2.5 inch pipe with exhaust... Do or Don't?

My local muffler shop is going to put on a 2.5inch pipe from behind my cat going to my OBX exhaust for $140, or just add the exhaust for $65. What will I be the gain of the 2.5inch pipe?



:monkeypis :badass:

well if your going turbo then 2.5" is good, but since you are NA, i wouldnt go above 2.25". remember, NA engines like backpressure, turbos dont.

If your going to get headers. cams. and others like that....go 2.5 if not.......dont worry about it.......!:alien: :bloated: :licker: :smoka: :flash:

I posted this over on the other forum as well. Thought you guys would get a kick outta it:


Strange this should come up. In the May 2002 Issue of Sport Compact Car under Letters, Page 26, Bill Luton, Pres. of Bill's Isuzuperformance.com did some dyno tests and Bill came up with a whole different picture on exhaust diameter.

You should really get this and read it.

He was able to prove that (or should I say recommend) the following should be the standard:

- 2.5" diameter for all 1.5 to 2.5 liter NA engines as the minimum size
- Any thing larger than 2.5" for 2.6 litre NA engines and larger

So with that said, I am still gonna keep my 2.25" piping and wonder if Bill was right about the 2.5".

And with that said, I will be keeping my 2.25" exhaust. I see it this way, Bill did not test the SR20DE's, but tested American compacts and a GEO (Isuzu). All engine manufacturers are different and different mods effect them differently. Especially exhausts and intakes. SO, he was able to prove his point for a Cavalier, Sunfire and GEO, but stick with what is being told about the 2.25" NA.

Originally posted by NismoPC
I posted this over on the other forum as well. Thought you guys would get a kick outta it:


i remember reading that in SCC as well. i don't understand his logic. i'd love to hear what he'd recommend for a 5 liter mustang. possibly a 5" exhaust? :D

SR20 with headers and exhaust - go with 2-1/4" piping. No dilemma there.

I went with 2.5" from my straight cat to the exhaust. I left 2.25" from my header to my cat. My car gets up and go a little quicker when I had 2.25" from my cat to my exhaust.:sun: :flash:

Originally posted by AznVirus
remember, NA engines like backpressure, turbos dont.

I'd really like someone to quote me a credible source for the concept that backpressure is good for any engine.

Please no "it's common knowledge" replies. That won't cut it. I'm a little bit .... ehhhh ... less youthful than most on the forum (bought my first car in '78). I've been "into" performance since the mid-seventies and been researching and tweaking ever since.

I've been an engineer for 11 years. My specialty is electrical engineering but I've got a background in mechanical engineering (and have studied fluid dynamics and thermo dynamics).

Any benefits coming from a smaller diameter pipe would result from increased exhaust gas velocity, not increased backpressure. Diameter is critical in header design for this reason. However, backpressure is just pressure. The higher the backpressure, the more restrictive the exhaust system, the more work must be expended to force the exhaust gas through it.

Backpressure is just another parasitic drain on engine output.

If someone can point me to a credible source to the contrary, then I'll change my view. Until then, my formal education and my 20+++ years of messing with cars say "it ain't so".

...my recommentation is to go with the larger diameter pipe.

It won't hurt your performance and it'll help your engine breathe a little better ... especially at higher rpm.

... and you won't have to worry about your exhaust becoming a performance bottleneck should you decide to further mod the engine down the road.

Originally posted by bump909
i'd love to hear what he'd recommend for a 5 liter mustang. possibly a 5" exhaust? :D

Not to be a smart ass... but he'd probably recommend a dual2.5 inch exhaust.

Now to be a smart ass.... What would you recommend? :)

Originally posted by killick


I'd really like someone to quote me a credible source for the concept that backpressure is good for any engine.

Please no "it's common knowledge" replies. That won't cut it. I'm a little bit .... ehhhh ... less youthful than most on the forum (bought my first car in '78). I've been "into" performance since the mid-seventies and been researching and tweaking ever since.

I've been an engineer for 11 years. My specialty is electrical engineering but I've got a background in mechanical engineering (and have studied fluid dynamics and thermo dynamics).

Any benefits coming from a smaller diameter pipe would result from increased exhaust gas velocity, not increased backpressure. Diameter is critical in header design for this reason. However, backpressure is just pressure. The higher the backpressure, the more restrictive the exhaust system, the more work must be expended to force the exhaust gas through it.

Backpressure is just another parasitic drain on engine output.

If someone can point me to a credible source to the contrary, then I'll change my view. Until then, my formal education and my 20+++ years of messing with cars say "it ain't so".
Maybe this may help.:D
"What all the hub-bub about exhaust backpressure? I thought I read somewhere that backpressure is irrelevant and it doesn't matter how big an exhaust pipe you use and that bigger is better."

Sam Strano (strano@ncentral.com)
Not entirely correct. I agree that backpressure isn't relevant; the less the better. The hitch is that pipe sizing is very important to the flow of the system. The bigger the pipe is the less pressure is there, however if the pipe is too big for the amount of air an engine can move, you'll lose power, even though there isn't any backpressure. If you run a 3 inch system on the car, the velocity of the exhaust gases is going too slow down, because the gases have to expand to fill the pipe. And slowing down the exhaust obviously isn't what you want."

http://www.se-r.net/faq/index.html

I believe he's staing that the main goal is to ensure and maintain a continous flow through the exhaust system. Perhaps that's why Nissan/Infiniti have put on variable-capacity mufflers on most of their cars, designed to relieve backpressure above a certain rpm by opening valves/ports located inside the muffler. Ex:The 95-99 Maxima/I30 engine made 190 hp. By putting on a variable-capacity muffler(along with some other minor tweaks;) ), they basically took the same engine and gave it a big boost in HP to 225 HP.:D
Come to think of it,:thinkerg: I'm going to make my car a guinea pig to see how much HP I can muster with the POP-intake, Hotshot Headers, '91 intake cam, and a 2000-2001 variable-capacity muffler on my 95 G.:toothless

Hmmm. Should be interesting.:evillaugh

Thanks for the post, Howard.

But I'd have to disagree with Sam on that one. The main drawback to large exhaust is weight (and the difficulty of bending the pipe appropriately to get it to actually fit). Gas expanding to fill the diameter of the pipe would be irrelevant... it does that whether the pipe were 2 inches or ten. At the exhaust tip, the "diameter" becomes infinite, the gas expands infinitely and the exhaust velocity slows to zero.

Remember, we're not talking headers here ... just the exhaust tubing downstream of the header collector.

If you swap in a pipe diameter that slows the velocity by half, we're still getting the same flow rate.

Think of it this way: We've got an engine running at a certain rpm, pumping out a certain amount of exhaust gas per second. We take the stock pipe off at the cat and put on something larger.

Even if the speed through the exhaust tubing is lower because the tube's larger, the amount going through there per second is still the same. Now if we go further upstream past the collector on the header the velocity at that point is the same as it was with our smaller tubing... same amount going through the same diameter primary tubes in the header. So the flow rate in the header (and therefore at the exhaust port of the engine) is unchanged by the larger diameter exhaust tubing.

What does change at the exhaust port is the amount of effort required to force that amount of gas down the tube. The larger diameter tubing offers less resistance to airflow (less back pressure), making it easier for engine to purge itself on the exhaust stroke.

That's cool, bro;). Not trying to start an argument, just stating some facts that I found and trying shed some light on the subject, that's all.:D

Y'all ready for this.

Back pressure is not good for power in ANY engine AND exhaust gas velocity is paramount to making power.

To explain these statements let's look at the physics of the system.

What we have is a column of exhaust gas moving down the pipe which has both mass and velocity. Therefore it has inertia/kinetic energy according to the following formula:

E(k) = mass X velocity squared or m x v x v

When the piston has slowed near the top of the exhaust stroke but the exhaust valve is still open - the column of gas travelling away down the exhaust system exerts a pull on the remaining exhaust in the combustion chamber by virtue of it's inertia.

This creates a low pressure in the combustion chamber which "scavenges" the remaining gas from the chamber and leaves a partial vacuum to aid cylinder filling on the subsequent intake stroke.

The magnitude of this partial vacuum is proportional to the inertia of the exhaust column which is proportional to the square of the velocity. Hence the importance of exhaust gas velocity. It not only exhausts the cylinder more effectively, it also aids in cylinder filling on the intake stroke.

A larger exhaust pipe diameter serves to decrease exhaust gas velocity and too large can decrease it enough to lose power. Power will be lost in the low revs first then progressively into the midrange and high range as pipe sizes get larger. Evidence of this can be seen in vueman's post above. While he increased the mass of the column by increasing some of the pipe diameter, this was more than defeated by the velocity loss in that part of the system and exhaust gas inertia suffered.

Tuned headers achieve the same scavenging effect but do it a different way. Headers take advantage of reflected pressure pulses (sound waves) to aid scavenging in much the same way as tuned intakes do(although in reverse). In fact, intake and exhaust systems can almost be considered as reversed versions of eachother.

I'm very sure about the above explanation and any questions, comments or criticisms are most welcome.

Oh, and here are some "credible sources"
http://www.isd.net/jhadfiel/exhaustsize.htm (make sure to read the blurb under the table)

Also go to http://www.speedoptions.com click on the pro-tips tab, go to page 12 and read The Exhaust System Part II by Mike Kojima. In fact, all of the tech articles there are worth reading before spending your (or somebody elses) hard earned money.

sounds dumb but ive heard from a couple of people say that too little backpressure will burn exaust valves . and that not enough backpressure will decrese torque . not sure of the credibility of either ..