Vroom (1) Vroom (2) Etc....
Rev counter or tachometer simple really, it counts the revs per minute at which the engine is rotating.
The measurement is made generally by a pick up sensor on the engine fly wheel however components such as alternators can be use to pick up the engine speed.
One revolution being a 360 degree rotation of the crank shaft.
Now as a single 4 stroke piston goes bang and produces power once every 2 revolutions you get a ratio of 1/2 a bang per rev * the number of cylinders or (revs per min*cylinders)/2 = no of power strokes per minute.
and what does all this prove? "beats us"
The main use for a rev counter of course should be to tell us when its time to change gear. If its approaching the red bit its probably time to consider changing up a gear, If you have got no gears left your probably about to be arrested. And if you are in the red bit and not paying any attention you may be about to blow up your engine.
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