Hey guys, unfortunately I need some G50 transmission help…

There are two tracks (Putnam and CMP) where I need to haul it down to 2nd gear in tight turns. I noticed back in April (at Putnam) that I was having a little grinding when upshifting from 2nd (at/in redline) to 3rd. I didn’t think too much of it and thought maybe it was because I wasn’t getting the car 100% straight before shifting (which obviously is sometimes really hard to do).

This past weekend (at CMP) it became much worse (I use 2nd gear in three spots there).

I thought, again, that maybe I wasn’t letting the car get straight enough before shifting but then I went out of my way to get straightened out and it would still PAINFULLY grind on the upshift.

There is NO grinding when DOWNshifting from 4th to 3rd (I heal/toe). And there is NO grinding during “normal” driving or in any other gear. It’s also seems to get worse the longer the session goes on.

A Porsche mechanic (but admitted non-expert in G50 transmissions) was paddocked next to me this weekend and this was his differential diagnosis: 3rd gear synchro overheating?? Bad carrier bearing?? Bad brass 3rd gear dog teeth?? I have VERY limited knowledge about what this stuff is…

Some background: the car is an ’87 911 and has 165,000 miles on it. To my knowledge, the tranny has never been rebuilt. The tranny fluid is brand new (Mobile 1). The clutch has less than 10,000 miles on it.

Another interesting fact to consider: Sunday afternoon, I was talking to a guy in my group with a 964… and he said he was having the EXACT same problem when upshifting into 3rd after Turn 14 (I don’t think he uses 2nd anywhere else)…

What do you all think is the problem and what is the best/easiest solution?? Thanks a LOT!
Jack

 

If you use the standard Mobil 1 75/90 oil, then that may be contributing to your problem. It is too slippery for the syncro to grab the gear and properly spin it up. You may want to run the Mobil 1 Delvac 75/90 oil which will work better. Obviously an oil change cannot overcome mechanical wear. Also, if the gear oil is getting hot, it will not work as well as when held around 200F.

 

Not sure about downshifting, but a little bit of a grind upshifting from 2nd to 3rd when still under a partial side load (tracking out of a turn) is not uncommon to 964's (realize your car isn't a 964 but 964's run the G50). Mine does it in specific corners at specific tracks and two other 964's that I know of that run the same tracks do it in exactly the same places (I've talked with the drivers of these other 964's to compare notes). There have also been several threads in the 964 forum on this topic. The theory is that when the drive train is loaded up under some G force, something is not lining up exactly right.

The solution that myself and some others use is to simply pause a breath in neutral when upshifting from 2nd to 3rd. My car never has this problem shifting hard and fast in a straight line. And it doesn't do it in every corner. Only certain ones. Some folks have mentioned changing to stiffer trans mounts to cure this. I haven't tried that. I just remember to pause briefly when upshifting in these corners. For what it's worth, I've had the third gear synchro replaced with a new one, replaced a slider, and had everything else in the gear box checked when that was being done. Still didn't solve the problem. I also run SHC gear oil (now Delvac). That didn't solve the problem either. Also, I don't have any problems when downshifting, or when upshifting in any other gear combination. Only happens when upshifting from 2nd to 3rd. And only when I get in a hurry upshifting coming out of certain corners.

 

When you make this shift the 3rd gear synchro needs some time to slow the input shaft and clutch disc down to the appropriate rpm for 3rd gear. You are probably shifting at redline, so the synchro ring has to pull the inertia of 2000 rpms (6600 rpm in 2nd down to 4600 rpm in 3rd) off the input shaft and clutch disc.

It is a big job do this and it requires a little bit of time. If you don't give it enough time, then you get the grind.

The corner loading should have no effect, except that I will bet that you tend to upshift faster when you are no straight.

There are a few fixes for this:

Make the drop from 2nd to 3rd gear smaller, which requires regearing the trans.

Decrease the inertia of the input shaft/clutch disc. Racing clutch discs have about 1/2 the inertia and totally solve this problem. Racing discs are also thinner and not so squishy so they fully release quickly, where a street disc takes more time to fully release.

Help the synchro work faster. The steel racing synchros work faster, and any synchro will work best with the right oil (Mobil Delvac or SHC 75W-90) kept around 200F.

 

What cooler setup would you recommend?

 

Not that I'm in disagreement with some of the statements but before you start down some of the suggested paths, what's your budget?

My opinion is that your synchros and/or dog teeth are getting worn to the point that they need servicing (replacement). Now, the matching revs discussion is important but I believe what you're experiencing now is beyond matching revs, better gear oil, cooler temps, etc. Through general wear and tear or your particular driving style, the tranny internals are getting worn and you're starting to hear/feel symptoms of that wear.

FYI, I have two trannys for my car (geared totally different for different length/speed of tracks), I've installed/replaced LSDs, I've replaced synchros and dog teeth more than a little (I have a 915....) and I've focused ad nauseum on entrance/exit speeds and relative gear ratios for corners.

The reason why I ask what your budget is is that, if you have a stock box, you have the opportunity to make one of the greatest changes to your car that you can make. If you need to drop the engine/tranny and go in and fix some things, re-gear it while you're there. Also, if you don't have a LSD, do it now as well. I can go on and on about this as can others but trust me when I say it's some of the best money you will spend.

Just my .02.....

 

Thanks for the helpful replies.

I am, unfortunately, on a budget. I had planned to weld in a cage this summer and my wife is having a baby in August (our first!). The addition of a (likely) multi-thousand dollar job is disappointing, to say the least.

I've tried to do some research since getting some of the above help. The oil change to Delvac sounds like a good idea. I've also read that G50s rarely overheat (only in long enduros is it ever an issue). Is this true?? I ask because the addition of a tranny cooler doesn't look like an easy or straightforward upgrade either.

Peter, the thought of new gear ratios "while I'm in there" is tempting but might not be possible for me yet... but maybe an LSD. Does PCA allow using other ratios "as offered by the factory" in the stock classes?? What is an estimate for a tranny rebuild (with steel synchros, etc)?? Vertex will send me a rebuilt G50 with OEM parts for $1700 (if I send mine as a core)... but a rebuild with OEM brass doesn't sound like it'll help me much. Where would you send a G50 to have the work done??

Thanks again for the help-

 

You can overheat a G50 fairly easily. The best thing to do is to use an IR temp probe on the case after a 90 minute enduro after the sits in the paddock a minute. This will give you an idea of the gear oil temp.

 

My G50 has the exact same issue. I've always assumed it was chassis flex of the Targa, so I've ordered (but not yet installed) a Wevo G50 tranny mount to better keep things in line. From reading this thread, I'm now assuming it is not going to have the desired result. Also, I agree with PedroNole about the gearing. I have shorter gears and it really helps the 3.2.

 

sjanes,
Yep, I have both the Wevo SS engine and tranny mounts... I love 'em but they won't fix your grinding. As sweet as the short gears sound, I'm keeping the transmission stock on my way to PCA "Stock" E class... Because you know, everyone else uses the stock ratios, too (ha-ha)...
Jack

 

The G50 does overheat but, IMHO, not going to happen in a sprint race and not what's causing your problem on a gear box with 150k+ miles on it. Skip the tranny cooler unless you're going to do some endurance (greater than 3 hour) races. I have two 915 boxes and one has a cooler and one doesn't. I've run the box without the cooler for 2-3 hours at a time without issue and the G50 is superior in terms of durability/heat.


Not sure about the Stock Classes and the gearing rules. If you can't do the gears, you should at least try to do the LSD.

I would say a tranny rebuild including synchros on all gears and probably 3 sets of dog teeth would be $2,000-$2,500 including labor. That's why I said "as long as you're in there....." The gear sets for the G50 are probably around $700-$800 (per gear) but they could be less. I know the 915 gear sets from Guard are in the $625 range. Ask Vertex if they'll do a rebuild with different gearing that you specify? As far as who can do it, any Porsche mechanic worth their salt can do your tranny.

 

Thanks everyone.