[Sa-WaitNoMustang900Turbo]

this is what ive been saying for at least two years now. I could never understand how backpreasure could = torque,i.e. I could never figure out the physics behind "exaust scavenging" nor had i seeen it for my self (i have only felt low rpm GAINS with loss of back preasure)

appearently many others agree with me

example:
http://www.uucmotorwerks.com/html_pr...torquemyth.htm

[stangliter04]

Peanut butter open headers time


* BTW good info.... I always thought the same.

[machy]

open headers is to dam loud

[Milner62]

lol yep open headers are loud. One reasion why im going with a custom setup exhaust system i put together myself which has larger tail pipes and muffles than the pipes before the mufflers

[itschristorres8]

open header?! wow i crack my exaust manifold(header) and it was only on one side and that was loud! open header is loud, idk id jus cut the exaust at the mufflers, thats loud too and nice sounding

[Screemin3o2]

It all depends on the cylinder head design cam design and a few other variables in my opinion. We hooked a built 355 nova on the dyno last week. It made 7 less hp when he ran open headers from full exhaust and his torque and horsepower curve weakened dramatically. Exhaust scavaging and backpressure are not a myth. If you have a decent knowledge of how airflow acts and you study scavaging and the backpressure effect, it does make since. I have been saying this ever since I studied deeply into engines and every other part of a car, everything is a science. If you add something to your car that scientifically will increase your power band, it will. If it does not, it is because there is an opposing force you have not recognized.
I.E. I bought a 75MM throttle and bolted it to my stock engine because obviously if it is bigger then it will provide more airflow. Then I began to understand airflow and realized how dumb I was to think if I did not match the port from my tb to my manifold or I did not increase every part of my induction system, I would still get a power increase. If anything I lost it and dramatically decreased the curve. You can't hook a holley systemax intake on your car and bolt it next to stock e7's and expect to gain any horsepower. It's the same with exhaust. Switch from a 1 5/8" headers to 1 3/4" or even larger on your stock engine and see how much more of a dog it is.

[mdvaldosta]

Backpressure assists with cylinder scavaging, therefore if done properly, will actually increase power and torque througout the curve. Typically, the less backpressure, the higher the torque curve moves in the rpm range (usually increasing peak horsepower and hurting peak torque).

Like screamin noted (the fugger knows his sh*t), theirs more to it than just backpressure.

[Screemin3o2]

I lost count of how many things in that article are wrong or don't make since. The guy is saying too little backpressure won't burn valves. Well hell, make it 0 backpressure and run an engine straight out of the exhaust ports with no headers or anything and see how quick it takes to burn a set of valves

[Sa-WaitNoMustang900Turbo]

carbs can act differently with changes in back preasure than a computer controlled engine, hell on my mower when i ran a strait pipe i had to retune the carb to run richer, i lost so much power off the bat, but when i adjusted the a/f it ran about 1hp stonger!(seemed like, no mower dyno lol)

sure you could see an increse in power with more back preasure, but in theory if your a/f is precise almost no matter what (as in a bmw), you should see an increase in power and torque with a drop in back preasure(no offense intended to mustangs v8s, but stock a/f test vs stock a/f test, bimmer wins all the way, making a drop in back preasure better on its engines)

remember IN THEORY, and as those who understand how engines work rather well, the smaller displacement/combustion chamber the more acurate and evenly dispersed the a/f will be adding to other obvious outcomes in tuning, also at low rpm speeds the a/f dispesion in any engine becomes more uneven making other obvious outcomes come true, especially on a performance engine with an agressive cam

also port matching reduces backpreasure by making the flow less turbulent (i nor this article ever said that increasing the diameter of stuff which mismatches port decreases backpreasure)

back preasure has no effect on burning valves, a/f does(or maybe an open block and COLD AIR hitting the valves does lol)

edit: now that i have re-read the aritcle I again see no place where he is technically wrong, he didnt say much, and the stuff he(and I) did say logically coincides with what you are saying(to an extent of course) whether you can see it or not

edit again: im not trying to argue, only trying to show the truth to people who can understand what it is talking about, many of your comments only confirm what ive been saying, yes the stuff associated with backpreasure is complex in some ways, but again, it is not the backpreasure that is directly causing the effects, it is the a/f!

[Screemin3o2]

If you run an engine straight out of the cylinder head, it will burn the valves because there is no vacuum to get the gases out of the chamber.

[Sa-WaitNoMustang900Turbo]

well, again im pretty sure its the cold air "burning" the valves on an open block, and that there is more preasure in the exhaust piping than in the ambient air despite the fact thae exhaust moves a lot faster(heat, moving mass of other air, and compression against walls cancels out moving fast) so there sould be more of a "vacuum" in ambient air, allowing the exhaust to be pushed out faster, in theory.

btw, nice avatar

[Screemin3o2]

I'm not understanding how you are saying cold air will burn a valve.

[Roadhawg]

You need a small amount of Back Pressure. This will prevent too much of the intake charge from being pulled into the exhaust system when both intake and exhaust valves are open.

[Sa-WaitNoMustang900Turbo]

well, on some cars(boosted and some with variable cam timing, new mustang GT included i think) at lower rpms and part throttle where the backpreasure would actually matter for valve overlap (fuel economy) there probalbly is relitivly little to no valve overlap, making backpreasure completely undesireable. Not to mention the carbed n/a performance cars which dont care in the least about fuel economy or low rpms where the a/f is not exact.

and do you mean how can cold air hit/affect valves on an open block, or do you mean what will cold air do to incredbly hot (metal remember) valves?

[Bish]

Man, I'm sitting here after reading all this and have come to one simple conclusion. . . . .

I don't know shit.

I'm going to have to reread this several times and maybe some of it will start making sense to me. I digress, for the time being I'm not changing anything until I understand this better.

Thanks for the awaking, it's greatly appreciated.

[ChrisJ]

I lost 2rwtq when I took my cats off but gained around 2rwhp...

There is such thing as back pressure...

[Screemin3o2]

I guess some people have different ideas of what back pressure really is and what it causes. That changes nothing. If something happens over and over again with different cars and the same part changes, etc. no matter how you look at it or what you call it, the way it works is still the same. No matter what, if you change the backpressure in your exhaust or wherever your power and torque curves will change therefore its not a myth.

[Screemin3o2]

Originally Posted by Bish Man, I'm sitting here after reading all this and have come to one simple conclusion. . . . . I don't know shit. I'm going to have to reread this several times and maybe some of it will start making sense to me. I digress, for the time being I'm not changing anything until I understand this better. Thanks for the awaking, it's greatly appreciated.

gotta start somewhere as long as you are interested in it. Just start at the beginning and keep reading. You think some of the best techs know everything just because they have experience? I guarantee they studied some of it sometime in their life.

[TENGRAM]

now i'll start by admitting that i only read most of these comments...not the novel posted by Sa-WaitNoMustang900Turb...so maybe someones already mentioned this.

backpressure is bad. period. exhaust velocity is very important and though back pressure can aid in exhaust velocity it is much better to have velocity without back pressure. appropriately sized exhaust is needed to produce maximum low end power and torque.

[SquirrelNutz]

What pipe size should I use for a street application?
In general, the following guidelines should be applied for street applications.

2"
tubing
up to 100 hp per pipe

2.25"
tubing
up to 160 hp per pipe

2.5"
tubing
up to 200 hp per pipe

3"
tubing
up to 300 hp per pipe

THIS IS FROM THE FLOWMASTER SITE.... I dont know where some of yall are getting your information but you do need backpressure there is no myth there! Too much backpressure is bad and too little backpressure is bad and that is fact!