This will be a daily driver, aiming for 250hp with a t3/t4 running around about 5-8psi. Is a 2.5" downpipe big enough or should I go 3"?

 

I wold go 3" just because no body is ever happy with just 250 whp. Boost is addicting and you'll want more power. I won't run less than 3" in any boosted setup.

 

I was content with my GSR when it was NA.
Anyone else? I'm sure it should spool faster even at 250, maybe?

I want to stay conservative so I don't break junk, and mild turbo seems better than a crazy NA build for power and reliability.

 

if your making a new downpipe save money and just make it 3inch that way you will always have it. then you wont need to be making a new one... 2 1/2 is small. every little helps.

 

If you already have a 2.5" set up, Just get a 2.5 electric cut out. Will help you get the gasses exited quickly and help with quicker spool.

Another benefit is being quiet on the street.

 

Quiet is good. But how would larger be quieter? I would think it would be noisier, like how a larger exhaust sounds like poo on a NA motor. Can anyone back up what EsotericImage said?

 

I think the 2.5" will be fine. A 2.5" isnt really a restriction until like 500hp. I do quite a bit of reading and somewhere over time i read (from a knowledgable source) that a 3" exhaust isnt even really needed until like 600hp.

 

Sorry to thread jack, but will a full 2.5 exhaust be good for his setup? Downpipe and all. I can either make one of these for free, or buy a 3 inch. thanks

 

It will run better with a 3"

Make sure when you tune it you have the exhaust thats going to stay on it.

 

IMO larger is louder.. pushing exhaust out a 2 1/2 hole is more zingy but not has loud as 3in. on any application i'd say bigger is more loud

 

you've read a no no. more like 350hp is when 3in should come into play.. A identical car/setup will make more with 3in rather than 2 1/2 if trying to make 350 and higher. i mean just look at dyno sheets. it shows. Also make sure the whole exhaust is 3in. not just a 3in downpipe going into a 2 1/2 exhaust

 

I think you will be fine with 2.5" for your goals. I've been running a 60mm exhaust for many years and pushing 20+ psi of boost. These guys seem to want to answer a question that you are not asking. Would we make more power with 3"? That answer is yes, but would you enjoy the car more? I know I wouldn't. My car is super quiet but I can eeeeeeeasily spin 3rd gear from a roll. Can pick up a nice lady for dinner and perhaps wail on a Vette on the way if I wanted to. There are other aspects to enjoying a powerful car you built than dyno numbers. Enjoy the ride.

 

Changing my DP and exhaust from 2.5 to 3.0in gained me almost 50hp with boost level the exact same on the same dyno on the same day. 247hp to 301hp a 9.5psi Drag kit on a stock del sol GSR. That was a good gain for me for $350

 

I know the 3" will make more power especially on a turbo car. But that doesnt nessecarily make it a restriction.

Its like if you have a turbo that will flow a certain amount of power. Say a 35r (65lbs/min give or take). When youve maxed that turbo out on a certain amount of boost and turning the boost up doesnt really do much for it. This turbo on a V8 is gonna become a restriction way before it will on something that wont flow as much air.

I still say 2.5" Ive got quite a few friends with good flowing turbo kits with hacked up exhaust that had to be welded like 5 times before they even got under the block. 1 of which has an exhaust that i swear probably goes to 2". Making 400+whp. Of course it will make more with a good flowing exhaust but it hasnt gotten to the point that the 2.5" is so small that its holding the car back from making anymore power.

 

if you dont wanna break junk i wouldent boost the lsv. strokes diff it deff is the way to go for short term power, but in my opinion lsv is a waste of time to boost and dd it, life span wouldent be up there as if it would be on a norm b16 boost setup.

 

What i was saying was to stay at 2.5" if you already have that set up. Use a electric cut out where the cat would be to have a free flow.

the 2.5" / 3" debate has been going on for years.

heres a good post of exhaust theory.

http://www.nsxprime.com/FAQ/Miscella...austtheory.htm

Now, it says pipe sizing should be 3" to 3.5 for 250-350hp which is what i was saying, instead of cutting everything up and making a new DP and cat back, just throw a cut out in it.

as far as being quieter, Im running 2.5" catless with a magnaflow resonator, and Vibrant sleeper muffler.I have a cut out for when someone wants to get cute on the highway, and still quiet on normal cruises down the interstate. Im ALOT quieter when i start it up in the garage, and when im at a red light next to the police.

Another thing I dont have is the droning cabin noise coming off some overpriced fart can exhaust.

Everyone can do what they want with their ride, im just giving you what route i went with.

 

You my friend Have no idea.! what do you mean short term power. a b16 isnt going to last any longer when a engine is built right. A b16 is the worst engine to ever have. its like a Sohc until it hits vtec. then it kinda starts to move!

 

I think I'm going 2.5 inch then. Thanks all!

 

I can see what he means a little bit. The better r/s ratio, and being a vtec block with the extra webbing...

But LS motors with supposedly "bad" r/s ratios are known to last a long time. And I'm using a GSR block. So....to heck with the B16s, my motor will be fine.