Torque drop with GT35R
 

I have a GT35R turbo setup that has been running great, but falls off on torque at higher rpms. Basically, torque starts taking a nosedive at around 6750-7000 rpm, though boost level is still maintained. My guess is that it is either due to my Greddy SMIC (relatively small) and a somewhat tortured intake path, or it is due to having a bone stock port motor. The torque drop is quick enough that the hp peak is about 6800rpm. Comments?

I should mention the GT35R has a smaller turbine at .82 A/R (or .86, I can't remember). However, I'm only running about 15psi so I wasn't thinking that was the "problem". Exhaust if full 3" with no cats.


-Andy

I think your exhaust A/R is too small.

No reason not to go 1.06 unless you really need the early power band.
If you're complaining about no power on top, then a 1.06 A/R turbine should fix the problem.


-Ted

It is the stock porting, not the A/R. Even with the 1.06A/R turbine the same thing will happen.

not true sir, the stock ports dont create high rpm power fade, just induce more lag and about 10 - 20% less OVERALL power than say.... a bridge. its all in the hotside.. yes get the 1.06 :D

I was under the impression stock ports will not create lag...the exact opposite actually...and flow less up top.

Port timing and sizing work just like cam timing and lift. The total combination determine torque curve. I have stock ports and a GT35R and my torque is only slightly better on bottom and top due to this turbo then when running non-seq.

Torque curve will have the same basic shape no matter what the power flow of the turbo unless it is at extreme end of the spectrum.

Can you post dyno sheet ? Maybe it's better if we see it. How about AFR at that range ?

I still say it's the small A/R. Ive seen dyno runs of a stock port 20b with non-seq stock twins make more torque than hp. The twins in this situation quickly ran out of their efficiency range creating a huge drop in power in the upper rpm's. Any single turbo with a small exhaust A/R will spool very early therefore causing the compressor to quickly spool out of it's efficiency range in the upper rpm's.

Its definitely your 1.06 exhaust housing.
Porting has nothing to do with it.
Even the 1.06 exhaust housing will drop off early.

well, an engine makes peak torque where it is the most efficeint (usually as you hit peak boost) and will drop off afterward. So, you may be pushin the stock ports pretty far.

There are guys here in aus that use Stock Import s4 blocks and make power above 8000rpm.
Its all in the turbo.

Only by being totally useless below 5000rpm, and having a lot of turbo lag.
I want to see totol engine specs and a dyno plot for this myth!

I have studied a lot of dyno charts at http://dyno.zeroglabs.com/dyno.php.

You have studied dyno charts. yet you know jack shit about cars.
there is more to a car then a dyno sheet.
You study the car. Not the dyno sheet.
Were do you get your facts from? Are they even facts or just myth? You havnt prooved jack shit yet?
Weve got cars here that have boost from 3000rpm and making power all the way to 8000rpm. The tuners here know what to do to get the turbos coming on early. And use the right exhaust front wheel combos.

You are seriously mistaken if you think Chuck doesn't know anything about FDs or tuning, LOL.

Every stock-ported engine dyno I recall seeing has hit peak HP by 7k rpm, unless, like Chuck said, was running a huge ass turbo with no low end.

Wasnt talking about Fds. Am talking about rotarys in general. If you can make peak power past 7k on a stock port engine your doing something wrong.

The thread starter has an FD, therefore the 13B-REW is the relevant engine of discussion.

Was this a typo or did you mean this? If you did mean it, you would be agreeing with what Chuck said!

sorry it was a typo.
CANT. is the word.

You're welcome to post some dyno sheets with full mod lists of stock-ported 13B-REWs. :)

I'd like to see the dyno charts and mod lists of the S4's that are making power that high. It's not that I don't believe it can be done. I'm quite sure it can, but I've just never seen it. I don know that the RE/GReddy drift FD as well as the Apex'i drift FD are on stock ports and they make power way up there in the RPM band. That was info that came directly from the mechanics at the Feb. D1 event in Irwindale and from a conversation with RE Amemiya himself at the December JGTC event. So if you wouldn't mind showing us what you're talking about rx72c, I'm sure we'd all appreciate it. Thanks in advance.

Zach

This thread stated off talking about the torque band, maybe rx72c is talking about horsepower which does peak later and higher in the revs.

Sorry guys, posted and then left town for a few days. Here's a dyno plot. Don't have AFR's from the dyo as the meter was reading like 1.5 high (known problem) and we were going off of another wideband. Shouldn't get over 12, though I only run 100 octane in any case.

To answer the question on turbine sizing, the car is used exclusively for autox so fast response was a priority. Probably never going to run much more than 15psi either, as it just makes for a bigger mountain to climb each time you are off the throttle.

-Andy

huh? imagine hitting 15psi sooner and then pulling harder!!

I was thinking that if the rate at which the turbo made boost was constant, it will take longer to hit a higher boost level.

-Andy

I've driven the twins before and after receiving RP streetport engine. The difference was nothing short of astounding.

My own FD single T66P w/ stockport started off w/ 0.96; laggy until the RPMs were wrung to 4400, then the acceleration exploded onward to the top. Changed to 0.84 and the power comes on 700 sooner. The 0.96 really needs the streetport engine. The bigger ports are just a whole different world.