Need advice on 2nd to 3rd gear crunching...

hunterpeaks · 09-11-2008, 10:13 AM

Hey- I have a 1988 - 911, with G50 tranny. It's a race prepped car made for PCA racing and has been raced for 4 years before I purchased it. It's got all the race goodies including suspension, cams, chip, headers, etc. I would estimate she's making 250- 260+hp.

I have a problem with the shift from 2nd to 3rd and getting some good crunching going on, when the car is full tilt out of a turn(so car body is tilted to left or right) and near 6000rpm and after a few hot laps. It just doesn't like those combinations. It won't happen when cold, and won't happen unless I am really pushing it coming out of a turn.

I have replaced the engine motor mounts, but not the Tranny mounts. I just got some solid tranny mounts from rennline and that's on the list for this weekend to replace.

Could the synchros be off? Maybe this is just normal for a 911 with G50?
What else could be wrong? Is it something I could correct?

Thanks for any help!

Hunter Allen

Peter Zimmermann · 09-11-2008, 11:03 AM

Quote:

Originally Posted by hunterpeaks

Hey- I have a 1988 - 911, with G50 tranny. It's a race prepped car made for PCA racing and has been raced for 4 years before I purchased it. It's got all the race goodies including suspension, cams, chip, headers, etc. I would estimate she's making 250- 260+hp.

I have a problem with the shift from 2nd to 3rd and getting some good crunching going on, when the car is full tilt out of a turn(so car body is tilted to left or right) and near 6000rpm and after a few hot laps. It just doesn't like those combinations. It won't happen when cold, and won't happen unless I am really pushing it coming out of a turn.

I have replaced the engine motor mounts, but not the Tranny mounts. I just got some solid tranny mounts from rennline and that's on the list for this weekend to replace.

Could the synchros be off? Maybe this is just normal for a 911 with G50?
What else could be wrong? Is it something I could correct?

Thanks for any help!

Hunter Allen

Sounds like a worn synchro assembly to me...

hunterpeaks · 09-11-2008, 12:52 PM

Quote:

Originally Posted by Peter Zimmermann

Sounds like a worn synchro assembly to me...

Thanks Peter. Only way to know for sure is to pull the thing apart, right?
Anything else it could be?

Hunter

tonythetarga · 09-11-2008, 02:21 PM

Can't speak to the G50 but my 915 had a similar issue. We replaced all 4 mounts but the thing that nailed it for me was to stiffen the rear motor carrier, the long metal piece that the motor mounts attach to. My mechanic, a racer and top builder recommended I weld small angle iron across the piece to stiffen it and keep it from flexing. The only place it made a grind for me was a 2 to 3rd upshift coming out of a tight corner that transitioned from uphill to downhill, so the car was definitely flexing at the only place left to flex, the driveline. After that mod, no more issue in my shift.
The mod consisted of a small 3/8" angle welded across the top of the bar. I have not taken a picture of it and now it is mostly hidden under the rear tin so I'm not sure if I can get a decent shot of it.

1987C1 · 09-11-2008, 03:29 PM

I have a 1987 (G50) and have the same issue.
When I am driving the car hard I sometimes get a crunch going from 2nd to 3rd.

The first time I noticed it was after 7 or 8 laps on the track.
I do occasionally get it on the street.

Could this be a common G50 issue? I have about 80K on the car and change the trany fluid every 2 years.

oly · oly's Garage

Peter,

Why would a worn synchro only be a symptom coming out of a turn?

I'm having the same problem with my 915 transmission and I'm hoping that changing out all four mounts (engine and transmission) will solve it. But if that doesn't, maybe my synchros are worn too.

Art

dentist90 · dentist90's Garage

+1 on engine and tranny mounts if they're still factory rubber.
On the other hand, you are probably 'pushing' the shift a little harder on the track than you do on the street or when cold, so a marginal synchro cone wouldn't be as noticeable until then.

hunterpeaks · 09-12-2008, 06:18 AM

Tony- Wow, that helps a bunch. Any pics would be awesome. How long was the piece of angle? A small 3" thing? Or a longer 12" piece? Does it extend across the entire length of the rear motor carrier? Or just 2 small triangle pieces on each end?
Any more help is greatly appreciated.
thanks.
Hunter

Quote:

Originally Posted by tonythetarga

Can't speak to the G50 but my 915 had a similar issue. We replaced all 4 mounts but the thing that nailed it for me was to stiffen the rear motor carrier, the long metal piece that the motor mounts attach to. My mechanic, a racer and top builder recommended I weld small angle iron across the piece to stiffen it and keep it from flexing. The only place it made a grind for me was a 2 to 3rd upshift coming out of a tight corner that transitioned from uphill to downhill, so the car was definitely flexing at the only place left to flex, the driveline. After that mod, no more issue in my shift.
The mod consisted of a small 3/8" angle welded across the top of the bar. I have not taken a picture of it and now it is mostly hidden under the rear tin so I'm not sure if I can get a decent shot of it.

Peter Zimmermann · 09-12-2008, 06:29 AM

Quote:

Originally Posted by oly

Peter,

Why would a worn synchro only be a symptom coming out of a turn?

I'm having the same problem with my 915 transmission and I'm hoping that changing out all four mounts (engine and transmission) will solve it. But if that doesn't, maybe my synchros are worn too.

Art

915s, as you know, are a completely different animal than the G 50. Driving/shifting a 915 on the track should really be done the same as when going for groceries, while the G 50 shifts can be done a touch quicker. Quick review - make sure that you do not have synthetic oil in your 915, and you do have it in your G 50. Usually gear "crunching" is internal, a problem with the synchro itself, but 915 linkage can contribute big time to shifting difficulty, while G 50 linkage is much better. Because the thread's issue is only with one gear (and it happens to be the gear that gets the most trauma in a track car), chances are very good (I think Dentist90 hit this nail on the head) that it's the third gear synchro.

tonythetarga · 09-12-2008, 07:37 AM

OK found a picture. Keep in mind we did this while the egnine was out so it was much easier to do.

hunterpeaks · 09-12-2008, 08:55 AM

Perfect. Thanks Tony. Exactly what I need.
Hunter

VroomGrrl · 09-12-2008, 09:46 AM

I will be following this thread & hoping to see your results if you make this mod.

I'm another one with a 2-3 gear crunch! My mechanic drove my car to check it out, and basically gave me the "nothing wrong, shut up & drive" speech" (very nicely!). But the crunch is definitely there, just upshifting from 2 to 3. And - in my car, the crunch lessens remarkably after the car is warmed up.

I have an 88 911, G50 trans. I'll have the gear oil changed in a few weeks when my car is in for some basic maintenance. We'll see if that makes a difference.

I have yet to test my car at anything near "speed," but I will be doing my first DE in October. I suspect any problems like this will be magnified once I start to push it. By then I'll have had the gear oil changed out, so...I'll just have to wait and see.

Let us know if you make that mod, and whether/how it works to solve your crunch, wouldja?