901 vs 915 gearbox & a short stroke 3.2
 

Just wondering how either gearbox will hold up to a 3.2 short stroke (a 3.0 with 98mm pistons), putting out about 240 hp.

Any ideas?

I'm currently running 240hp on a fresh 901. I've heard they are reliable up to 300 hp, except in first gear. Don't dump the clutch in first gear, or you might break it off. Also, you'll need a good clutch to hold the torque if you are running a 215mm flywheel. the stock clutch will not hold it or last very long. I would assume a 915 would be no problem, since it was built to support more torque than the 901.

Good luck,

JP

The limiting factor in the gearboxes is the torque, not the HP. As the engines grew larger (torque is directly proportional to engine size), Porsche redesigned the gear boxes to deal with the increase in torque. This is why 930s got special gear boxes even though they put out about the same HP or less then RSRs -- because they made significantly more torque.

Initially I'd uses the gearbox that you have and take it easy in 1st gear. If it wears out too quick for your taste, you can just replace the box with a later style box at that time.

Here are the specs, The original 901 had a max input torque of 19mkg(138 lb-ft), this was increased on the B series in '69, due to a new die cast Al. case, to 20.5 mkg(148.5 lb-ft).

I wouldn't do it. There was a recent post on Rennlist from a fellow that regretted the decision to use a 901.

My 914 has a 3.2 probably putting out 250HP, wont know the torque till I get it on a dyno

The 901 has a short gearstack

So far the gearbox seems to be holding up well, no sparkly surprises when I check the gear oil.

First is used to to get the car in motion, no dumping the clutch or things like that

I do however have a spare gearbox ready to install if the box in the car grenades

There are a number of extremely powerful race cars running 901's. Otto has one - a 3.8RSR monster 914-6; something over 360hp in an 1800lb car. The trans has survived multiple seasons of track duty. So, it can be done.

But a street car sees a whole lot more 1st gear launches, and overall miles. The general concensus seems to be the 915 trans for high-torque longevity.

I'm with the recommendation to "try it" and then decide. 901 trannies are relatively cheap so if you blow it up, you would always have the option of swapping in another one, and you would still be well ahead of the conversion cost to a 915 or later unit.

Thanks guys for your help. :)