How accurate is the HiPerMath throttle body equation?
An early 2000's Chevrolet Corvette with an eight cylinder, 5.7 liter (348 cubic inch) engine, with a stroke of 3.47 inches and bore of 4 inches, which makes its maximum horsepower at 6000 rpm, uses a 78 millimeter throttle body on the real world street going versions. The HiPerMath equation:
with the number of cylinders, bore and stroke filled in
calls for a 77.9981 millimeter throttle body. The equation matches reality. In race form, the Corvette teams use a 90 millimeter throttle body. 77.9981 x 1.15 = 89.6979 (which is 15% bigger than the street version). The formula matches reality for racing, also. HiPerMath has data for engines used on the Bonneville Salt Flats to motivate a car to 200 mph and other high performance engines. The equation is accurate for those engines, as well. This equation can be trusted for real world applications for engines 2.0 liters or larger. For a 2.0 liter engine producing its maximum horsepower at 6000 rpm, the formula calls for a 46 millimeter throttle body. Many owners of this size engine use 65 to 75 millimeter throttle bodies. These large sizes are unnecessary. World Rally Championship cars use a 2.0 liter engine with a 44 millimeter restrictor plate and produce over 600 horsepower. Using a throttle body which is larger than the one called for by the equation will most likely slow the air down and cause a loss of power at lower rpms, unless you are using a turbo. Turbo charged engines may benefit from a larger throttle body and plenum. HiPerMath has no data on these modifications, so the equation does not take turbos or supercharging into account. Using multiple throttle bodies is trickier. There will typically be a large loss of power at lower rpms and a modest gain at higher rpms. Engines with multiple throttle bodies typically idle at 1000 rpm or more, because the air moves too slowly at the typical 700 - 800 rpm idle setting to keep the engine running smoothly. Air velocity not only affects the mass of air which can enter the cylinders of the engine, but the distribution of fuel in the air. Higher air velocity mixes the air and fuel more throroughly, which creates better fuel distribution, which equals more power, since a higher percentage of the fuel will explode in the cylinder.