eli5 rev matching

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Make it simple…..what exactly is “rev matching ” how do you determine the RPM for the speed you’re going?

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5 Answers

Anonymous 0 Comments

Rev matching is the process of adjusting the engine speed while changing gears, so that both sides of the clutch are turning at the same speed when the clutch is being engaged.

Imagine the opposite: having the engine at idle and releasing the clutch. As the clutch engages, the engine and gearbox speeds are pulled together until they are turning at the same speed. Done quickly, this is generally accompanied by a sharp jerk as the vehicle speed rapidly adjusts.

The advantages are a smoother clutch-in and somewhat less clutch wear.

As for how you know what the right RPM is, that’s a matter of experience with the particular vehicle.

Anonymous 0 Comments

Car manuals used to have a shift table telling when to shift. According to speed and and tach meter.

The point of rev matching is to have the engine close to the same speed as the transmission when you clutch in.
If the speeds donc match, you will lose (a small amount) of time and risk losing traction in slippery conditions.

To determine what Rmp to rev to…. Just look at the tach when you up shift. If going 50km/h in 2nd speed is when you shift to 3rd and the Tachmeter shows 4000rpm. Then any time you downshift into 2nd gear at 50km/h you should try and have the engine at 4000 rpm.

Anonymous 0 Comments

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.

Anonymous 0 Comments

When I was floating gears in a semi truck, it became a second nature to just do it by ear and feel. I learned by shifting in the low range of gears in the parking lot of the truck driving school I was in. I would watch the tach and see how much of an rpm difference there was between gears and double clutched it until I learned to float (clutch to release the gear, clutch to enter the gear, but never all the way to the floor cause big trucks have a clutch brake there). You do shave the gears a little when you’re learning, but that’s one reason it’s three thousand dollars for a three week course.

Anonymous 0 Comments

Jokes on you people. I just throw it into neutral while coasting and then when time to start accelerating again I just put it back into whatever gear I usually have it at that speed