Big turbo and exhaust diameter

[Paris turbo]

Hi guys! I am going to switch to a bigger turbo for my 1nzfe ! thats a gtx2863. my power goal is 270whp . for what exhaust diameter should i go for? 3inch or 2.75? i also have a 38mm external wastegate open dump.

[kou]

2.5'' exhaust is supposed to flow 500hp.

[fnkngrv]

I am moving bigger here soon from 2.5" since I should be producing more power. I am currently sitting around 250 to the wheels on my street tune. I have also heard that 2.5" can flow up to 500hp however the key to that is back pressure. If you are running a log style manifold I would go 2.75", but if you aren't looking to break 300 then no need to. You very well could get away with 2.5" still however.

[malibuguy]

No no no back pressure!

2.5 is good to about 250 wheel. After that 3"

[fnkngrv]

Also make sure that all ingress and egress points are smooth transitions. No sloppy welds internally causing turbulence because that will in turn increase the effect of any back pressure present.

[Paris turbo]

Originally Posted by kou

2.5'' exhaust is supposed to flow 500hp.

yeah but the question is ''how much power is left on the table?"
my exhaust maker says that a 3inch is too big and i will lose torque down low. doesnt the turbo spool faster with a bigger exhaust???? thank you all for your answers i am going for the max efficiency

[kou]

I guess you have to decide if you want all the power in the higher revs. You could do a 3'' downpipe then transition to 2.5''. from what I have heard the down pipe should be 1.5x the size of the turbine exit hole then a smooth transition to a smaller size pipe to keep up velocity.

[fnkngrv]

Good point Kou. Also if you plan on doing cutouts be aware that it will pretty much eliminate all back pressure however as Kou said all of your power and response will be in higher than normal rpms.

[cali yaris]

2.5 is good to about 250 wheel. After that 3"

this is not correct. I got 357 whp with my 2.5" exhaust, and it wasn't what was getting in the way of more.

[fnkngrv]

Originally Posted by cali yaris

this is not correct. I got 357 whp with my 2.5" exhaust, and it wasn't what was getting in the way of more.

I can't recall.....did you have a log style manifold at that power?

[kou]

Originally Posted by fnkngrv

I can't recall.....did you have a log style manifold at that power?

I believe he was using the manifold that was sold at MI but with external wastegate.

[malibuguy]

Well in most cases 250 is about the reasonable limit for 2.5" done plenty of testing to verify this. Did you have an open WG ?

On a turbo car...the idea is the pressure differential from the manifold to the downpipe is what drives the turbine. There is such thing as too large. The rule of thumb is use the next size up diameter tubing from the diameter of the exducer of the turbo. If recirculating the WG go another size.

Now to contradict that statement...we developed a 3" downpipe for the new ecoboost mustang for another company. The stock downpipe at one point is 2.25" for a good bit. They make 310flywheel. Needless to say with a 3" downpipe they pick up solid power and then with a tune to work with the extra airflow made even more power...like stupid power.

My tercel makes 130/174 with 2.25". Going bigger made no difference. Going smaller i lost power across the board.

[fnkngrv]

The outlet of my turbo is 3"

[CrankyOldMan]

Having just purchased a 2.5" res-back setup from Malibu, I feel my manhood is inadequate... Unless we're just talking about a stock 1.5 with a Blitz.

[Blown_xa]

3" even at lower boost and power levels will still be more efficient than a 2.5". Even when I had a tiny turbo and less than 200whp, I switched from 2.25'' to 3'' and it was a 17whp improvement. My xB ( which I sold 2 months ago) was 319whp with a 3" exhaust, I then put back the 2.25" Borla to quiet it down for daily use and it was pulling teeth trying to tune it past 270whp. Huge difference, and also because on the same wastegate spring we couldn't get the same boost with the boost controller maxed out... do to the exhaust back pressure.