what is over-rev??????

The power that your bike makes after the your peak HP. What that means is when you over-revving your bike, you are revving it past the RPM where it makes the most power.

over-rev is a good thing. It is how long the engine makes good power past the HP peak. Over-rev lets you streatch a gear a little bit longer, and sometimes can allow you to not have to shift right before the end of a straight, or between a corner and a jump.

Over-revving is not really a good thing. Top-end life is inverse of rpm. The higher you rev, the less life a top end will have, but than again, these are race bikes, not XR's

Masterphil's description is correct. Here's another way to explain it:

Did you notice that when you look at a dyno graph of an engine's power, it curves upward to a peak, then starts to curve down again? The portion after it reaches the peak is the overrev. On some engines, the line on the graph will stay fairly flat. The flatter it is, the more overrev the bike has. The more sharply it drops off after the peak, the less overrev it has. That is because when the curve is flat, the power is not dropping off as quickly. Overrev is a good thing, because it gives you the flexibility to hold the throttle on and rev it a little further instead of shifting. At least in certain situations.

On a related note, the way the curve on the dyno looks will show if it is the type of bike people describe as having a "powerband". The dyno graphs of some bikes are fairly "flat" from low rpms to high rpms. They will curve upward, but the curve is not very pronounced. An example of a curve like this would be a Harley, or in dirtbikes, a 450. Other bikes have a very pronounced curve with a steep curve going up. Two stroke 125's are often like this, especially old ones before power valves were invented. These are bikes with the steep curve are the ones that are said to have "powerbands".

On bikes with the very pronounced curves, when the bike is at low rpms, it has little power, and does little more than make noise. As the rpms increase, the curve becomes very steep. This indicates the bike is increasing in power rapidly at this point. A steep curve shows what people call a "hit." On engines like this, the curve will ramp up sharply to its peak. This portion of the curve where the bike is making so much more power is what's called the powerband. On motors like these, it is important in keeping the rpms in the the range where the bike is making power, the "powerband."

This has nothing to do with how much power the bike makes overall. A bike with a wider, flatter curve can make more power than a bike with a sharp, steep curve. The flatter curve is often easier to ride as well, because the power does not come on in a burst like a peakier bike.

Hope that helps.