3.2 low pressure turbo - data point

[IROC 912]

Hi All,

I've never has my car on a dyno before so had the opportunity this weekend with a local car group and a dyno day. The car is a 1966 912 with a low pressure (7psi) carrera 3.2 engine that had a stock rebuild when I purchased the car about 4 years ago.

Specs include GT3076R turbo with 0.63 hotside, aem infinity 6 with COP ignition (Turbokraft), Tial wastegate, 7:31 magnesium 915 with uprated clutch only, and 60lb injectors on 91 octane gasoline. I'm running stock heat exchangers and a 930 J Pipe, along with a home made Zork tube.

I've only tuned the car on the street getting the AFR's as close as I can with a stock carrera 3.2 interpretation of an ignition map for the ECU. The dyno operator was saying there was wheel spin at the upper rpm range because they couldn't strap the car down properly with the strapping. Not sure if this is the case or if it is the clutch slipping, as I have no clutch slip on the street, only tire spin.

Any comments would be appreciated as I am not sure if spending money on additional tuning would be worthwhile. I'm working on AFR tuning only at the moment and the car is a weekend warrior only, not a daily driver.

Results of this session were about 290 RWHP and about 300 FtLbs RWT.

https://youtu.be/BF9inOxeJbQ

[Tippy]

So, I’d say you have a small turbo made for high boost. These units from Garrett really shine when going over something like 1.5 to 2 bar of boost. Meaning, at low boost you’re running, it’s not very efficient.

2nd, 3.2’s have pretty big ports from intake, heads, and headers. IMO, T4 sized turbos are just fine for these motors. T3 is probably better suited for stock 930’s. With a T3 60mm turbine and a very tiny 0.63 A/R turbine housing, you’re leaving a lot of higher end hp on the table.

Simply put, you’re choking the crap out of the top end of that motor. The turbine is fairly restricted and I bet you’re seeing a decent amount of back pressure. I’m sure it spools like lightning though!

Make sure your exhaust post turbo is totally free flowing. No chambered or bends through the muffler. Straight from inlet to outlet would be ideal IMO.

If you want it to wake up, an old school 60-1 or 62-1 would be a sweet turbo for it I’d imagine. It would much more on/off and laggier, but when it hit 0.5 bar, it’d bring a smile to your face!

[copbait73]

Nice looking car. I’m sure your weight is near 2200# and that 7:31 tied with 300HP makes for rapid acceration 0-100. HP sounds good for mild 7psi.

[IROC 912]

Thanks for the feedback.

I'll look at some of the suggestions you posted Tippy for the future, as I just used what I had access to. I know there are compromises in the build but I need to tend to the 915 gearbox first before adding any more power.

I have a G50/01 and aluminum 8:31 915 in the workshop but both will need some work to stuff in the back end of the 912 The 915 will need strengthening and the G50 will need shortening. I think I see a slippery slope ahead

It's a fun car to drive with the 7:31 915and 235/50/15 rears but highway driving to get to the track sure sucks.

Cheers,

Sandeep

[Tippy]

I’m not dogging the GT30 at all, as it’s a nice compromise between lightning quick spoil and peak hp.

But, for it to hit 500hp on our engines, it’d probably need up to 2 bar of boost if I had to guess without looking at the comp maps.

I recommended the older style turbos because they deliver much, much more CFM at lower pressure ratios (boost) than the GT30 giving you a little more power up top. Also, the 60-1 and 62-1 turbos can come with a “P” trim turbo and larger AR housings for additional top end too.

You’ll sacrifice low end torque putting around, but once it lights, BAM, you’re scootin’!!!

[greglepore]

Debating this myself-have an old school on a Protomotive stage 1 kit-going back and forth with Todd he suggested a gt3582. Any thoughts?

[IROC 912]

I don't have any experience with that turbo but am guessing it is 1 step up from the GT3076R I am currently running. The pressure ratio map at 0.5 bar on the GT30 runs through the 76% efficiency island so its working well for me at the moment.

My car is not built for high speed with the 7:31 R&P and I prefer to autocross and do shorter tracks than high speed ones. The GT30 is awesome on spoolup and seems to be done by about 6000 rpm, but its time to shift then anyways.

If I had a different gearbox and intentions for the car, perhaps the GT35 would be the way to go. I could always try a 0.82 AR hotside vs the 0.63 I am running now, but it looks like my clutch will need attention during the winter

[TurboKraft]

Very nice!
Your numbers are right in line with what I would expect for a low-boost 3.2 Carrera.

Yes, the GT30R is a little small for your high compression 3.2L, and the 0.63A/R turbine allows for very quick spooling which explains the decent torque you're getting from 2500-5000rpm. It is a restriction in upper revs, however -- look at the nose-dive your torque curve takes at 5,000rpm.

If you would like to see a back-to-back comparison of a GT30R vs. a GT35R, here's episodes of the video series we did with Speed Academy, rebuilding and tuning their 930:
Ep.13 GT30R dyno: https://www.youtube.com/watch?v=KBve2yqdHME&t=1s
Ep.14 GT35R dyno: https://www.youtube.com/watch?v=Pg_Dp_j9li4

[IROC 912]

Thanks for all of the feedback everyone. Looks like I have some shopping to do .... but gotta decide on a gearbox plan first. Damn turbos and 915s

Cheers,

Sandeep

['76 911S 3.0]

Nice numbers! I have a 0.82 hot side from a 30R I sold a while back, let me know if you are interested.

[quattrorunner]

Originally Posted by IROC 912

Thanks for all of the feedback everyone. Looks like I have some shopping to do .... but gotta decide on a gearbox plan first. Damn turbos and 915s

Cheers,

Sandeep

you seriously need to stay low boost or upgrade transmissions.

[Rawknees'Turbo]

^^^

Brandon knows what he's talking about - bisch broke two 915s before switching to a 930 box.

[IROC 912]

Appreciate the feedback. That's the awesome thing about these forums ... someone has already gone down that road and has the broken parts to prove it.

I'll keep the setup "AS IS" for now, knowing what needs to be changed for improvement down the road and determine a budget as well.

Cheers,

Sandeep

[puddy]

Hey Sandeep, I have 355 RWHP and a 915 trans and have beat on it at the track for 4 years. Previous owner also did so. I know its days are numbered. If you're not doing big smokey burn outs and dropping the clutch, your trans may last alot longer than you think.

[IROC 912]

Thanks for that note Chrs. I've been meaning to contact you offline regarding that gearbox.

Cheers,

Sandeep

[quattrorunner]

Originally Posted by puddy

Hey Sandeep, I have 355 RWHP and a 915 trans and have beat on it at the track for 4 years. Previous owner also did so. I know its days are numbered. If you're not doing big smokey burn outs and dropping the clutch, your trans may last alot longer than you think.

I suspect it's a crapshoot. Both mine broke second gears in similar ways, coming smoothly on boost. was running .8bar boost.
That being said I'm jealous of those who still have 915 ratios.......

[IROC 912]

I've spoken with a racecar / engine builder who is known in the Toronto area for building winning cars and a customer of his runs a 911 SC with 3.3L turbo putting 385 RWHP to the ground through a built 915. The car runs in the VARAC vintage racing series that makes a stop at MOSPORT / CTMSP on June 15 to 17 weekend.

Will be going to that race and speaking to the driver to get his opinions on the 915. It looks like it is working for him as well but want to discuss the kind of maintenance that 915 gets, seeing as it is a race car.

Cheers,

Sandeep

[quattrorunner]

There has been much discussion. In the end it comes down to $.
It takes part$ to make the 915 more $trong and even then I couldn't get guarantee$ that the gear$ would la$t. Ask Matt Manson he makes great gears and limited slip diffs for these transmissions.

[quattrorunner]

And wevo make great parts as well. I'd love a g50-01

[IROC 912]

Agree on all the point$ above.

I have a 915 7:31 in the car now, a 915 8:31 on the bench and also a g50/01.

The G50/01 will require shortening and a new clutch / FW /PP combo. I have a plan for a cable operated remote master cylinder to control the hydraulics but need a firm quote on the package.

The 915 8:31 will need parts as well so once a quote for this has been determined, I can decide which way to go. Hoping to recoup some of the funds by selling the other two gearboxes. I'm hoping the G50 works out

Cheers,

Sandeep