Exhaust back pressure is _always_ bad (long)

[kpt4321]

I agree with Jesse. I have never ever heard of this before. Sounds hella fishy.


....Kyle T.

 

[thehyena]

that's odd.... i improved performance on my 1.8 by having the exhaust go 2 1/4 to a 2" cat back and then open back up at the end with a 2 1/2" outlet and finally the muffler.

 

[kpt4321]

Originally posted by thehyena
that's odd.... i improved performance on my 1.8 by having the exhaust go 2 1/4 to a 2" cat back and then open back up at the end with a 2 1/2" outlet and finally the muffler.

WTF is that supposed to mean? That was not worth anything.


....Kyle T.

 

[mrmadmax]

Well, I might as well add my $0.02 considering that induction and exhaust was basically my engineering thesis in college.

1) Toss control. I have learned to never say never in the world of engineering, but I'm gonna call BS here. NO self respecting engineer would design a connecting rod that has to rely on cylinder pressure to offset the naturally occuring dynamic forces on the connecting rod. This is simply urban legend. If he did, he'd look like an idiot when the first shift occurs at 7 or 8000 RPMs. What happens at 8000 RPMs when you close the throttle? The manifold and chamber pressure go below atmospheric (vacuum), and the exhaust pressure goes to atmospheric. EVERY TIME.

That would certainly lead to a lot of broken rods if we needed back pressure or cylinder pressure to hold things together. Like Enigma_Man said, the rod is more than adequate to handle the stress.

2) The reason you lose power when you remove your N/T exhaust is because you are actually INCREASING your backpressure in the process. To understand this, you need to consider the split second timing of the pressure pulses in the exhaust. An exhaust system, when operating at the correct speed range will actually LOWER the pressure at the port below atmospheric. This occurs for a split second during the cycle, and, when timed correctly, leads to a momentary below atmospheric pressure condition. This happens not only because of the slug of air moving down the pipe at high velocity, but more because the pressure waves that travel at the local speed of sound in the pipe are timed with the mechanical operation of the engine. If you remove the exhaust system, you inadvertently "tune" you exhaust system to a MUCH higher RPM range than you can safely operate, so you raise the exhaust port pressure to atmospheric, and you lose power.

Further, there is MUCH more advantage to be had than just the fresh charge during scavenging; for instance, during overlap, the piston is moving slow and can do little to move the incoming air charge. By scavenging during overlap, we can get the intake charge moving BEFORE the piston begins to move downward. This initial movement creates a pressure wave that has effects inside the intake manifold as well.

If really want to get trick with your exhaust tuning, I suggest installing a high speed pressure transducer at the exhaust port and several other locations in the exhaust, and viewing what happens on an oscilloscope relative to the cam sensor signal. It's really interesting to see how closely the timing of a pressure wave to the engine cycle is related to torque output.

For those of you who know this stuff already, I have left a lot out of my explaination, and you know what I'm talking about. A thorough explaination takes many pages of text. For those that do not, there are several good books (on an engineering level) that explain what is going on, and how we take advantage of it in the real world. Look for titles published by SAE international.

 

[thehyena]

Originally posted by kpt4321


WTF is that supposed to mean? That was not worth anything.


....Kyle T.

it generates a whole hell of a lot of backpressure

 

[kpt4321]

Originally posted by thehyena
it generates a whole hell of a lot of backpressure

WTF is "it"? From now on, you're banned from posting in this thread. STFU.


Nice post mrmadmax! You did a very good job explaining all of this stuff, it's good to have someone else on our side.


....Kyle T.

 

[LightningGSX]

Originally posted by DV8tion
If you can't find it on google it cant be real.
I only said it helps, I did'nt say it was responsible for keeping the rods together.
Exhaust manifold pressure should stay fairly close to intake manifold pressure, but in reality exhaust pressure stays higher because of the use of restrictive small AR turbine housings.

Why should the exhaust manifold pressure and the intake manifold pressure stay fairly close?

 

[Kevin Jewer]

If your exahsut mani pressure gets significantly higher than intake mani pressure there are several problems, not the least of which is having to run a lot more boost to get a little more flow. And more pressure (at the same flow) means more heat (and most likely more turbo required). Often times the limit of adding power via nitrous oxide is also the point where the exhaust to intake pressure ratio gets way out of wack. Perhaps one of the other two knowledgable guys can elaborate.

 

[LightningGSX]

I understand the effects of int/exh pressure differentials,but what I was wondering is,I can see upsides and downsides for a higher ratios,lower ratios, and a 1 to 1 ratios but I can't see how no pressure differential would be best in all cases.

 

[LightningGSX]

If that makes sense to you, I can't explain it very well.

 

[kpt4321]

Nope, it does not makes sense.

You don't want no pressure differential, you want as (relatively) high a pressure on the intake side as possible. By that I mean that you want the pressure ratio across the chamber to be as much as possible.


....Kyle T.

 

[LightningGSX]

Originally posted by kpt4321
Nope, it does not makes sense.

You don't want no pressure differential, you want as (relatively) high a pressure on the intake side as possible. By that I mean that you want the pressure ratio across the chamber to be as much as possible.


....Kyle T.

You obviously didn't read my post to well.I said I COULD NOT see how equal pressure would be best.

 

[MNGSX]

Actually you want the intake pressure to be above exhaust pressure.

Why?

It means the turbo is working and the exhaust is'nt restricing the increased flow on the way out.

An engine is an air pump that burns fuel.

So higher intake than exhaust helps the air thru the entire engine and out the tail.

 

[LightningGSX]

Isn't that what kpt4321 said above?

 

[kpt4321]

Originally posted by LightningGSX
Isn't that what kpt4321 said above?

Good to see that at least one person reads what I post.

....Kyle T.

 

[LightningGSX]

Well if you would of read my post I said that too.Yet kpt4321 and mngsx both said I was wrong.What's up with that?mngsx has a habit of not reading posts and posting info that has already been said.