This is all true. Especially the 3" being too big. Remember that the larger your exhaust the lower the backpressure and less low rpm torque. The larger diameter allows more exhaust flow and allows the engine to rev more freely increasing high rpm horsepower. 4 bangers already suffer from poor low rpm torque just due to their lack of cylinder size.

A smaller diameter exhaust decreases exhaust flow and, to a point, increases low end torque, while slightly decreasing peak horsepower. Remember that HP is (torque(ft-lbs) x RPM) / 5252. 4 cylinder engines (such as the much talked about 230hp S2000 engine) make large HP numbers by turning insane RPMs while 8 cylinder engines make high HP numbers by making high torque numbers. The trick is finding a nice median between High RPM HP and Low RPM Torque to produce the best all around performance.

 

Exhaust cutout FTW!!

But seriously according to a recent post any supercharged car under 350 horse is best with a good 2.25!! Like GMPP's!! People report 12 horse gain with it, I'm not saying they aren't seeing it with others but I have only seen one before and after dyno of a cat back install and that was with GMPP's.

IMO. Our stock exhaust is good if you change out the muffler, it seems to be the biggest restriction. Thats what my cutout is for!!

 

I have a Megan racing 3in turbo muffler. it is loud but i took the resintor out. I will getting my new JBP vortex header, downpipe and intake next week. i hope to get it put on it a couple weeks once the snow melts. but i have no complaints about their products. i had a Megan muffler on my Z24 as well.

 

I like this thread because i was thinking of what to do for an exhaust since its my next mod. And this has answered alot of questions.
Do you have any more info, or can you explain this in more detail to me? It makes sense if thats the size GM is putting out but would just like some more details to better understand. thanks

 

you need to have a certain exhaust velocity... you don't want to go too small, because it will be restrictive, and you don't want to go too big, because the exhaust won't flow properly... imagine this scenario... you're trying to drink your soda, and you're pinching your straw to be smaller than it is... harder to drink, right? now, imagine that your straw is 2 inches wide, it's just as hard to drink... same concept... you want a good velocity for the power that you're putting out... i wouldn't go over 2.5" untill you're putting out some serious power....

 

well i say to hell with everyone, its ur ride ou put what you want on it, if you want 3" go with the HAHN system header down pipe and exhaust. $1460. in my opinion, its hard to mix and match different headers down pipe and exhaust. size and fnl fitting is always an issue.

 

but we're trying to say that 3" is too big, and you will not gain as much power as a 2.5"... why would you tell him to get a 3"?

 

The horsepower is BHP. This graph was used in publications by magnaflow, flowmaster and one other company that i cant rememeber.

 

im not telling him to get it but if hes set on getting it let him, yes give your opinion but dont make him feel like an ass, especially when everyone start hounding on him. let him decide and whether he goes 2.5 or 3" thats his choice. thats what i hate bout this forum, right away people go against what other people want to do. thats when **** talking and animosity start. all im saying is dont taint his decision.

 

thats way too expensive for exhaust IMO

 

alright that makes sense. I like that scenario with the straw was a good way of describing it. Thanks
So now if i do a gm extruded manifold, and gm catback, what to do for a good aftermarket downpipe? ca 2.5? even tho the exhaust is 2.25?

 

3" is honestly bigger than you really need. You're going to start having a lack of exhaust velocity and it will shift your powerband upwards.

 

so are you guys saying that you should get a header with a 3 in collector

 

It's different w/ a supercharged car. A downpipe wont give you more that 3 or 4 hp keep it stock. header + catback will yeild the most gains.

 

like others were saying, buy your header, down pipe, cat and muffler and take it to an exhaust shop and get it done for alot cheaper

 

stock cat is very restrictive... and HEAVEY... holy hell, my downpipe felt like it weight 2 or 3 times as much as my cobalt addiction one... i did have 2 cats though...

 

x2, as well there is a 2" bottleneck at the o2 sensor for the 2.0L which had got to go.

 

i swear by my Corsa Touring 2.5" Catback system--low mellow purr at low RPM and sounds like a freakin ANIMAL at high RPM, even the butt dyno notices a slight improvement. not mindblowing...like a pulley or slappin' on a turbo, but slightly noticeable. i paid around 650 for the system and about 150 for the install

 

see now this is what i would like to compare , your 800 dollar corsa exhuast or my 350 crush bent 3 in exhaust

 

When it comes to exhaust flow and performance, crush bends are just totally the wrong way to go. You'd end up having a smaller total output of flow at the bends because it would be smaller in diameter than the rest of the piping.

 

ok so say i get the gm extruded and honed manifold, CA 2.5 downpipe with high flow cat and then gm sport 2.25 catback. will there be any problems going from 2.5 down to 2.25? This seems to be the setup in my mind that i will go with

 

the point of getting an aftermarket exhaust is to eliminate restriction... but going from a 2.5" pipe to a 2.25" pipe, you're basically creating the restriction that you're trying to get rid of... unless you're doing it ONLY for the sound, then it doesn't matter... performance wise, it will work best if they're all the same size...

 

I see what your saying. I was thinking that. How bout, gm extruded and honed mani, ca 2.5 downpipe, and magnaflow 2.5 catback? would that work better?

 

would be better, yes...

 

then there it is.... now i just have to find the best prices....