902/16 Transmission question

[VWLoosenuts]

Looking to see what kind of HP can the 902/16 transmission handle.

[cgarr]

Thats the hill climb box i belive. I have that in my 912 at about 125 hp. I would not go much over 160 they are pretty light but maybe someone has one in something other than a 912

[Matt Monson]

Depends on the weight of the car. Porsche put 250hp+ through the mag boxes in the 71 ST, but released the 915 the next year. The weakness of a 902/16 will be the 6:29 cwp. It is nowhere near as strong as the 7:31.

[VWLoosenuts]

Originally Posted by Matt Monson

Depends on the weight of the car. Porsche put 250hp+ through the mag boxes in the 71 ST, but released the 915 the next year. The weakness of a 902/16 will be the 6:29 cwp. It is nowhere near as strong as the 7:31.

I’m looking at close to 300hp. So probably best I go with a different unit then huh?

[Evans, Marv]

I have a (1974) 915 in my '69 w/ a 2.7 that puts out maybe 200hp. I don't think I'd go with the 902 even with my set up.

[jpnovak]

Originally Posted by VWLoosenuts

I’m looking at close to 300hp. So probably best I go with a different unit then huh?

The pinion gear won't last... As Matt said the 6:29 CWP is one of the weak points. You will be limited to around 175 ft/lbs of torque. The 7:31 is rated under 200 ft/lbs.

What kind of engine build? What does the torque curve look like? High rpm power will last longer than low rpm torque.

This will also require an oil cooler to keep the gear oil below 250.

The other issue with that much power is the cantilevered 1st gear. It sits outside the intermediate plate and is known for shearing if you put too much stress through it. This means no clutch dumps or hard starts with that much power.

[Matt Monson]

Originally Posted by VWLoosenuts

I’m looking at close to 300hp. So probably best I go with a different unit then huh?

Yeah. 300 is the upper limit of the 915. The 3.0 RSR's made that kind of power and once the Turbo Carreras hit the 915s started blowing up. But with a bearing plate and side cover, I see lots of racers in the 300 range on their 915s.

It's not just the ring and pinion. 901/902 gears are several mm thinner than 915 gears. The wider gears are much stronger.

[confused]

I have put 200hp through 901 boxes plenty of times. Mag or Alloy. The 911S was in that ballpark. All these people building 300hp monsters these days need bigger transmissions. The 6:29 ring and pinion trans you have here has value to someone with an early motor that has wild cams in it.

In Tony Adomowicz's Tran Am champ car for 1968 they used an experimental 5 tooth pinion gear from Porsche racing department. The point of the pinion was so tiny that it got very hot. So the team developed a trans cooling system with a fan mounted in the rear seat and an electric oil pump. Tony told me the only way he could beat the big hp trans am cars was to jump out ahead of them at the start, so he basically reved the ***** out of his 906 spec engine to get the power out it right away.

These days there is so much torque if you go with any modern engine you don't need to rev near as much but back in the day it was the only way to get power and it took a better driver to find the power.

[VWLoosenuts]

Thanks for all the great info. Looks lime I will be selling my transmission.

[Matt Monson]

Is this for your 912E project? What happened to your original 923 gearbox?

[VWLoosenuts]

Originally Posted by Matt Monson

Is this for your 912E project? What happened to your original 923 gearbox?

it is for my 912E

Unfortunately it didn't come with it or the motor. Would the 923 trans hold up to that HP?

[Matt Monson]

923 is 915 based. And it has really nice ratios. But the clutch is set up for a type IV.