Is there a way to make Porsche 915 trans stronger?

[Diggem]

Just checking to see if anybody has some tips on how to make a 911 Porsche 915 transmission stronger or more durable so that it can take more torque and horsepower? I think the particular trans that I have is a 1979 Porsche 911 SC transmission.

[RarlyL8]

To a point, yes. I've run these boxes behind 930 engines and got them to withstand up to 350whp for several years. Just haw much they can take depends on how bad you beat on them. I always let the turbo do the work so very easy on the trans in that regard. Mods were WEVO bearing retainer combined with reinforced side plate. Limited slip helps as well.

[jamesjedi]

more info:

915 Side Covers

[Matt Monson]

The two gears that get really hammered, just because they are used the most, are 2nd and 3rd. If you are going to significantly increase the power going through them, it is wise to replace those 30-40 year old gears with brand new ones.

[spuggy]

Originally Posted by Matt Monson

If you are going to significantly increase the power going through them, it is wise to replace those 30-40 year old gears with brand new ones.

And Matt's not just saying that because he sells gears...

My tall 2nd gearset suited a 930 well; I loved it, knew it was old and treated it carefully (roll throttle on slowly enough it'd be time to shift before hitting either full throttle or boost).

Wasn't enough; it did this commuting one day:





Should probably clarify how much more you mean by "more torque and horsepower"; anything significantly over 350 with a 915 puts the emphasis more on "when" than "if" it'll break. There's a point at which babying the transmission and living in fear just gets old...

[pampadori]

Has anyone shotpeened and cryo treaded the gear stacks to make them stronger? if it even made them just 15% stronger, that would be a nice extra margin.
Edit: after reading a bit more, it sounds like the best bet is heat treating, cryo treating, and then shot peening - in that order.

In a previous life i used to work for a place that sold dog-box gear stacks that could handle about 3 times the power of the factory gears could on turbo charged nissan race cars. We sold these new straight-cut dog gear stacks for something like $3,000 a set. Could you imagine getting something that inexpensive for a 901 or a 915?!
I just discovered a damaged 5h gear slider and went to price a new one...$970 for the one slider! WTF??

Anyhow, call https://www.300below.com/ and ask how much it would costs to send the gear stacks to them to get treated. Might be cheap insurance.

[Bill Verburg]

While the gears can break I've found that the main issue is just keeping the shafts properly aligned
to that end the Wevo girdle(aka bearing retainer plate) and strongest side plates(probably Wevo w/ 930 bearings) that can be fitted are the first things to address.

The 915 mg trans come oe w/ a steel bearing plate so it's mostly the later al. case 915s that need the it.

[r lane]

I have a 915 with an oil cooler on the side plate. I don't suppose anyone makes a mod of that. Wondering if the additional material and waffling has any strength benefit. I am in the process of changing gears and now would be the time to be preemptive. Bob

[Sboxin]

I agree with Matt and Bill and James - -
We have been using a modified 1978 SC 915 in my son's race car which has a 3.8L modified 964 engine with about 300 crank HP, dyno shows 250 RWHP and 255 RW ft lbs.

California Motorsports in Lake Havasu AZ did the rebuild/modifications and replaced the 3rd gear set with Matt's gears. They also replaced the side plate with a CMS and turbo size bearing - and bearing retainer plate also. We use a cooler with an electric pump and fluid cooler ducted through the rear floor. We are going on our 5th race season and so far all is good . . .

Regards,

CMS shop -that's a 935 transmission on the bench behind Shawn

[Mahler9th]

Of course sharing knowledge of your application details might be helpful when seeking advice.

I have about 20 years of experience with a mag case 915 in a racing application. Engine displacements between 3.6 and 3.8 liters, with big sticky slicks. Tire sizes 16 inch diameter and 13 or 14 inch wide. Very big, very sticky.

Engine outputs between ~325 and 370 bhp.

Lots of local friends with similar set ups.

I like the one piece bearing retainer. I use the WEVO. I have a friend that has about 40-50 years experience with these things (as a technician and racing driver, including IMSA) and he doesn't think you need the retainers for mag case 915's because they have steel inserts. He thinks it is more of a nice to have with mag boxes.

I have used a few stock gears and a few Beagle en Engineering gears, but my favorite, by far are those supplied by Guard Transmission. As Matt says, in racing apps some gears tend to get used up. Guard has always made fine products. We are lucky to have them.

I used a Guard TBD DIFF for years and I loved it. I just recently removed it (I hope to re-sell it if it can be refurbished) and am now using an excellent Guard LSD DIFF. Really love that thing. Much stronger than a factory-style ZF.

I have run a factory alu side cover, and now run a WEVO side cover.

WEVO make excellent products. The founder/engineer has tons of pro racing experience including IMSA and some Indy car. He is an absolute expert, as is Mr. Monson.

One thing that folks are sometimes not careful about is their ring (aka crown wheel) and pinion. I use the factory-style 8:31 units. It turns out they last longer if treated better (e.g., staying off berms), just like other transmission components!

[spuggy]

WEVO bearing retainer, stiffer side-plate are givens over 300HP.

An LSD is stronger than an open diff - which can apparently break at higher power levels. With a working open diff, losing traction and having one rear wheel steer the car also gets to be a safety issue.

Direct on-gear lubrication doesn't hurt at all - the factory did in on all race boxes I'm told - and all 915s have provision for spray bars that just need to be drilled out. WEVO used to do an electric pump kit IIRC. Late factory mech-drive cooler pump side-cover is fairly stout, although the best side-cover of all is undeniably a billet one with the 930 bearing.

Later 915s apparently got stronger, so preferable to the earlier - e.g. SC - ones.

Clutches that can hold 350+ ft/lbs at the wheels are another topic - the solutions are unlikely to improve the driving experience in traffic. (RSR-style Sachs competition pressure plate and a 6-puck clutch worked for me - only took me 5 clutch swaps to arrive there). Expect pedal pressure to increase tremendously, and replace anything - actuating levers, cable brackets etc - that are original/old, because they will just break anyway.

I ran a mag box with a 270 WHP CIS 930 for a while. It was comprehensively trashed inside when disassembled; intermediate shaft outer bearing race just came out on the shaft because there wasn't any case left to hold it in...

The factory apparently (Starke, I think?) gave up on the 915 for hi-po use when they found they couldn't make it last 24 hours with the "baby" turbo RSRs...

[Joe Bob]

Wevo stuff is what I use....Matt also has a very good reputation.

[Tippy]

How to make a stronger 915? Replace with a G50.

[Matt Monson]

Just an FYI for people, we are the originators of the bearing plate. We made them for Wevo before they had their own local manufacturing. We've continued to make them under our own brand ever since.

[Geary]

For anyone who believes cryo treatment to be the bees knees, Matt may have a story to share ..

[stownsen914]

So I have a question about use of the beefed up side covers on mid-engine 915 applications. If I've understood correctly, for a 915 in rear engine application, the side thrust from the differential is toward the side cover, so the benefits of a stronger side cover and the heavier duty 930 bearing are pretty clear. What about in a mid-engine application since the differential is flipped to give opposite rotation? I believe in that case the side thrust is toward the other side (not the side cover). Would a beefed up side cover help in that case?

[Mark Salvetti]

Originally Posted by Matt Monson

Just an FYI for people, we are the originators of the bearing plate. We made them for Wevo before they had their own local manufacturing. We've continued to make them under our own brand ever since.

Mine is a Guard bearing plate, I think I bought it in 1998 or thereabouts.

Mark

[911tracker85]

How to make a stronger 915? Replace with a G50.

easy for you to say....

I have a 78 SC/930 conversion now into the uphill side of a full restoration. guy who had this built in 1990 stuffed a twin plug rebuilt 3.3 turbo motor but left a 915 gearbox. I sold the 915 to a friend who rebuilt it, in pretty bad shape with damaged gears. surprised it did not blow up, but clearly this car was never tracked as it still had stock SC suspension and brakes. all that will be upgraded.

due to the space constraints of a non-turbo tub, planning to use a 930 4 sp but need either a 76-77 or have it rebuilt and the bell housing shortened.

from some discussion it sounds like you can put a lot of money into a 915 but can only make it so strong. based on the engine rebuild plan I am expecting close to 400 hp/tq.

I have this thread marked to circle back for updates.

[spuggy]

Originally Posted by 911tracker85

due to the space constraints of a non-turbo tub, planning to use a 930 4 sp but need either a 76-77 or have it rebuilt and the bell housing shortened.

SBH 930 boxes have significant rarity value. Not all long-bell 930 boxes can be shortened- and they weren't cheap to start with/not getting any cheaper.

If you need it, you need it. For the money and effort required (of which the transmission itself is only a fraction), may as well go G50.

Although 930s happily pull tall ratios, hard to beat the fun factor of being in the right gear - although I guess it really depends on how you want to use the car.

Oh, re-gearing an N/A G50 to "better" suit a 930 not exactly cheap either.

Originally Posted by 911tracker85

I have this thread marked to circle back for updates.

There's threads here from 10-15 years ago with the same topic/reasons/conclusions. I'd love to see a magic way to make a 915 live with 400+ RWHP, or a Subie transmission conversion that could deal. Either seems unlikely.

[Matt Monson]

For many years guys were putting takeout used G50 NA gears into 50/50 and 50/52 boxes because the NA gears are shorter. For a turbo I wouldn’t regear an NA g50. For a v8 I would.