For all the time and effort we spend on making power, few people really take tuning to the next step by optimizing the transmission's gear ratios to the engine's power curve. It's not nearly as important if the car is new and has an abundance of power but, in the case of Project Corolla, gearing can mean everything on the track.
We rarely take apart transmissions here at SCC simply because there is rarely a need. Discounting Coleman's rally beater Datsun 510 (which has an engine and transmission swap), Project Corolla is the oldest project car in the ranks still running on its original transmission. This means the track beating we've been giving it has brought the stock gear box near the end of its service life. The slop and bearing material in the transmission oil was proof.
So we need a fresh transmission. We might as well freshen up the differential at the same time since the OEM LSD had pretty much worn out years ago. If this were any other car, this could have been taken care of with a trip to the junkyard and a weekend parts swap. That's the problem with old cars. Even with the piles of T50 transmissions that Toyota produced for the Corolla and various other vehicles over the years, reasonably low wear parts are just too difficult to come by. The added craze of SR5-to-GTS Corolla conversions has all but depleted the Corolla parts bin. The only reasonable option left was to rebuild the transmission. As the adage goes, if it's worth doing, it's worth over doing. So we've decided to undertake building our stock gearbox with all new internals and a TRD gear set originally designed for spec racing back in the days. The rear axle would also be rebuilt and refurbished in addition to getting updated with a TRD 2-way clutch-type LSD.
The main reason behind dumping the stock gears wasn't because of wear or age. In fact, other than syncro and bearing wear, the original gears were still pristine. Gear spacing was the culprit of our problems. Understandably, Toyota designed the T50 transmission with street use in mind. This means, like most 5-speed gearboxes, second gear tops out at 60mph for the all important 0-60mph performance number. First gear was squeezed in somewhere between that, slightly on the tighter side, to let drivers launch from a start with some pep.
Toyota's engineers picked fourth gear as the direct 1:1 drive, since one of the gears had to be. Third gear split the difference between second and fourth, and a slightly overdriven fifth was thrown on top of that so that the car would run at reasonable engine speeds while on the highway while not being too overdriven where the engine would just fall on its face.
Track conditions are different. Unlike the street, standing starts are almost non-existent, and engines rarely spend time in the lower half of the RPM band. This makes a tight first and second gear very ineffective. Our recent cam and manifold changes have also shifted the power curve further up the RPM band, making tighter stock first and second gears even less effective. Most low speed corners were still too fast for the stock first gear and second gear never wrung out far enough to avoid an up-shift mid exit.