Enter the engine efficiency (VE), the CID, RPM, and speed into the calculator to determine the engine air intake diameter.

Engine Intake Diameter Formula

The following equation is used to calculate the Engine Intake Diameter.

D1 = SQRT ( VE * CID * RPM * 144 * 4 / (S * 88 * 3456*pi))

  • Where D1 is the engine intake diameter
  • VE is the engine efficiency
  • CID is the engine size
  • RPM is the rotations per minute
  • S is the speed (miles/hr)

What is an Engine Intake Diameter?


The engine intake diameter, also known as the inlet diameter or bore is the diameter of a cylinder at the point where the piston travels through the cylinder head into the combustion chamber.

The engine intake diameter is one of the factors that affects valve type, size, and location.

An engine’s intake diameter is an important factor in the design of an engine. Because of this, there are a few different ways to measure it. Engine intake diameter may be based on the physical size of the engine’s intake manifold, which is called its bore.

Alternatively, the engine’s intake diameter may be based on the size of the cylinder head ports. This is probably more common for larger vehicles that use carburetors or throttle-body fuel injection systems because their intake diameters are defined by their throttle body valves.