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spuggy spuggy is online now
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Join Date: Jul 2003
Location: Glorious Pac NW
Posts: 2,828
Originally Posted by Rich911 View Post
Well where do i start?
Here's a good place - welcome!

Finally I'm using a 915 3.2 box. Will this stand 450/500lb/ft of torque? or will it rip it to shreds?
I very much doubt that it'll work long-term. I think it'll end in tears.

My high-mileage mag-cased 915 (with at least two prior rebuilds we think) was internally looking pretty sorry for itself after 25,000 miles in front of a mild 930. I replaced with a late 3.2 alu cased 915, and had Mike Bainbridge (MBEngineering on this board) convert to an internal spray bar kit, taller 2nd and 5th gears and fit a WEVO bearing retainer and an LSD with variable lockup.

You'll be glad you spent money on that last item under deceleration, whatever you're using as a gearbox.

If I'd had more time (discovered the box was shot after motor rebuild commenced), I'd have used WEVO's diff and intermediate cases as well, with the stiffer bearing housings (I had a WEVO side cover with the 930 output bearing, but had to use the factory oil pump cover). WEVO were working on a modular cooling kit too, dunno if it's completely out of R & D/available yet.

I'm making 395 HP & 350 ft/lbs at the crank - and that's entirely not by accident, heh.

300-350 ft/lbs is generally regarded as a realistic limit for a 915, and you still have to avoid certain scenarios that will break the 915 (2nd gear is apparently the weakest, and you can peel all the teeth off it).

In an older thread on this board (actually in the 911 tech forum, which pre-dates the 930 one), Hayden (of WEVO) and others make pretty clear two things you will want take on board;

a) pouring money into a 915 is no guarantee that you can make it live longer at power levels significantly higher than it was designed to take (e.g. above 300 ft/lbs) - it's a crap shoot at that point. If you drive reasonably, you can get a lot of fun out of it for a few years, or you could break it pretty much at will.

b) the one thing the 915 really cannot withstand is massive torque at low RPMs.

The stock 930 is a torque monster, and a tweaked one doesn't make less torque anywhere. An EFI'd and blown 3.2 will be too.

Twin-plug and flame-ring it while you're in there, you'll only regret it later if you don't. Aftermarket fasteners for rod bolts are a given, right? You'll probably want the 965 fully-finned cylinders, and should use the pistons that go with them, because your stock 3.2 compression would yield something like 16:1 with stock boost otherwise...

If you accept that you can break your gearbox at will with one prod of the right foot and drive reasonably, you may get some use out of a 915, but you should be thinking of options.

At that power level, you are out of options on the clutch; I've got a 915 Sachs competition cover coming, and the stock friction plate will work for me.

You, however, will need a very much stronger cover than that, which will have a "very heavy" pedal. You also will have to run a puck clutch to handle that torque - and these are generally regarded as "not streetable" due to the harsh engagement.

The six-puck versions are less unacceptable than the four-puck, but the word is that neither is good for a streeted car. You might want to clarify your intended usage; my car is >99% street.

So your plan thus far is do-able, but probably needs some work to refine...

You're in Oxfordshire? You need to talk to this chap called Bob Watson in Banbury, he's built and worked on a few 930s.... PM me if you need a phone number.

Helix Autosports (also in Banbury) can talk to you about a competition clutch, but if you're determined to make that power, I think you should be making plans to switch over to either a 930 box or the G50 now. Both use a 240mm clutch, which makes the limits of the 915 225mm clutch irrelevant. The G50 conversion is nicer, but involves major work to earlier cars, if I understand the process correctly.
'77 S with '78 930 power and a few other things.
Old 07-28-2007, 03:21 AM
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