This weekend after a 40 minute stint on the track, we saw our transmission temps getting to 247F. That is on a day were it was about 45F ambient and we had the pump and cooler running.

Doing some research, the answers I've heard on safe temperatures are all over. The oil is good to 300F according to the manufacturer and some folks say they run there, but that seems really hot. Most folks say they like to be around 225F.

We just rebuilt the transmission and set the R+P to the factory backlash setting of .007", but we're going to open the transmission up and loosen things up a bit.

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where do you have your gauge sender mounted?
I would expect there to be some temp variance between cars with temp probes in different locations.


I'm running a vdo gauge
temp probe is in the speedometer drive hole.
no data logging on my trans
but I'm seeing 240 - 250F
that is with a cooler and pump also

we started loosing transmissions
which prompted the cooler
I don't have any "before" data
I do intend to experiment further with my cooler air flow in an attempt to lower my numbers.

brant

Ours is mounted near the drain. This is the third transmission in 6 years - the old pinion gears are really ugly. I don't have any before data either, but this made me pretty scared of what it was doing at Watkins Glen last summer when it was 95F out!

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remind me...
how much motor/torque are you running?

I have lost 3 in about 3 years, but everyone of mine has been the intermediate plate bearing...

Stock 3 liter but it's always a 2 driver car.

We killed the main shaft and pinion shaft bearings as well.

As an FYI, pinion shaft bearings are getting really hard to find.

I can believe it
and you have lots of torque compared to me...

we are a 2 drive car almost always
I think the 2 driver car really compounds the heat problem
going out to start a session on a thermally maxed out drive train brings extra endurance needs

The problem is not the 'tight' backlash, it is the 3 liters pushing it. If you do not like the damage that you are seeing now, just wait until you see the damage from 'loosening' the backlash. Porsche changed to the 915 trans when they went from 2.0/2.2l to 2.4l in 1972. That is the tip off. When we look at what the factory used in racing back in the day, the '901' really did not make it very long, especially as the engines got bigger. I have seen folks with 2.4/2.7l cars go through gears annually when used on the track regularly.

We've been running this setup for over six years, so I'm pretty familiar with how it kills things. I was wondering if anyone else had temperatures they ran or similar experience.

What trans oil are you running ?


my race trans builder switched me over to synthetic about 12 years ago
I know its not supposed to be good for the syncro's...

but he did it for the bearings not the syncro's

b

I've used Amsoil and Mobil 1. The Amsoil seems to do better. This year I'm running Miller's Nanodrive oil. It has some neat properties and I think it should work pretty well. Time will tell.

Some questions and some observations. First off, where is your cooler and second what is the temp drop across it? The trans cooler needs cool and clean air, not hot and dirty air from under the engine which happens when folks put the cooler under the rear bumper. If you run the car on a dyno, you can shoot the oil line temps across the cooler since the fans dyno shops use will generally replicate the outside air flow. Does the shifting get harder as the race goes on, if so then the internals are expanding and they will fail possibly. Do you use the transmission to help brake the car, if so the change in load will cause accelerated wear and failure. Wayne Baker would always say "brake like hell and then down shift".

Observations from 10 years of vintage racing, I noted the 3L + sized cars used a 901 transmission a year if no cooler and every 2 years or so if a cooler was run. If the transmission was upgraded then there would usually be a year added but the extra expense would cause the whole thing to level out. I found shifting would get hard when temps went over 300 degrees which is why I went to a cooler. I also found that having the transmission inspected and any parts replaced yearly saved me from the failures which are always MORE expensive! Finally I found the JWest shifter reduced the wear and tear on the transmission significantly but they are no longer around. The best and also most $$$$$$$ is to install a Fortun 6 speed or similar and they last forever.

All good points. The rear trunk is cut out and the cooler is mounted basically above the passenger inner CV joint. There is no fan on it right now, but I'm going to add one before the next event. I'm also going to put an air temp sensor back there to see what air temps we're putting through the cooler.

There is no engine braking going on. We've been getting 2-3 years out of a transmission with no teardown in between. We've been doing ok compared to most, but we can still do better.

As far as I know that shifter is still available:

http://www.jwesteng.com/porsche/914/rennshift_914.htm

Here is a picture of how we did mine and the cooling duct. Under the car I added a baffle in front of the cooler outlet so there would be a low pressure area and help suck the hot air out. The intake is in the front of the right side GT flare in a high pressure area and to help when we were tooling around under caution I installed one of those small boat type bilge vent axial fans AND a mister assembly that was actually a JC Whitney windshield washer kit and a spray nozzle. The little water tank was good for about 15 minutes running and was switched to the fan and I noted once it was turned on, the temp reading (sensor from Jim Patrick in speedo drive hole) would drop approximately 35 degrees in less than a mile! The only issue was cars behind me would see some "steam" coming out the cooler outlet and back way off which turned out to be a very good thing!

I added a fan to the cooler and the high we saw now was 237 on an 80 degree day. Much better.

As a note, we are measuring the temp near the drain, not in the speedo drive area (I plugged that so it doesn't leak). I'm guessing our temps would be hotter than somone reading it back there since the speedo drive so far from the heat source.

can you post a picture of which fan you used?

I did this up real quick. The cooler is 5x9 so a regular fan won't work. I'm going to do something a little nicer and mount the fan right on the shroud.

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so its one of the inline bilge type fans then... where did you get it?

I got mine from Roush, but you can get them all over. Some are Cool Shirt brand. You can also search for bilge blowers. These look pretty cheap http://pitstopusa.com/c-132241-brake-syste...ooler-fans.html

I am curious to know the tolerance spec of what I would call the front bearing/housing etc.

Not the intermediate plate...that can be replaced.
The front one that is in the case and holds the pinion shaft.

I wonder if there is a better solution there...
A better and more modern bearing and potentially a machined sleeve for the case.

Seems the flex is in that shaft versus the Ring/Diff setup which seems pretty stable, and can be beefed up with new plates etc.

I am asking because I have never seen anyone address that point of the system...
I have seen new side covers, better bearings.
I have seen new intermediate plates etc.

I have also seen ovaled out cases that are now considered junk.

Not an easy solution, but seems a larger bearing seat to spread the load, and possibly a more modern bearing using more modern metal would tighen up that point.

Seems like your failures are due to that pinion gear moving sideways.

rich

I'm no trans expert but majority of the solutions have been presented - such as: engine size, main topic..oil temp, proper back lash and pinion. As for a larger bearing to distribute the load, maybe some one has already thought of that. But if my memory I correct, there' s not enough metal between the bearings ( main shaft &pinion) for this.
To my knowledge, the 934 / 935 were eating up ring and pinion years ago until the race teams used Swepco trans fluid. I just had my spare 86 915 rebuilt by a Porsche race team. The tools to set the ring / pinion cost $6k...the tech asked me to use Swepco..I brought 201 and 210 to his shop and he used 210. His comment was, 210 is a heavy oil, but it will run cooler....this was in light to my question on my 80% locking Guard unit. Based on the pictures above, i would exam the gear sets closely- they most likely will have small bits of metal pressed into their face. This will cause an uneven mesh of the two gear sets....i've been down that path -. Just my .02 cents

Ps..I opted for a new aftermarket side cover too

just another data point:

I took my cooler set up which normally runs a 3 inch flexible tubing to the cooler from an underbody aluminum scoop I had built....

Over last weekend, I disconnected the scoop and zip tied to the hose to the drivers side window roll bar. not protruding into the air stream but close to the edge of where the side window would be... so definitely close to the air stream, but no scoop nothing effective in the air stream

20degree F drop in trans oil temps
the highest I saw on a moderate day was 205 - 206F range