915 transmitions

[1ricardo]

Dear friends

Is there any difference in the 915 transmission fitted on a 1978 911SC and the 915 transmission fitted and 1983 911SC?

I have a 3,60 engine that i would like to couple with one one those 2 transmissions, whichever is best

Thanks for the help

[0396]

To my knowledge, same trans.

[Matt Monson]

No important differences. Either one I would want freshly rebuilt to run against a 3.6

[KTL]

I would want to use a one-piece input shaft and pinion shaft bearing retainer (like the 930 transmission uses) to provide the best chance of keeping the shafts coupled together. The benefit is arguable but I think its wise to use something as inexpensive as this part when you're exceeding the design torque capacity of this transmission by using a 3.6 engine.

[Spumato]

Did the 3.6L engine not use the G50 transmission as a stock tranny? I think the reason that Porsche switched to the G50 was due to the HP limit had been exceeded by the time the introduction of the G50 happened. You may want to do some research, but I think you'd be throwing your money away as I feel the 3.6 may out class the capabilities of the 915.

[Trackrash]

A heavy duty side plate would be a good idea.

If you are tracking the car an oil cooler will help it live longer.

[gliding_serpent]

Originally Posted by KTL

I would want to use a one-piece input shaft and pinion shaft bearing retainer (like the 930 transmission uses) to provide the best chance of keeping the shafts coupled together. The benefit is arguable but I think its wise to use something as inexpensive as this part when you're exceeding the design torque capacity of this transmission by using a 3.6 engine.

I am interested in hearing the discussion against the benefit of these plates. Nothing is a guarantee, but I am curious to hear the other side of the story.

[Trackrash]

Porsche uses one piece bearing retainers on all their newer transmissions starting with the G50.

Only down side is possibly cost and slightly more work to assemble. Bearing retainers are commonly known to wear so if you need to replace one it's a no brainer going to the one piece.

[KTL]

Yes the G50 was developed in advance of the future 3.6 cars. It made its first appearance in the 959 and '87 Carrera. It's definitely much stronger than the 915. The power shafts in the 915 are rather dainty compared to the G50 gearboxes.

That said, a lot of people have been able to keep a 915 together in 3.6 cars. Some of them race cars.

To further what Gordon said, the original individual retainers are not all that rigid (rather thin) and they allow the fit of the pressed in shaft bearing (a rather light press fit) to loosen. So you then have a bearing that is slightly floating in the retainer and thus not supporting the shaft very well. I suspect this loose fit in the retainer can contribute to a common problem where the adjacent pinion shaft bearing race loosens. This is the bearing that is actually inserted into the diff housing case bore. The loose pinion bearing race is most common in the aluminum transmissions.

https://www.youtube.com/watch?v=xmNmxm_AT0c

CMS and Wevo make very nice one piece retainers

http://www.wevo.com/Products/TransmissionProducts/WevoTransmissionProducts-Internal_XT032BearingRetainer.htm

CMS PORSCHE 915 BEARING RETAINING PLATE – California Motorsports Inc

These 930-style singular retainers are a much beefier part that has more material around the bearings, to provide better support around the OD of the bearing. Plus the singular piece acts as a more stable foundation that allows both bearings to rely on all 10 mounting points in the face of the diff housing. Whereas with the individual retainers, the input shaft bearing only has 4 mounting points to secure it and the pinion shaft has 6 points.

I don't think there's any strong argument against using the singular retainer. In other words, there's no downside to using it besides it costing you money. My opinion is, if you find that your two individual retainers are no longer capturing the bearings? Why would we replace them with the same part again? Especially if you're going to subject the trans to hard use. The hard use is pretty common in my experience. These cars have so much to offer from an enjoyable driving experience. It's a shame if people baby them too much. They can withstand a lot of hard driving over many years.

[gliding_serpent]

Cool, i asked because one well known transmission builder (gary f) did not feel they were needed. I can't say I agreed. I sleeved my bearing race and went with the guard retainer plate and diff side cover for my 3.4l build. My car will put down about 260hp and have HPDE aspirations so I figured it was a smart choice.

[safe]

The G50 must be a lot stronger, it weighs almost twice a much as a 915 (mag case).

[Matt Monson]

Originally Posted by Trackrash

Porsche uses one piece bearing retainers on all their newer transmissions starting with the G50.

Actually the 1 piece plates were first used on the 930 and the G31 924 Turbo "snail" gearbox. The later is actually 915 based. The idea for a 1 piece plate was not original, and was just something that we borrowed from other gearboxes already in production. You can take an original 930 plate and put it into a 915.

Regards,

Matt Monson
Guard Transmission llc.

[k9handler]

I did the 3.6 install and used a 915 tranny...had to cut a notch near the crank sensor to make it fit. Was not that difficult.