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915 trans with 400 hp turbo motor

Hi guys the car I recently bought has a 915 trans. It's aparently stock from the late 80's. From what I read online it sounds like the trans probably won't last long with the power that I have which is approx 400 horse power with the engine work done to it. What are your experiences with this? Is it a ticking time bomb as is?

Thanks.

Old 10-12-2014, 03:12 PM
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I know that during the manufactured years of the 915 they are not all the same,
I know the later model 915 trans are stronger than the earlier ones,
Plus if any previous history avail on box you never know it might have been beefed up by a po?

How much stronger i personally dont know
In the end theres one way to find out..
You could go for a quick drive late at night with a friend following and give it to her (the car) , if she doesnt break then you know she will handle it (the trans)

Many people say this and that and the other is no good and you have to upgrade etc..
But i think if it Isnt broke then why fix it? when a change could give you problems in other areas, happened to me,, everything worked fine until i started on the slippery slope, i shoulda stayed on the hill lol

ive got a 915 an early one in my euro 930 with 350hp at 0.8 under normal conditions but for about two months i had problems with my wastegate and was boosting to 1.5 bar on a regular basis, apart from severe whiplash the gearbox was fine.

theres a guy around here with 450hp and a 915 and he tracks it on a regular basis works fine

And theres guys with stock 234hp that have blown 915 trans lol

So my answer is with a question

How long is a piece of string?
Old 10-12-2014, 07:24 PM
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I have quite a bit of experience with 915 trannies behind a turbo. A stock Euro 930 will destroy one if you abuse it. To make them last you must:
> rebuild with bearing retainer, LSD, heavy duty side plate
> never dump the clutch, let the turbo do the work
> add oil cooler if tracking
> max durable power level is 350HP to the wheels.
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Old 10-12-2014, 07:34 PM
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Puddy, Robby is a nice guy, but he tends to talk crazy (like in his post above) , but to answer your question, no the 915 in stock form does not stand a chance against 400 HP (note that Porsche found it would not handle the torque provided by the stock engine, and thus, did not use it in the Turbos), and even when modified, the 915 transmission must still be babied from standing starts or it won't last.

Last edited by Ronnie's.930; 10-12-2014 at 08:36 PM..
Old 10-12-2014, 07:41 PM
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Ok thanks for the info guys! The trans was one of the last years of the model, an 1986 is what I was told. I know this trans has been in the car for at least 10 years. The PO did a lot of track days and racing with it but he always blipped the throttle on up shifts and matched revs when down shifting. So I think it wasn't abused very badly at all. He said it had all the wearing bits replaced 5000 kms ago.

The only upgrades that know of are a short shift kit, LSD and a center force clutch. There are twin oil coolers on the motor but I'm not sure if there's an oil cooler for the trans, i'll have a look. I just got that car last week and I'm moving in 2 days so I've been busy packing and I haven't had much time to really get under it and check thing out.

I'll definitly take it easy on the trans. I'll never dump the clutch and always shift it as nicely as I can and hopefully I can make it last

Cheers, chris

Last edited by puddy; 10-13-2014 at 03:41 AM..
Old 10-13-2014, 03:02 AM
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Euro Gearboxes have Oil coolers built into them

One would think then that they would be capable of Higher HP than the US counterparts?
If so how by much does anyone know?


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Old 10-20-2014, 06:01 PM
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The last 915 I built for the Red Rocket was an '86 with all the goodies; bearing retainer, HD side cover, custom 5th gear and OEM Euro oil cooler. The trans came out of an '86 Carrera M491 widebody. Worked very well and lasted just fine with no standing drag starts. Let the turbo do all the work.





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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 10-20-2014, 06:21 PM
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Quote:
Originally Posted by Ronnie's.930 View Post
(note that Porsche found it would not handle the torque provided by the stock engine, and thus, did not use it in the Turbos)
Starkey says that the factory could find no modifications that would make a 915 last for a 24 hour race in the first forced induction RSRs - and so designed the 930 box. Which is basically a beefed-up 915 with 4 speeds.

Apparently not all 915's were 5 speed - although almost all US early 911's took the 5 speed option until the factory eventually made 5 speed the standard/only option.
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Old 10-21-2014, 03:14 AM
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I must have the best 915 ever built! my 84 Carrera has been turbocharged for 5 years and I beat on it like you wouldn't believe. I drop the clutch spinning first into second all the time, heck the other day I did four donuts then grabbed second spinning the whole time till 3rd gear lol. borrowed time I guess
Old 10-21-2014, 11:31 PM
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You must have skinny tires. I use 315s and 345s. One launch on a concrete drag strip pad and the side plate shims were crushed.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 10-22-2014, 03:43 AM
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Quote:
Originally Posted by Matter View Post
I must have the best 915 ever built! my 84 Carrera has been turbocharged for 5 years and I beat on it like you wouldn't believe. I drop the clutch spinning first into second all the time, heck the other day I did four donuts then grabbed second spinning the whole time till 3rd gear lol. borrowed time I guess
That is not a testament of your transmission strength but how your car has poor traction which saves your transmission.
Old 10-22-2014, 06:23 AM
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So you just skip 1st gear and use 2-5? I was going to send my '84 915 out to get the 7:31 replaced with a 8:31, plus the bearing retainer & side plates. Would a oil cooler help? Wevo makes a RSR style oil cooler system with the spray bar over the gears.
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Old 10-22-2014, 07:07 AM
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Quote:
Originally Posted by ficke View Post
That is not a testament of your transmission strength but how your car has poor traction which saves your transmission.
Agreed.

I'm still using the 915 gearbox but I'm more careful since it's my second on the turbo. I don't ever standing start launch hard without using the turbo for the motivating force. Even though I don't launch mine I munched the second gear while at around 4krpm in second gear full throttle. It wasn't a prepped trans and was a mag case 915/61 but showed no signs of stress till the gear exploded.
It's expensive to build the trans with mods mentioned above, seriously expensive and I'd add anothern option that might help. It too is expensive....G-50.
I don't think there is any free lunch with this issue. The cheepest is to just keep buying 915 geqrboxes and replace like brown bags when you're done.
If you have wider that 225 tires I think you're playing with fire.
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Old 10-22-2014, 07:47 AM
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I snapped the mainshaft of my old 915 in half... Ended up going the G50-50 option, camber boxes, motorsports syncros, different gearset, etc, etc... It cost me about 12K back in the day. I recently spoke to Patrick Motorsports, and they said that a G50-50 box conversion (that actual transmission, not just a regular G50), would be a good 25K now!!!!!!
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Old 10-22-2014, 07:57 AM
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Yes im only on 265 on the rear right now, but will be running 315's next season on 18x12 fikse wheels then the 915 will have no chance. My car is light and has very stiff suspension as well which has also helped the transmission. Ive been very impressed with setup as i run 11lbs boost on stock 3.2l and neither engine or trans have given my one ounce of trouble....
Old 10-22-2014, 12:10 PM
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Well I'll be sure to be gentle on my 915...
Old 10-26-2014, 07:11 PM
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Be gentle, for one poster in here it's just a Matter of time until BANG!
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Old 10-26-2014, 08:17 PM
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10 years of track days? That gearbox is a time bomb. When it was refreshed they should have timed out 2nd and 3rd gear at a minimum. The ring and pinion is likely on its last legs too.

If it were my car I would pre-emotively pull it and rebuild it with upgrades and the above mentioned gears.
Old 10-26-2014, 08:41 PM
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Quote:
Originally Posted by Matt Monson View Post
10 years of track days? That gearbox is a time bomb. When it was refreshed they should have timed out 2nd and 3rd gear at a minimum. The ring and pinion is likely on its last legs too.

If it were my car I would pre-emotively pull it and rebuild it with upgrades and the above mentioned gears.
Hi Matt, what do you mean by "timed out" out 2nd and 3rd gear?
Old 10-27-2014, 02:33 AM
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Quote:
Originally Posted by puddy View Post
Hi Matt, what do you mean by "timed out" out 2nd and 3rd gear?
In the racing world, we place a service life on engine and gearbox parts. When they have been in the vehicle for a certain amount of track time, we time them out and throw them in the bin and start over. Everyone who races times his engine. Not as many guys are as diligent with their gearbox and time and time again I see it bite them in the butt when they mow down a row of gearteeth like a stack of dominos falling down.

Furthermore, there is frequently this attitude of,"I'm just doing track days and DEs, I'm not racing. It's not the same." True and not true at the same time. I would say that trackdays put 75-80% of the stress on components that racing does, which is significantly more stress than when we daily drive a car. It's really bad with the GT3 guys. They blow up 3rd gears all day long in the 3rd or 4th season of track days.

See the attached picture (sorry for the poor focus, it's a cell phone shot). See the shadow right at the base of the tooth? That's the tip wearing through the heat treatment at the root of the gear. I call it "chopping down the tree". It's how they almost always fail. That gets deep enough and then you get the dominos. This one isn't really bad, but was bad enough that we yanked this gear and consider it junk except for a nice set of used synchro teeth. I suspect you've got at least a couple of gears in there that look like this. This particular picture is a 5th gear from a car that was a commuter and say a lot of highway miles. You can get the wear on any gear with enough use, but the most used track gears are 2 and 3.

Regards,

Matt

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Old 10-27-2014, 11:00 AM
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