Normally (naturally) aspirated racing engines usually operate over an engine RPM range. Tuning for performance is often needed especially with air density changes. That usually involves two different engine speeds that are key to high power. These are the torque peak RPM and the horsepower peak RPM. Engine design establishes the specific engine speeds for both.
Note: A normally (naturally) aspirated engine designed to operate at only one engine RPM may have a torque peak and horsepower peak at or near the same RPM. That design would establish optimum features making both better. However, the engine would perform poorly at other RPMs. As a result, the application is limited. Engines for power generator operation or liquid pumping with RPM control are examples.
Torque Peak RPM
The torque peak of a racing engine is the RPM where air pumping through the engine is optimum. That is established by intake size and camshaft timing. Typical intake port velocity peaks at 400 feet per second. Slower port velocities at lower RPM are less efficient due to air and fuel separation. Essentially the ports and camshaft timing are too big for that lower speed, and torque is lower. Higher port velocities at higher RPM are also less efficient due to the increased pressure drop from pumping losses. Essentially the ports and camshaft timing are too small, and torque is also lower.
Horsepower Peak RPM
With higher engine speeds however, the number of torque pulses per second goes up. That overcomes the reduction in torque up to a point. That is where the horsepower peak occurs. Above that, torque is so low that the increased torque pulse frequency cannot overcome it. Some engines also reach other RPM limits such as valve float, an electronic rev limiter, or pumping losses from exhaust backpressure or simply small ports, valves, and valve timing.
Different Engine Speeds Example
Methanol racing engines are common in circle sprint car racing and drag racing. Racing camshaft timing and large intake ports / valves make torque peak values typically around 5,000 RPM. Horsepower peak values are achieved at a lot higher engine speed, typically around 7500 RPM. Torque at 7500 RPM is lower, but horsepower is greater due to the higher torque pulse frequency.
Fuel System Delivery Example
Fuel system tuning considers both engine speeds. For methanol engines, torque peak air/fuel ratio is typically 4.8 to 1 with a volumetric efficiency around 100%. Horsepower peak air/fuel is typically 5.2 to 1 with a volumetric efficiency around 85%. Fuel injection jetting is unique for both engine speeds with a smooth transition from one to the other. In addition, jetting must be adjusted for different air densities. Trial and error tuning can be difficult because of the two key engine RPMs, the transition, and differences from air density changes.
More information is in our books: High Horsepower Tuning for Mechanical Fuel Injection, Jetting for Racing Mechanical Fuel Injection (for Small Block V-8s), Jetting for Racing Mechanical Fuel Injection (for Big Block V-8s) 2nd Edition.