Exhaust[ing] question.

[jknappster]

I've been struggling with this topic for some time so please put me out of my misery. I would like to achieve (eventually) 600 NA ponies out of my gen 3. Like most, I like to purchase parts with the end in mind. My struggle, can this be accomplished using the stock exhaust manifolds and cats but upgrading to a Corsa cat back specifically the 2.5 inch version.

Areas that cloud my thought: what size pipe is the Gen4 stock system? Since the side exhaust system is so short, is there really measurable flow difference from a 2.5" to a 3" Corsa cat-back at the 600NA level? Are both Corsa versions 3" throughout or is the 2.5 version just have a 3 inch tip?

The end project, like "project ******** 700R" will have ported heads, cam, etc.

 

[345s-bspinnin]

The 2.5" exhaust muffler on the Gen IV is identical to the Gen III, except for the crossover. It as been shown on multiple occasions that cat-back don't gain any power over the OEM mufflers. There are several Gen III/IV 600whp+ NA running on stock mufflers. As a matter of fact, Dipapa is pushing somewhere around 650whp on his Gen III heads heads/cam car with stock mufflers!!!!

 

[jknappster]

Good deal! When you say mufflers are you referring to the complete system from the manifold to tip?

 

[AbsolutHank]

Nah, just the mufflers.

You'll need HF cats for sure, and just find some used 08 tips to eliminate the crossover and keep the mufflers factory.

 

[345s-bspinnin]

Good deal! When you say mufflers are you referring to the complete system from the manifold to tip?

By muffler, I mean just the muffler (resonator). The headers are different between Gen III/IV. Post muffler, the Gen IV just has the turnout pipe, while the Gen III has the cross-over with H-pipe.

 

[JonB]

NO PROBLEM, so long as you have HF cats as mentioned... you will actually get a torque bump with the 2.5" as shown in numerous Belanger tests....

Out of your misery now?!

 

[purplesnake]

NO PROBLEM, so long as you have HF cats as mentioned... you will actually get a torque bump with the 2.5" as shown in numerous Belanger tests....

Out of your misery now?!

You better "splain it to him Lucy", I mean Jon, why he may be better off with the 2.5, over 3.0.

 

[Sonoman]

NO PROBLEM, so long as you have HF cats as mentioned... you will actually get a torque bump with the 2.5" as shown in numerous Belanger tests....

Torque bump... I believe that, 4-stroke engines need a certain amount of backpressure for optimum torque (depending on a lot of other factors). I remember once having an exhaust restrictor blow out of one of my motorcycles, and the performance of the bike was immediately less snappy/torquey. It sounded more powerful, but it certainly felt less powerful, and even a butt dyno is pretty accurate when you have such a sudden change not separatd by down-time, temp changes, etc. I would think less exhaust restriction would benefit high-end HP, though, especially if a re-tune is done with the less restricted setup.

 

[jknappster]

NO PROBLEM, so long as you have HF cats as mentioned... you will actually get a torque bump with the 2.5" as shown in numerous Belanger tests....

Out of your misery now?!

Close, getting less painful. So gen 4s come with high flow cats since their 600 ponies.

 

[JonB]

Close, getting less painful. So gen 4s come with high flow cats since their 600 ponies.

NO !

The G4 cat-back is less restrictive and cooler due to NO CROSSOVER vs Gen 3. Gen 3-4 cats are the same.


I think the torque bump got 'splained by the bike post above! Some backpressure is needed to preserve torque. That torque might be high, mid, or low range, depending on several factors. Proven on dynos.

In hard-core, championship autocrossing, (under 1 minute runs mind you) those who have experimented have found slightly FASTER times with 2.5" than 3" equivalents... Torque Bump! Drags Too, per Belanger, and I did attend one such test in AZ.