So what are the criteria that determines the diameter exhaust you should use?

Seems there must be some logic behind different downpipe sizes (2 to 3"), so what about exhaust and test pipes?

Im running about 400 DD rwhp. I have a stock manifolds, Injen SES exhaust (2" ID I think??) with Helix test pipes and am seeing 15psi. I wanna go to a FI specific exhaust and reduce my PSI's while hopefully picking up some HP.

Are there some flow guidelines to reference between a 2.5", 2.75", and 3". I think like an american that bigger is better, so I'm leaning towards the 3" (AAM, FI, or APS).

what say yee?

Thanks

 

I say that bigger is better when dealing with FI, but the minimum size for power output goes up with how much air you're flowing. The motor is a big air pump, and when you're compressing air into it, it needs somewhere to go, and the larger diameter the exhaust the more efficient the system will be at higher RPMS when the turbo/blower flowing to its peak potential.

That being said, I think on our motor most 2.5" true dual exhausts are good for basic builds up to 550-600whp or so. True dual 3" are good for anything beyond 600whp. Anything under 2.5" true dual or 3" single is not an FI exhaust and will only hurt power delivery.

The argument that you make more torque with a smaller exhaust is kind of misleading as well. I say this because when the motor makes a little more low end torque, the top end is suffering to a greater extent. The tradeoff is far from worth it.

Just my take...

 

After the turbo, opened up as far as possible is the best. Back pressure on the turbine reduces the work out of the turbine which as you can imagine hinders the spool rate.


Look at large gas turbines in electric plants or ships.

 

Please keep in mind, he's Vortech

 

Missed that.

Nevermind me.

 

you can put the biggest exhaust possible on the car and it'll help some, but you're still going to be bottlenecking at the engine itself with the stock exhaust manifolds/heads/cams. That's the area I'd bet that would be the biggest improvement for hp/psi.

the optimum setup is one that doesn't change diameter all the sudden. You're better off improving the flow evenly all throughout instead of focusing on only one end of it

the intake manifold/engine/exhaust manifolds is where air is going to be the most dense and directly impact your psi reading

 

Couple things to keep in mind. Not all exhausts of the same diameter flow the same necessarily. Muffler choices and exhaust design will impact flow. And I agree with sentry65, with the cavaet that the length of the restriction is also important, and the length of the cat-back exhaust outstrips the pre-cat piping in length.

 

I'm gonna have to agree with sentry. No sudden changes in diameter, smooth bends and 2.5 true dual or a single 3'' will serve 98% of the people on this board.

 

Ya i know i definitely have flow issues. Making 15 psi with HP similar to lower PSI blower setups.

Its the heads, manifold or exhaust, or a combo of all 3.

Replacing my exhaust and test pipes with something larger is easiest.

Manifolds, i dont really wanna change bc who keeps a blower forever and doesnt go turbo? Also, headers never show that much improvement on the dyno. So are they actually flowing better then the stock manifolds??

Cams and Head work is outrageously expensive and would only be an option during a motor build.

engineering is alot of math so there seems there would be specific guidelines on diameters, etc to use.

 

With a SC, you will probably be just fine with a good, straight through design, single 3" system or even a straight through dual 2.5" system.

Stay away from anything with chambers or baffles.

 

For the very least do some headers & hfc, there will be nice gains to be had with a retune.

 

I'm just doing resonated test pipes and keeping the stock exhaust with a Vortech and a 3.12 pulley. I'm hoping this gives me good mid range torque and keeps the sound at somewhat of a reasonable level. I have heard from more than a few that going too big with a supercharger will lower your mid range torque and make it even more boring to drive. You are flowing a lot more air than I will so if I had to make a suggestion I would say go with the Borla and resonated test pipes. Shouldn't have any rasp and should have a nice deep toned exhaust note.