Engines: output

Now that we know how engines work, its time to consider how well a particular engine works and why. For that we need to introduce some terms.

Displacement. Displacement is total volume inside the cylinders of an engine if every piston was at its bottom-most position. So if you have a 4 cylinder engine and each cylinder is 500 cubic centimeters (cc's) the total displacement is 2 liters (500 * 4 = 2000 cc's). The more the displacement, the more air and fuel can be taken into the engine, resulting in more power. Displacement is in general an excellent measure of how much power an engine can make.

Torque. Torque is a measure of force. Its an instantaneous measurement, meaning it doesn't take into account time. You can think of torque like trying to turn a wrench; the torque is the force you are applying to the handle. We express torque in terms of force over a distance; typically the force of 1 pound over 1 foot, aka ft-lb (foot-pounds). The amount of torque an engine makes varies based on how fast the engine is turning.

RPM's. Revolutions per minute. This is how many times the crankshaft of the engine turns in one minute.

Horsepower. Horsepower is how we measure the power of an engine. Hp = torque*revolutions-per-minute / 5252. The '5252' is a constant, and it also means that at 5252 rpm's, torque and hp will always be equal. Since the amount of hp is based on how fast the engine is turning, hp will almost always increase as rpm's increase. In terms of how fast our car can accelerate, hp is the most useful figure.

RWHP or BHP. Rear-wheel horsepower or brake horsepower. This refers to horsepower that was actually measured at the wheels of a car using a device called a dynamometer, (or dyno for short). This tells us how much power is actually getting to the ground. This is less than the horsepower at the crankshaft of the engine since there are frictional losses in the transmission and differential.

Redline. The maximum rpm it is safe to spin the engine at. For a production car, this is typically 6-7k rpms. For an F1 car, it is capped at 19k rpms.

F1 teams aren't too keen to publish their dyno charts, so at right are a couple sample dyno plots from a production car engines. The chart has two lines, representing torque and horsepower. Sometimes it can be hard to know which line is which, so a tip is that peak horsepower nearly always occurs at a higher rpm (farther to the right) than peak torque.





Looking at the power curve can tell you a lot about the characteristics of the engine. In the top dyno plot, the torque smoothly increases and retains a nice flat curve in the meat of the powerband. A flat plateau in the torque curve, and a nice linear diagonal line for horsepower means that the engine will respond proportionally to throttle input. In the second graph, a turbocharged engine from the MkIV Toyota Supra shows an engine with a 'peaky' powerband. Because of the character of the turbo, this engine doesn't make nearly as much power at low rpms as it does at high rpms. The torque curve jumps abruptly as the turbo spools up. We'll discuss turbos more later.

Anyway, because the engine is making its best power in a narrow range, it would benefit from a gearbox with closely spaced gears, shifting more frequently to keep the rpm's up. In that respect, this engine is similar to an F1 engine. Both like to be rev'ed high and kept high to make the best power.