First important thing to understand what rev-matching is to understand gearing of the car. We know that in first gear, torque is greater hence the lowest top speed. In first gear, we can reach up to 30-40 km/h quickly but cannot exceed than that. In second gear, torque is comparatively lower than that in first gear, but relatively greater top speed. So taller gears have less torque and higher top speeds, and shorter gears have more torques and lower top speeds. Hence, for any given vehicle speed, any transmission gear will put the engine at a different rpm.
Connection Between Engine and Transmission
There is a link between engine and transmission through clutch disc, flywheel and pressure plate shown in the figure below:
There is a clutch disc which is splined internally to the transmission. So whatever the gear the car is in, it will be rotating with that. So in first gear, clutch disc will rotate very quickly and in sixth gear, it will rotate slower. Since it is splined to the transmission, therefore it is always going to rotate with the wheels until or unless the car is not engaged in any gear (neutral).
It is bolted up to the flywheel. It always rotates whatever the engine speed is. There is a contacting surface which sandwiches the clutch disc between itself and flywheel.
Now let’s say we are in fourth gear, and we want to put the vehicle in second gear. To do so, both clutch disc and pressure plate are engaged together so they are both rotating at the same speed, we are in fourth gear, and the clutch is engaged, and so we want to go down in second gear. So we disengage the clutch and then we put it into second gear. Now clutch disc is rotating much faster than the pressure plate. So what we do is, we tap the throttle, and we bring pressure plate up to the speed at which clutch disc is rotating at, and then we mesh them together, and we have a very smooth transition.
Why is it important to match speed between the engine and transmission before meshing them together?
Mainly there are two reasons:
- We are going to have less wear. If clutch disc speed and engine speed match up, we are not going to have wear on clutch disc by meshing them together.
- We are not going to upset the balance of the car. In racing, if the vehicle is not rev-matched then the engine will send that braking torque to wheels, and so car’s balance will be upset.
We hope this will have increased your knowledge about your car and helped you in some to improve your day to day driving.