Whats the correct orientation of the top vent on a 915 trans when set up for use in a 914?
The bottom of the vent tube is cut on a 45 degree angle and the cut is aimed away from the ring gear at a particular angle. However, reversing the R&P changes the relative position of the angle cut to the ring gear. The vent tube must also be repositioned for it to work properly in a 914.
If the angle cut orientation is wrong, gear oil will be flung into the vent tube, temporarily sealing the vent and pressurizing the gear case as it heats up, causing oil to go past the flange seals and make a mess on the outside.

We had this happen in a 912E once when the customer's local mechanic had filled the trans thru the vent hole and screwed it back in all the way without regard for orientation.

So the question is, which direction does the hole in the top of the vent aim in a 914?

You would think it would be the same as the 914 trans, angled towards the battery. Can you compare the breather with a 914 breather? I am also installing a 915 trans in my 914 and I am debating on whether to use the breather or the existing puke bottle in the trunk.

Its located on the centerline of the 915 case, whereas the 914 is off to the right side.
However, I think you might be right about aiming the vent hole toward the battery.
That's what we're going to try next.

as you can see, the cutoff and hole aim away from the flying oil or forward as mounted in the 911. Reversing a 915 causes oil to fly the other direction towards the bell housing.

In a 914, since you are flinging oil the opposite direction, the vent has to be rotated 180 deg. The slash should aim forwards away from the spray coming off the ring gear. That means regardless of the car model or the direction the tranny is installed, the slash always aims to the front of the CAR You might have to put a slight angle on it to keep the vent facing away from the oil sling (perhaps 20 degrees)
I hope this sounds clear.
Click to view attachment

Thanks Paul!

why not just cut the inside end of the tube square? what is the need of the angled cut?

The cut, located properly creates a low pressure zone as the wind from the ring gear passes by. Air stays coming in and oil doesn't get pushed out. When oil gets thrown at it , the slash acts as a shield to the opening so oil can't easily get flung out.

Chris,
I'll have to concur w/Paul. Especially the cross section photo of the 915.

When I first got my 914/6 back on the road back in '09, I set the vent hole orientation the same way. Tightened it all the way down.

Giving the car a shakedown drive, after the FIRST 5 MILES, I pull back in for a check underneath to check things out, and the whole side of the 915 transmission was soaked from leaking oil. I first though an axle flange seal was out of wack so I pulled it all apart for a check but all was fine. Wiped everything down, and went back for a ride, and same result. Oil everywhere. Except this time it looked to originate from the vent hole.

So, I read in a Haynes manual at the time, how the orientation of the vent is important for oil retention, so I started backing it out 1/8 of a turn at a time. Then driving the car a short distance and measuring he result. Each time, it leaked less and less.

Eventually the vent hole quick leaking completely with the hole approximately at the 2 o'clock position when looking down on it from overhead.
Doesn't leak a drop now.

Not sure if the setting is correct by any means. But it is working for my application..

Ron