ok, so this is a JDM s4 13bt, not that it really matters. I got a 4 inch fart can, and 3 inch piping, currently with 2 cats WELDED on cuz the previouse owner is ghey n stupid. Anyways, since i gotta cut and weld a section back on anyways.

Does anyone have any experience with a 4" and 3" exhaust? I've heard if it being over kill to have a 4" but since piston turbo engines have like a 3" exhaust, i was thinking since the rotary relatively runs and produce more power then a 2.6 litre piston engine, maybe it would need a 4" to have absolutely no back pressure.

I want literally like no backpressure at all. Noise and legallity isn't much of an issue. I'll be mainly using this car for track. As for ground clearance, i'm not so worried about it i have an idea up my sleeve, but it'd take quite some custom fabrication though. This is basically going to be turbo back all the way.

any inputs?

 

A little back pressure can be a good thing. It can help with spool. A 3" exhaust is more than enough.

 

on a fairly stocksetup then 3" is gonna be plenty. However i did see a big difference in going to 4".

 

Any actual numbers or is it just the butt dyno?

 

hm...well for now it's fairly stock, but i'm going to up the boost on this thing until i blow the turbo, so i can get a bigger one along with other stuff you do when u get bigger turbo.

hm. i guess glass man is right. i forgot it helps spool a lil. guess i should just cut the cats out and replace with another 3 inch until i run it at higher boost. still got lots of restoration to do on this biatch heh

 

I felt like the spool was way faster when I had the DP disconnected..

 

oops. double post

 

3 inch will be plenty enuff until you get a larger turbo and engine management. 4 inch will most likely give you a boost spike and smoke your engine not your turbo.

 

"A little back pressure can be a good thing. It can help with spool."

"hm. i guess glass man is right. i forgot it helps spool a lil."

If both of you are reffering to back pressure after the turbine as in your exhaust system your 100% incorrect. Any back pressure AFTER the turbine (as in V-band back) will SLOW spool and DECREASE over all power potentual THROUGHOUT the engine power band. Going bigger will make the turbo spool quicker, be more responsive, and yield MORE power everywere in the power band. You just have to make sure you have enough wastegate flow to keep up. The bigger the exhaust the bigger your wastegate system must be to be able to maintain boost levels your looking to keep it at without boost creeping.

So, do you need a 4" system? As long as your wastegate system can exhaust enough energy away from your turbine you'll see quicker response and make more power per same boost pressure verses a smaller system. Seeing as though your running mostly stock stuff, I'd have to agree with whats been said so far and that 3" will suffice for what you have.

But as far as turbo exhaust systems go, bigger will ALWAYS be better if performance is what your looking. Yes there is a point were all your doing is making more noise without any performance gain, but you cant fit a drainage pipe under your car anyways

~Mike..............

 

There are no dyno numbers to what i have said besides the fact that i saw boost a few hundred rpms sooner and lost traction in 2nd gear. Where as before with the 3" system i had traction in 2nd. Also i found out the hard way that i didnt have the wastegate for a 4" system and would get crazy boost creep after i went up in size.

 

Why dont you just go 3.5" and call it even =)

 

I find a 3.5" DP, and a 3" exhaust is a good combo

 

Why the transition? It will still only flow as much as a 3" dp.

 

What turbo and wastegate were you running?

 

You can run a smaller pipe the farther back you go (to a point). As the exhaust cools it decreases in volume. It's super hot coming out of the turbo, so larger there is better.

 

see sig

 

3" wont really be any restriction at all until you're over 400rwhp and then it still flows pretty good.

stevekan said about a 2 rotor making 470rwhp that his 4" hurts his torque curve and he would have a better torque curve and not lose any flow by going to a 3.5"

 

Actually it spools faster, the greater volume of the 3.5 leads to faster spool, theremainder being 3" yeilds a better torque curve. Plus That's what I had, so I work with it

 

what does the transition look like? how many inchs to go between the two sizes.

 

transition is "JDM Style" About 4 inches of concentric pipe diameters welded, and ground smooth. I'll be taking a bunch of pics in about two weeks when the car is "done", before I go to Button Willow, just in case I wreck it LOL.

 

My 3.5" --> 3" transition was a smooth, albeit short piece of swedged pipe from stainlessworks.net.

http://dave.newbern.org/gallery/album01/aek

 

I'm a little rusty with this, but if I remember correctly a 3 inch pipe doesn't become *restrictive until around 900hp? (based on fluid dynamics)

 

My transition is perfectly smooth and is about 4" or 5" long. Looks just like a cone or something. I ordered it form burnsstainless.com.

 

I'm having a hard time seeing how the size of the pipe will effect the torque curve. IE, I can't think of an instance where smaller would actually help. It's all about chamber filling right? So bigger exhaust= less back pressure post turbo= allows the turbo to spool better= fills the chamber faster etc. I don't see how decreasing the pipe down stream would help anything. I agree that it might not hurt based on the cooling exhaust.... unless the larger pipe would slow the cooler air to a velocity that became restrictive?

As for the size of the exhaust based on power, I think most people, myself included would use a large DP for spool characteristics, not top end. A vast majority of people making big power are using vented WG's, in which case the size of the DP is virtualy irrelavent for peak power b/c the energy is being dumped through the WG's anyway.

-Chris

 

BACKPRESSURE DOES NOT HELP WITH SPOOL!!!! can someone please end this interenet car forum myth. Please go put your cat back on your car and tell me if it spools quicker.

With that said I'm real stupid and run a 5" DP that transitions into a 4" rear section. If I could run a full 5" I would but at the moment it just isn't happening.