915 good with 320bhp?

[jph83]

Hey everyone, i'm hoping to carry out a 993 3.6 varioram conversion on my '83sc this winter and would like your thoughts on retaining the 915 gearbox.

The engine will be modified to produce approx 310-320bhp (hopefully!) and the brakes and suspension are already upgraded to an acceptable spec. The car will be used mainly on the road, with the occasional trackday... Nurbergring here i come!

The box is gonna be rebuilt but what non-factory/uprated parts would you recommend i use, if any, bearing in mind the increase in HP and tourque?
Should i go for a ZF plate-type LSD or a torque biasing Quaife ATB?
Finally, should i change the final drive ratio?

Many thanks in advance, Jon Hawkins.

[Wil Ferch]

Like so many things.... "It depends"....

Look at it these various ways...
1.) In the mid 80's, Euro Carrera'a can be had with or without cat. Cars without cat had 231 hp and had a trans oil cooler. Cars with cat had "enough" lower hp ( to about 207) that the factory deemed trans coolers unnecessary.
2.) Early 1974 Carrera RSR's had 330 hp with the 915 trans...but "longevity" was probably based on whether it could live through one 24-hr race. Our expectations are higher for a street or street/track car.

So...using a 915 won't give you catastrophic failure...it's not *that* weak...but it will be stressed...

---Wil Ferch

[RickM]

Talk to the guys at Protomotive. They've built many cars with much more HP while utilizing the stock 915. However, a higher HP normally aspirated application would be different from a turbo.

[ttweed]

A 915 should be adequate for your purposes if you cool it and use the later 8:31 R&P, which is stronger than the earlier 7:31. The one-piece billet bearing retainer would be a good idea, too. Whether you use a ZF or torque-biasing diff depends on your application. Most dedicated track cars use the ZF. For autox and street driving, the Quaife is better.

[KTL]

Ask wevoid.

[ChrisBennet]

It's not so much horsepower as heat and torque that stress the 915.
-Chris

[jph83]

Thanks for the replys guys, so i'm thinkin it might be a good idea to run a trans oil cooler, like that used on the 3.2 carrera, is this a straight forward conversion on the SC type 915? Any guys out there done it to their box?

[wevoid]

jph83,

Here is a recent thread where the subject was discussed.

3.6 Conversions, are any changes to the transmission recommended?

For a road car upgrade, a 915 in good serviceable condition will be fine with the motor you propose.

Ensure the internals are fresh and that the pinion roller bearing is not shagged out of round and your 915 will give good service with a 3.6

If you are going to do track work, a cooler would be a good idea to extend the life of the transmission.

Regards

Hayden

[jph83]

Thanks for the link Hayden, much appreciated. So now assuming the internals are all good, a cooler is a good way to go to compensate for the extra heat produced during track work, but not a necessity for street driving?
Has anyone got some max speed figures for 1st thru to 5th when runnin a 7.31 R&P and a 8.31 R&P assuming tire sizes are stock. Note this is a euro 915 if this makes any difference.

Thanks, Jon Hawkins.

[wevoid]

Jon,

For max speeds;

What stock tire size?
What version 915, look underneath it will have 915/?? to identify which stock ratios.
What max rpm?

Dont even consider a 7:31, it will come back to haunt you. If you want to alter through-the-gear-speeds, use alternate gear ratios with the 8:31.

Regards

Hayden

[Bill Verburg]

For street use even a mg case 915 w/ 8:31 will be fine for 964 levels of torque, I checked with several factory engineres about that.

For a 3.8 that will be tracked additional strength and cooling is desireable. I am using a 915/67 in front of my 993RS engine.


You can use the Wevo parts to strengthen this if it becomes necessary(at least that is my plan).

For track use a ZF is best because of the additional benefits of actuation under trailing throttle, this will help braking.

The net gearing depends on tires as well as trans and diff. Additionally what is great at the track may not be so desireable in a street app. I always felt that my 964 engine was overgeared for street use and ok for track use w/ stock ratios and 25.7" tall 275/40 tires, for the track a shorter/closer 3,4,5 would be desireable.

The same trans w/ the 993RS is geared very nicely. Even undergeared in a lot of situations, closer 3,4,5 would still be nicer for the track.

Here is a chart of the 915/67 stock gearing/diff ratio & 275/40x17 tires(25.7"OD)