ATARI
SO i hear back pressure is good, back pressure is bad

whats the deal guys...

one thing i can think of

is people trying bigger pipes to reduce backpressure and that ends up killing their flow rate

but what about straight pipe?
cutouts?
no cat?

wouldnt that reduce back pressure

ATARI
wow no help...

Cooked Rice
Really it shouldn't be called back pressure. Cause back pressure = loss in performance. Basically a "proper-free-flowing" flowing exhaust exhibits properties which give it a "scavenging affect" basically your exhaust becomes a area of low pressure (a vacuum) that helps suck exhaust out of the cylinders. This is only achieved if the exhaust is the correct size which allows gas to be accelerated as it exits the engine, hence creating a low pressure in the exhaust. if your exhaust is too free flow/large, it loses it's ability to affectively accelerate the gas in the exhaust and cannot create a low pressure to "scavenge" the gases from the cylinders. Smooth bends, no cat, but proper exhaust diameter (not too big, not too small) help in the scavenging effect. If you've ever felt your exhaust, you'll know it pulsates. Think of each pulse as a ball of energy leaving the engine via the exhaust. Trailing each ball, is a vacuum, which in turn helps suck the next "energy ball" out of the cylinder. If your exhaust is too large, you lose the "air seal" for the energy balls and cause turbulence(and turbulence causes back pressure as well), too small and you get back pressure.

EM1FTW

:werd:

89s1
Agreed. A properly sized exhaust will get the gasses out as quickly as possible while keeping them as hot as possible.

If the exhaust is too large the exhaust gasses cool and expand becoming a blockage for the exhaust behind it.

Most of this goes out the window with a turbocharged engine however. :)

Expansion and anti-reversion chambers are a whole 'nother ballgame too.

ShermanEF9
People who think exhaust not leaving the system, and putting pressure backwards into the cylinders is a good thing, they should picture it this way... Is it a good thing when you're taking a shit and it ends up going back inside of you? same can be said if you sit down and it just all falls out... thats just where things get weird, and where you should probably rethink your lifestyle.

ATARI
okay,

also how do you determine the proper length?

say if you didnt just make it the length of the car, but you could make it any length

i hear you put chalk on the exhaust pipe, and were the chalk stops burning off is where the exhaust should end

texasnick
Backpressure being a good thing is a retarded myth, and overused by ricers IMO. It's all about the flow rate of exhaust gas out of the motor not being hindered by the exhaust.

Remember fluid mechanics;

Q = v*A

where Q = Fluid Flow Rate
v = Fluid Velocity
A = Cross Sectional Area of what the fluid is flowing inside of.

Q will have a maximum value at a specific RPM. If your exhaust is too restrictive, it will not allow for this maximum Q. You will only get, say 95% of the maximum. You can increase the diameter of the exhaust piping to help achieve this maximum Q out of the exhaust, but past the point where you actually reach this maximum you will experience less performance.

Why?

You have reached Qmax. The motor is incapable of achieving a higher flow of exhaust gases out of it. Therefore, Q becomes constant. Now, what happens when you have a constant Q and you increase A? v decreases.

As far as pressure goes, the higher the velocity of air coming out of the engine is, the lower pressure is exerted back onto the engine, ie backpressure. The trick is to maintain this Qmax while having the highest v possible, reducing or eliminating back pressure.

Basically, you put an exhaust that is too big (in Diameter) on your car and the exhaust flow coming out of the motor isn't high enough to warrant it. Exhaust velocity decreases and puts MORE pressure back onto the motor.


The only thing length has to do with these, is frictional losses of the fluid and possibly the ambient temperature of the air around the exhaust tip.

I'm done work for the day, if you all couldn't already tell LOL

Toma
Lot of factors. Back pressure can be bad, or it can not affect things, or it can help....

It is all i the dynamics of how the cylinder fills and empties. If the cam timing at a certain rpm is set-up a certain way, exhaust back pressure may not matter.... ie, you could unclog the exhaust and not gain anything untill you went with more aggressive lobes...

Or if you were over cammed in certain situations, the extra back pressure could actually help keep charge in the cylinders from passing through, and trapping slightly more cylinder pressure.

Of course, in an ideal world, the cam and exhasut would work together, and you would want as little pressure in the exhaust as possible,,, hell, a vacuum in there would make the best power.

On MOST stockish cams, back pressure does not help.... you would want as little as possible, thought it MAY NOT help, it certainly wont hurt.

962 kid
I have a fantastic article on exhaust design that I might scan and post up. A properly designed exhaust will be acoustically tuned to use pressure waves to basically siphon the next exhaust pulse out of the cylinder. The volume of the primary runners should equal the volume displaced by each cylinder, and the length of each runner is dependent upon the speed of the pressure wave, or the exhaust temp. From there, diameter is easy to determine.

When you hear people say that NA engines need a little backpressure, they're mistaken about their terminology. Exhaust backpressure is never a good thing unless you have a rare situation like Toma described.

Right answer, wrong explanation. The exhaust gas is not used for scavenging at all, the pressure waves created when the exhaust valve opens is. When the pressure wave reaches the end of an open pipe, a wave is reflected back towards the engine. The goal is to have the wave hit the seat of the valve just as it opens.

Also, as 89si said this all goes out the window with turbocharged motors.

ATARI

alright thanks