While Honda engines and chassis have gained near legendary status for their performance and reliability, the company's transmissions have gotten something of a bad rap. If the RSX's transmission has any real flaw, it's that the company installed certain equipment that encourages performance shifting without accounting for the effect that such treatment might have.
The RSX's manual transmission works much the same as any other. Power flows through the input shaft to the output shaft via a gear-set that determines ratio. Between each gear are smaller gears called "synchronizers" that serve as an in-between point for the power flow, allowing the driver time to match engine RPM to output shaft RPM by either throttle rev-matching or clutch dragging.
Although synchronizers do a great job of increasing shift smoothness, they are a weak point in the transmission. Power-shifting (shifting through the gears faster than the synchros can compensate) forces the synchronizer's teeth to grind together (which is where that grinding noise comes from). Those teeth are much smaller than the gear's teeth, so they will be the first thing to snap off.
The RSX's transmission is no more inherently inclined to grind than any other Honda transmission, but Honda may have bit off more than the transmission could chew with the shifter. Sporty RSXs use a very short shift lever, which encourages faster shifting and decreases the driver's mechanical leverage while shifting gears. Too-fast shifts with too little force behind them are perfect for producing gear grind and subsequent synchro failure. Compare the RSX's transmission to the non-synchronized units used in big trucks; truck transmissions use very long shift levers that provide a lot of slow torque to provide smooth shifts without synchros of any sort.
The simplest way to prevent synchro breakdown in an RSX transmission is to resist the temptation to shift like a lunatic. The fact is that power-shifting feels cool, but is unlikely to provide any meaningful gains in acceleration. Army Scout Snipers have a saying that applies as much to your transmission as it does their Barrett .50 cal rifles: "Slow is smooth, and smooth is fast."
As painful as it might be to admit, the simplest modification you can perform to increase transmission longevity is to replace your RSX's short, sporty shift lever with a longer unit from any base model Honda Civic. The only other solution is to install a straight-cut gearbox (a.k.a. "dog box"), a transmission that uses perfectly straight gears instead of the RSX's angle-cut gears. While the best of these boxes are capable of handling more than 1,000 horsepower, they're incredibly expensive ($4,700 to $6,500), and those gears howl like a demon at highway speeds. Still, if you're making more than 300 horsepower, you have the money and don't mind a constant, ear-piercing whine, then this might be the route for you.
