915 and a turbo

[sundevil64]

What would be hp limit for a later 915 (84-86) transmission on a 930 motors.

If a 84-86 915 had a bearing retainer and lsd with the thicker side plate do you think it can hold up to about 390 hp from a twin plug,efi, turbo motor with a k27 HFS turbo?

Thx,

John

[getz]

My 915 is doing well with a non intercooled 3.0L that dynoed about 350 hp to the crank. I have never dropped the clutch in first however.

[spuggy]

Originally Posted by sundevil64

What would be hp limit for a later 915 (84-86) transmission on a 930 motors.

If a 84-86 915 had a bearing retainer and lsd with the thicker side plate do you think it can hold up to about 390 hp from a twin plug,efi, turbo motor with a k27 HFS turbo?

Thx,

John

Your estimate for an EFI motor seems low. I made 370RWHP single-plugged on CIS with a K27/HF, Kokelyn & GHLs.

My built 915/73 with WEVO bearing retainer, late factory oil pump cover (driving internal forced lubrication spray bars, as per factory race boxes), 40/65 LSD held 370 RWHP for quite a few years. Still in good shape when removed and sold; 99% street-driven, with some track days. If I did it again (which I never will), I'd probably use the WEVO side cover with the 930 bearing and an electric pump to drive the internal spray bars.

You can still break a 915 anytime with those power levels, it just takes a moment's inattention or an outbreak of stupidity. Hayden (of WEVO) says there's basically nothing you can do to make a 915 reliable at those power levels. You can make it live longer/a long time by observing some simple rules:

Never drop the clutch.
Always roll the power on (high torque at low RPM will kill the box)

Yeh, I got really tired of babying the 915 and granny-shifting it.

The last one gets especially annoying with a hybrid turbo that builds boost low - my K27/HF made 6PSI @ 2400 RPM, but I never went full throttle below 3500-4000 in 2nd - which usually meant you ran out of time in 2nd and never actually went full throttle in any gear below 3rd...

Building that 915 (not replacing all the bearings, seals, forks, synchros/brake bands while you're there is just dumb) is also pretty darned expensive; it's about twice the cost for a built G50 short-bell conversion - which is over a much better idea/investment IMHO.

If you do go 915, use the late axles as used on the 491/G50 cars with the rare-as-rocking-horse-poop 915 108mm flanges if you can get them, the CVs on the axles for the 100mm flanges aren't as strong (don't live as long as you'd like).

The stock 915 clutch (or even the Power Clutch, which *IS* a stock clutch, cherry-picked to be top 10% of spec) won't hold that power either. Anything over 1/3 throttle around max torque; it'll break free. In fact, I think the Power Clutch slips with 300 ft/lbs (930 RoW) - otherwise, why would they only last 15,000 street miles...

You need the Sachs Sport clutch (I sold mine via the classified here; part # on the pressure plate) - closest thing you can get to the (long-NLA) RSR 915 pressure plate.

Might as well rebuild the pedal box, replace the cable and every bracket/actuating arm before you fit it - the increased pressure WILL break them all, one-by-one, leaving you stranded...

I used a puck clutch - don't think that's necessary, and it's quite a PITA to live with, especially in traffic. But I was on the 4th or 5th clutch job at that point, and just wanted to drive it for more than 2,000 miles without doing it again...

[Rodsrsr]

Agreed on all points. I've been running a rebuilt 915 behind my turbo 3.2 for a few years now and its still fine, but no first gear launch and 1/2 throttle in 2nd. So, yes it will work but its no fun. A G50 is right around the corner for me.

[quattrorunner]

I'll input as well. I'm not as nice as rodsrsr to mine. I never launch in first, I go full in second gear and am on my second 915/61. Last box I broke the second gear itself. Everything else looked great.
My first box had high miles (190000) and lasted about 8000 miles boosted.
I have about 350 lb ft of torque if guess.

[RarlyL8]

The short answer is no, the 915 will not hold up in front of a 400WHP engine.
I ran the 930 engine in an SC for 15 years with the final version being 350 ft/lb torque. That is really max for what the WEVO enhanced 915 will take, and even then you must never launch hard. The turbo has to do the work not the tranny.
Beyond that, a 400WHP engine is beyond the gearing of the 915. That level is better suited for the 930 4spd. First is too short and you'll rip the teeth out of second.

[sundevil64]

Thank you all for your input. I guess I knew that answer, but was hoping for something new.

It looks like I will either have to park the motor until I can get an early 4 speed for a carrera installation or a G50. Of the two I would rather do the 4 speed as I love my 4 speed in my 930 albeit at highway speed that keeps me in a little boost at 80 mph.

I have a euro 915 out of my 84 that I thought I might use, but that would mean there would be one less euro 915 in the world...

[RarlyL8]

The 930 4spd will fit in your Carrera, it just sits further back in the chassis. I believe in '84 they were still using the same tub for the 930. If you have a 930 now then it may be more different and fun to build/tune the turbo engine in your Carrera to match the 915. Reduce the boost level to keep the torque below 350 ft/lb and don't run sticky tires. Add a limited slip the the tranny and you have an incredibly quick and fun car!

[mreid]

I have Spuggy's old built 915 living happily behind a 300hp 3.6 out of a 95 993. Love the asymmetrical LSD and while I baby 1st, it works just fine with full power in the mid-range.

[sundevil64]

Originally Posted by RarlyL8

The 930 4spd will fit in your Carrera, it just sits further back in the chassis. I believe in '84 they were still using the same tub for the 930. If you have a 930 now then it may be more different and fun to build/tune the turbo engine in your Carrera to match the 915. Reduce the boost level to keep the torque below 350 ft/lb and don't run sticky tires. Add a limited slip the the tranny and you have an incredibly quick and fun car!

It's funny you mentioned that. It was discussed amongst friends to pull the 4 speed from the 78 930 and put in a built 915 as the 930 has less hp than the 79 930 efi'd twin plug motor with 993 HE's SC/964 cams, k27 HFS and an m&k twin w/g set up with your hooligan muffler.

The only thing is the 78 930 sunroof delete is pretty much a survivor and I don't want to mess with it too much.

I may have to find a 4 speed or keep the power down to accommodate the 915 in the mean while. I will have to track down an lsd as that seems crucial for equal distribution of torque to alleviate some of the stress from the side plate.

John

[Tippy]

You know John, I'd hate to have to baby a car in fear of busting the 915. I'm kind of in the same boat I'd admit with my G50 in 1st gear since I don't have a billet diff cover. But I never worry about it on the street with street tires.

Seems like a 915 would hold you back unless you never want to increase horsepower and always be on your mind when you mash it in 1st. But that's just me. . I want to ride in it though.

[RarlyL8]

Ooff, yes if you have THAT much of a build foretabout the 915!
At that point I would consider swapping engines instead of trannies. Everything is reversible.

[sundevil64]

That would be kind of fun to pickle the original motor and tune the 79 930 turbo motor to around 420hp and put it in the real 930. The 4 speed would certainly hold up to that and would be a hoot!

I'll have to give that some thought.

[sundevil64]

Originally Posted by RarlyL8

The 930 4spd will fit in your Carrera, it just sits further back in the chassis. I believe in '84 they were still using the same tub for the 930. If you have a 930 now then it may be more different and fun to build/tune the turbo engine in your Carrera to match the 915. Reduce the boost level to keep the torque below 350 ft/lb and don't run sticky tires. Add a limited slip the the tranny and you have an incredibly quick and fun car!

Aren't there differences in 930 4 speeds after 79? I thought they were longer? As in too long and needed shortening of the bell housing and the input shaft? I may be wrong.

[RarlyL8]

Yes the '79 930 4spd will sit about an inch further back in the chassis than the 915 which places the engine in the same OEM location as the 930. All that machining/expence is only needed if you want to retain 915 length and chassis placement of the engine.

[quattrorunner]

Ooff, yes if you have THAT much of a build foretabout the 915!

At that point I would consider swapping engines instead of trannies. Everything is reversible.

Agreed

[Matt Monson]

It is fairly uncommon to find people ready to fork over the dough but it is possible to make the 915 even stronger by buying a new mainshaft and at a minimum 1st-3rd gears.

The newest 915s are are over 25 years old. The oldest are 40. A lot has changed in gear design and metallurgy in the ensuing years. And the stock 915 1st is really short. Redoing it and mating the low gears to the 930 engine will both increase reliability and performance. But it will cost you around $4000 in just parts.

[Uncle]

I thought the main issue with the 915 was case flex once you start to make a little power and more to the point torque. More so than the gears being weak.
IE:You load up the outer case this places load on not only the ring and pinion but the main shaft. The next thing you know you have a pile of broken 915 at the other end of the clutch. By not giving it full throttle in 1st and 2nd you're not loading the case up as much. Then because you have the mass of the car already moving you can show it some more gas in 3rd.
You can't really fight case flex other than a billet side cover and even then once you are making good numbers it's still a band aid fix.

[sundevil64]

There was a manufacturer that made a "cradle" for lack of a better word for corvette transmissions. It essentially looked like an outer skeleton that supposedly prevented transmission flex. I saw this a Corvettes at Carlisle. I had wondered if this was possible or for 915, but given the availability of the g50 it just may not be worth it. I wish I had taken a picture. It was interesting if nothing else.

[RarlyL8]

Yes you can make a cradle for the 915. A buddy of mine did this when testing the durability of the unit in front of a 400WHP turbo engine. 4 transmissions later most problems were greatly reduced. Here are the weak parts we have seen after half a dozen forced failures:
> Side plate takes all the stress non-LSD. Need heavy plate and LSD to reduce.
> Ring/pinion bearing will crush. Need bearing retainer.
> Second gear is weak. Replace with stronger.
> Wheel hop will destroy the crush washers. Need heavy t-bars/shocks.
> Speed shifting under boost can crack the case. Build cradle.
All this can be done for not a tremendous amount of money. If abusing the tranny keep a spare. You can have 6 spares for the price of a G50 conversion.