Exhaust backpressure

[Pharaohabq]

I was talking with some friends today, and considering what size tailpipe and header we might go with, and one of my friends was saying that you have to have some restriction in your tailpipe, or that we would start blowing out rings in the engine. I'm not so sure about that, have any of you heard that there's such a thing as too large exhaust flow, to not have enough back pressure?

Maybe I'm wrong, but it seems to me that the better an engine exhales the more power you can have.

Thanks, Phar

[michaelp]

Back pressure is restriction to flow...you dont WANT back pressture, but you don't want a to large of an exhaust either. Finding the optimum sized exhaust could be hard for some people, but you want just the right size for superior FLOW, with no back pressure, but not to large which will hurt flow.

[evildky]

"backpressure" actually refers to velocity, basically at some size the pulses sort of pull each after then out, and once yiu get to large you loose this sort of suction, most N/A Z's run up to about 2.5" only those who can "tune" exhaust with timed headers etc should venture larger, and for the turbo's anything over 3" is a waste

[Pharaohabq]

K great that pretty much explains that , thanks a bunch, as usual. Phar

[AZ-ZBum]

and for the turbo's anything over 3" is a waste

That will depend on how much power you plan on making. There is a reason turbo diesels run huge exhausts.

There is a certain size to horsepower relationship. You won't find a 1000hp car running 3" exhausts. More like 4" or bigger.

But I agree with everything else you've said. I made a page a LONG time ago trying to explain why backpressure was bad.

http://www.az-zbum.com/backpressure.shtml

[evildky]

I did rather assume he wasn't gonna make 1000hp, I know a guy running a single 4" on an S30, of course it's hooked to tuned headers on a SBC, and the guy is a pipeline engineer, sounds unlike any small block I've ever heard, car runs 10.52 in the quarter

and I must disagree to a point on the turbo, I think that as long as you have exhaust pipe it is important to maintain the exit velocity, a dingle 4" pipe on a stock turbo sprint I fell certain would generate less power than 2"

[Perrenial Badass]

Doubtful. Backpressure is a restriction. The 4" exhaust can flow more than the 2" exhaust, so it will always be capable of more power. It may be loud and obnoxious when it does it, but it will do it.

As for the "exit velocity"... I think that you don't fully understand what goes on in the exhaust pipe. Every time that all the exhaust valves are closed, the exhaust stops moving. More exhaust comes out and the old exhaust moves forward. The "exit velocity" has nothing to do with it. Actually, the only way I can understand your thinking is to assume that you think that a high "exit velocity" would spool the turbo faster.

[evildky]

ok first of all, on a turbo car once you get past the turbo there should be no "pulses" it should all be pretty well smoothed out by the turbo so the valves closing no longer factors into the equation, but as the volume of spent exhaust exits the turbo from what is usually a very small hole compared to the diameter of the pipe you create a low pressure zone (basically suction) this helps to pull the gasses through the length of the exhaust pipe, but at some point the volume of air can get lost in the volume of the pipe, which was my point witht he turbo sprint (have you seen the turbo on these? looks like a motorcycle turbo) when dealing with turbo exhuast generally speaking, the quicker andeasier it is to get out the better but when you have 10 feet of pipe to go through keeping it all flowing smoothly and without restrictions is more important than reducing the pressure to the point that the gasses expand to the point of tumbling over themselves or so to speak

none of this has to do with spooling the turbo, thats a whole discussion unto itself

[Perrenial Badass]

On turbodiesels, I have seen exhausts with short expansions right off the downpipe that do create suction. This pulls the exhaust right out of the engine and provides a boost in power and torque.

Over a very small RPM area. This is often tuned to provide extra towing power where the truck needs it the most. But the suction that you need to think about is that behind the exhaust at the engine. The exhaust cannot physically move away from the turbo unless the air between the engine and the turbo moves. The "air pressure" in the pipe... or lack of it... will overcome any motivation to exit the pipe. Because there will be nothing to fill the space where the exhaust used to be.

That said, this suction has less effect as you get closer to the end of the pipe. At, say, the muffler, outside air can enter the exhaust system. Since the exhaust is hot and wants to expand and rise, it will exit and outside air can enter the pipe. But there is no air inlet at the exhaust side of the turbo. Unless, considering the age of our cars, we have rust holes in the piping. At which point, you'd be right, the exhaust will go right out that hole.

There is such a thing as too big of a pipe. But not because it causes you to lose power, but because it is inefficent for your size motor... well, exhaust flow... needs. Usually, a overly large exhaust for a car makes it peaky, whereas a correctly sized unit will make the most power in the midrange... although the peak power gain will most likely be less than the larger unit.

[evildky]

the exhaust gasses before the turbo are pressurised, it's this pressure that spins the turbo as opposed to leaking out around the turbine wheel

the exit hole I was refering to the the hole in front of the turbine wheel on a stock style T3 turbo, on T4's fitted with external wastegates or Holset HY/HX series turbo the exit hole is abotu as large as the pipe diameter but I know the turbo didge diesels still step the diameter up for the 3"ish opening to 4" ish downpipe

efficient = more powerinefficient = less power

I think we need a turbo sprint and some 4" pipe and a dyno to lay this thing to rest

[Perrenial Badass]

That doesn't mean that if the exhaust gases are traveling faster that the car will make more power. But you are right, a dyno would settle this.

Although... I am sure that tests to this effect have already been conducted, and the results are probably somewhere on the internet.