Gearbox for 911 Turbo Racecar ?

Cookie Porsche · 08-30-2005, 08:23 AM

Hello,

As you may have seen we're building a Turbo engine for our racecar. NOW we need a gearbox. We need 5 gears and it must be able to handle 400+ rwBHP.

1) Do I need a G50 (too heavy!).

2) Can a 915 5-speed be modified to handle this power on track?

3) Is there any NON-PORSCHE (yirk) boxes available, maybe even sequential types?

4) What clutch setup do you recommend. Will it fit the standard SC / 915 flywheel?

4-speed is for tractors, too limited. I think the G50 is too heavy.

Regards, Tom

beepbeep · 08-30-2005, 04:47 PM

Quote:

Originally posted by Cookie Porsche
Hello,

Hello!

Quote:

As you may have seen we're building a Turbo engine for our racecar. NOW we need a gearbox. We need 5 gears and it must be able to handle 400+ rwBHP.

1) Do I need a G50 (too heavy!).

Well, it's a 5-speed Porsche transaxle...a fairly good one.

Quote:

2) Can a 915 5-speed be modified to handle this power on track?

No, not in reliable fashion.

Quote:

3) Is there any NON-PORSCHE (yirk) boxes available, maybe even sequential types?

Yes, Hewland makes nice transaxles...they cost a lot. You might find used ones somewhat cheaper.

Quote:

4) What clutch setup do you recommend. Will it fit the standard SC / 915 flywheel?

Standard SC/915 flywheel is too small, you should step up to 240mm clutch.

Quote:

4-speed is for tractors, too limited. I think the G50 is too heavy.

Well it's your opinion. 917/30 and 935 had 4-speed transmissions. If you find G50 too heavy just call Hewland and open up your wallet.

Quote:

Regards, Tom [/B]

WydRyd · 08-30-2005, 07:40 PM

Fit a G50 and lose weight elsewhere! The G50 will handle anything you throw at it

Cookie Porsche · 08-31-2005, 01:35 AM

Well, hrmmph...

If I go the G50 route should I just change 3+4+5 to lower ratios - no sense in changing 1+2 with that kind of power...

OR WHAT SAY YOU?

I currently run a GT LSD diff in my 3.8/915 combo - anyone with ideas for the Turbo/G50? same?

Merv; If you saw my current car it will be difficult to remove weight on the pack - unless I leave the car unpainted...

What clutch for the G50? WIll the RS pressure plate (light weight) and a button-clutch hold the Turbo?

??????????????

Tom

edbaus · edbaus's Garage

If this is a race only car, what do you need a first gear for. Put a first gear on a 930 trans that is more like a second gear, then shorten the others up as well (lengthen 2nd, maybe 3rd stays stock)

It is a little more for the custom main shaft and the car will not autocross very well, but for track work what do you need a first gear for.

I should take my own advice.

Ed Baus
GT4s

Cookie Porsche · 08-31-2005, 05:11 AM

So true - very good advice....

But a 930 4-speed will cost almost the same as a G50, or not?

And the clutch setup on the Turbo 915 weighs like a ton (+/-)?

Also a bit tired of shifting the tractor g/box from the 1960's....

My current 915 runs almost 65mph in first so I really like that idea - rolling starts only.

Tom

edbaus · edbaus's Garage

Assuming early car (torsion bar car)

the torsion bar tube gets in the way of the g50. So, you either need to have the g50 modified to have a shorter length (money) or (if you are not running torsion bars anymore) have the center section of the tube cut out and a loop bridged over the trans to retain stiffness.

Ed

edbaus · edbaus's Garage

Oh, money on the 930. I got a 930 with limited slip. in as used condition for $1500. I will go through it myself, but all in all not a bad price. The prices I have seen on the g50 have been about that for a non lsd unit.

Ed

Cookie Porsche · 08-31-2005, 07:30 AM

Ed,

Thanks. No torsion bars - it's a race car and everything is cut out - I can fit almost anything in there! (Car is running 993 multi-link suspension and no tube/rear seats, etc.).

Were do you suggest I look for the 915/G50? I was quoted some $2200 without LSD! $1500 sound alot better.

Regards
Tom

KobaltBlau · 08-31-2005, 08:18 AM

If the car is running 993 multi link rear suspension, you must have that ring frame (that the links attach to) back there as well. in this case, it would be a pain to fit a 915 or 930 box, I think. The G50 should be easiest to fit with the 993 subframe, and you won't get a hewland set up for near the same price. G50 makes the most sense for you, you could even do a 6-spd G50.

KobaltBlau · 09-05-2005, 08:46 AM

what's the latest?

Cookie Porsche · 09-05-2005, 09:13 PM

Andy,

In between building the car I take breaks to make money (!).

I may have located a G50 (from a 3.2) locally and will go check it out. Seems the way to go and this is the earliest G50 around - hopefully it is smaller than the later types?

Is there any pros or cons btw the different G50 boxes?

As you mention I have the 993 rear subframe in my car, but all mountings are new (car was a 72!). Currently running a very trick 915 but this will not handle a Turbo as mentioned above.

I would change some ratios and of course fit a LSD. Any other mods????

Thanks. Tom

air-cool-me · 09-07-2005, 08:44 PM

just a thought... what about a trans from a 993? you have the multi-link... that sounds like a large project...

just thinking outloud about something i know nothing about..

-nick

Cookie Porsche · 09-07-2005, 09:24 PM

Nick,

The 993 suspension (multilink) is independent from the engine and box - so I can fit anything. Currently running 3.8 + 915 but now I am going Turbo and would like a stronger box.

915 4-speed seems a step backwards. G50 early version is smaller than the later 6-speed (i.e. lighter). Since I'm changing ratios any G50 should do.

ANYONE with suggestions as to what to do to my G50? Differences btw the various G50's?

There must be plenty IQ out there....

Regards
Tom

beepbeep · 09-08-2005, 04:28 AM

G50/52 is most desireable one. It was fitted to turbocharged cars and can withstand very high torque numbers (I know of one fitted to 800Nm TT 3.6 engine that survived lot's of dragracing and a trip to Nürburgring).

At very high power, OEM clutch diff isn't enough and it's best to fit Quaife mechanical differential.

WydRyd · 09-08-2005, 05:37 AM

Quote:

Originally posted by Cookie Porsche

What clutch for the G50? WIll the RS pressure plate (light weight) and a button-clutch hold the Turbo?
Tom

Tom, I have this same dilemma. Your best option for the G50 is either a Spec Stage3+ Clutch kit, or a Kennedy Engineering kit, with 930 Spring Centred Disk.

These will hold upwards of 600lb/ft of torque

KobaltBlau · 09-08-2005, 12:40 PM

Quote:

Originally posted by Cookie Porsche
The 993 suspension (multilink) is independent from the engine and box ...

is it really? I mean, does the trans crossmember mount to the car and not touch that subframe at all, even on a "real" 993?

Cookie Porsche · 09-08-2005, 09:35 PM

Quote:

is it really? I mean, does the trans crossmember mount to the car and not touch that subframe at all, even on a "real" 993?

Yep. Totally independent.

That's why it is such a lovely mod for a race car. Too involved for a street car.

I'm not having much feedback on the choice of G50 though - were are all the gearbox experts?

WydRyd. Tell me about the Spec Stage3+ Clutch kit, or a Kennedy Engineering kit - never heard about them...

Regards
Tom