[spuggy]

Originally Posted by Tmo

the only question being whether it will be strong enough for my engine.

A built 915, treated with the appropriate respect/fear that comes from the knowledge of the consequences of being stupid, can last just fine...

My warmed-over CIS 930 made 370HP/350 ft/lbs - at the rollers - with 6 PSI of boost @ 2400 RPM, and 80% of peak torque @ 3000 RPM. I drove that tune for ~6 years with a 915.

But it's potentially still a grenade, and you just get tired of driving around the limitations, the unsuitable N/A gearing, and the compromises - like the wildly anti-social puck clutch & RSR pressure plate.

Originally Posted by Matt Monson

It’s always the street car guys who burn up 5th gear, with oil starvation. A spray bar is not an extraneous mod.

Amen to that. The 915 in the car when I got it had a very fetching dark blue colored 5th gearset when dismantled "for a look" - apart from zero dog teeth on 1st and a shift that felt like a stick in a bucket of rocks, it had seemed "kind of OK". Until we looked inside, saw the carnage/spun bearings etc.

Figured oil slosh from the nose cone under boost did the heat damage. Sourced/built up a mid-80's RoW 915 - and used the cooler pump to drive internal spraybars.

Originally Posted by Tmo

The engine should produce around 300+/- HP and apx. 350 TQ,

That's basically ballpark for a stock, unmodified, RoW CIS 930, with slightly more (50 ft/lbs) of torque. Which I'm just gonna SWAG your motor probably won't make down low in the RPMs.

So, basically, not uncharted territory, as plenty of guys have run CIS 930s with 915s, or low-boost SC setups. Some last for a long time.

As you're building a box up, you might consider 86-up 3.2 Carrera "attached stub axles" (used with both the 915 and G50 from that MY, if I'm not mistaken). These have 108mm CVs and 6xM10 bolts. The earlier (74?-85) 6xM8 bolts on the 100mm CVs are lot more prone to breaking or backing off. Ask me how I know... Some folks even have issues with them on SCs...

You can't use the earlier still (pre-74?) 108mm 2 dowel/4xM10 bolt flangeswith an SC trans because they're coarse spline. And the "only used for 10 minutes before going G50 with 86.2" 108mm 6xM10 915 86.1 flanges were rare used and NLA from Porsche last I looked 15 years ago...

Pretty sure Matt said somewhere that you can use G50 108mm flanges in a 915 if you shorten them internally to suit. Remove the wheel stub axles - hub splines are the same from '69 through '89 (at least) - the "attached stub" axles bolt right up to the 108mm flanges.

This gives you turbo-spec axles - but shorter, to fit 911 trailing arms.

The wheel/hub arrangements on 930 trailing arms are different than 911, no idea about those.

Originally Posted by Tmo

I drive with mechanical sympathy, but I'd like as much strength as practical to back it up. I don't drag race, and I'm not prone to hooning.

Well, you say that now

But eventually you'll realize that turbos are really easy to squeeze lots more power out of. And then you'll realize the 915 will always be your limiting factor...

BTDT.

The costs of rebuilding a 915 properly aren't going down - never mind building it to be as studly as possible, or re-gearing to suit your transplanted motor.

Another thing to consider is that the gears in your SC transmission will be asked to deal with substantially more torque than intended by the factory when they were shiny and new - but now they're 40 years old.

Just sayin' - because my 930 ate 4 teeth off 2nd gear (and cracked the base of the 5th tooth/moved it sideways). During a commute to work - it just decided it'd had enough...

There's a reason race teams age parts like gearsets out.

And there's only so much you can do to a 915 - as the factory realized with the 2.1L Turbo RSRs because they stopped using them.

I don't know the power characteristics of the GM motor/what it likes, but you may well find the ratios in the 915 are not a great match. Probably too short-geared, and so far too buzzy to be ideal would be my guess. So you may also want to consider at least a tall 5th, for freeway cruise.

I suspect you'll probably end up wishing all the gears were taller - because you're making 350 ft/lbs with a turbo, gearing intended for an N/A car making 200 ft/lbs likely won't be a great match.

It's very easy to spend quite significant money on a 915 - and it may end up not being ideal for what you really need. It will always be the limiting factor for the maximum power you can make, or the way you can drive the car, dictating how you launch, roll in the throttle etc.

A SBH G50 with carefully selected gearsets is the best way to get a strong 5-speed matched to the characteristics of a turbo motor. And pretty much necessary over a certain power level, if you don't want a 4 speed 930 box.

As Matt says, building a Porsche hot rod isn't cheap.

350 ft/lbs is probably right around the upper limit for the stock clutch, too. Might work, might not. Might depend where you make that figure.

Sachs Power Clutch was said to be a stock clutch cherry-picked to be in top 10% of spec. Mine seemed to hold 300 ft/lbs at the flywheel well enough. I think the short service life was at least partly due to PO liking to roll @ 40-50 MPH and build boost against the brakes before releasing - felt like a steam catapult launch and was a lot of fun, but couldn't have helped...

If the clutch doesn't hold. then you're into trying various aftermarket options - many of which, in the 915 space, aren't intended for turbos. Sachs tend to quote their ratings with a pretty substantial "safety margin" - but many aftermarket vendors spec their clutches for pretty much exactly what they'll hold...

I can make a recommendation for a Sachs (ZF Race Engineering) pressure plate that held 350 ft/lb at the rollers with a puck friction plate for me (last resort after 3-4 others were dismal fails) - but it does make life quite miserable in heavy traffic. And it will break all the old/original parts (actuating arm, brackets etc) one-by-one until everything is new, so skip the breakdowns and figure on replacing those when you fit it...

As far as LSDs go, for my $0.02 it's hard to beat a 65/35 asymmetric clutch LSD, set to about 80 ft/lb - and the GT LSD is pretty much the gold standard. You'll thank yourself every time you back off and the rear doesn't float around vaguely for a few seconds until it settles, like it does with an open diff or a torque bias. You get used to the turn-in push very quickly and just compensate.

Both the torque bias and the clutch pack LSDs will help with breaking traction on a greasy road when you don't mean to. But only the clutch type nails the rear end down on decel.


You may not have considered other changes which are probably desirable when you add 50% more power...

Torsion bars would probably benefit from being stiffer, to control both squat and weight transfer (which can be quite extreme). I find 21/30 works well for me, with the front being supple enough that it's still comfortably streetable. The more linear the turbo is the better for weight transfer - but the rear will still squat under boost, do need to control that. Or at least accommodate how deep into the flares the tires will actually go under power.

3.2 calipers/rotors up front will work - but probably are the minimum you should consider. You'll probably find the feel (and pedal travel) not especially confidence inspiring. And you'll likely wish they had more thermal reserve if you go for repeated stops from speed. I was pretty happy to get rid of mine...

17" wheels let you run cheap, sticky hi-perf tires. There's a wide selection in 225/45/17 & 255/40/17, some of which are both reasonably priced and perform very well.


HTH.