DFWX

While this is a question I raised in the past, it does seem increasing relevant due to increasing numbers of 928s being boosted. Receiving rather dismal projections of how much horsepower and torque a 928 drivetrain can withstand, I dramatically downgraded my 928 project car's motor agenda. At the core was going to be an early 32V running S4 pistons (for 7-1 compression) - and use an intercooled Eaton SC and turbo combination which combined with a rather excellent stand alone engine management system and a secondary small racing fuel tank would have made enormous hp - and had rounded up basically all the components for this.

Once you start playing with compression, boosting, engine management and are not constrained by pump gasoline only, the amount of horsepower ANY 5 litre motor can make is And that is without factoring in any other options such as nitrous oxide.

While I may in the future do to such a project, a more important goal is to have my 928 actually running and 400 or so on a turbo 16V Euro works fine for that.

So, again, what do the members here think is the ceiling on horsepower that the torque tube, transmission and differential can take? 500? 600? 700?
I prefer 5 speeds for the fun of them, though automatics may have some strength and acceleration advantages...

Mark O

Kaz

This Russian company built some kind of monster horsepower car and the article mentioned the company went with the automatic transmission from a 928 due to its ability to handle the 750lbs of torque their engine was putting out. Don't remember what the horsepower was. I think among 5 spds the 91 transmissions are thought to be the strongest.

You might also search the archives as it's been debated time and again.

Vilhuer

I think Russian car used BMW V12 and GT gearbox meaning it's manual. GTS box is stronger still but even it starts to develop problems eventually. Late model automatics can handle larger torque numbers when rebuild with AMG parts. It's tq which kills boxes, not hp. TT is weakest link in the chain.

mark kibort

I think anderson is bordering on the limits, being that he has lost a couple of the main shafts of the GTS Gear box. i think he is around 475hp and 450ftlbs of torque.

Im sure , since he is beating up on the transmission at max output for over several hours a race weekend, its safe to say, that the transmission is probably able to keep up with more torque and hp if its more intermittant in use. what that number is????

MK

Evil Patrick

Well, The Bastard puts 400 ft lbs to the ground and that's through an 85/86
BW 5-speed with early posi retro-fitted [thanks, Mark Anderson / 928 Int'l!].

400 to the ground means there's about 460 going in (if you use the 15% rule).

Only problem I have? Wheel spin. Lot's of it!

Aside from just focusing on the tranny, the rest of the drive train needs to be up
to the task as well. From the flywheel to the lug nuts. Since I'm not stock until the
intermediate shaft and forward clamping collar, I can't speak to the 928 flywheel,
pilot bearing, clutch disk/s, pressure plate or throwout bearing, but the torque
tube assy and back is all Porsche 928 "stock", with the exception or the retro-
fitted posi.

DFWX

As in the past, I'm hearing 450 hp is about the max. - which is very restrictive on engine development. Mark O

sweanders

450 is obviously not the max sine Tim and Cheryls car has 515 rwhp. The DEVEK white car also had over 500 rwhp when I drove it and it worked well.

hacker-pschorr

Tim Murphy's car is around 550rwhp - he recently installed a GTS gear box.

Todd has not put his car on the dyno yet - I'm willing to bet for a street driven 928 he's topped anything we have seen. If not, he will shortly. So far he has not has any transmission issues - he is running a custom gear venter reduction drive unit so his torque tube has been modified to accept this. Tim's car is a 5 liter with 15lbs of boost (I think) Todd is currently running a 6 liter with the same boost, maybe more if he's turned it up since I last spoke with him. Todd is also running drag radials on the stock 16" wheels.

tammons

I think the most recognized limit is 550 - 600 hp, but its torque that breaks things. Even at 450 if you drive it hard or race you could still break parts. 500 hp is plenty enough to get you in trouble. Beyond that takes major upgrades. The guys down here that built the 950 hp S4 had a custom ceramic clutch setup, custom driveshaft couplers, a bigger custom chrome molly driveshaft, but could never keep the transaxle together. They went through 3-4 transaxles. I think the key there is to have a custom unit built with 930 gears.

Funny the 924 turbo car used 930 internals and can handle around 750-850 hp. Too bad they never got serious with the 928.

IcemanG17

There are several 928's with more than 500rwhp & 500rwtq......but almost all are 5 speeds and dedicated track cars....so they are far lighter than a stock car, but they do have much wider rear tires too...... Torque is the killer & it does seem the automatic is slightly stronger than the 5 speed....especially when it is built with the AMG internals and a special torque converter....but that won't help the diff-TT-halfshafts-etc? I've heard the AMG tranny is rated to 1000hp? Steve Pappin put one in the GTAR with a custom torque converter....

heinrich

As much as you give it. In other words if you're the sort of driver who needs a clutch every 20k miles, then not a lot. If you drive in a way that doesn't stress the drivetrain, you will have a heck of a time overloading the drivetrain and even the stock clutch.

pmotts

Bingo

Fabio421

I agree with H. It will break much sooner if you are doing NHRA style drag launches than if you drive like a grandma. Also once you get up to speed, say 3rd gear or higher, it should be less stresssfull on the drivetrain.
This has been discussed here before and I thought that the conclusion was somewhere around 600 HP. I'm sure a search will yield the threads.

H2

One of our brethern from Alaska swears by the auto trans (on his twin screw)..claimed that it was far better to deal with as a 5-speed would make for more difficult traction. Then again, I don't think he makes too many jack-rabbit starts either. However on the twisty track, winding out, his car was awesome! And torque probably more even.

Harvey

Mark Anderson

Maybe the real question is at "how long will it last and a certain torque level." If you look at a track like Road America you are at WOT for 80% of the time. A typical weekend has about 2 hours of track time so you get about 96 minutes WOT with little or no wheel spin to lessen the load. I can't imagine how long it would take to get that amount of WOT on the street. That's like doing 0-100 9,216 times.