Dave W.

Well then, that's a lot of lag. I'd check for boost/exhaust leaks for sure.
Typically a 4th gear pull on the dyno helps the turbo spool up sooner. The lag in lower gears would only be worse.
What spring pressure is in the wastegate?

RajDatta

Dave, this a 968 6 speed with very tall gears. I don't think I have any exhaust leaks but would need to confirm. wastegate is set to .8 bar spring pressure if I remember correctly.

RajDatta

I am leaning towards going back to factory headers and cross-over to begin with and take it from there. I remember better torque with the factory setup. Darn, I had a perfect working setup that I sold on rennlist with the proper GT30 flange.

333pg333

When did you or your mechanic last open up the MBC? I know there are a lot of solid suggestions already listed but while we found the MBCs to be quite good when new, they built up a bunch of crap inside them and needed to be cleaned / serviced to run properly. I'd also check that.

You are leaving a buckload of driveability on the table Raj going off that dyno chart. That sheet is back from 2009. It's now 2013 (well down here it is). Might be time to go back and retune. How do you actually tune your AFRs?
I ran a Tial/Garrett .82 turbo preceded by the .63 on my mostly stock 2.5 and I can't see why you won't get that .82 to work MUCH better. I'd be reluctant to slap on the .63 without diagnosing why you've got so much lag now. The .63 will help but I think also mask the problem.

rlm328

You may also want to take a look at the wheels in your turbo to see what condition they are in. The turbo is 8 years old from your description and it maybe getting long in the tooth.

Paulyy

I have a gtx3076R w/ .82, technically speaking my turbo cold side is larger and hot side is larger then yours and i get maximum torque at 4k rpm.

I'm not sure why it's so laggy but these turbos are designed for 3" exhausts.

Yours looks like it just struggles after maximun torque as there's a flow restriction somewhere not allowing more air.

Yes if you go smaller a/r you'll gain spool, ~300-400 rpm sooner.

RajDatta

Agreed and point taken. The car is a garage queen, so even though the dyno's are 4 yrs old, it equates to maybe if lucky 1k miles of use. The turbo is only taken out on special occasions.
It would be one thing if I ran the car a lot, which would make things gunk up, but I can't imagine 1k adding that much to require checking the boost controller. I will definitely check things though. I agree, there is a lot left on the table, which is why I am posting here.
I think going to a factory style header and cross-over would definitely be going in the right direction. I was not there when they tuned the car but the idea was to get the gases real hot to help spool things. Atleast that is what Chris had told me to do. Obviously, the shop didn't do as planned.
I know for sure I wont find much more hp without making wholesale changes to my intake and exhaust (which under the current circumstances is not an option). Might as well get some of the drivability back with getting a much better torque curve.

RajDatta

Not sure how your setup is much different. We both have .60 A/R on the inducer and .82 A/R on the exducer, so they are very similar. The turbo came with a 4 inch housing on the inducer originally, which I swapped for a 3 inch housing.

The flow restriction is on both sides, but probably mostly on the intake side.

Smaller A/R to .63 and factory headers and cross-over seems to be be in line. That should get me back to a much better powerband.

RajDatta

Well, Chris chimed in and he confirmed that the setup was designed to make max tq at 4500rpm, which is in line with what it originally did with me. Add to that a SFR header and a bit on the tuning end and we have this situation.
I remember speaking with him about his previous setup where he was getting wheelspin in 3rd gear at 80mph. He designed this setup to allow his peak torque to hit at a much higher rpm. Unfortunately he never ran this setup but instead sold it. Since this setup was designed for a track car, it is not ideal for the street. It is time to make it more streetable.
I will keep you guys posted on my progress.

333pg333

Under the circumstances you describe I think you're onto the right path. If you really only want to run 1 bar then going back to stock headers/X-over and a smaller turbo will certainly work. To my mind your current setup should be more tractable than it is but it also sounds like it is designed to run on high oct fuel at min 20psi. That would be interesting.

RajDatta

That is exactly what Chris had planned, run 1.3 bar at the track, aiming for 450rwhp and 4500rpm max tq peak. Unfortunately, that is not how I plan on using my car, so I must modify it for my use.
I guess better late than never. All in all, it still works very well, but as everyone noted, there is plenty to be gained by making tweaks. This is probably opposite of what most people on this site are looking for, but my goals are different.
On to finding a header, crossover and then modifying it to fit the GT turbo. I had it all and sold it. I will also go ahead and order the .63 A/R exducer housing.

Paulyy

sorry the rear is the same except mine is TiAL SS housing V-Band option, the coldside is difference since mine is the GTX 11 blade compressor wheel.
Ind: (gt) 57mm / (gtx) 58mm
exd (gt) 60.20mm / (gtx) 60.80mm

It's 100-200 rpm difference in spool.

Thom

Raj, since you are focused on originality I would suggest sourcing a stock K27/11. This would allow you to use stock headers too.
For what you are trying to achieve a GT3076R is IMO a waste, and with a smaller hotside on a 3.0 it will be working even further than its sweet spot than it already is with the retstricted inlet.
You were commenting about being "past that age" earlier - I would say that the real point is about having an efficient engine, regardless of how fast you want the car to be. With an efficient engine the car will be as fast as it can be without necessarily feeling it. Efficiency should prevail on how fast the car feels, IMHO.

Happy new year!

nick_968

Interesting thread. My first observation is that you say you do not have any WOT runs recorded on the dyno which makes the charts you have hard to interpret. Moving to a smaller hotside will of course improve spool. I am not sure the issues here would be caused by the intake. The exhaust (not sure on the diameter here) although it may restrict things a little will certainly not kill the power curve dead at 5k rpm, but again if it is not a WOT run then its hard to know. I have seen the charts of more than one high power car running on a factory exhaust including the first of the 3.2 UK cars and they do not flatline at 5k rpm.

RajDatta

Specs on my turbo:

Model:GT30
Turbocharger Part #:705867-0002
CHRA Part #:700177-0015

COMPRESSOR STAGE
Hsg. A/R: 0.7
Wheel Trim:56
Model: T04S
Wheel Diameter:2.25/3.00
Horsepower:510
Comp Flow lbs:55

TURBINE STAGE
Hsg. A/R:0.82
Wheel Tim:84
Wheel Diameter:2.16/2.36

Can someone help me identify exactly what this turbo is? It is made in Japan. Based on everything I see, these were not sold in US and were popular with Japanese cars. Chris bought this from a supplier in Australia.

Is it time to ditch this for a new turbo? Its easily 10 yrs old but has been run for only 2-3k miles and I hate to get rid of something just because..