Make it simple…..what exactly is “rev matching ” how do you determine the RPM for the speed you’re going?
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On a low gear, the engine needs to spin faster to move the car at a certain speed than a high gear would need.
When you shift gear, you normally lift off the accelerator pedal, press the clutch pedal, change gear, release clutch and press accelerator. In the process, the engine would lose speed because you lifted off the foot from the accelerator. This is fine for upshifts, because the higher gear would need the engine to spin slower anyway. But, for downshifts, this is the opposite of what needs to happen: the engine should be spinning faster for the lower gear but it’s actually slowing down.
On small engines, it’s usually not a problem. As the clutch engages, it will bring the engine to the correct speed. But, in very low gears, and especially with bigger engines, due to the higher engine torque, it’s harder for the clutch to pull the engine speed up. That means wear on the clutch. Even worse, the clutch could fully engage and the engine might “win” the arm wrestling and impose its (gear-ratioed) speed on the car wheels. Say, the car is travelling at 30 mph and wheels are rotating for that speed. You downshift, engine wins and now wheels are rotating at the wrong speed relative to the ground. That means the wheels are spinning on the ground, you have lost traction and that can be very dangerous. This is an extreme example, but it can happen in the right conditions and I experienced it personally.
Rev-matching is the process of bringing the engine speed at the correct target speed for the next gear, so when you release the clutch there’s no slipping and arm-wrestling. In practice, this is mainly a concern for sport cars, not much small cars, though rev-matching would reduce clutch wear in either case.
Rev-matching is traditionally done by pressing very shortly the accelerator pedal to rev up the engine a bit (while the clutch is disengaged). On modern sport cars, rev-matching can be done automatically by the engine computer. When doing it manually, you’ll never be as precise as the computer. There’s usually about a 1000 rpm difference between gears, so that’s what you’re trying to bump up the engine speed by. But it’s not an exact science and it doesn’t matter that much. Even if you’re off by 200 rpm, you’re still much closer than with no rev-matching at all.
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