thingo

Do you know how many blades on the compressor wheel? I think yours may be the seven blade, an older technology wheel.

That does explain some of your results, I would expect better performance with a current catalogue gt3076

RajDatta

Bingo Thingo, that is exactly what it has. It took an Aussie to identify an Australian sold turbo . It has 7 blades in the front offset by 7 blades in the back. Chris was running these turbo's 10 yrs back.

RajDatta

I can neither confirm nor verify if the runs were done at WOT since I wasn't there but looking at the charts, one would believe that the runs were at WOT, since it makes full boost.

TurboTommy

While I would agree that the .82 A/R might be only a little too big for what you want, I don't think it's the major issue causing the apparent sluggish boost rise. Especially because the exhaust wheel size itself is actually on the small side; and it's a 3.0 liter.
Something else is up. Some thoughts:
Maybe the dyno chart is not representative of what's actually happening in reality.(You said it felt fast; usually that means good area under the curve)
Your exhaust is not modded? You mean just a regular exhaust with cat and everything? This would definitely reduce spool-up performance on a 3.0L.(probably significantly)
I can't see the intake being an issue if you have a MAF, unless there's a bottle neck somewhere.
If they are 4 into 1 headers; they could move the power band to the right a bit.
Get a wideband. I've learned to never, ever, ever blindly trust aftermarket tuning. It has nothing to do with the tuners credentials. It has to do with your specific car. Things can change. You need to be informed. Stupid AFRs (somewhere in your rev range or throttle positions) can play havoc with the overall performance.

thingo

I would lose that turbo and get a present day gt3076, stay with the .83 housing and see how you find it.

Rogue_Ant

That turbo should be a great fast-response turbo on a 3.0L...

TurboTommy is correct - something is not right... Don't replace the turbo (even an archaic 60-1 would spin-up faster than your dyno is showing). Find the issue, and fix-it.
With it being that laggy, tuning is not the cause.
There must be a physical issue: boost controller not working, wastegate not sealing / opening early, large pre-turbo exhaust leak, plugged cat, cam timing retarded (significantly) or very late closing intake valve, boost-leak (massive), diverter valve not holding, in-correctly routed ISV, throttle not opening all the way (or even close), ect.

333pg333

+1...I'm sure there is a physical cause for this apparent lack of onset of boost. Also agree on getting it re-dyno'd and also adding a decent wideband if you don't already have one. There might be a really simple solution that saves you a heap of time and money and headache.

Thom

A "present day" GT3076R has the same specs as the specs Raj mentioned.
This turbo won't spool properly without a proper inlet and a 3.0 engine won't let this compressor flow as freely as it could in upper rpm with a stock-sized exhaust.

With my previous T4R-based Garrett/KKK hybrid, featuring a cold side very similar to that of a GT3076R, I was initially running a stock exhaust and the airbox without the snorkel.
While I did get reasonable spool, say 15psi a little past 3000 rpm, the engine just did not breathe properly in upper rpm when running more than 10 psi of boost.
Replacing then the airbox with a MAF pipe + cone filter improved the spool a bit, say seeing 15psi at 3000 rpm or even slightly earlier, but the engine still did not breathe properly in upper rpm until I replaced the exhaust with a full 3". With the stock exhaust and any more boost than 10 psi the engine felt it was loaded as if it was trying to pull the car braked with an open parachute behind.

333pg333

Sorry, can we clarify what exhaust is on this car? Raj says that it's a "Factory 968 turbo S intake and exhaust". What exactly is that guys? (Thom, I hold you as one of the pre-eminent experts on 968 turbos.)

thingo

All GT3076R turbos are not the same, and there is a good reason why garret does not sell this turbo. It is an old style compressor wheel on a modern turbine wheel,they are mismatched.

nick_968

I think this is a good question, as Thom says a small exhaust will choke things but not as much as the 951 style air box intake. If the engine can't pump the air at higher rpm that will certainly hold things back but it may not entirely explain the late onset of boost. I had major problems with spool on my 3.0 only cured by using a snap on leak detector and working my way through 3 sets of factory headers until I found a mint straight set with no cracks! Made a big difference. There may be more than one bottle neck in your system. Check for leaks to rule that out and then try and match a turbo to your setup (if after all of this you still prefer not to change anything on the intake & exhaust side of your setup).

There was a guy here in the uk selling a new turbo off one of the original rs cars spares kit a while back. It was a kkk hybrid, maybe that sort of thing would work better for you. As others have mentioned the tuning may also be a factor don't rule this out but fault find on the mechanical side first. Maybe Thom could answer this but did the factory RS cars still stick with the original airbox system or did they have something less restrictive? I know that the one in the UK was running a 3 inch exhaust as I almost bought the spare one for my car.

RajDatta

Guys, these are all great questions and recommendations. I agree with what some of the recent suggestions are.

1) Put the car back together as is and put it on a different dyno

2) See what kind of numbers I get. If the numbers match, diagnose what the issue is

Throwing $$ at it might not be the best way to start. Also, putting it on a different dyno should give us some prespective on how accurate the tuner's dyno is. Thanks guys for all your suggestions. In the meantime, I will post a picture of the factory turbo S exhaust.

Raj

RajDatta

Garrett did sell these turbo's. They were sold back in early 2000-2002 and were used quite extendsively in Japan and Australia and sold under HKS as well. This is well before Honeywell bought them. These units are made in Japan and very high precision units. I don't think anything is wrong with this turbo. It is probably not setup for a street car since the designer himself told me he had it set to make max tq at 4500rpm, which would be high for a street car. He planned to use it in his racing car, since he was dealing with too much wheel spin with the smaller .63 A/R.

I like the idea of Dyno'ing the car 1st and see where things stand.

Thom

The factory 968T airbox is the same as the 951, with the same rectangular restriction before the AFM.

The 968TS exhaust is an odd beast. If I remember correctly the downpipe is the same doubled-skinned unit carried over from the 951, then the (single) piping goes into a shorter but surprisingly larger cat, then into a triangular backbox like on the 968 NA from which dual piping goes the rear muffler, then we have this dual piping going to the twin tail pipes.
I would say that the total section of the twin pipes out of the triangular backbox and going through the rear muffler would make a total section larger than the single piping of the 951 single piping out of the 951 cat, but in between the downpipe and the triangular back box, cat included, I'm not sure if the single piping is as large as 3". It might be slightly larger than the 951, but it just might, and if there is an actual difference it may not be significant. When I last looked at such an exhaust, it definitely looked smaller than my 3" Lindsey exhaust where it bends alongside the cross pipe then blends cleanly into the huge cat. Will be interesting to see if Raj's original exhaust is similar.

Raj, isn't your turbo actually an HKS GT3037S?

RajDatta

As promised, exhaust pictures.

Downpipe is 2.5 inches and once the 2 pipes split, it is 2 inches thru a small resonator and then on to a big muffler.







Raj