Torque vs. HP Revisted

[Tommy]

The best short answer above is, IMHO, scottj's. As he has got more race track experience than most here, I value his opinion.

From an engineers perspective, the burning fuel in each cylinder's power stroke serves only to apply a pushing force to the piston. That force is transferred through the connecting rod to the offset arm of the crankshaft creating a brief rotational torque. (Force of piston X Length of crankshaft arm = Torque on crankshaft). That torque is transferred through the rest of the drive train to the wheel(s). When the drive train tries to rotate the wheel, friction between the tire and ground usually makes it easier to move the car forward than to slide the tire on the ground. In this instance, the rotational torque of the wheel is converted to a linear push against the ground. (Torque of wheel / Radius of wheel = Force pushing car forward). ... As Newton taught us, applying force to an object will cause it to accelerate per the formula: Force = Mass X Acceleration. So, the theoretical rate at which a car will accelerate is directly related to the torque created by the engine.

BUT, we don't drive our cars in a vacuum, so as we gain speed the air begins to push back against us. Consequently, a car producing a steady torque will accelerate from 10 to 20 MPH more quickly than from 80 to 90 MPH. And ultimately we will reach a speed where the push back from the air and everything else will equal the push forward from the engine causing the acceleration to slow to zero (i.e., top speed).

Recall that each piston produces a pushing force and subsequent torque only momentarily during its power stroke. If we increase the number of pistons or the RPM we can increase the number of power strokes from the engine per second. This is what horsepower is all about. Horsepower can be expressed as: Torque / Time or Foot-Pounds per Second. It is a calculated number derived from the average amount of torque produced by an engine over time. This relationship makes it obvious why engine horsepower typically increases as the RPM increases, up to a point.

The horsepower number most often quoted for an engine is its peak horsepower at a specific RPM. IMHO, that is useful information only if you plan to spend most of your driving time at that RPM (e.g., NASCAR Daytona). Otherwise, saying your car performs well because it has a 600 HP engine is kind of like saying a person should be able to fix your car because he has an IQ of 150.

For my use, I value my car's ability to accelerate over a variety of speeds more than it's top speed. For that reason, I had my engine built with a big flat torque curve from 3,000 - 6,000 RPM. I want the engine to last, so I limited it to 6,000. If I were a real competitor like scottj, I would have moved the torque curve higher up the RPM range.