hello all, I want to do a "good thang". I am building a strong Ls/Vtec setup. I will be pushing some solid air with a modified [mangled] B18C1 intake and Type R valve train + camshafts. Bowl /seat/chamber work. I am starting the exhaust with a long pipe 4 into 1 "Dynatech" header with a 2.5" collector.
This setup is going into a 91 hatch. I want to get ideas about the exhaust back from the header. I need ability to absorb engine punches. I have to use a cat converter.
This will be a daily with "mad" stealth ability when I need some extra "candy". Should I go up to a 3" cat to slow down the velocity for exhaust cat cleansing? or should I reduce from 2.5" to increase the speed and get through the cat? I am stymied by my own ignorance. All ideas greatly appreciated.

 

i take it your ls/vtec is going to be non-turbo, there for i would make sure that you have Some type of back pressure. Your not going to want a lot of it because your purposly trying to flow a lot of air through the head, but remember that hondas like a little bit of back pressure unless your going with the turbo setup then you won't want anymore back pressure than the turbo is applying. hope that helps you make your decision.

 

backpressure is a myth, or so i've been told :dunno:

 

you dont want backpressure. you want to keep exhaust velocity up.

 

:werd: what i thought, but DM has been up my ass lately

 

ok well if u guys believe that backpressure is such a myth, take your stock motor & put a huge straight pipe on it. you will see that the car doesn't perform as well.

bottom end torque will be lost

 

thats not what im saying. what i am saying is that you need the exhaust to be properly sized to the engine's specs. too big and your velocity goes down. too small and your choking it.

 

I'd really love to see some 1/4 times with a consistant driver and a dyno sheet of a car with:

- a fully straight back pipe
- no cat + fag muffer
- 2.0" exhaust with good muffler
- 2.5" exhaust with good muffler
- 3.0" exhaust with good muffler

 

This is the guideline i go by when choosing an exhaust:
2-1/4" up to 210HP @ the flywheel (about 180-185 whp)

2-3/8" (60mm) up to 235HP @ the flywheel (about 200-207 whp)

2-1/2" up to 265HP @ the flywheel (about 225-235 whp)

2-3/4" up to 325HP @ the flywheel (about 275-285 whp)

3" big for big HP (Forced Induction: > 275 whp)

 

That's probably an awesome way to put it. The sizes for horsepower range.

My GReddy Evo2 for my Accord is 60mm, so I guess I'm not at optimal size (about 135whp lolol) but it sounds good.