How much can a 901 transmission handle?

[ZCAT3]

What is a reasonable power limit for a 901 transmission?

[jluetjen]

It's more a question of torque (rather then HP) and how it's used. Abusing 1st gear will significantly shorten it's life. If you're gentle with first gear I've heard of people hooking it up to small-block V8. I doubt that it would last for 200K miles in the latter example.

How big of an engine do you want to hook up to it?

[Bill Verburg]

Search is a wonderful tool
look here

[ZCAT3]

Thanks Bill. Of course I did a search I guess it is a matter of the right search terms - 901 and power did not prove very fruitful.

[sammyg2]

The posts so far are accurate, but we can take it a step farther. Obviously extreme shock load can break parts so you shouldn't rev it all the way up and dump the clutch in 1st gear, eventually even a 1.7 liter type 4 engine would break a 901 if you did that.
1st gear is not supported on both ends by bearings, it hangs off the end and is only supported on the inboard end (same as reverse). That means that under extreme torque the shaft can bend or flex a little, causing the gear mesh to go bad, which can break gears. The rest of the gears are suspended between bearings so thay can take more, that's why 1st gear needs to be babied with a big engine.
The other limitation is the output shaft (or pinion shaft). It is kinda skinny and under heavy load it can bend (flex) to the point where it tries to climb the ring gear. That can cause the gear mesh to go bad and things to break. Again, the same problem, the pinion shaft is only supported on the inboard end of the pinion gear.
Some people say that flex can also happen in the magnesium intermediate plate. That's where the bearings are. If the plate flexes it can cause the gears to move and that's bad. They sell aftermarket heavy duty aluminum intermediate plates but I am not convinced that they are worth the money.
I ran a chebbie 350 with a 901 that I rebuilt myself for over a year. I only used 1st gear to put it in the garage or on a trailer, but I spun the rear stickies many times in second (and third, and 4th). It never complained at all and I was dumping well over 350 ft. lbs. through it and my shifting style on the track was anything but gentle.
I figure that combination with my driving would prolly only last about 20,000 miles or so but that translates into 5 or 10 years based on how many miles I drove it.
If you are running just under 200 ft. lbs. of torque you should be able to make one last close to 100,000 miles (if you are easy on 1st gear).
At around 250 the lifespan might be half that. At 300 it might be halved again or even less.
Porsches numbers are very conservative because they have to be. they need to sell something that will last a very long time, otherwise they suffer from warrantys and a bad reputation.

[ZCAT3]

Thanks Sam. I am thinking of dropping a 3.2 into my 69 911S. This is a street car and I am not a stoplight racer. I have a similar powered engine in my track car (a 71 911T) and the PO fried the 901 in that car with this setup - the car has about 225 RWHP and maybe 190 RWTQ. So he replaced it with a prepped 915 that works great at the track but is not what I would want in my street car.

I had never driven a 901 car until I got the 69 and I really like that transmission. It is very smooth, yet has a nice pure mechanical feel to it. Ideally I would be able to mate this with a 3.2 and not have to rebuild it every year.

[Joe Bob]

I beat the crap out of a 901 with an LSD which is supposed to be a killer with a 3.0 and 964 cams.....I ALWAYS burned it in first gear.....I had an extra trans ready to go and this one just wouldn't give it up. Reminded me of an old girlfriend.

Finally blew up when gently shifting from first to second when slowly cruising looking for an address....was able to make it home w/o first or reverse gear...pulled it, swapped the LSD and went at it again....