back up. I just though I wanted as much advance as I could get w/o pinging, but even though I'm not pinging, advacning the engine more dropped my MPG. It still seems like I'm trying way too hard to get 10MPG, which isn't that impressive....I mean...does compression affect MPG that mucH? 10.7:1 isn't that ridiculous....
02-25-2004, 11:44 PM
#17
Supreme Member
Join Date: Sep 2001
Location: Decatur, Illinois
Posts: 2,906
Likes: 0
Received 0 Likes
on
0 Posts
What heads do you have? One thing that you have to realize, is that w/ the agressive ramps of the extreem energy cams you get real high dynamic compression without a high static compression. I dont remember the exact results, but I tried figuring dynamic compression w/ the xe268. It didnt take much static to get to the upper limits of the dynamic compression you should run on pump gas. I know that if you talk to comp cams they will tell you that 9.0 is optimal w/ the 268, but they say you can run up to 9.5.
I assume you have aluminum heads right. You should be able to run about a point higher so inbetween 10.0 and 10.5 would probably be reccomended by comp. Your 10.7 should be doable, as long as you are carfull w/ the timing as it seems you may be figuring out. How much base and mechanical timing are you running?
As far as gaining mpg by lowering the timing, it is possible you were getting slight detonation even if you werent hearing pinging. Also, detonation will ruin plugs so you may have to change plugs since you got that problem fixed before you will see the mileage you should.
Ben
I assume you have aluminum heads right. You should be able to run about a point higher so inbetween 10.0 and 10.5 would probably be reccomended by comp. Your 10.7 should be doable, as long as you are carfull w/ the timing as it seems you may be figuring out. How much base and mechanical timing are you running?
As far as gaining mpg by lowering the timing, it is possible you were getting slight detonation even if you werent hearing pinging. Also, detonation will ruin plugs so you may have to change plugs since you got that problem fixed before you will see the mileage you should.
Ben
02-27-2004, 05:17 PM
#18
Supreme Member
Join Date: Jan 2003
Location: Woodbury, NJ
Posts: 1,059
Likes: 0
Received 0 Likes
on
0 Posts
Car: 87' Iroc
Engine: 350
Transmission: 700R4
Iron protopline vortec heads.....everyone I talked to said I could run 10.7:1 on iron heads....its too late now, the engine is in my car.
I'm just wondering what I should do as far as vacuum advance.
I'm running about 32* timing @ WOT. That seems okay, makes power and doesn't ping, it's my vacuum advance timing I can't get to work with me.
I have an accel 71000 series probillet distributor, its says it's adjustable. I've been told that the adjustment only changes how much vacuum it takes to reach full advance, that I have no control over how much advance I actually get, only when it comes in. My manual says I get 10* @ 10", but someone told me that is distributor degrees, which is half the engine speed. So at 20"wc I get 20* from my vacuum advance. I can't change the 20* I get, I can only control when it comes in.
So How much vacuum advance do I want? How can I control EXACTLY how much vacuum advance I get? I get 13" WC vacuum at idle @ 650-700RPM initial w/o vacuum advance plugged in. I'm pretty happy with my WOT timing, 8* initial, with 24 mechanical. But im getting 8 MPG and I can't seem to get that happy spot with vacuum advance. My car doesn't ping...but i have a feeling i have too much vacuum advance still
I'm just wondering what I should do as far as vacuum advance.
I'm running about 32* timing @ WOT. That seems okay, makes power and doesn't ping, it's my vacuum advance timing I can't get to work with me.
I have an accel 71000 series probillet distributor, its says it's adjustable. I've been told that the adjustment only changes how much vacuum it takes to reach full advance, that I have no control over how much advance I actually get, only when it comes in. My manual says I get 10* @ 10", but someone told me that is distributor degrees, which is half the engine speed. So at 20"wc I get 20* from my vacuum advance. I can't change the 20* I get, I can only control when it comes in.
So How much vacuum advance do I want? How can I control EXACTLY how much vacuum advance I get? I get 13" WC vacuum at idle @ 650-700RPM initial w/o vacuum advance plugged in. I'm pretty happy with my WOT timing, 8* initial, with 24 mechanical. But im getting 8 MPG and I can't seem to get that happy spot with vacuum advance. My car doesn't ping...but i have a feeling i have too much vacuum advance still
02-28-2004, 04:42 PM
#19
Supreme Member
Manifold vacuum is the vacuum that exists UNDER the carb's throtle plates. There is vacuum present any time the engine is running except at WOT where it drops close to zero.
Ported vacuum is taken from a source JUST above the throttle plates. At idle the port sees no vacuum becuase it is not exposed to the vacuum that exists in the intake, below the throttle plates. Open the throttle slightly and the the throttle plate passes over the port and exposes it to the manifold vacuum that exists in the intake. From that point forward it supplies vacuum almost exactly the same as a full manifold vacuum port.
Centrifugal advance is supplied by way of springs and weights inside the distributor that are attached to an advance mechanism. As RPMs rise the centrifugal advance supplies more and more advance up to whatever it's limited to supply. Usually around 20-24*, depending on the distributor and how it's set up. It usually "maxes out" around 3500-4500 in a stock factory distributor. Aftermarket distributors or ones that have be "recurved" for a performance application typically bring in full advance more quickly- by 2500-3200 RPMs or thereabouts.
Vacuum advance works independently from from centrifugal advance. The advance supplied by one is ADDED TO the advance supplied by the other. Under certain conditions you can have as much as 45-55* of actual advance when the centrifugal and vacuum advance are both fully "in." At WOT it's more typical to max out at about 30-36* total advance since the vacuum advance supplies zero (becuase there is almost no manifold vacuum to activate it).
Ported vacuum is taken from a source JUST above the throttle plates. At idle the port sees no vacuum becuase it is not exposed to the vacuum that exists in the intake, below the throttle plates. Open the throttle slightly and the the throttle plate passes over the port and exposes it to the manifold vacuum that exists in the intake. From that point forward it supplies vacuum almost exactly the same as a full manifold vacuum port.
Centrifugal advance is supplied by way of springs and weights inside the distributor that are attached to an advance mechanism. As RPMs rise the centrifugal advance supplies more and more advance up to whatever it's limited to supply. Usually around 20-24*, depending on the distributor and how it's set up. It usually "maxes out" around 3500-4500 in a stock factory distributor. Aftermarket distributors or ones that have be "recurved" for a performance application typically bring in full advance more quickly- by 2500-3200 RPMs or thereabouts.
Vacuum advance works independently from from centrifugal advance. The advance supplied by one is ADDED TO the advance supplied by the other. Under certain conditions you can have as much as 45-55* of actual advance when the centrifugal and vacuum advance are both fully "in." At WOT it's more typical to max out at about 30-36* total advance since the vacuum advance supplies zero (becuase there is almost no manifold vacuum to activate it).
07-15-2004, 02:35 PM
#20
Supreme Member
Join Date: Jan 2002
Location: Cinnaminson, NJ
Posts: 1,537
Likes: 0
Received 0 Likes
on
0 Posts
Car: 89 Formula
Engine: Carbed 5.7
Transmission: TKO-600
hey guys i am bringing this post back from the dead instead of starting a new one. I read it and found it interesting, so today I go outside check my timing and its a 12 degrees, i hook the vacume advance back up to ported and its 12 degrees, then I hook it to straight manifold vacume and it idles at 1200 so i knock it back down to 800 and check the timing and its at liek 35 degrees at idle. Now isn't that way to much to be safe?? Any input would be helpful, also I will post my sig so you can see what motor I am runnig if it makes a difference, I run 12 degrees initial and 36 degrees total. thanks
07-15-2004, 02:59 PM
#21
Supreme Member
Join Date: Jun 2001
Location: Charleston, SC
Posts: 9,550
Likes: 0
Received 2 Likes
on
2 Posts
Car: 91 Camaro Vert
Engine: 02 LS1, HX40
Transmission: 2002 LS1 M6
heh, its funny that you brought this to the top today.. i just picked up a crane recurve kit along with my jetkit today...
http://store.summitracing.com/partde...N=120%20300419 (same thing as that, but i bought it locally)
http://store.summitracing.com/partde...N=120%20300419 (same thing as that, but i bought it locally)
07-15-2004, 06:16 PM
#22
Senior Member
Join Date: Dec 2001
Location: Coquitlam, BC
Posts: 896
Likes: 0
Received 1 Like
on
1 Post
Car: 86\92 Mutant
Engine: 355CI 430HP
Transmission: T-5 with mods
Axle/Gears: 7.625", Eaton Posi, 3.73
Originally posted by 89formula#1
hey guys i am bringing this post back from the dead instead of starting a new one. I read it and found it interesting, so today I go outside check my timing and its a 12 degrees, i hook the vacume advance back up to ported and its 12 degrees, then I hook it to straight manifold vacume and it idles at 1200 so i knock it back down to 800 and check the timing and its at liek 35 degrees at idle. Now isn't that way to much to be safe?? Any input would be helpful, also I will post my sig so you can see what motor I am runnig if it makes a difference, I run 12 degrees initial and 36 degrees total. thanks
hey guys i am bringing this post back from the dead instead of starting a new one. I read it and found it interesting, so today I go outside check my timing and its a 12 degrees, i hook the vacume advance back up to ported and its 12 degrees, then I hook it to straight manifold vacume and it idles at 1200 so i knock it back down to 800 and check the timing and its at liek 35 degrees at idle. Now isn't that way to much to be safe?? Any input would be helpful, also I will post my sig so you can see what motor I am runnig if it makes a difference, I run 12 degrees initial and 36 degrees total. thanks
The same at cruise. The engine is at light load and the engine is throttled back. Charge density is decreased, it takes longer for the flame front to fully combust with a less dense charge ( Do not confuse "less dense" with being lean. Totally different principle ). Add in a leaner mixture at cruise ( light load ) and engines can actually require up to 40 to 45 degrees of advacnce ( at cruise ).
However, 35 degrees advance ( static and vacuum ) is a bit much though. Is it actually 35 degrees? Recheck it. You should be aiming for between 27 to 30 degrees combined static and vacuum advance at idle, depending on fuel quality
Note: you should not use a stock vacuum can when using manifold vacuum and a recurved distributor. This has been covered before.
HEI cans have a lot of advance, typically 20 degrees. Get a Crane can which can be adjusted from 5 degrees of advance to 15 degrees of advance. This is adjusted with the adjustable cam. The " rate" is adjusted with the diaphram spring screw accessed through the vacuum tube.
Check your initial timing with a timing light, vacuum advance disconnected. Make sure that no mechanical advance starts before 1,000 rpm. Set your static at 12 degrees at idle, then plug in vacuum advance and reset idle speed. You should now have 12 + 15 degrees for a total of 27 degrees. Run the diapharm tension as light as possible without getting " pinging " under load .
Edit: I see you are running " Vortec heads " These have a much smaller combustion chamber and are of the fast burn type. They require roughly 5 degrees less total advance than " Regular " heads ( 64cc and larger ). Drop total to 32 max mechanical ( vacuum advance disconnected ).
Aim for about 25 degrees combined advance ( static and vacuum at idle) 10 static plus 15 advance. You might even try limiting the vacuum can to 10 degrees. Thus 20 degrees combined at idle. Vortec heads don't need as much advance as noted..
Last edited by Chickenman35; 07-15-2004 at 06:32 PM.
07-17-2004, 06:47 PM
#23
Supreme Member
Like Checkenman said, and I posted originally further up, stock vacuum advance cans typically give a LOT of vacuum advance. 22-26* is not at all uncommon. For a performance application that's going to have the initial advance up in the 12-18* range (vs. 4-6* for most stock motors) that's way too much vacuum advance. For a performance application 12* of vacuum advance is plenty. If you run a functional EGR system 16* is a good starting point.
Like Mr. Dude posted, the Crane advance kit is one way of limiting the vacuum advance. If you use the Crane kit and limit the vacuum advance as far as it is capable of you will be left with about 12* of vacuum advance.
Like Mr. Dude posted, the Crane advance kit is one way of limiting the vacuum advance. If you use the Crane kit and limit the vacuum advance as far as it is capable of you will be left with about 12* of vacuum advance.
07-21-2004, 01:40 PM
#24
Supreme Member
Join Date: Jul 2001
Location: Pueblo Co
Posts: 3,974
Likes: 0
Received 0 Likes
on
0 Posts
Car: 1989 C4
Engine: L98
Transmission: 700r4
Axle/Gears: 307
Havent visited the carb board in quite awhile but since this is back up near the top.
I did try full manifold vacuum instead of ported. I noticed no gain and was more prone to stall from a stop and had to use a bit more gas to take off. So if there was a gain it was lost in take off. Ported seems to work better for the bird and the truck, the Malibu is still attached to full (OEM) but I should have ran ported and full when I had the engine on the dyno. Probably wouldent have seen anything different but still would have been interesting.
I did try full manifold vacuum instead of ported. I noticed no gain and was more prone to stall from a stop and had to use a bit more gas to take off. So if there was a gain it was lost in take off. Ported seems to work better for the bird and the truck, the Malibu is still attached to full (OEM) but I should have ran ported and full when I had the engine on the dyno. Probably wouldent have seen anything different but still would have been interesting.
07-21-2004, 03:41 PM
#25
Moderator
iTrader: (14)
Join Date: Mar 2000
Location: Littleton, CO USA
Posts: 43,169
Likes: 0
Received 36 Likes
on
34 Posts
Car: 82 Berlinetta/57 Bel Air
Engine: LS1/LQ4
Transmission: 4L60E/4L80E
Axle/Gears: 12B-3.73/9"-3.89
You shouldn't see any difference on the dyno unless you're doing part-throttle runs. The difference would probably be in BSFC, rather than raw power output.
The Proform main body isn't drilled for the timed port (haven't looked into whether it is even possible). Switching to manifold vacuum with a GMPP HEI distributor (using the vacuum can that came on it), it seems to like that better than it did with the Holley ported vacuum.
The Proform main body isn't drilled for the timed port (haven't looked into whether it is even possible). Switching to manifold vacuum with a GMPP HEI distributor (using the vacuum can that came on it), it seems to like that better than it did with the Holley ported vacuum.
07-23-2004, 08:25 PM
#26
Supreme Member
Join Date: Jul 2001
Location: Pueblo Co
Posts: 3,974
Likes: 0
Received 0 Likes
on
0 Posts
Car: 1989 C4
Engine: L98
Transmission: 700r4
Axle/Gears: 307
Originally posted by five7kid
You shouldn't see any difference on the dyno unless you're doing part-throttle runs. The difference would probably be in BSFC, rather than raw power output.
You shouldn't see any difference on the dyno unless you're doing part-throttle runs. The difference would probably be in BSFC, rather than raw power output.
08-16-2004, 04:46 PM
#27
Senior Member
Join Date: Nov 1999
Location: Omaha, NE
Posts: 803
Likes: 0
Received 0 Likes
on
0 Posts
Hey guys got a question for you. My ZZ4 came with a 20 degree can that is all in at 7" of HG. On really hot days my engine will detonate or "ping" under light throttle. Engine temp of 220 degrees F or so. I know, a little hot but not unreasonable. I tried a MR.Gasket (Mr. Crap in my opinion) Adj. Vacuum Canister. This canister only changed the RATE of 30 degrees advance. HOLY Crap! 30 Degrees in just the canister, way too high! The only reason it stopped my detonation was that timing did not even come in till 5" of HG. I have ordered a Pertronix Adj. Canister that claims to be adjustable for both rate and total amount of timing between 0 and 22 degrees. Although, I don't know at what vacuum level the diaphram starts operating. Hoping the instructions show more detail. I also though of using the Echlin 1810 Can with 15 degrees of adv. at 8" of HG because it is pretty much the ZZ4 can with 5 less degrees. My question is do you think I should leave the ZZ4 can with 20 degrees in and just get a Be Cool radiator to get temps down, use the Echlin Can or the Pertronix can? I would have thought GM knew what they were doing but what can I say, it "pings" at times. I really need to see the Pertronix rate charts to ensure full advance at idle. Anybody know these rate values? Any input is appreciated.
08-16-2004, 06:39 PM
#28
Member
Join Date: Jul 1999
Location: Tucson, AZ
Posts: 272
Likes: 0
Received 0 Likes
on
0 Posts
Car: '82 Camaro
Engine: 383
Transmission: TH-350
Originally posted by CamaroMike
I tried a MR.Gasket (Mr. Crap in my opinion) Adj. Vacuum Canister. This canister only changed the RATE of 30 degrees advance. HOLY Crap! 30 Degrees in just the canister, way too high!
I tried a MR.Gasket (Mr. Crap in my opinion) Adj. Vacuum Canister. This canister only changed the RATE of 30 degrees advance. HOLY Crap! 30 Degrees in just the canister, way too high!
HTH
Thread
Thread Starter
Forum
Replies
Last Post
