Confuzed1

iTrader: (4)

I looked uo a neat article on how to do this here:
http://www.fordmuscle.com/forums/tec...d-engines.html

I understand all the reasoning behind it, but I'm kinds skiddish about drilling into my 600 dollar Pro-Systems 750 DP carb. So I call up the Holley tech line to see if they offer a baseplate part# I can just buy and bolt it on to my existing carb.

I was suprised when they said they don't sell the baseplate!! He did send me a PDF file of how to modify my exisiting one. This is unlike the one on Fordmuscle - and the way Holley says to do this, I'll need to use a mechanical advance distributor afterwards. I'd kinda like to keep my vacuum advance. Which one is correct, or are both correct?? Will I need to switch to a mechanical distributor if I modify my baseplate doing it either way? This is confusing me....

Apeiron

It says right on that sheet you can keep your vacuum advance if you connect it to the manifold instead of the carb.

Confuzed1

iTrader: (4)

I don't see that on the sheet. Where do you see that?

It seems to me, they want me to convert my timed vacuum port that's on the carb into a power valve vacuum port, so the timed port coming from the carb which my vacuum advance should hook up to is no longer available.....

The only reason for doing any of this is for draw through roots style supercharger applications. (sorry, forgot to mention that) - so the power valve will see true intake manifold vacuum when I tromp on it to allow the power valve to open and augment fuel - just like it does on a NA car with the carb sitting on top of the intake.

Will my vacuum advance work properly if hooked up to intake manifold vacuum at idle? Could it cause surging?

Street Lethal

iTrader: (16)

The vacuum advance is supposed to connect to a full manifold vacuum port on the carb....

Apeiron

You're right, it doesn't say that, I read it wrong.

Yes, that's what they're doing, turning the vacuum port into a power valve port.

Do you have a reason for not wanting vacuum advance at idle?

Confuzed1

iTrader: (4)

Street Lethal - Ahhhh, I was editing when you responded! lol
-I know people choose to hook up their advance to full manifold vacuum. But every carb instruction manual I have says the vacuum advance should be hooked up to the timed vacuum port though...The timed port must be there for a reason....what would it be there for?

Apeiron - After calling a couple of dyno/tuning shops, some say I shouldn't run a vacuum advance at all!

-I know I'll most likely end up limiting total advance to 28-30 degrees as it is to prevent from detonating. I'm new at the supercharger thing though. Just concerned that running full manifold vacuum might cause other issues like surging at idle, etc. but I'm not opposed to it. I just always thought people ran them to manifold vacuum when using long duration cams and low manifold vacuum at idle. I have 14-16" of vacuum right now at 900 RPM's and my initial is at 16 BTDC. I've limited my total to 30 degrees all in by 2800 RPM's using one of the bushings that came with the distribtor.

Street Lethal

iTrader: (16)

Remember, when setting your base timing, whether fuel injection (EST) or carb, the advance needs to be disconnected. This immediatelly tells you that timing is being advanced at idle. Also, at WOT, there is 0 vacuum, so a timed and/or delayed port becomes irrelevant in reference to advance. Some people choose to lock they're timing, and I believe they might use that port for some reason or another, but timed vacuum ports are usually plugged....

Apeiron

Surging at idle is rarely a problem, and only happens in some cases where the vacuum at idle isn't enough to give fully advance, like when you're using a stock canister with its long, lazy advance rate.

Confuzed1

iTrader: (4)

Street Lethal: I'm not arguing with you, but just so you know I'm not making this up or anything.... I just looked back over my instructions for both my Holley carb as well as my distributor so I cut and pasted the following from them:
From Holley - From Accel - -But I know most guys around here just connect it to manifold vacuum. That way, when big cams are used you can set your initial advance low enough to where your starter will stll turn the engine over without "kicking back" (initial too advanced) and right after the engine starts manifold vacuum and timing will increase which will allow a big cammed engine to idle more smoothly. But that was never the intended reason for vacuum advance, at least on most stock cars. They tell you to ensure the hose is disconnected to ensure nothing could possibly (but not likely) interfere when setting your initial advance.
-But all that being said, stock engines (I assume) most likely use a much more conservative mechanical timing curve, and could use the additional 10 degrees of advance at light load to get more power....and anyone please correct me if my thinking is wrong on this, I'd really like to know!

Apeiron - I see what you're saying. And if it won't surge at idle, I may have to experiment....It may turn out that I really DON'T need vacuum advance at all with a blower. But everyone agrees that referencing the PV makes a blower more streetable. So not having a "timed port" on this carb might not be an issue at all....

Apeiron

In the dark ages, engines all used manifold vacuum for the advance. Timed vacuum was introduced for emissions reasons, with an accompanying change in the rate and amount of vacuum advance.

Street Lethal

iTrader: (16)

I know your not making this up, but Apeiron hit the nail right on the head. It was intended for emissions purposes. Here's a snippet from an old, but good, article about that very thing....

Enjoy....;


TIMING AND VACUUM ADVANCE 101

"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"....

- Corvette Enthusiast October/November '03

Confuzed1

iTrader: (4)

Good article Street Lethal!!

Yeah, I'm from the "ported spark" generation. When I was 16, only nice cars out there were the 150 HP smog era 350's. And I've actually dealt with cars that had a tuning sticker on the hood that said to set the timing at 2 degrees ATDC!! (I think it was an old buddy's 455 Pontiac)

So of course, the overthinker that I am - I need to ask questions! It does appear I wasn't far off at all as I read the last paragraph on your article. On a full manifold vacuum advance setup. It's allowing added advance at idle on a modified engine, or stock engine for that matter - but the vacuum can setting is higher on the stock engine advance before it "deploys". So how do I set up my vacuum advance to work well?? I'll throw my situation in:

-The engine seems to idle relatively smooth at 16-18 degrees of initial advance. (turning dist.) The timing mark is not to be trusted as accurate. The tab is most likely off. I use it as an indicator only.
-Getting 13-15" of vacuum at 900 RPM's (checked w/gage)
-Mechanical advance on this dist. will yield 22 degrees of advance, less if I use bushings to limit it. I have the 16 degree bushing installed. (can limit to 16,12,8 using bushings)
-The existing vacuum can on my dist. is adjustable. Presently set to give 10 degrees advance (crank) at 10" of vacuum.
-Bear in mind, this is a supercharged application with maybe 4-5 lbs. of boost, so I'd like to start out conservative by limiting total advance at WOT to 28-30 degrees total at WOT.

What's a good starting point??

Damon

Here's what the timing curve on my roots-blown small block looks like right now:

Initial: 8*
Max centrfugugal: 20* @ 2800
Total timing: 28* @ 2800

Vacuum advance source: full manifold vacuum port @ carb
Max vacuum advance: 16*
Range: starts at 6" vacuum, ends at 12"

The important thing is WHY I use that curve. Obviously, I don't want to be running a lot of timing wide open- I use 28* as a "safe" setting. It might take a little more, but I've blown up too many engines by pushing the envelope.

With the typical 20* centrifugal advance mechanism in my HEI that leaves me at 8* initial- no real way around that short of modifying the centrifugal advance mechanism (which I don't feel like doing). So that's where I work from on the centrifugal side. The rest of the tuning is in the vacuum advance.

Given the LOW 8:1 compression ratio of this engine it will take TONS of advance in the low RPMs, light throttle. And with only 8* initial advance..... well, the choice of running the vacuum advance line to a full vacuum port on the carb was a no-brainer. It bumps the idle advance up over 20* for a real smooth and steady idle and the keeps that same high level of advance as you step off idle- it drives really sharp without having to barely breathe on the gas pedal pulling away from a stop sign. Of course, when I step through on it, that vacuum advance goes away and I'm left with the "blower friendly" modest advance curve. I also made sure that the vacuum advance was down to ZERO before the lower intake manifold stepped into boost. That's why there's no vacuum advance below 6"- that's just above the vacuum level at the carb that is present when the vacuum/boost gague in the lower manifold snuggles up close to the zero line (on MY engine with MY combination of parts! May not be exactly the same on yours).

This setup WON'T WORK if you have a big lumpy cam. Mine is very mild so I still have plenty of vacuum at idle and light throttle to work a vacuum advance canister. If it's a big lumpy cam you're probably better off with locked timing. Although I'll share that I'm currently experimenting with locked + a small amount of vacuum advance hooked to a ported vacuum port on the carb. But this experiment is on a different engine.

Theorizing about another way to do this..... I suppose you could run your vacuum advance line to a port UNDER the blower- where it will see both boost and vacuum. Frankly, I might try that myself some day. I didn't do it yet becuase I had concerns about what it would do to the diaphragm in a vacuum advance can that was never designed to hold boost, only vacuum. But it would certainly simplify the vacuum advance tuning! You wouldn't need to be nearly as picky about the advance rate in the canister then. You could set it real low (like beginning to advance at only 2" of vacuum) and it would still be back to zero before you step into boost.

That's the main point of all this vacuum advance tuning- you don't want ANY vacuum advance at all if you're engine is getting into boost. But you DO want vacuum advance if you street drive the car. Vacuum advance STILL DOES ALL THE GOOD STUFF ON A BLOWER ENGINE THAT IT DOES ON A N/A ENGINE. You just have to be more careful where you hook it up and how your tune it.

Apeiron

That's Lars Grimsrud... always read anything he has to say.

Confuzed1

iTrader: (4)

Apeiron - Hey, if I knew it all, there'd be no reason to ever post here! It helps!! - and I admitted long ago I don't know alot about everything...just a little...lol

Damon - Thanks for like, what? the 30th time? You've been a great help through this whole build!

So I wasn't far off when I said I need a total of 28-30 degrees of mechanical advance in by 2800 RPM's then. I think I'm advanced too much now since I'm running 18 initial plus 16 mechanical from there (=34 degrees total) - I heard no detonation - but I didn't open it up all the way either nor have I hooked up the vacuum can - yet. But remember, I don't trust my timing tab as accurate, so I'll have to tune this by ear a bit I think.

I'm not running a radical cam - at least I don't think so....224/236 dur @.050, 504/520 lift at 113 LSA. But I'll have problems trying to adjust the carb at just 8 degfrees BTDC to idle at all,,,,do I make my carb A/F adjustments with 8 initial and say, 10 degrees added via the vacuum advance hooked up? I'm sure I gan get a smooth idle at 18-20 degrees of advance. I think that's the only way I could get the carb adjusted. Does that sound right to you?

But I'll need to see what kind of vacuum I have at the carb base - I was using the port below the blower before to determine what vacuum I had.

The car will be 90% street driven, so I want it set up as if it came this way from the factory if possible. I'm tired of killing every bug within a block of me every time I run the car. It gets old and makes me and the car smell like a gas station. That's why I got rid of running big cams that idle like crap NA, and went with a blower. My thought process may be a bit warped though...

Damon

Yes, that's right. Mine doesn't idle so well at only 8* either. But with the vacuum advance on top of that pushing it above 20*, she's very smooth.

Set the base timing where you need it, then plug in the vacuum advance. Assuming you have a reasonable amount of engine vacuum it will immediately pull the vacuum advance canister in fully. You'll be at a stable amount of timing at idle now. THEN go set your idle speed and mixture for best idle like you normally would.

Confuzed1

iTrader: (4)

OK, tried what you were talking about Damon -

I adjusted the carb where it was holding 14-15 " of vacuum at idle at the carb base - it was idling high, but I then backed the initial timing down to around 8 degrees, at the idle went to around 700 RPM's - not bad I thought. Then I plugged in my vacuum advance, and the idle shot way up!!
I took my timing light to it, and it advanced it all the way up to 38 degrees advanced!! - Do I just need to adjust my vacuum canister down with the allen wrench or what? I don't believe it was "factory set to advance 10 degrees at 10" of vacuum". I expected it to advance, to around 18-20 degrees -but not near that much! So do I adjust the can to where it only gives me 10 degrees or so of advance at 14-15" of vacuum? Where should I go from here?

Apeiron

That's pretty normal if you're using a stock emissions-era vacuum advance canister, they have a lot of advance. For the adjustable one, it may be hard to tell if all that advance is from the vacuum canister, or if there's extra centrifugal advance from the increase in speed as well.

Don't use the advance to control the idle speed.

Confuzed1

iTrader: (4)

I have an Accel Street billet distributor - brand new. It has an adjustable vacuum advance canister. I should change it out for the crane advance? What do you mean by "Don't use the advance to control the idle speed?" - If the vacuum can is advancing the timing at idle, (which it will when plugging it into full manifold vacuum) the speed will increase on it's own, there's no way around that.

I guess I'll try decreasing the advance using the adjustable vacuum can I have...

Apeiron

I forgot when I typed it that you had an adjustable advance. Can you limit the amount of your vacuum advance?

Don't use the advance to control the idle speed means don't retard it just to bring the idle speed back down. Adjust the idle speed on the carb after you've got the timing set up.

Confuzed1

iTrader: (4)

Ahhh-OK. I set my initial at 22 advanced, because it's happy there. I set the idle at around 800. THEN I retarded the timing to around 8 -10 degrees and plugged in the canister - I wanted it to kick the timing back up to the original 20-22 degrees or so, then I would have readjusted the idle.

Instead, it kicked the timing way up to 38 degrees advanced on me.

Apeiron

Don't get too hung up on the vacuum advance, it's only for cruising and part-throttle acceleration. Get the centrifugal advance set up so you've got no pinging at WOT, and then experiment with the vacuum settings so that it doesn't ping under part-throttle.

blown_zed

Holy cr@p!! That last paragraph is the best advice I've read yet!!

Blower engines like a lot of initial -that can mean anything from 6* up to 20*. They don't want much more than 32* ever. Back off from that 25-28* max to keep all the pieces together. I broke a piston at 34* total and never saw it coming.

Set up your initial and mechanical first and foremost. Try to have it all in between 2000 and 2800 -whatever make your motor happiest -2500 works for me.

Once you have those two dialed in you can play with adding vacuum advance to control idle temps, fuel economy and part throttle pings. But remember by the time you hear your blower motor pinging it's probably already too late.

Jury's still out on the ported vs manifold vacuum debate for me. I'm willing to try both and see but someone will have to explain to me why it's good to have all your initial advance plus whatever your vacuum pot throws in at idle speeds.

I can see that it takes a constant amount of time for a flame to travel across a combustion chamber at a given engine load and I can see that at higher RPM there is less time for that to happen so the process needs to be started earlier hence the need for advance as engine speed increases.

Connecting your advance pot to full manifold vacuum advance will give maximum vacuum advance at idle and less as the throttle blades open and manifold vacuum drops. (Though it's true your mech advance compensates this drop off.)

The port for your "ported" vacuum source is just above your butterfly valve. Ported vacuum is created by air moving through the carburetor. This means that if your vacuum advance pot is connected there, you've got NO advance at idle. I don't see that this is a bad thing.

At a few millimeters of throttle opening, the edge of the butterfly valve moves above the ported vacuum source- this exposes that port to whatever vacuum is in the manifold (which is typically decreasing now that the throttle is beginning to open) but more importantly it also increases the airflow directly next to the port opening that provides your ported vacuum. Since moving air is less dense that stationary air your ported vacuum source sees maximum vacuum as the combination of manifold and venturi effect sources at part throttle.

Part throttle is what vacuum advance is all about. As the throttle blades begin to open more fuel is admitted to the combustion chamber. But part throttle is used mainly under lower loading conditions. Under light loads cylinder pressure is not at maximum so fuel burns more slowly than it would at maximum cylinder pressure. This is why you typically see detonation under high loading conditions. Going up a hill under full throttle for instance, creates maximum cylinder pressure- under these conditions the fuel all wants to burn at once or rather explode- this is your pinging or detonation. Since the cylinder pressure is not high under part throttle/light load it takes longer for fuel to burn -the lower the cylinder pressure the longer it takes for the flame to travel across the combustion chamber. So under light loads and part throttle you need a little more advance to allow this to happen. Ported vacuum fills the bill here though I can see manifold vacuum doing equally well since manifold pressure is reduced at part throttle too.

Once the throttle blades open all the way, manifold vacuum goes to near zero and your ported vacuum also drops off. True moving air is still less dense than stationary air so air moving past the port must generate some vacuum but without the partly closed throttle blades allowing manifold vacuum to increase and the partly closed butterfly directing air down the wall of the carb where the port is located, the ported vacuum source drops right off.

This is not really a big concern now because now cylinder pressures are high and your combination of initial and mechanical advance is timing the combustion- hopefully right in that sweet spot between where its not detonating early and not happening so late as to waste the heat by melting down the exhaust valves and headers.

So there you have it- in theory ported vacuum supplies no vacuum at idle, maximum vacuum at part throttle and very little vacuum if any at WOT.

Now if anyone can explain to me why more timing advance than your initial advance is required at idle (e.g. full manifold vacuum) I'm all ears.

I mean I guess I can see it all balancing out under part throttle and full throttle conditions, but why haul the vac pot open at idle? It doesn't make sense to me.

regards

Zed

five7kid

iTrader: (14)

I don't remember seeing this last year. Lars works for the same company I do. He recurved a distributor for me several years ago.

Anyway, I don't have any personal experience with super charged applications, but I do agree with the manifold vs. timed vacuum advance stuff.

blown_zed

I hate articles that say things like "these stock factory engines ran like crap". Thats' BS. I'm not sure it was Lars who wrote the article posted above. Factory engines always run great provided they remain factory-They're generally smooth and quiet with good idle characteristics, throttle response and enough vacuum to operate whatever they need to operate-like brakes. Start messing with their various systems or fail to understand and maintain them and yes they'll run like crap- are they always the most efficient engines in terms of power and economy? Maybe not - but that's no reason to go pulling the ported vac line off your emissions era engine and drilling holes in your intake manifold to hook it up to full manifold vacuum- There's a lot of things to consider before you do these kinds of things. How much advance does your pot give you, how much vacuum do you need to pull it on and at what point is it off? What's your initial timing and how about your mechanical advance curve? Can you adjust your carb at all? Have you made any other mods?

Peak engine efficiency happens when maximum cylinder pressure is achieved between 15 and 23 degrees ATDC. Factory engineers are not unaware of this and their designs are intended to make this happen at most engine speeds and loads. They also consider emissions and fuel economy.

I just found this page which is great:

http://www.corvette-restoration.com/...o_articles.htm

Its a series of articles that explain the nitty-gritty of the ported v manifold vacuum advance debate. Essentially, the gist is that there is no right or wrong way to hook up your vac pot. Let me say that another way- There is no one way that is always preferable to the other. Or said another way, manifold vacuum is not always better than ported vacuum for controlling your spark advance and neither is ported vacuum always better than manifold vacuum. Which you use depends entirely on your particular engine's design characteristics.

So if your engine is designed to run on ported vacuum and runs fine and does not generate too much heat at idle ported vacuum will assist with maintaining idle quality, reducing fuel consumption at idle and reducing hydrocarbon (unburned fuel) output as well at CO (partly burned fuel) output. You'll be setting a good example for future generations and your kids will breathe easier.

But, if you have a highly modified engine and have trouble with overheating at idle you can reduce exhaust temps (exhaust port temp, header temp, underhood temp) by finishing the combustion process earlier in the cycle so the spent gasses are not still on fire when they leave the combustion chamber. In order to do this you simply need to start the burn earlier so it has plenty of time to finish before the exhaust valve opens. You can do this by going with manifold vacuum which will pull your vac advance to the max at idle.

There can be no 'performance' gained by swapping from ported vacuum to manifold vacuum because ported versus manifold vac advance affects ONLY idle. Once those throttle blades crack open a bit your ported vacuum source sees manifold vacuum so the two are essentially the same thing.

Hope this helps.
regards
BZ