Exhaust size

[MCBst]

So I did do a search on this, I was wondering what would be better in terms of exhaust size? 2.5, 2.75 or 3? I read in what I searched that 3 is mostly useful for going for a larger turbo later on, but is there a benefit to going with the smaller size? I know exhausts arent big power makers but I just was curious the pros and cons for the sizes (if any). My downpipe and exhaust are my next upgrades and I just want to get all my research done before I pull the trigger. Thanks for the help!

 

[TheStig]

3" - Pros: Most you can flow and/or likely ever need to flow. Cons: None.
2.75" - Pros: Bigger than 2.5". Cons: Smaller than 3"
2.5" - Pros: Uh... it weighs less... Cons: Its the most restrictive by design, no getting around that.

EDIT: I should mention, this does not mean 3" is the answer. But its your best option if all other things are equal (IE a 3" Cobb exhaust versus a 2.5" Cobb Exhaust, always go for the 3"). It comes down to quality, price, sound and maybe some brand preference.

 

[CanadianGuy]

3" - Pros: Most you can flow and/or likely ever need to flow. Cons: None.
2.75" - Pros: Bigger than 2.5". Cons: Smaller than 3"
2.5" - Pros: Uh... it weighs less... Cons: Its the most restrictive by design, no getting around that.

EDIT: I should mention, this does not mean 3" is the answer. But its your best option if all other things are equal (IE a 3" Cobb exhaust versus a 2.5" Cobb Exhaust, always go for the 3"). It comes down to quality, price, sound and maybe some brand preference.

I would add that if you want 3" also change the cat to be 3" for the flow.

 

[MCBst]

Ok, I was just curious because I heard from somewhere that you could lose back pressure or what not.

 

[Coolnezzz]

3" cons: louder, some people don't like that. Most of the 3" exhausts have a drone, especially from 3-4K RPM.

 

[BrokenSVT]

Backpressure. *giggle*

Backpressure has "uses" in N/A builds, but the term is essentially a bench racer/hotrodder old wives tail. In forced induction applications, backpressure is the devil.

 

[MCBst]

Thanks for the quick response guys! I guess 3 inch looks like the way to go.

 

[FORZDA 2]

Go with the largest down pipe available(3") and from there it really doesn't matter. I run a 3" uncatted (cp-e) DP with a 2.5" exhaust. Unless you're running a giant turbo with 40psi boost the 1.6L engine can't move enough air for any larger exhaust.

 

[AMontgomery]

There is also the matter of the physics of the way hot air vs not so hot air move through confined spaces. In some extreme cases, a larger pipe will flow less efficiently than a smaller pipe as hot air is less dense than cool air and a larger pipe cools down faster than a smaller pipe. The is definitely a point of diminishing returns. Though, I highly doubt that a 3" reaches that point on our cars.

Perhaps someone with a fluid physics background can chime in with more info.

 

[GAbOS]

Exhaust can be either too big, or too small really. In a perfect world, you would want an exhaust size that changes based on engine rpm. Ferrari actually uses two runners per cylinder in some cars with a valve opening and closing the larger of the two based on rpm.

"Velocity" is what you want from an exhaust system. The faster you can allow the gasses to escape, the better. Pressure has a lot to do with velocity though. I'll use the same analogy as was used to explain it to me once; a water hose.

If you allow the water to flow without an attachment it slowly flows out. Now, put your thumb over the end and restrict flow, creating back pressure. What happens? Increased velocity. There's all sorts of crazy equations for this.

You don't want as much pressure as you can put in it but you also don't want to eliminate it altogether.

 

[BrokenSVT]

Except that in a turbocharged application, the turbocharger IS the nozzle/attachment/thumb. It's the representation of backpressure to the engine. Sure, you need to evacuate the exhaust gases and sure, velocity plays into that. Explain that to the many dump-directly-off-the-turbine housing setups out there.