Weird 915 Race Gearbox Problem: Need the braintrust..

[Geneman]

Ok Peeps this has me and others totally stumped..... and driving me friggin crazy...

I was at Sebring last week for SVRA event..beforehand i had Kevin Wheeler and the boys from KMW swap my spare gearbox in....( the other was aging a bit and grinding a bit and would not stay in 3rd). Both are short geared , professionally built, plumbed for external cooler , and had a temp sensor logged on my AIM system etc.

The spare was a ZERO hours, never had oil in it, properly stored one from a very well known Porsche gearbox dude. Clutch and everything else were fine including linkage. I changed fluid from Mobil1 synthetic to a non synthetic custom mix that the boys from 901Shop ( see below) have found to be best for 915 gearboxes.

This box shifted fine in the pits .. but as soon as i got it on track, it would completely freeze up .. this happened every single start i tried it. I could not even get one warmup lap, under just warmup conditions, it became stuck in neutral. lever would go back and forth in neutral but would not move remotely forward or back into any gear of 1-4. I could only get it into 5th , and the ooch it back to the pits ( with throttle and clutch..)

Even stranger , when i got it back to pits, after it sat for 10-15 minutes, it would then shift again ( with or without having motor running in neutral during that period). this reversal happened gradually and allowed getting 3-4 back , then 1-2 came back a little later. literally this then felt normal. then when i went back on track, the exact same thing happened ( almost on the exact same section of track). For instance this almost always seemed to occur after the hard rights at T10 and T13 (where there are also alot of bumps). (i know this because i ended up pulling off course each time at the wall safety car gaps at T14 or the new pit entry at T16).
BTW. these were very gentle shifts during warmup : of course proper rev matching heel-toe, and nowhere near the type of shifting i would do in a race.

It seems as something is getting misaligned or jerked out of place inside the gearbox. The linkage was not loose or binding ( In fact worked fine for 5th and revers). The shifter ( standard wevo shortshift). was solidly mounted with no play and there was no play in the shift tube or connector to the trans. I even llfted the shifter unit up from the tunnel mount with a few thick washers to create more clearance for the rod movement in the tunnel... did nothing. All parts of the shifter were fine, including the nylon cup etc. The motor mounts ( both trans ands motor mts are yellow stomski: semi solid) both fore and aft are fine, nothing seems loose. There are no seeable craks in the torsion tube.

THis does not seem to be temperature related. Same thing happend if the trans temp oil was 90 degrees or 150 degrees,. Moreover, I was told categorically by the 901 Shop guys ( Scott and Brady ) that "915 boxes WILL NOT SHIFT if you use synthetic oil... they learned this lesson the hard way" . I was very happy to hear this sunday so i changed the mobil 1 synthetic to their custom mix ... but the same thing happened.

One more thing that is totally strange. I took the car out onto rte 98, (the sebring access rd) and ran it hard up and down the road at the 50-70 mph ( its registered insured and plated). up and downshifting for 35-40 minutes, hot gearbox etc,. AND IT SHIFTED FINE FOR THIS WHOLE TIME!.... So the problem only occurs on course at sebring under pseudo race conditions.

This i cannot explain other than the bumps and hard turns on track vs off. ITs maddening. i did not complete 1 lap in that whole SVRA event... even the announcers on the cool-as-hell live feed on the SVRA App. noted that... "the green RSR from RJR Racing seems to having some mechanical problems this weekend..."..

Any thoughts?

[Jonny042]

That's a bummer. Sorry about your race weekend.

It almost sounds like the interlock is keeping you "stuck" in nuetral - you can experience this if you put the lever between gears in any 915 car - place the lever exactly between 1st and 3rd and gently feel around, you'll feel a bit of a notch, try pushing the lever into gear and you'll feel the lever move a small amount and comes up against a hard stop. That's the interlock doing it's job.

I wonder if it's a coupler adjustment problem? Of course that doesn't explain why it happens at random..... maybe a loose coupler clamp? I have one that needs WAY too much torque before it tightens up, and it can slip and throw things off. I have a WEVO billet clamp on order. This could be even more likely if you have a wevo shifter because the spring tension is applied through the coupler at all times when in 1st, 2nd, and 5th/reverse.

Also, is there any chance you have a Wevo gateshift in the box? Again, coupler misalignment could be the culprit there and given that it's a very exact control over shifting, it needs to have the shifter adjusted perfectly.

[Jonny042]

Just to add - there are interlock parts in the trans that if it's not reassembled properly, could be on the wrong side of the shift rails in the mid-case. Not sure what the symptoms of this would be, but wonder if the little detent capsules are bouncing around uncontrolled somehow and led to the problem only on the one bumpy corner?

[Geneman]

Thanks Jonny . I have the Stomski precision shift coupler. the one pictured with the set screw. The set screw is conical and seems to take up any minute play there is in the whole in the shift shaft.. Its hard to believe there would be a MORE PRECISION coupler out there. but who knows. i will try anything.

have no idea what the internal setup is in terms of gating ... will ask my gearbox builder. but i doubt its wevo.... nothing saids about it on invoice.

thanks very. much for your comments

[Jonny042]

Originally Posted by Geneman

Thanks Jonny . I have the Stomski precision shift coupler. the one pictured with the set screw. The set screw is conical and seems to take up any minute play there is in the whole in the shift shaft.. Its hard to believe there would be a MORE PRECISION coupler out there. but who knows. i will try anything.

have no idea what the internal setup is in terms of gating ... will ask my gearbox builder. but i doubt its wevo.... nothing saids about it on invoice.

thanks very. much for your comments

Is that your actual setup? If so the blue billet clamp is probably a good upgrade. My coupler clamp is the stock one and over time they can stretch and become less effective.

You would know if you had a gateshift, I think - they are not cheap. You can tell by looking at the bottom of the trans, the fork cover is billet so its visually apparent.

Sorry I can't help more..... I know what a letdown that can be. Lots of work and expemse to get to a race weekend only to never take a green.

[Matt Monson]

Bent shift rod from someone in it's past life lowering the gearbox and engine while it was still attached to the linkage?

[blue911rsr]

I would take a real good look at your chassis sounds like the torsionbar tube might be
coming loose and letting the trans mount moving around witch would misalign
your shift linkage . A lot less stress on the road that might explain that.

[Geneman]

MATT it did look like the shift rod, the part i could at least see in the tunnel, was warped just a bit because it looked to be slightly bowed as you pushed lever back and forth in neutral. but it does not explain why it shifts fine in the pits,.. .. i am looking for a tower type 915 shifter ( fabcar? hargett?) to get the rod out of the tunnel , so i can exclude that variable and also see that the rod is straight and turning properly. also, raising the shift rod a bit ( in case it was jamming on somnething in the tunnel) using the thick washers did not solve the problem.

[Geneman]

i agree blue. something is moving. problem is i cannot tell if its internal or external to the gb.

[Geneman]

Matt. its possible that the KMW boys bent the shift shaft while swapping boxes in the paddock.. always a bit awkward to do. ( i did not have this problem with the original box in there...that was a whole different set of problems. i think)

[Matt Monson]

Originally Posted by Geneman

Matt. its possible that the KMW boys bent the shift shaft while swapping boxes in the paddock.. always a bit awkward to do. ( i did not have this problem with the original box in there...that was a whole different set of problems. i think)

I doubt it. Not only are they professionals, but it would be on removal, not install.

[Walt Fricke]

Can I assume that on track you simply could not move the lever fore and aft (except in 5th, and reverse when back in the paddock)?

You would not expect an experienced gear box rebuild shop to the the interlocks (detents) wrong. It is easy to do that if you are working from a parts manual or shop manual in your own back yard - you can get the funny little slugs on the wrong side of things depending on the angle you have the case at on your stand while reinstalling the shift rods. Been there, but whatever issue it caused me wasn't intermittent, that I recall.

You could experiment by removing the two external bolt plugs and removing the springs and whatever else a magnet can fish out (look at a parts diagram to see what these interlock bits look like) and then putting the plugs back to keep the oil in place. If you are careful shifting you won't get stuck in two gears at once. If you have a really bumpy local road, and it replicates the problem Sebring created with things as they are, and removing the springs improved things, that could be a clue - but all that's improbable.

I am having trouble seeing how a bent shift shaft would cause this issue. The 5/R shifting is the closest to the nose cone, and depends on a flat finger on the shaft fitting a rather shallow notch on the shaft the shift for for 5/R. So I would think a bent shaft would be most apt to screw up 5/R. Problems with that are what lead to the "locked in reverse and 1st" problem.

I once forced a shift into 1st when angry, which bent the forged part of one of the internal shift rods where the dongle engages one of two slots. Left me with fewer gears (no 3/4, as I recall) until I took off the bottom plate and fixed things with a crow bar. Silver lining was I then understood how the linkage worked in there. For 5/R the dongle is rotated beyond where the two notches for the two shafts are located, and swings free, letting the finger do the work.

While the rotary action (moving the shift lever right/left in neutral) is unaffected by the dongle, the fore and aft action (which is how the dongle moves the shift shafts) is created by the fulcrum fork which is attached to that 4 stud bottom plate. Hard to imagine the nuts inside holding the fulcrum to that plate are loose after a professional rebuild, but you might want to drain the oil, remove the plate, and see what is going on inside.

Again, none of this makes sense as an intermittent issue.






The last I think is the reverse/5th lockout, not the one with bolt plugs. Not usually removed as par of a rebuild. The ones on the side are. For what it is worth.

Shift forks loosening on their shafts? But that doesn't correct itself. Shift shaft alignment? Again, not self correcting or in any obvious way bump dependent.

Good luck

[1970 911]

Originally Posted by Geneman

i am looking for a tower type 915 shifter ( fabcar? hargett?) to get the rod out of the tunnel , so i can exclude that variable and also see that the rod is straight and turning properly

Looks like one just came up for sale.
https://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/1086739-fs-hargett-shifter-915-a.html
Dean

[stownsen914]

Sorry for the bad luck geneman. Sucks to go to Sebring only to have this happen.

If you want to distinguish between a problem with your shifter vs. one in the trans, disconnect the linkage from the trans while it's locked up and see if you can shift it by hand using the rod on the nose of the trans. Might be tough I realize since the problem goes away so quickly.

You might be able to trace the issue by disconnecting the linkage and playing around with it while cold.

[stownsen914]

Also, bad engine or trans mount moving around too much, or one of them came loose?

[175K911]

Originally Posted by stownsen914

Also, bad engine or trans mount moving around too much, or one of them came loose?

I was about to make the same suggestion. Only 4 bolts to check, I've had the rear engine mount bolts come loose

[Matt Monson]

Originally Posted by stownsen914

Also, bad engine or trans mount moving around too much, or one of them came loose?

Yes. My suspicion is a "compound" issue where the bad part isn't bad enough to cause the problem by itself.

[Mahler9th]

I see you have some good discussion going here. I will think about it and if I come up with something I will certainly chime in further.

Does not sound like a shifter problem. So unless you simply just prefer something different from what you already have for non-related reasons, I might stand pat on the shifter itself.

I will re-read all of your posts. And if you want to discuss off-line, feel free to send me an e-mail. I think you still have my e-mail address.

[Geneman]

Thanks Mike and everyone. Really great analyses. Will let you know what turns out. Frank

good advice on the shifter unit... likely not the culprit.

[shoooo32]

The intermittent portion of the problem leads me to believe it's chassis related. Something is binding or moving during cornering that will not allow it to shift.

We see this a lot with the 80's tube chassis race cars where the engine/trans is a stressed member. When the engine comes loose from the tub, the car won't shift. Nothing wrong with the trans; fix what's broken and it shifts fine!