How pipe size affects performance

[Natty]

OK, I know that exhuast gases exit the exhuast valves and then go through the exhuast manifold through the exhuast pipes etc. I hear that when you get rid of the exhuast gas, it makes it easier to cram more air into the intake valves for combustion. Makes sense. But what is it that you want to do to get the best perfromance? Do you want to releave pressure or do you want to move the most gas at the highest speed?
Some of you may see where I am going with this. I think some basic physics applies here. Continuity says that the same mass flow entering the pipe is the same as it leaves. I forgot the actual equations though.
Basically, as you increase the area or the pipe (2.25 vs 2.5 for example), you decrease the speed of the gas. In the end though, the same amount of gas moved at the same rate. Of course, this assumes that density of the gas doesn't change.
So, according to this, pipe size doesn't matter.
So can anyone show me where I am wrong and what you are trying to do with the gas to get better performance.
Thanks,
Jeff

[sykikchimp]

Your also forgeting the bends in the pipe also effect exhaust gas velocity due to turbulence of the gas within the pipe.  I do know that exhaust velocity is faster at higher RPM which allows the Motor to actually take advantage of the Larger piping, thus the increase in High end horsepower.  Not sure about the loss of torque though.  heres an idea though
Have you ever noticed that Cams and larger exhaust have the same kind of power adding trends..  Larger pipes increase high end, and lower low end, whereas Longer duration cams allows better high end and less low end ?  That says to me it has something to do with the Pressure within the intake manifold.  On an N/A engine the Air is pulled in by the piston moving, while on a turbo engine its shoved in due to an expansion of space and rise in intake pressure.  Without the air moving out past the exhaust valve at a specific speed you may loose velocity of the intake air, which doesn't matteer on a turbo engine.  Thus the larger piping decreases the speed at which the air can leave, thereby decreasing the speed at which the air can enter.  Over all this lag is defeated in the end by higher RPM's, and higher pressure's that can again take advantage of the larger pipe's velocity.


---There are NO facts in this.  Just an idea.  I'm sure there are people around that know exactly why.  However I feel I'm probably on the right track.

p.s. that had to be the most INCOHERENT post I've ever placed.

[DuffMan]

You want to remove the gas as fast as possible. That's why you dont want piping thats too big. Backpressure is always bad, even for a little 1.6 honda.

EDIT: Here's a good article that explains it in depth. <a href="http://www.magnaflow.com/05magazine/05sportc.htm" target='_blank'>http://www.magnaflow.com/05magazine/05sportc.htm</a>

(Edited by DuffMan at 6:26 pm on Mar. 4, 2002)

[sykikchimp]

a certain amount of backpressure is Good in an N/A motor. &nbsp;It is Bad in a Turbo Setup

[Natty]

This is what I have been hearing:
&quot;An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque&quot;

I just don't buy it. An exhaust of low velocity and high area is just as effective as an exhuast pipe with high velocity and low area. The same amount of air is moved at the same rate. Of course, this is in theory and I am going on straight lines for now.
I can't take this. My teacher was a rocket scientist/chemist. I'll have hime wrap his head around this.
Jeff

[DuffMan]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote </td></tr><tr><td id="QUOTE">Quote: from sykikchimp on 6:29 pm on Mar. 4, 2002
a certain amount of backpressure is Good in an N/A motor. &nbsp;It is Bad in a Turbo Setup
</td></tr></table><span id='postcolor'>

This is a common misconception.

Natty - you answered your own question. It has to do with the scavenging effect. As for what the scavenging effect is and how it works, well do a search on goggle.

[Fuzzy Ewok]

Yeah, read the first couple pages of that article...you really answered your own question. &nbsp;While the exhaust valve is closed, high speed exhaust will create a negative pressure area right next to the valve as the exhaust moves away. &nbsp;The faster the exhaust is moving, the higher negative pressure is generated (which is why huge, oversized pipes wouldn't work).

As the exhaust valve opens, the exhaust gasses inside the chamber are not only pushed out by the exhaust stroke of the cylinder, but pulled out by the negative pressure outside the valve (which is what is called the scavenger effect, I believe). &nbsp;In turbo applications, there's a huge amount of exhaust gasses that need to be moved, so bigger piping is required and is able to be properly utilized.

[White240sx]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote </td></tr><tr><td id="QUOTE">Quote: from Fuzzy Ewok on 8<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/wow.gif" border="0" valign="absmiddle" alt=':0'>5 pm on Mar. 4, 2002
As the exhaust valve opens, the exhaust gasses inside the chamber are not only pushed out by the exhaust stroke of the cylinder, but pulled out by the negative pressure outside the valve (which is what is called the scavenger effect, I believe).</td></tr></table><span id='postcolor'>

The negative pressure tail of a positive sound wave that is ejected during the exhaust event causes that negative pressure. When that sound wave reaches a cross-sectional disturbance (the collector in a header), it is reflected backwards. If the pipe length is &quot;correct&quot; it will arrive back in time to &quot;pull&quot; out lingering exhaust gases. So the header is the most important part of the exhaust system when trying to increase VE. The rest of the exhaust system only increases VE (aftermarket system of course) by alleviating restriction or backpressure. So in conclusion the difference between 2.25&quot; and 2.5&quot; is nominal at best. A larger exhaust pipe decreases velocity, as well as temperature, which creates a cooler, denser gas to move out of the exhaust system, but over all the flow is theoretically increased. A small pipe increases velocity, but decreases flow. For optimal performance, there are just to many independent variables to really get a conclusive number. A good methodically designed header and a 2.5&quot; exhaust would probably be a sufficient compromise between flow and velocity for people who are running a high output N/A motors.

[Fuzzy Ewok]

Interesting...that's more detail than I'd ever heard on that aspect of it. &nbsp;<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=':)'> &nbsp;Thanks!

[Gismo R]

Basically, an exhaust pipe that is to large is just wasted space. With piping that is has a &quot;perfect&quot; diameter and a &quot;perfect&quot; path for exhaust to flow, every particle in the exhaust would travel through the pipe parallel to the pipe, while not sacraficing power or torque. However, nothing is &quot;perfect&quot;. So with piping that is too big particles may &quot;swirl&quot;. This could end up slowing the exhaust flow.With piping that is too small, it would take too much force from the piston to push the air out. This would take away power. This is pretty general but maybe it helps.

[Jeff240sx]

I've talked to a few women in about this very subject!
I asked if pipe size REALLY effected performance.
The general conclusion was YES.
I asked why...
They said that the positive pressure in the compression chamber was better, whilst decreasing pipe size decreased back-pressure, allieviated restriction, and reduced overall combustion in the chamber.
White240sx was absolutly correct when he said with a larger pipe &quot;overall the flow is theoretically increased. A small pipe increases velocity, but decreases flow.&quot;
-Jeff