915 2-3 Shift

[gab997]

February experienced an issue at Sebring going from 2nd to 3rd. While going out of T3 and out of T7 I would upshift to third and it was like the gear was not there and would have to shift into fourth, once it went into 1st and was able to catch it quick enough not to do any damage. The only thing that would let me shift into third was slowing the shift WAY down and would go into third. I have taken the car out on the road and hurried the shift to see if I could replicate but could not.... What could have caused this at the track, increased temps, case stress?? Any input on what to look for would be appreciated.

Good driving,

Greg

[RSTarga]

Try changing out to stiffer transmission and engine mounts. Your chassis is twisting under racing conditions and this is affecting your linkage.

[gab997]

Originally Posted by RSTarga

Try changing out to stiffer suspension and engine mounts. Your chassis is twisting under racing conditions and this is affecting your linkage.

Mounts installed and suspension stiff. Previously have run Sebring many times and has not been an issue until last event.

[RSTarga]

Then check your couplings. Have you changed your transmission AND engine mounts to the stiffer types? Do you have a roll cage to stiffen the chassis? Something is moving around under corner loading.

[Matt Monson]

You've worn out your 3rd gear synchro and possibly your 3/4 slider as well. 3rd gear gets the most use and abuse on the track. Hot gear oil and stressed conditions are the first place the wear shows itself.

Now there's an outside chance you need to change your gear oil and this will let you limp along a while long. I find that most people are religious about changing their engine oil on their track car but often think once a year is fine on gear oil. If your gear oil has more thn 3 or 4 race weekends on it, it could be the root of the problem. But I suspect it's really just that you need to open up your box and refresh 3/4.

You aren't describing typicla street car shifting issues. You are describing track shifting issues.

Regards,

Matt Monson
Guard Transmission LLC

[Eagledriver]

I had problems with my 2-3 upshift for years during races. I blame heat. The gearbox gets very hot (280 or more on the track, especially with a LSD). When this happens the case expands more than the shift rods and the other steel parts. Alignement gets comprimised and you have a missed shift. For me, I would feel like I was hitting a gate instead of a gear.

I installed a do-it yourself tranny cooler and haven't had a problem since.

-Andy

[kent olsen]

Interesting! At Sebring I noticed (with my 915) a reluctance to shift from 4th to 3rd. It's like the alignment is off. If I slow down my shift and or double clutch I have no problem. I played around with the linkage to make sure it was tight and I'm shifting straight into 3rd but it's still there. Maybe it's time for a transmission rebuild?

[gab997]

Originally Posted by RSTarga

Then check your couplings. Have you changed your transmission AND engine mounts to the stiffer types? Do you have a roll cage to stiffen the chassis? Something is moving around under corner loading.

Yes transmission and engine are tranny mounts solid. No cage just brey krause harness bar. I have heard that there are a couple components-pinion bearing retainer and side cover to stiffen up transmission. Are there any other recommendations to stiffen up 915?

[gab997]

Originally Posted by Matt Monson

You've worn out your 3rd gear synchro and possibly your 3/4 slider as well. 3rd gear gets the most use and abuse on the track. Hot gear oil and stressed conditions are the first place the wear shows itself.

Now there's an outside chance you need to change your gear oil and this will let you limp along a while long. I find that most people are religious about changing their engine oil on their track car but often think once a year is fine on gear oil. If your gear oil has more thn 3 or 4 race weekends on it, it could be the root of the problem. But I suspect it's really just that you need to open up your box and refresh 3/4.

You aren't describing typicla street car shifting issues. You are describing track shifting issues.

Regards,

Matt Monson
Guard Transmission LLC

Sound like the best thing to do is drop the engine and tranny and see what 3rd looks like. I just changed gear box fluid with 201 before the event, I usually change it after every 4 events.
When I do open drop the tranny, I heard there are a couple items that could be installed to stiffen up 915. Recommendations on this.

[DRACO A5OG]

Originally Posted by RSTarga

Try changing out to stiffer transmission and engine mounts. Your chassis is twisting under racing conditions and this is affecting your linkage.

+1, going to swap to Cab Mounts. I was gunning it thru an Freeway Onramp and it off cambered and I was rough getting into 3rd.

All other times she is fine but she does not like it when she is twisted and trying to up shift to 3rd. If I straighten her out it is fine but the chassis twist does make it tougher.

[gab997]

Originally Posted by Eagledriver

I had problems with my 2-3 upshift for years during races. I blame heat. The gearbox gets very hot (280 or more on the track, especially with a LSD). When this happens the case expands more than the shift rods and the other steel parts. Alignement gets comprimised and you have a missed shift. For me, I would feel like I was hitting a gate instead of a gear.

I installed a do-it yourself tranny cooler and haven't had a problem since.

-Andy

That's exactly what it felts like here. Could you let me know what/ how you installed your cooler.

Thanks,
Greg

[euro911sc]

If it feels like you are hitting the gate instead of the gear then you are jamming either the decel block or accel block between the synchro and the dog tooth hub. Happened to me years ago. The block actually stuck there eventually and I lost 3rd gear completely as the block would not allow the syncro band to shrink and click into the slider. If I slowed down the shift to a crawl I had no problems. Required a complete tear down, new dog tooth, new synchro, new blocks for 3rd and all was well.

I have since read Speed Secrets and realized that the milliseconds gained with a speed shift is not worth the risk of blowing the trans up. My whole view of shifting technique has changed. Not to say I shift like a turtle, but I am more interested in a gentle correct shift than a fast one. YMMV!

-Michael

[Porsche84]

Greg

Where are you located? I live in Notrh Port FL and might be able to help with the 915

[RSTarga]

If you don't have this problem in a straight line, this is not a transmission problem. This is a linkage alignment problem caused by chassis twist. Check your bushings, but if you have a stiff suspension, solid tranny and engine mounts and no cage you are twisting your chassis under cornering load.

[gab997]

Originally Posted by Porsche84

Greg

Where are you located? I live in Notrh Port FL and might be able to help with the 915

Live in Naples, you have a shop or you work on them yourself?

[Porsche84]

I do have a shop but it is not a business. I do like working on the 915 trannys. I would be happy to help with yours if you need to take it apart. Some of the comments about the car flexing, affecting shifting are valid, though. I had an issue with a loose rear mount at Sebring, which affected shifting. easy to fix, after I found it.

[Eagledriver]

There are threads on here about installing coolers. A quick recap on what I did. Installed a tilton pump on the bottom of the parcel shelf between the carbs. Installed a cheap Summit racing cooler in the deck lid intake (about an 8 by 12 size). Installed a pipe fitting in the bottom of the differential cover for an outlet and one in the top of the differential cover for an inlet. Connected the whole thing together with AN hoses (dash 10 I think). Have to be careful to deconflict the fittings on the diff cover from the CV joints. Only took a few hours to install (no tranny drop) and cost maybe 400 dollars.

-Andy