How bad is 3in exhaust on n/a engine ?
 

Stock 240sx with ka24de, i wanna get a catback, almost all the catbacks are 3in, How bad you think the torque loss with be ? I was thinking it woulnt be so bad considering that its stock headers and stock cat, there is probably enough back pressure from those.. Any thoughts or experience specifically with a 240..

Thanks

I went from a 3" to a 2.5" on my old car with a KA and didn't notice much of any difference other than a loss of torque in the upper range

allright im gunna go 3in. do you know if a 3in catback bolts up to the stock cat ?

Just curious... For what reason(s) are you using for going with a 3" cat-back?

Its what is widely available. Most are 3in since lots of people run sr20 or ca18 engines
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Have you done any calculations or modeling? I think it will do more good for your application (not to mention more immediate) than asking.

Modeling ? Calculations ?? No. I dont know formulas for airflow based on pipe size. But thanks for that....
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you will get all the needed back pressure from the manifold down pipe and stock cat.

2.5 is more than enough for an NA imo.

I would suggest that you have an exhaust built for you. Use our muffler and tip and have our suggested exhaust shop make a 2.5" exhaust for you. It will make you very happy.

Just because a size is widely available or popular, it doesn't mean it's ideal. I had full 2.75" piping on my old CA18DETT (yes twin turbo). My suggestion would be a 2.25 or 2.5, max.

I Know its not ideal but i can pick one up cheap.. I might just fork out the money and get a 2.5..

Thanks for the input.

2.25 to 2.5 for N/A

The 3" catback won't really make a difference because the stock manifold and cat will be the restriction anyway. The one thing that might cause problems (depending on the catback you use) is where the 3" catback meets the stock cat. Sometimes they don't seal up nicely and you can get an exhaust leak.

Yeah, that's my main concern is getting one that will mate with my stock cat with out problems. Although, the flange is huge on the stock exhaust.

LOTS of misinformation here about max 2.5" or whatever.
There have been many dynos on a 4cyl engine making more power with a 3" exhaust. Bigger 4cyl like 2.2L and up, not civics. I run a 3" exhaust myself on my prelude and I had a 2.25, 2.5 and then I finally ended up with a 3". You would probably not benefit as much because of your stock header and exhaust so you should change those too if you want more power. Tuning will be everything though.

I wouldn't say that it's misinformation. By my suggestion of 2.5" max, I'm trying to give him the best of both worlds. Smaller = better low end torque, Bigger = higher top end. By going for something in the middle, you sacrifice a little on each end, but retain most of the rest. Yes, the stock headers and cat will help with torque, but if he decides to upgrade them at some point, he'll be back at square one if he wants to go for more lower end torque.

This theory of smaller exhaust = more torque has been thrown out the window for a few years now. Yeah I know it makes sense but its been dyno proven already that a 3" exhaust makes power through the entire power band. This was done on a bone stock h22. There are a lot of preludes out there running 3" exhausts now and I know this isnt an h22 but it is a 2.4L motor.

yeah, the stock headers are pretty good too, comes stock with a decent size 4-2-1 header.. I think im gonna go 3", there is just way more of them on ebay :p

Lots of guys running N/A KA24's on Zilvia.net have gone from a 3" to a 2.5" because the added backpressure of the larger piping decreases their max WHP. Also dyno proven.

well here are a few threads just for you to read up on. 3" exhaust makes power over a 2.5" even on an NA k20.
http://www.honda-tech.com/showthread.php?t=2341715
http://www.honda-tech.com/showthread.php?t=2197695
I know where you are coming from and a lot of people don't believe it. It is really up to you. I speak from experience and not just listen to what all the people say. My friend also runs a 3" exhaust on his pretty much bone stock ka24 and although its not very fast, its fast for what it is. Another reason the people that have shown dynos of just bolting on a 3" exhaust and losing power is 99% because thats what it was....bolt on. You need to tune it to the same AFR as it was doing with the 2.5" exhaust too.


I also run a 3" exhaust on my prelude....can't say it doesn't work. It ran 12's...NA
It also made more power on the dyno although I can't find the dyno right now.

I also went as far as looking on a 240 site to read up about 3" exhaust on NA KA24.
http://www.240sxforums.com/forums/ka...ade-guide.html
Seems to me like....nothing but gains.

A lot of people just go straight to 3" despite any losses in the lower range because they're likely either going to go KA-T, SR, or RB.

run a 3"

it'll be fine. and loud. but fine.

i did go 3inch, picking it up tomorrow.