[curtis73]

RPM is important in two main ways; 1) engines breathe only enough air to support up to a certain RPM. THe higher the RPM the more air it ingests. Imagine breathing through a straw, you'll only be able to support a certain amount of breath. Then switch to a 2" tube and you can breathe alot faster. So, in many engines, airflow is what restricts top RPM. 2) if you rev it too fast, things blow up. The pistons will literally break off the rods and the engine will self-detruct from the inside out.

Every time you get a combustion event inside the cylinder, the engine is providing power. In a 4-cylinder engine at 2000 rpms you have 4000 combustion events per minute. At 5000 rpms you have 10,000 combustion events per minute. Higher revs means more fuel releasing its energy as power per minute. So, what happens is, more revs mean more power, up until the point that things like the intake, heads, exhaust, and other internal parts start becoming a restriction to airflow. A power curve is bell-shaped. It starts at zero power at zero RPMs, then climbs to a peak (you'll often see horsepower numbers advertised as something like 150 hp @ 5500 rpm) and then it will drop off again above that peak because you're running into where airflow is restricted. So, one of the ways to make more power is uncork the airflow, but then you have to rev to (for instance) 6500 to get that power. You have to be sure the engine is capable of 6500 RPMs without exploding.

Thats a simple explanation, but you'll pick up more info as you go.

Also, not all 2-cylinder engines are 1-liter. The size of the piston and the stroke of the crankshaft can make nearly any displacement. I have a 2-cylinder engine in an old tractor that is an 8.2 liter.