I figure if anyone here recognizes this stuff it'd be you guys who put loads of hp on AWD cars on 1/4 mile races, input greatly appreciated.

I came out of a hairpin on the racetrack, I'm in 2nd gear, got on the gas.
I have a Carbonetic diff combined with Hankook Ventus RS3 19" tire setup (997 GT3 sizes), traction is nothing but astounding so it doesn't break loose a lot and it didn't this time either, and the 2nd gear just gave in.

No noise, no "pop out". Just stopped driving in 2nd gear.
All other gears fine so no shift fork issue or other.

Pre-diagnose from a respected tranny builder: Dogring/drive gear weld broke off in the seam. To me, this is unheard of that these boxes are so weak, or is this just a weak link I didn't know about or did I just have really bad luck on this one?

The tranny has a total of 10k miles, driving style is very smooth.
Car makes about 520hp crank.

 

Odd but the old fix was to change the slider and have a new dog ring welded to the old second gear. Some of the early ones did not work so well. If you are going into the box get the GT2 bearing retaining plate it is made from steel instead of iron. I feel with the lsd and good grip you will need it.

 

Thx!! The box was opened and worked on 450 miles ago, thoroughly, so I'm a bit surprised this happened. I'll let the guy open it up again to see what's going on and what parts might have been "cheaped out" on.

 

by no means expert, but i have had my previous car's g'box opened/repaired twice. the inherent weakness in the box ( as explained to me ) IS the 2nd gear dogring.

 

I know/knew the adjustment of the dogring was really important, to not chip the dogs and have the gear pop out, but that the ring separated from the gear is just unbelievable. As mentioned, my car doesn't really make that much hp.

 

the way i've understood the issue is that if shims aren't used to change the stacking position of the 4th gear on the pinion shaft allowing a different placement of second gear in the stack ( whew ), then this weakness will always be present, and that the dog ring once worn the dreaded 2nd gear popout will, or at least can, result. as to the dog ring itself snapping off? it's a known weak spot, so who knows.

as to 520hp at the crank, mines at comparable figures and that's enough for me i think we're right about the point in increased hp to where stresses that might not be problematic at stock 420 hp, can now contribute to premature wear esp on clutch/trans or whatever. but other more experienced folks might know differently.

 

Well I did not have the pop out issue that's for sure.

The tranny builder (builds for 24H Dayona Porsches) said it sounded like a separation of the dogring and 2nd gear. We'll send it back to him and see what went down.

Are there different quality gears? Like cheap knock off parts that could have gone into the box, or does it happen on stock cars as well? 1/4 mile people should know IMO, running tons of hp and sticky tires, they'd be the most likely to have seen this issue?

 

I am not sure how they weld the dog ring on but I do remember looking at the stock parts and thinking the weld looked like it was friction welded on.

 

Most gearbox manufacturers do friction weld dog rings to the gear stock much like how turbo manufacturers weld turbine wheels to the turbine shafts, however some are laser welded at the seam as well and in some older applications they where heat shrunk onto the gear collar(old tech) Application specific really, but never really looked myself when inside a G50 or G96

Be a very rare event to have the dog ring slip on the gear, but there's a first for everything. I'd expect the selector hub to strip the teeth off the dog ring first before actually slipping.

Have you noticed any change in throw on the lever? You very well could have bend the shift fork just enough as to not engage the selector hub over the dog ring in 2nd gear, yet still have ample throw for first. other gears would not be affected due to separate forks and linkage rods for them.

Time will tell for certain. I'm interested in knowing the root cause for sure, as I still rebuild a boot load of these gearboxes for customers.

 

If the 2nd gear in question was the original gear and hasn't had the dogteeth replaced on it, the failure that you are describing is somewhat rare. We don't hear about very many of them breaking the dog ring off. Lots of dog teeth get damaged, but you mention no pop-out issue, which is the usual symptom of damaged teeth.

Now if the gear has been repaired and had new dog teeth welded onto it, all bets are off. I have seen the repairs actually compromise the strength of the gear itself by removing the temper from the gear. This is the result:

 

I'm not sure the signs or symptoms here, but this is what mine is doing.

Only with full throttle in 2nd gear, 1.0 bar, and at about 5K there's a hard thump in the rear of the car. I immediately back off and everything seems fine. I attribute it to a hard cutout to keep from overreving (tach needle is moving pretty quickly and I'm looking at the road when it happens).

Shifts and pulls fine around town in all gears, no change in shifter pattern or feel.

Another 2nd gear full throttle and a lesser version of the same. A thump around 4-5K then picks up and moves on, shift to 3rd, no further problems. Full throttle pull in 3rd no issues.

I've had motorcycles pop out of gear and its a loss of acceleration and rev to redline. Didn't notice a big free-rev after either of these incidents. Can't say I was totally attentive on the first.

Car has a tune (1.0 bar boost) and nothing more for engine mods.

Is this what you're calling "gear went out"?

Any ideas on a reasonable repair cost if it is a gearbox issue?

 

Hows your tranny and motor mounts?

 

Matt, I did not realize how little material there is there to transfer the load through until I seen it myself. It is only a few mm

 

Rennfab and GTgears and all here, thanks very much for really good input.

Tranny and motor mounts are good.

The 2nd gear stopped driving plain and simple. No thud, no "hard" kickout, so nothing like when a gear hasn't been engaged enough, it just stopped driving with no noise and the engine just revved up as in neutral.

So the weld might be a plausible cause. It really sucks b/c it's expensive due to all the labor to fix this kind of failure.

In the best of worlds it could be only a fork having a slight bend, but then again I have 7x24 hour races under my belt in Porsches and 5 GT endurance seasons in GT Porsches + tons of other racing, and in this time I have never bent a fork.

When I test shifted into 2nd position directly afterwards, I felt the shifter passing the synchros (as usual), but then moving the shifter further to engage the gear, there's just no drive.

Is the fork/sleeve really so narrow that it can pass the synchro well and clear but then not get to engage the dogring?

This box had low miles and then got new gears installed through a popular vendor.
500 miles was put on it (different car). Then it was overhauled by a very merited Porsche racing gearbox builder since the end nut on the 5-6th gear axle came loose and shredded 5th and 6th gears. It turned out that whoever installed the close ratio 5 and 6 gears for this vendor re-used the old axle end nut and it was not loctited down so the whole gear stack started moving and the gears grinded against eachother.

During that repair, also 2nd gear axle end nut was inspected and the 2nd gear synchros were replaced since they were worn. After this, the box got installed into my car, broken in softly and now has 450 miles - and 2nd gear not working anymore.

Overall the tranny has 15k miles. The last 950 miles with new gearsets.

Either way, trans has to come out and be opened all the way to access 2nd gear.

Cost assessment?

 

Can you tell me more about the regear? Was a motorsports 2nd gear (and mainshaft) installed when the gearing was changed or did the gearing stick to 3rd through 6th?

Stock, your 2nd gear is fixed on the mainshaft, along with 1st. If you've got a motorsports mainshaft and slip fit 2nd gear it's going to cost more than if it's a stock 2nd gear. If it's stock, the 2nd gear idlers are now available directly from Porsche for around $450-500. If you need to buy a new 2nd gearset complete it's $1100-2000 depending on vendor.

Figure $1000+/- labor once the box is out of the car. If you aren't removing it yourself, there's more labor there. And lastly there's the ??? parts that are composed of "what else did I mess up when this happened?" stuff.