3.5" exhaust too much?
 

Is running a 3.5" full exhaust to much for a street ported gt35r rx7? I'm used to running only 3" on my EVO's and STI's.

Thank you

No 3.5" is good helps spool and keep egts lower, especially if you reroute the wastegate.

Thank you

I run whatever the size of the turbine housing size is all the way back. In my case it's 3"

Like said above, running the larger diameter exhaust will help with spool time, response, and power. If you're having an exhaust made than I would go with the 3.5" (or even a 4" that steps down to 3" with a 3.5" midpipe).

Typically 3" exhausts are MUCH more abundant and they tend to flow plenty for street cars. Personally I had PFS make me a full 4" exhaust (I'll be having Speed1 fix it soon enough, since PFS cut corners on the DP and the DP/MP connector), it's fairly loud but much less than when I had my WG dumping to the atmosphere..

Larger is better on a turbo car.... Or less is better.. With the exception of a short megaphone to be the ideal turbo exhaust....(if I remember correctly) However there is probably point where too large is pointless... No pun intended..

Ummm wrong, stop saying stupid shit on here please

I have a 4" DP tapered to 3.5" at the midpipe all the way back and I'm on the stock twins. Will be keeping the setup when i go single.

I have a 4" exhaust from turbo to exhaust tip. I had some guy at an exhaust shop custom make it.

NO need to get hostile

He has a point though. On a turbo, bigger is always better but it gets to a point where there is no more benefit obviously. 3.5-4" downpipe with a reduction somewhere down the line is the ideal street setup for performance and sound.

thewird

I said that cause first off, that guy said this which is dumb, then he also said in another thread that all oil is the same just in different bottles, just stupid answers with no research, proof, or anything to back up his claims, this is a forum for advise and learning and teaching and he is doing nothing but wasting time and space by saying dumb shit like that, and it's not just him, it's alot of people but I just happened to read his two replies back to back and it's annoying...

Haha gotcha. ;)

With a open dump wastegate, Would the size of the exhaust really benefit much from a 3.5" or 4" size (from an efficient 3" size)?

Wouldn't your flow up top effectively be the wastegate + exhaust size in that case?

Ultimately, it comes down to your power goals. If you have a midframe turbo with 15psi or less with pump gas, a full 3" will do fine. However, as you begin to make power north of 400whp, a 3" begins to choke the system.

Yes, venting to the atmosphere helps free some of that choking. This is why you hear of individuals picking up 30whp from venting their wastegate to the atmosphere with a 3" exhaust. But really, the underlying problem is the exhaust is too small. When shooting for bigger power, more size is needed. Rotaries benefit from a large hotside. This is is true from turbine to exhaust tip.

It would. By how much depends solely on how much pressure the system is already seeing. Again, it really comes down to power goals. Apart from noise or emissions, there really is no downside to a 3.5-4" exhaust on a turbo rotary.

I made 454whp @ 23psi on a T3 35R. This is with wastegate plumbed in a full 3". Spools was pretty good but my EGT's were quite high. I was absolutely stretching the capability of the system. I since upgraded to a T4 p-trim 35R, dual wastegate manifold, and full 4" exhaust. I have yet to dyno, but I am confident I will see over 50whp from the same boost level.

I have a 3" or 80MM that I run all the way back from my single turbo (T4 GT3574R) and I tuned it up to 17PSI or 14PSI sea level. I didn't feel much resistance/die off up top. But I have an open dump wastegate.


I am not sure of the power I am making, but by looking at how much fuel I am using (low 70's% injector duty cycle) and I have 5,300CC's of fuel running 40PSI base pressure. I have a street ported motor, I am guessing around low to mid 400WHP.

If you are guessing a 50WHP increase at the same pressure, that is a HUGE difference. But you should only see that at the upper rpm range.

If you have your car mainly as a street driven car, I don't see a 3" being all that restrictive, especially with an open dump wastegate.


Another thing I see a lot of people doing are running larger diameter downpipes and then to smaller diameter midpipe/cat back.

I figure that you would want the opposite of this. You would rather mate a larger cat back size than a downpipe or midpipe as the exhaust slows and cools it expands.

Perhaps I have it backwards???

Yes, 50whp total increase. However, spool and midrange power will jump dramatically. Unfortunately, I changed up more than the exhaust, so there isn't an ideal measure of how much it actually helps the system alone. But, I had a nice long discussion with my tuner, and he said the very same thing. 3" works great for 400whp, but as you go up, it just does a horrible job. His words, "we did one that made 430whp at 20psi, changed nothing other than a 4" exhaust and it did 481whp at the same boost level". This guy doesn't BS, or tell stories. This of course had me researching and looking into the correlation of bigger power and exhausts on turbo rotaries. They are like pea's and carrots. ;)

I made 386whp @15psi on the same setup. Power didn't come close to falling off with the 3".

As far as bigger to smaller goes, that is correct. You must remember that it isn't just the heat, but the velocity of the exhaust gases. I personally chose to use 4" all the way back. With proper resonators and mufflers in place, noise will not be a huge issue.

What size outlet does your turbo have?

You do have it backwards... As exhaust cools is contracts... Taking up less space... Resulting in smaller diameter needed the further from the turbo you are...

Yes, however, there is a fallacy in this argument in that as gasses cool they become significantly more dense and more viscous, increasing the losses in a given pipe diameter, especially in the bends.

Sorry, old thread.

IIRC, in conclusion, the bigger diameter is a good think all exhaust long, in front because of gasses volume, and in back because of gasses viscosity.

Are you guys stepping up the downpipe size? My EFR 8374 only has a 3'' exit. I'm having a super hard time looking for exhaust, downpipe, and mid pipe greater then 3'', 80mm. I guess I can do the full custom route. I'm just wondering how much more beneficial it will be, plus I love the way my Greddy TI-C looks. =(

my single TDX61 has a 3" exit but the DP is 4" tapering down to 3.5" to match a full 3.5" turboback exhaust, atmospheric dual wastegates. Not installed yet...