what will happen if i ran open headers
04-29-2003, 10:07 AM
#1
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Car: 89 Camaro RS
Engine: 5.0 tbi
Transmission: street-strip 700R4
what will happen if i ran open headers
i was wondering that will happen if i ran open headers ?because on my friends 85 z28 hes running open headers and his car is alot faster then mines rolling so if i ran open to will i get more power? to beat him or will i loose power and loose to him again my motor is an 89 tbi 5.0
04-29-2003, 10:26 AM
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Car: 1999 Yamaha Banshee
Engine: 379cc twin cyl 2-stroke stroker
Transmission: 6 spd manual
Axle/Gears: 14/41 tooth
You will get a ticket, that's what. Power gain? Maybe, maybe not.
04-29-2003, 01:50 PM
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Car: 89 Camaro RS
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After I did my SLP headers I had to drive home with open y-pipe. It ran like ***. No back pressure= no power for motors without alot of hp.
04-29-2003, 02:00 PM
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Originally posted by 89fastlookinRS
No back pressure= no power
No back pressure= no power
it has NOTHING to do with back pressure, it has to do with the lack of volicty and scavanging. you are missunderstanding the whole purpose of exaust.
04-29-2003, 02:22 PM
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Car: 89 Camaro RS
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From what I've seen it does. If you completely open up the exhaust on a 120 hp motor you will lose power. But if you run open headers on a 400+hp motor, that motor flows enough air flow to not need backpressure. As to why you don't want chambered mufflers on a car that is turbo'd or super charged. You want straight through flow. Sorry if you think I'm wrong but I've seen it happen many a time.
04-29-2003, 02:43 PM
#7
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Originally posted by Dewey316
why do people keep saying this?
it has NOTHING to do with back pressure, it has to do with the lack of volicty and scavanging. you are missunderstanding the whole purpose of exaust.
why do people keep saying this?
it has NOTHING to do with back pressure, it has to do with the lack of volicty and scavanging. you are missunderstanding the whole purpose of exaust.
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04-29-2003, 03:01 PM
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yeah man you're missing his point though, you may lose some power at certain points but it isn't because of a loss of backpressure it has to do with the scavenging characteristics of the exhaust system. Do a search on backpressure there have been some very long posts about it and one post finally put to rest all the talk of needing backpressure, check it out, its a good read.
04-29-2003, 03:50 PM
#9
K ya I ran open headers for a day, smashed by the school everybody looked it was great...till the cops came, luckily one of the people that work at the school was cool and told me to leave quick because some white lady is calling the police. It sounds ****in bad man, the thing is you can f up a valve. Not because of no back pressure and that bull **** sucking it or what not. Its because if you run em open for a long enough time (as in non stop not for years) ur engine gets hot, and when you shut off ur car all the cool air rushes up into the heads and since the valves are still real hot and the cold air hitting it they expand and break (something like that) so thats the only real problem with running them open. Short trips are fine tho :lala: . People will get irritated hella fast (I dont care but maybe u do) so just throw it in neutral once you get to ur block. And running open headers you will have a better power gain then full exaust (is it just me or do every NHRA car run open. Think they would do it if there was a power loss. And yes its loss of velocity that loses power not no back pressure. But headers are so short i would not worry about u loseing velocity at all. So yea youll probably see some power increase how much I cant tell you. Have fun kids 
04-29-2003, 08:53 PM
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Car: 1999 Yamaha Banshee
Engine: 379cc twin cyl 2-stroke stroker
Transmission: 6 spd manual
Axle/Gears: 14/41 tooth
Originally posted by Dewey316
why do people keep saying this?
it has NOTHING to do with back pressure, it has to do with the lack of volicty and scavanging. you are missunderstanding the whole purpose of exaust.
why do people keep saying this?
it has NOTHING to do with back pressure, it has to do with the lack of volicty and scavanging. you are missunderstanding the whole purpose of exaust.
Originally posted by DevilsAddvocate
white lady is calling the police.
white lady is calling the police.
And just for the record, I drove around for 3 months straight open manifolds and y-pipe (if you can even call it that) without one problem with my valvetrain.
miacamaro305: Check this out
04-30-2003, 12:24 AM
#11
Originally posted by brodyscamaro
Correct
But a black or hispanic lady would have let it fly?
And just for the record, I drove around for 3 months straight open manifolds and y-pipe (if you can even call it that) without one problem with my valvetrain.
miacamaro305: Check this out
Correct
But a black or hispanic lady would have let it fly?
And just for the record, I drove around for 3 months straight open manifolds and y-pipe (if you can even call it that) without one problem with my valvetrain.
miacamaro305: Check this out
04-30-2003, 03:21 AM
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I have a desktop dyno... I ran MY numbers... (but it's different with YOUR CAR because I have HEADS/CAM/INTAKE/INJECTORS) but from small tube headers w/ y-pipe and mufflers I have 320 HP... with small tube headers WITH OUT pipes or mufflers... the numbers are 342 HP... torque stays relatively the same... (increase of 8 ft-lbs of torque)..
It's a solid difference of 22 HP on my car... THAT'S WHY I GOT AN ELECTRIC CUT-OUT
(but I only use it for the track, and when I wanna **** of people,... (ALSO it's fun when starting up your car in a parking garage), ESPECIALLY when r1cers come up to me and start revving... I FLIP the SWITCH, at idle... (where it doesn't make A huge difference) AND THEN TACH IT OUT TO 5000... after they rub there ears.. they roll up their windows and look away...
On your car, it'll probably ONLY make a difference of 10 HP or so...
ONE BIT OF ADVICE.. .if you leave them open... and wanna drive on the highway... BUY EAR-PLUGS
It's a solid difference of 22 HP on my car... THAT'S WHY I GOT AN ELECTRIC CUT-OUT
(but I only use it for the track, and when I wanna **** of people,... (ALSO it's fun when starting up your car in a parking garage), ESPECIALLY when r1cers come up to me and start revving... I FLIP the SWITCH, at idle... (where it doesn't make A huge difference) AND THEN TACH IT OUT TO 5000... after they rub there ears.. they roll up their windows and look away... On your car, it'll probably ONLY make a difference of 10 HP or so...
ONE BIT OF ADVICE.. .if you leave them open... and wanna drive on the highway... BUY EAR-PLUGS
04-30-2003, 08:39 AM
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Car: 89 Camaro RS
Engine: 5.0 tbi
Transmission: street-strip 700R4
okay thanks alot but im already going to get my hedmann headers next week and 2 40 series from flowmaster to run true duals
04-30-2003, 09:23 AM
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Originally posted by 89fastlookinRS
Sorry if you think I'm wrong but I've seen it happen many a time.
Sorry if you think I'm wrong but I've seen it happen many a time.
basicly any back pressure = pumping loss for the engine. period.
now if you run exaust with very large primaries (such as 1 3/4" primary headers on a 305) the exaust cannot maintain velocity in the large primaries at low RPMs, and because of this the "backpressure" is effectivly increased, and the engine has to work harder at pumping the exaust gasses out of the cyl. this extra power used to pump the exaust out is used up, and can't be transfered to the wheels, that is why you feel the lack of low RPM torque.
04-30-2003, 09:37 AM
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kool everyday i learn something new i plan to be a mechanic i have my own shop in like 5 years but only for gm car like montes t/a z28 and corvettes ok
04-30-2003, 10:13 AM
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Car: 1999 Yamaha Banshee
Engine: 379cc twin cyl 2-stroke stroker
Transmission: 6 spd manual
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Originally posted by txhotRS
I have a desktop dyno... I ran MY numbers... (but it's different with YOUR CAR because I have HEADS/CAM/INTAKE/INJECTORS) but from small tube headers w/ y-pipe and mufflers I have 320 HP... with small tube headers WITH OUT pipes or mufflers... the numbers are 342 HP... torque stays relatively the same... (increase of 8 ft-lbs of torque)..
It's a solid difference of 22 HP on my car... THAT'S WHY I GOT AN ELECTRIC CUT-OUT (but I only use it for the track, and when I wanna **** of people,... (ALSO it's fun when starting up your car in a parking garage), ESPECIALLY when r1cers come up to me and start revving... I FLIP the SWITCH, at idle... (where it doesn't make A huge difference) AND THEN TACH IT OUT TO 5000... after they rub there ears.. they roll up their windows and look away...
On your car, it'll probably ONLY make a difference of 10 HP or so...
ONE BIT OF ADVICE.. .if you leave them open... and wanna drive on the highway... BUY EAR-PLUGS
I have a desktop dyno... I ran MY numbers... (but it's different with YOUR CAR because I have HEADS/CAM/INTAKE/INJECTORS) but from small tube headers w/ y-pipe and mufflers I have 320 HP... with small tube headers WITH OUT pipes or mufflers... the numbers are 342 HP... torque stays relatively the same... (increase of 8 ft-lbs of torque)..
It's a solid difference of 22 HP on my car... THAT'S WHY I GOT AN ELECTRIC CUT-OUT
(but I only use it for the track, and when I wanna **** of people,... (ALSO it's fun when starting up your car in a parking garage), ESPECIALLY when r1cers come up to me and start revving... I FLIP the SWITCH, at idle... (where it doesn't make A huge difference) AND THEN TACH IT OUT TO 5000... after they rub there ears.. they roll up their windows and look away... On your car, it'll probably ONLY make a difference of 10 HP or so...
ONE BIT OF ADVICE.. .if you leave them open... and wanna drive on the highway... BUY EAR-PLUGS
Hey buddy check out this advice: DD2000 isnt accurate.
04-30-2003, 10:21 AM
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Originally posted by brodyscamaro
Hey buddy check out this advice: DD2000 isnt accurate.
Hey buddy check out this advice: DD2000 isnt accurate.
04-30-2003, 11:46 AM
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Yeah, my bad. I use the word backpressure more to describe what you're talking about with scavenging and all. I am in complete agreement with ya, I just went about the wrong way of saying it.
04-30-2003, 12:05 PM
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who needs back pressure. Ive ran with it open. Not much different then what i have now. Just really REALLY loud. I just have a muffler hooked up to my y-pipe and thats it. Do yourself a favor and get the proper sized headers and full exaust for your application. I just have a flowmaster 40 series and its way too loud. Everyone i know makes it a point to complain about the earsplitting noise level.
04-30-2003, 12:47 PM
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I have a pre cat cut out on my car. i've run this consistently with and without the cap on and there is a marked 2 tenths difference in my runs. The only thing i can figure that requires backpressure is the egr system. running on track with the cut out open hasn't affected my car in over 8 months of running it. I only run on track like this, mind you.
04-30-2003, 02:35 PM
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Car: 1999 Yamaha Banshee
Engine: 379cc twin cyl 2-stroke stroker
Transmission: 6 spd manual
Axle/Gears: 14/41 tooth
I do not know of ONE person, online or in person, that has had this problem. People online talk about it like it's a real threat, and the fact is nobody has proof that this happened. When you turn the car off the headers are still above 1000* more then likely, how "cold" is the air inside the primaries really going to be?
04-30-2003, 02:44 PM
#22
None, but how many people do you know that run open headers? I havent seen it but my manager (old school hot rodder) tells me thats why when ever you go to the track and stuff where the real dragsters are at they always stuff sox or what not into the collectors after a run.
Last edited by JesasaurusRex; 04-30-2003 at 03:19 PM.
04-30-2003, 02:53 PM
#23
About 10 years ago I emptied my cat,and removed the muffler,my car is louder at higher rpm's for sure, but runs great eversince!
At cruising speed in o.d. it's not a problem at all!
The dark,thundering sound when accelerating is just great!!
I don't care about horsepower,it still has more than enough!!
Even the anual inspection here in The Netherlands is no problem!
They just measure emissions when idling,and dispite the non functional cat,the car is still "clean"according to Dutch laws!
They also take the car for a test ride,and every time the inspectors come back with a smiling face!
They love the V-8 sound as well!
At cruising speed in o.d. it's not a problem at all!
The dark,thundering sound when accelerating is just great!!
I don't care about horsepower,it still has more than enough!!
Even the anual inspection here in The Netherlands is no problem!
They just measure emissions when idling,and dispite the non functional cat,the car is still "clean"according to Dutch laws!
They also take the car for a test ride,and every time the inspectors come back with a smiling face!
They love the V-8 sound as well!
04-30-2003, 03:28 PM
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GUYS.. I know that desktop dyno is NOT accurate, i figured that out when I was looking up the "pre-set" dyno's and a 5.0 mustang with STOCK HEADS, an H-pipe and a big cam was running 550 HP and 670 ft-lbs of torque... I know they respond well to mods... BUT NO WAY... that's why I said "i plugged in the numbers" not THIS IS WHAT MY CAR RUNS.... have you guys heard about this other one... it's called "performance calculator" you can plug in tranny-type, gears, rear, suspension.. intake, air cleaner... IT SEEMS AWESOME, except the $29.99 price-tag... I'll pay a little more than that for an ACTUAL DYNO
OPEN HEADERS... hey MIACAMARO... you should still look into electric cut-outs
OPEN HEADERS... hey MIACAMARO... you should still look into electric cut-outs
04-30-2003, 03:30 PM
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Car: '92 Rally Sport
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Backpressure is BAD! No matter what the circumstances (unless you want your stock EGR valve to not throw a code 32). Read Ed's post here: https://www.thirdgen.org/techbb2/sho...threadid=33246
05-01-2003, 02:26 PM
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Car: 1999 Yamaha Banshee
Engine: 379cc twin cyl 2-stroke stroker
Transmission: 6 spd manual
Axle/Gears: 14/41 tooth
Originally posted by txhotRS
GUYS.. I know that desktop dyno is NOT accurate, i figured that out when I was looking up the "pre-set" dyno's and a 5.0 mustang with STOCK HEADS, an H-pipe and a big cam was running 550 HP and 670 ft-lbs of torque... I know they respond well to mods... BUT NO WAY... that's why I said "i plugged in the numbers" not THIS IS WHAT MY CAR RUNS.... have you guys heard about this other one... it's called "performance calculator" you can plug in tranny-type, gears, rear, suspension.. intake, air cleaner... IT SEEMS AWESOME, except the $29.99 price-tag... I'll pay a little more than that for an ACTUAL DYNO
OPEN HEADERS... hey MIACAMARO... you should still look into electric cut-outs
GUYS.. I know that desktop dyno is NOT accurate, i figured that out when I was looking up the "pre-set" dyno's and a 5.0 mustang with STOCK HEADS, an H-pipe and a big cam was running 550 HP and 670 ft-lbs of torque... I know they respond well to mods... BUT NO WAY... that's why I said "i plugged in the numbers" not THIS IS WHAT MY CAR RUNS.... have you guys heard about this other one... it's called "performance calculator" you can plug in tranny-type, gears, rear, suspension.. intake, air cleaner... IT SEEMS AWESOME, except the $29.99 price-tag... I'll pay a little more than that for an ACTUAL DYNO
OPEN HEADERS... hey MIACAMARO... you should still look into electric cut-outs
06-03-2003, 12:24 PM
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Car: 2005 BMW 545i
Engine: 4.4L N62B44
Transmission: 6spd auto
Axle/Gears: Rotating
Originally posted by DevilsAddvocate
It sounds ****in bad man, the thing is you can f up a valve. Not because of no back pressure and that bull **** sucking it or what not. Its because if you run em open for a long enough time (as in non stop not for years) ur engine gets hot, and when you shut off ur car all the cool air rushes up into the heads and since the valves are still real hot and the cold air hitting it they expand and break (something like that) so thats the only real problem with running them open. Short trips are fine tho :lala: .
It sounds ****in bad man, the thing is you can f up a valve. Not because of no back pressure and that bull **** sucking it or what not. Its because if you run em open for a long enough time (as in non stop not for years) ur engine gets hot, and when you shut off ur car all the cool air rushes up into the heads and since the valves are still real hot and the cold air hitting it they expand and break (something like that) so thats the only real problem with running them open. Short trips are fine tho :lala: .
how is a muffler going to stop air though?
also think about it
you headers should still be very very warm when you shut the car off
that warmth is going to be transfered to the air
so by the time the air does get up there it is going to be warm
think about transfer of heat for a moment and you should see that
06-03-2003, 12:55 PM
#28
I onno i have a chambered muffler (flowmaster own3z :hail: ) and I doubt air will easilly just make its way back up through the entire tubing. Just the more pipe there is the harder it is for the cold air 2 get to ur heads simple as that.
06-03-2003, 01:09 PM
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Car: 2005 BMW 545i
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Transmission: 6spd auto
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Originally posted by DevilsAddvocate
I onno i have a chambered muffler (flowmaster own3z :hail: ) and I doubt air will easilly just make its way back up through the entire tubing. Just the more pipe there is the harder it is for the cold air 2 get to ur heads simple as that.
I onno i have a chambered muffler (flowmaster own3z :hail: ) and I doubt air will easilly just make its way back up through the entire tubing. Just the more pipe there is the harder it is for the cold air 2 get to ur heads simple as that.
you still shouldn't have to worry about warped/cracked valves
hot air should have more pressure then colder air
so even with just open headers you air shouldn't even be going in there
and if it does
it should be warmed up enough not to do anything
06-03-2003, 05:51 PM
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this is what itll do
1.) you will get made fun of cause it will sound like a farm truck
2.)a ticket from the local police department
3.)it does nothing to help hp infact it will probably rob horsepower cause there won't be any back pressure
4.)ill smack ya for doin it
1.) you will get made fun of cause it will sound like a farm truck
2.)a ticket from the local police department
3.)it does nothing to help hp infact it will probably rob horsepower cause there won't be any back pressure
4.)ill smack ya for doin it
06-03-2003, 05:59 PM
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Car: 1999 Yamaha Banshee
Engine: 379cc twin cyl 2-stroke stroker
Transmission: 6 spd manual
Axle/Gears: 14/41 tooth
Originally posted by Dennis91RS
3.)it does nothing to help hp infact it will probably rob horsepower cause there won't be any back pressure
4.)ill smack ya for doin it
3.)it does nothing to help hp infact it will probably rob horsepower cause there won't be any back pressure
4.)ill smack ya for doin it
06-03-2003, 06:37 PM
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i guess some people just arent reading the whole thread... i ran open headers and they sucked....like 0-60 in like 20 minutes...well not that bad but you get the point, if you just sit down and recollect the points made above, anything that restricts the movement of just about any moving part in your valvetrain is going to reduce power made, so therefore back pressure with makes it harder for those components to move would be bad
06-03-2003, 06:46 PM
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Car: 88 Camaro SC
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Axle/Gears: 9-bolt w/ 3.23's
its the lack of exaust gas scavanging that kills it, not back pressure. No piping after the headers means that the pulses of exaust wont be able to pull as much of the residual gasses out of the cyl.
06-03-2003, 08:31 PM
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it is a good thing. Less pressure on the piston means less work will be lost to moving the exaust gasses. Lack of scavanging is not. Thats one of the reasons that 1 3/4 in headers dont work well on smaller distplacement V8s is that the velocity is too low to help with scavanging.
06-04-2003, 05:14 AM
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txhotRS: Could you do me a favor and give me a little more insight on these electric cut-outs you have? I've been wanting to run open heads occasionally just for amusement/get back at some nieghbors I have beef with and that would work perfectly! PM or E-mail me about if you get a chance. I'd apreciate it!
Also, I need to have one of my ***** buddies read this thread. He is DEAD SET on the fact that backpressure is the whole reason that our cars perform the way they do. Think he might find this interesting, or kill me, one of the two...
Also, I need to have one of my ***** buddies read this thread. He is DEAD SET on the fact that backpressure is the whole reason that our cars perform the way they do. Think he might find this interesting, or kill me, one of the two...
06-04-2003, 10:33 AM
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i know, i cannot beleive how many times i have tried to explain this and people STILL argue about it.
it is simple physics, if you even made it through high school physics you should understand this. it is very simple.
back pressure = less HP, why it does is two fold.
1) pumping loss just like the AC running, there is a certain amount of power it takes to push the exaust gas out, just like there is a certain amount of power needed to turn your AC compressor.
2) any exaust left in the cyl. when the exaust valve closes, is taking up space. there is X number of cubic inches in the cyl. if you have Y amount of exaust taking up space you can only fit X - Y air/gas into the cyl. next time it is on its intake stroke. and we all know more air/gas = more power.
in fact for the guys who are convinced that backpressure is good. in the best case your exaust is so well tuned, and the scavanging effect is working, you should actualy have NEGATIVE pressure in the primary as the exaust valve opens, and it will actulay SUCK the exaust out. that is right what you really want is so little back pressure it goes into the negative, the last thing you want is POSSITIVE pressure, because of the 2 reasons stated above.
if you still think back presure is a good thing, do me a favore, run stock manifolds not headers, because the stock manifolds will perform much better, as they will help you create all the HP making backpressure that you so think you want. better yet, why not block off the exaust ports completly, then you will have all the backpressure your heart desires, i am sure you engine will procuce such astronomicly huge amounts of power, you will hardly be able to contain yourself.
it is simple physics, if you even made it through high school physics you should understand this. it is very simple.
back pressure = less HP, why it does is two fold.
1) pumping loss just like the AC running, there is a certain amount of power it takes to push the exaust gas out, just like there is a certain amount of power needed to turn your AC compressor.
2) any exaust left in the cyl. when the exaust valve closes, is taking up space. there is X number of cubic inches in the cyl. if you have Y amount of exaust taking up space you can only fit X - Y air/gas into the cyl. next time it is on its intake stroke. and we all know more air/gas = more power.
in fact for the guys who are convinced that backpressure is good. in the best case your exaust is so well tuned, and the scavanging effect is working, you should actualy have NEGATIVE pressure in the primary as the exaust valve opens, and it will actulay SUCK the exaust out. that is right what you really want is so little back pressure it goes into the negative, the last thing you want is POSSITIVE pressure, because of the 2 reasons stated above.
if you still think back presure is a good thing, do me a favore, run stock manifolds not headers, because the stock manifolds will perform much better, as they will help you create all the HP making backpressure that you so think you want. better yet, why not block off the exaust ports completly, then you will have all the backpressure your heart desires, i am sure you engine will procuce such astronomicly huge amounts of power, you will hardly be able to contain yourself.
06-04-2003, 11:51 AM
#41
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Originally posted by Sleipnir
i guess some people just arent reading the whole thread...
i guess some people just arent reading the whole thread...
No, you absolutely cannot have too little backpressure. That is one of the biggest and dumbest fallacies that exist in relation to 4 stroke IC engines. Why? I'll tell you.
Pumping Losses The concept of backpressure means that there will be a high pressure area at the exhaust port. When the valve opens, the escaping exhaust gases have to push against that high pressure area. How can a parasitic loss be good for your engine? The only engines that NEED backpressure are various small engine designs, mostly 2 stroke. The problem with them is they have the tendency to be too effective at driving the exhaust out, loosing compression. I assure you that on a big 4 stroke engine such as an SBC you want the exhaust to have the free-est path possible. In fact, it would be ideal to have a low pressure zone at teh exhaust port. A low pressure area would help to pull the exhaust gases out. Freeing up more power that your engine would have wasted pushing the gases out...which brings up the next topic beautifully.
Scavenging We've heard this before, but what does it mean in relation to our discussion of exhaust. Easier for me to use an example. So your engine fires and now it's on the way up on the exhaust stroke. Exhaust doesn't come out at a consistent rate, it comes out in pulses. Each pulse is a high pressure area, and as it moves, it leaves alow pressure area behind it. Aha, there's our scavenging. You want that low pressure area to be at it's peak when the exhaust port opens on the next exhaust stroke. Thats another reason why headers make better power than manifolds. besides just flowing better, instead of all the pulses being dumped in a log fighting with each other, the tubular runners allow the exhaust pulses to stay seperate and create a nice low pressure area behind it. This is also where tuned and equal length headers come into play. Tuned headers are sized such that the length of the tube corresponds the speed of the exhaust pulses so that the low pressure area is maximized at certain rpms. No surprise that short headers are better for high rpms than longtubes.
Problems can surface if you use too large of a primary diameter, loss of torque. The morons are quick to spout 'you lost backpressure and thus torque.' Next time you hear that you will smile and know that that person failed physics in high school. The problem with using too large of a primary is this. The exhaust pulse only has so much gas and energy in it. If the tube is too large, the pulse expands to much, losing energy and thus velocity. When it loses velocity, it can potentially stall and stop moving in the tube, or at least slow down. aha! Too large of a header actually CAUSES backpressure, and thus lost power. We feel this power loss as a loss of torque because usually this effect is much more pronounced at low rpms as much less gas is moving.
The same principles apply to the entire exhaust system, from primaries to collectors to pipes to mufflers. I am too tired to explain it all, books have been written on these topics. I have just scratched the surface, but hopefully you all understand a little better why their is no such thing as good backpressure. I know some of this has been a little oversimplified, but it think it gets the message across.
thank you please drive through...ed
Pumping Losses The concept of backpressure means that there will be a high pressure area at the exhaust port. When the valve opens, the escaping exhaust gases have to push against that high pressure area. How can a parasitic loss be good for your engine? The only engines that NEED backpressure are various small engine designs, mostly 2 stroke. The problem with them is they have the tendency to be too effective at driving the exhaust out, loosing compression. I assure you that on a big 4 stroke engine such as an SBC you want the exhaust to have the free-est path possible. In fact, it would be ideal to have a low pressure zone at teh exhaust port. A low pressure area would help to pull the exhaust gases out. Freeing up more power that your engine would have wasted pushing the gases out...which brings up the next topic beautifully.
Scavenging We've heard this before, but what does it mean in relation to our discussion of exhaust. Easier for me to use an example. So your engine fires and now it's on the way up on the exhaust stroke. Exhaust doesn't come out at a consistent rate, it comes out in pulses. Each pulse is a high pressure area, and as it moves, it leaves alow pressure area behind it. Aha, there's our scavenging. You want that low pressure area to be at it's peak when the exhaust port opens on the next exhaust stroke. Thats another reason why headers make better power than manifolds. besides just flowing better, instead of all the pulses being dumped in a log fighting with each other, the tubular runners allow the exhaust pulses to stay seperate and create a nice low pressure area behind it. This is also where tuned and equal length headers come into play. Tuned headers are sized such that the length of the tube corresponds the speed of the exhaust pulses so that the low pressure area is maximized at certain rpms. No surprise that short headers are better for high rpms than longtubes.
Problems can surface if you use too large of a primary diameter, loss of torque. The morons are quick to spout 'you lost backpressure and thus torque.' Next time you hear that you will smile and know that that person failed physics in high school. The problem with using too large of a primary is this. The exhaust pulse only has so much gas and energy in it. If the tube is too large, the pulse expands to much, losing energy and thus velocity. When it loses velocity, it can potentially stall and stop moving in the tube, or at least slow down. aha! Too large of a header actually CAUSES backpressure, and thus lost power. We feel this power loss as a loss of torque because usually this effect is much more pronounced at low rpms as much less gas is moving.
The same principles apply to the entire exhaust system, from primaries to collectors to pipes to mufflers. I am too tired to explain it all, books have been written on these topics. I have just scratched the surface, but hopefully you all understand a little better why their is no such thing as good backpressure. I know some of this has been a little oversimplified, but it think it gets the message across.
thank you please drive through...ed
Later,
Harry
06-04-2003, 01:27 PM
#42
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When dealing with exhaust systems on our TBI cars there is another item to understand. The EFI systems are speed density (SD) which uses a volumetric efficiency (VE) table. This VE table is used in the fuel calculation for the injector pulse width. A higher VE value equals more fuel. The VE table is factory programmed into the MEMCAL (an EPROM) of the ECM.
What happens when you reduce the backpressure is that the VE of the system (intake/engine/exhaust and everything in between) increases. Meaning more air will pass through at a given RPM & manifold pressure.
However, the VE table in the ECM has not been changed to correct for this increase in VE. Hmm, now the engine is running leaner then it was originally programmed to do. And may not run as well as it previously had.
Interesting.
Another tidbit: the ECM also compensates for a change in barometric pressure. Such as when the elevation increases from 'going to the mountains'. This compensation is in the form of increasing the VE to add additional fuel as the barometric pressure DROPS.
Seems backwards. But it isn't. The reason is that reduced barometric pressure decreases the atmospheric pressure on the exhaust outlet(s). Thereby increasing the VE of the system.
This all gives everyone mod'ing thier cars a good reason to get into PROM burning.
RBob.
What happens when you reduce the backpressure is that the VE of the system (intake/engine/exhaust and everything in between) increases. Meaning more air will pass through at a given RPM & manifold pressure.
However, the VE table in the ECM has not been changed to correct for this increase in VE. Hmm, now the engine is running leaner then it was originally programmed to do. And may not run as well as it previously had.
Interesting.
Another tidbit: the ECM also compensates for a change in barometric pressure. Such as when the elevation increases from 'going to the mountains'. This compensation is in the form of increasing the VE to add additional fuel as the barometric pressure DROPS.
Seems backwards. But it isn't. The reason is that reduced barometric pressure decreases the atmospheric pressure on the exhaust outlet(s). Thereby increasing the VE of the system.
This all gives everyone mod'ing thier cars a good reason to get into PROM burning.
RBob.
06-04-2003, 06:33 PM
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has the world gone mad!!!! all motors need some kind of back pressure to run its a common sense fact
06-04-2003, 06:49 PM
#44
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Originally posted by Dennis91RS
has the world gone mad!!!! all motors need some kind of back pressure to run its a common sense fact
has the world gone mad!!!! all motors need some kind of back pressure to run its a common sense fact
06-04-2003, 10:28 PM
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Ok, now it makes perfect sense!! I read the entire thread 1MEAN92RS posted a link for and it only backed up the argument here. Gah this is awesome!! I even started a thread back when I first registered becuase I was unsure if the stock exhaust bends were there for backpressure or not. Can't wait to get my true-dual setup!!! :hail: :hail:
:hail: :hail:
06-04-2003, 10:39 PM
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so basicaly no open headers right???
im just going with the flow here, hey if it runs better open then leave em open if ya wanna go deaf, if it runs better with pipes ten put em back on, that simple
im just going with the flow here, hey if it runs better open then leave em open if ya wanna go deaf, if it runs better with pipes ten put em back on, that simple 
06-04-2003, 10:49 PM
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Lol, you just may be right on that one, the fact still remains though, this thread has answered a LOT of questions! (At least mine, heh ) Here's that thread I was talking about. Don't mind me in there, that was like my 2nd post ever and I didn't know what I was talking about (making it sound like I do now....)
Stock exhaust bends......necessary?
) Here's that thread I was talking about. Don't mind me in there, that was like my 2nd post ever and I didn't know what I was talking about (making it sound like I do now....)Stock exhaust bends......necessary?
06-05-2003, 08:35 AM
#50
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OK, I think we shoudl actually make a sticky on this.
Backpressure=bad, velocity=good
People commonly confuse backpressure with velocity. You want to have a very high velocity without causing any restriction in the exhaust. Therefor people confuse going too big with the exhaust and effectively slowing down the escaping gasses, aka losing velocity, as reducing backpressure. You do not under any circumstances want any backpressure at all. It just makes the engine work harder to pump out the gasses. What you don't want to do though is go too big on the exhaust, which will actually slow it down, for example, running 1 7/8 inch headers with dual 4 inch pipes on a stock 305 or 350, that would do nothing but slow down the escaping gasses and reduce the velocity so much that it would in effect have more backpressure. So please stop confusing velocity with backpressure!
Backpressure=bad, velocity=good
People commonly confuse backpressure with velocity. You want to have a very high velocity without causing any restriction in the exhaust. Therefor people confuse going too big with the exhaust and effectively slowing down the escaping gasses, aka losing velocity, as reducing backpressure. You do not under any circumstances want any backpressure at all. It just makes the engine work harder to pump out the gasses. What you don't want to do though is go too big on the exhaust, which will actually slow it down, for example, running 1 7/8 inch headers with dual 4 inch pipes on a stock 305 or 350, that would do nothing but slow down the escaping gasses and reduce the velocity so much that it would in effect have more backpressure. So please stop confusing velocity with backpressure!