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.

Look at how much torque you have at 3k and below. Looks almost 20bish. Thats an easy way to tell how fast your turbo is spooling. Go with a larger exhaust A/R to move your power peak a little higher in the power band.

https://www.rx7club.com/attachment.p...hmentid=120697


Personally your power band looks excellent for a daily driven street car.



Edit: Did a search and found Artguy's M2 ball bearing set-up that runs sequentially.

HP graph
http://www.conceptart.org/dyno/ArtguyM2hpsheet.jpg

Torque graph
http://www.conceptart.org/dyno/ArtguyM2torquesheet.jpg

WoW!

Hmmm after looking at the graphs, you may have a different problem. AMRX7 you lost over 100 hp from 7k to 8k while Artguy only lost about 40 hp in the same rpm range. I don't how if Artguy's engine is ported? I know I'm comparing too completely different setups, but you wouldn't think a single would drop off like that (especially compared to twin turbos).

Well that make a difference as opposed to running street gas.

Artguy's engine was ported, pretty good size port too from what I remember. As I recall, he later found out that that engine had poor tuning and had mediocre compression, which explains the relatively poor low-end. Perhaps Jason can chime in and set the facts straight.

In any case, that is a pretty quick drop off in the GT35 dyno above...a combination of conservative tuning, backpressure, and stock porting, perhaps?

I know I'd still be damn happy with that powerband. That sucker is rocking just above 3k rpm with a table-flat torque curve after that. For a street-driven car, that looks awesome!

that drop off at high rpm is a stock ported motor and a small turbine side, although the high rpm drop off looks a little more then usual... low end torque and boost responce looks pretty good by the dyno graph.

IIRC, it is probably still too rich on the top end. We did lean it out some on subsequent runs, and it picked up about 20 ft-lbs at 8000rpm, but there was no change until after 7500rpm.

Thanks for the responses. I'm just trying to figure out what to do next. I like the response of the small turbine, but would like to extend the powerband out just a bit and make more like 400-420whp. Sounds like porting may help, and I want to build the motor properly instead of relying on a stock reman anyway. If that doesn't do it, upgrading the turbo is easy.

-Andy

Resurrecting old thread with some updates. Since this thread originally started I at one point had the motor ported (good sized street port), and just recently stepped up the turbine housing to 1.06 A/R. Need to get all the dyno plots in one place, but basically with my T3 GT35R setup and....

stock motor w/.82 A/R turbine at 15-16psi
-about 375whp
-hit full boost around 3700rpm (need to go check that number)

large street port motor w/.82 A/R turbine at 15-16psi
-about 400whp (and much more on the top end, about 80hp more than stock ported motor where it would fall off)
-full boost by 3800rpm

large street port motor w/1.06 A/R turbine at ~16psi (boost control was hard on this dyno, small roller dynojet)
-about 410whp
-full boost by 4200rpm

The power gains with the porting were very noticeable especially on the top end. Spool and low end torque wasn't quite as nice, but net was much more power in the 7Krpm+ range.

I was *hoping* the larger A/R turbine might be what the system now wanted, but was disappointed with results. Spool obviously suffered by about 400rpm, and top end power was almost the same. Since this is an autox only car, I will probably go back to the smaller .82 turbine.

-Andy

In Kyle's car (Nocab72...Turbo Magazine article) we were only able to get into the 460's at 17psi before we started hitting a wall. Same turbo with T3 (1.06AR Turbine) but on LARGE street ports. How "LARGE" we don't know since the engine has not given a hint of needing to be torn down (not driven very much).

I don't have a dyno chart from that session handy but I remember the same general shape of the curve...maybe without as much of a drop as with your .82AR.

I will have to side with our Aussie friend here and state that in tuning cars you have to keep in mind the whole car. I wouldn't call someone ignorant over anything, but everyone on this thread has been wrong before...and will again. It's nice to see people sharing information and debating in a civilized manner. Adding to the general knowledge pool.

Tony

Yes, I wasn't too happy with the larger 1.06 housing. I expected to lose some spool, but was hoping to see more top end gain. The spool loss was large IMO, and the top end gain was almost negligible. There were some dyno-dyno differences here and to get a perfect picture I need to go back to the same dyno to confirm.


In any case I was concluding that my restriction is somewhere else in the system. A few things I can think of...

-port job isn't that large
-manifold design --- it has short runners (about 4.5" long)
-wastegate plumbing --- they route back into the downpipe about 16" from turbo outlet, but at 90deg angle (obviously not good for flow)

-Andy

Another possible restriction is the intercooler. I'm using a Spearco core that is rated at 470hp and 700cfm. I might be maxing it out by 8000rpm or less on the cfm. If the hp number is crank hp that would make sense too since I seem to have trouble getting much past 400whp.

-Andy

Andy,
Why are you trying to make my job harder. We've been having a nice "even" fight lately. Trust me, you can't use much over 400hp. ;-)

Haha, I know Erik especially at HPT where it is hard enough to use 300whp. We need an RX-7 AWD kit.

In any case I think I am done in the power dept for awhile. I have a very responsive 400whp setup now and to make significant power gains would probably mean new I/C and all new manifold and exhaust (and maybe turbo and maybe port job...;-) ).


-Andy