What does RPM tell you about a engines performance?

The only relative info from "RPM" to PEFORMANCE is what rpm your engine makes most torque and most horsepower. However, this info is very revelant in several ways.

First, with a tachometer (rpm guage), you can tell how fast your engine is spinnng relative to the speed at which you are actually moving.....on an AVERAGE engine for example, you don't want to be moving at 75mph in first gear at 10,000, rpm's too high, overheating and engine siezure will occur. Then you don't want to being be at 25mph in 5th gear at 1500rpm, this is "lugging" the engine, oil starvation, overheating, knocking, clunking, all kinds of bad things.

Next, every engine, produces its own peak HP and TQ at different rpm's. In racing, you use the tach's rpm reading to know exactly when to shift for max performance. Each gear will produce HP & TQ at a different rpm, so the bike or car is put on a machine (dynomometer or Dyno) to measure what rpm both HP & TQ fall into. For racing, drag racing - you want to shift at your max TQ point, for high speed road rcing, you want to shift at the max HP point....however.....there are 2 factors involved in this....shifting point rpm drop....when you shift gears, the rpms will drop slightly AND each higher gear produces most TQ at a slighty higher rpm...SO....what you actually want to do is while on the dyno, you measure in 1,2,3,4,5,6th gears, when you hit the max HP or TQ rpm, you shift and see see what the specific drop in rpm's is, say you shift at your max TQ point 5000rpm and your rpm's drop 1000rpm at that point, now you also take into consideration that the next higher gear make max TQ at 6000rpm....you should shift at 6500rpm....you take that 1000rpm and ADD it to the 5000rpm where your max TQ was, and divide the 2 gears max rpm difference by 2 this will keep your engine running at 5500rpm, the closest possible point of max TQ so that when you hit that next gear, you're as close as possible to the point where you are making the most TQ for that new gear, without over-revving the engine in the previous gear, causing a loss in power and speed.

This is why mainly drag racers have "shift lights" to signal exact shifts points, instead of the rider relying on the tach....0-200+mph in under 6 seconds, you need your eye on the track, not a gauge.

I know all this sounds a little complicated, but in time, it gets to be second nature. Know EXACTLY where to shift in a race is the difference between winning and losing!!!!

All it tells you is how fast the crankshaft is rotating. It has to do with predicting the power or performance of an engine. The harleyguy did a good job of explaining how a tach can be used but for ages automobiles didn't have tachs, even when equipped with manual transmissions.

Tachs are not much different than temperature gauges on water cooled engines. Some actually read the temperature, others have a normal range but no actual temperatures and others are simply a red light that only comes on when the coolant is too hot, but none tell you anything about the engine's actual performance.

If you know the shape of a torque curve in a graph for an engine then you can predict the horsepower with the rpm – horsepower is calculated by multiplying rpm by torque and dividing by 5252 (when the torque is measured in ft lb) – so, in a graph including torque and horsepower, the two lines will cross at 5252.

That is only part of the story – gearing and loss through friction will affect any predictions.

-If you allude to the so-called interpretation of the bore / stroke ratio:

Develop short-stroke power and torque in the upper rpm ranges ..

Long-stroke engine (Harley Davidson) to develop the power and torque in the lower rpm ranges ..

appendix:

-I`ve got an old Moto-Guzzi Le Mans I wich includes both characteristics,-because it`s more a square Huber..than a long stroke engine..take`s a lot long time to come up to the high speed limit..but than it races long distance .... as you expect from an old tractor can also ..

way to much is put on RPM. it is only a gauge to tell you when to shift

so you don't over rev the engine. after a certain rpm the engine actually

looses power. in the average street car they are automatic transmissions

and the Tach, has no purpose, just something to distract the driver.

...yah,, tachometer only indicates the speed of the engine, but if you want to relate it to the car's performance, there is a certain speed in which the engine produces the maximum torque or power but it does not go beyond the red zone,

...so if you want to get most of the engine power you have to keep the engine speed at this mark,,the red zone is used to get the maximum engine speed before shifting,,,

Nothing, other than the speed at which the crankshaft is rotating.