skydog

Next time anyone tells you anything about "back pressure", try and get them to explain the concept to you. There is no such thing.

Instead of trying to conceptualize back pressure, think of it this way.

There are two things going on in your engine's plumbing (intake and exhaust). They are 1) a standing sound wave created by the periodic opening and closing of the valves on both the intake and exhaust side, and 2) the flow of air/fuel into the engine and exhaust flow out.

So, we have a sound wave and some mass of gasses flowing in the exhaust (and intake).

The sound wave is caused by the opening and closing of the valves. The frequency of the sound wave is controlled by the speed at wich the valve opens and closes (valvetrain RPM). To review some highschool physics, a standing sound wave is a series of "pulses" of alternating high and low pressure zones that travel past some fixed point. Also to review, when these pulses reach the end of the tube, they get reflected back up the tube. We can use these high- and low-pressure zones to help us out. If we can get a low pressure pulse to arrive at the exhaust valve just as it opens, it can help us pull the gasses out of the cylinder a little more efficiently. Also, on the intake side, if you can get a high pressure wave to arrive at the valve just as it opens, it will help us get a little more charge into the cylinder. We can "tune" the exhaust and intake runners by changing their lengths (or diameters).

As far as flow through the tube, we want to balance the speed of the gas so it is not too fast, and not too slow. If the gas is required to flow too fast, there will be an undue ammount of friction and turbulance in the tube, and we will be robbed of some power. If the gass does not flow fast enough, the gass will not have enough inertia. The inertia of the gass is important because it determines how much gass we can get into the cylinder after bottom dead center (the point at wich the cylinder is no longer pulling on the gass). The more velocity, the more intertia, and the more gass will squeeze itself into the cylinder after the intake stroke and before the valve closes. We use the diameter (and length) of the tube to tune it for velocity. For a given mass of gass traveling down a tube, the larger the diameter, the slower the velocity.

Now, for low end torque:

We want a longer pipe to accomodate the longer (lower frequency) sound wave (caused by low RPM operation) AND we want a smaller diameter pipe to keep the velocity of the gasses up. That sounds like a stock exhaust to me, right?

For more top end power:

We want relatively short pipes for the higher frequency sond wave, and larger diameter to let the gass flow with less restriction becase the high rpms will help keep the gass velocity up. Hmm, this sounds like short intake runners (think itb's) and large 3" diameter turbo exhausts.

Yall have some pride! Back pressure is a redneck camaro and mudslingin' concept. Dont use that term anymore!

symmetry

iTrader: (1)

You've definitely got too much backpressure built up

skydog

Sorry if I sound irate...

Blahman240

what are you talking about?

modoman

iTrader: (6)

its just a term.

bamb00

iTrader: (14)

i think i need more backpressure to go faster, how do i get it again?

mkingk2

iTrader: (1)

lol, I'd give you a rep point but don't know how

SOHC

^^^ haha i dont know how either

so its okay to have a 5" exhaust!?!

wooh! im getting mine today

wonderz

iTrader: (18)

anyways,
does backpressure = back pressure, which back means the rear section, aka exhaust, which mean, exhaust pressure?

since generally when ppl use the term backpressure, they are talking about the exhaust.

DaRk_MauL

iTrader: (7)

yep backpress is a bad term because you dont want this you just have a little
resistans.Larger exhaust piping less resistans more high flow of the exhaust!
correct me if i am wrong.

TheSmuggler

iTrader: (11)

I seriosuly CANNOT belive some of the responses to this post.... jebus this site is in trouble.

bamb00

iTrader: (14)

maybe if you have some backpressure you'll understand

CivicsRdBest

And here I was thinking it was that pain from the uncomfortable seat...

TheSmuggler

iTrader: (11)

I seriosuly can't think of a better description of bad information. We NEED backpressure on our N/A cars... how do you think your car get's ANY torque at the low end, if you but a 3 inch exhaust on your N/A car your RUINING it's power, and NO it does not help that much if all in the top end... So in general, no backpressure is not for reneck mustang owners, a certain amount can greatly benifit or harm a low producing n/a car, or any car for that matter.

skydog

Ok

Go back and read the original post again. I state that for low end torque, you need a smaller diameter, longer tube.

Now, I know the 3" diameter thing is a little much, but that is an extreme case to show my point. You might use that on some mid displacement six cylinder highly turboed car. Besides, that application would be for high end horsepower...



To theSmuggler-- Please explain what backpressure is. WHAT IS BACKPRESSURE?

skydog

I think when people say "backpressure" they really mean something like "an exhaust system that uses a relatively smaller diameter pipe within reasonable design parameters that promotes an adequate gas velocity to enhance cylinder evacuation after top dead center of the exhaust stroke in the lower portion of the operational RPM range of the engine."

hilosponger

Now isnt BACKPRESSURE just a little easier to say instead of sounding like you graduated from Harvard when you just barely graduated high school...?