Sounds like a Mk series box.
If this is a "slide-on" gear, and if you look at the amount of material between the root of the gear tooth and the top of the layshaft spline, you will see there is not a lot of material there. The more steel, the better it will take the pounding.
In many cases, customers have little idea on the age of some of their gearsets. Could this be a case of it being used (often and hard for Autocross) past it's time?
I am not a fan of some "brands" of gears because I feel they will not hold up as well. That may be a case.
Which R&P ratio? I think with a 9:31 you can run a "16 something" for second gear if you want the common 60 mph or less second gear. (I run a 17:34 but that's just me ). Buy a STRONG 16 or 17 from Keith.
You have to life the gears, or inspect them for pitting at the root of the teeth regularly. That plus the fact that a 15 tooth layshaft gear is just getting very small if it's a regular gear. You will break them with sticky tires and a hard shift into what is really a first gear ratio. Far better choice is a hubbed second gear, that has the spacer hub built onto the layshaft gear and is far stronger, also twice as expensive. When they built the MK 5 option they only offered integral first gears and hubbed second gears.
I hope I am not confusing 4 speeds and five speeds with the hubbed second gear idea...
One thing that autocrossers can do is what is common in drag racing - have the gears drawn back to RC 54-56, down from the RC62 that they currently are. This will make them slightly more ductile and able to withstand the shock loads longer.
Makes the gear teeth wear out faster if it ever goes back to road racing, though.
Chances are that the gear was just old. Chances are you can assume all your gears are old. Chances are 90% of the racers out there have no idea of the age of their gears. Brian is right. Life the gears. Keep records of how many miles/hours/events on them. Perform NDT on them once a year. It's easy and cheap, especially relative to tearing up a gearbox.
Charlie Warner fatto gatto racing
In 15+ years of running Solo with a MK9 box FF, I have broke three (3) 15:xx gears. Two were used of unknown life. One was a new 15:37 (2.47).
After breaking the new 15:37, I decided that move up one gear to a 15:36 (2.40) for the reasons noted above; the layshaft gear of the 15:37 just does have enough material to withstand the continual abuse in a Solo application.
As I found with the other two 15:36 gears, you need to lifetime these gearsets. That is, the best advice would be to buy a new, REM treated, M-series gearset and plan on replacing after ~200-250 runs (~3 yrs).
In 20 years of running FF's in autocross, I have broken gears twice, maybe 3 times. They were all 15-37's. Pru is right about REM treating. But I ended up going to 16-xx gears and never had any more problems (knock on wood). If you have a 9:31 RP, you shouldn't really need a 15-xx anyway. As my car started handling better, i kept going up in gear ratio. So you could argue that improving the handling of your car saves you money in gears.
I also keep track of the age of my gear sets. Admittedly most are used and so I can't know their true age. But at least I have a record of when I obtained them and how much use they've had while in my possession.
BTW, I have one of my broken 15-37's on my desk at work that I use as a paper weight.
If you are going to continue on with Autocross as your main event, than you would find a 9/31 CWP better suited for gears (1-4) longer life. The ratios you would use will need to be increased about 3 teeth (total tooth count) to see the same speeds, which adds material where you need it.
You might find that an approximately 66 mph at 6800 rpm second gear (17:34 with a 9:31 and either a 16:35 or 16:36 with a 10:31 according to this chart http://www.taylor-race.com/pdf/GearConversion2.pdf ) will give you the extra "beef" you need with the 10:31 if most of your courses are not "real slow" or don't have great traction. Your 1st gear appears to be good for about 45 mph so for the real slow corners just go back to 1st.
With this gearing you may find that the "upshifts" from second to third are far fewer and occur after you are actually on a "straight" instead of still in a corner. Make the spread between second and third pretty tight. Then have an "overdrive" fourth for any really quick "non SCCA" courses. Afterall, how quickly do you really want to accelerate between say 80 and 100 in a parking lot?
I've run this gearing for many years and used to be reasonably competitive before a long "layoff induced layoff from National level autocrossing).
As to competativeness. I am willng to be as competative as the car owner wants to be. Only cuz I usually have him covered as far as time. (and I know he reads this thread)
In reality niether one of us as aspirations of winning a big shiny trophy. But it is more to run this car in a parking lot as opposed to say an H-stock car.
We really would like to compete in an event where we are not fussing with the car all day.