10 Things You Always Wanted to Know About Engines

This article is copied from the September 1999 issue of Hot Rod Magazine

By Ray T. Bohacz

The ideal intake manifold allows fast mixture delivery, low frictional flow losses, uniform mixture distribution, minimal fuel-film accumulation, limited heat transfer from the engine, and, if desired, takes advantage of inertial supercharging from resonant tuning.

    This is quite a task, especially when the device must fit under the hood and between the cylinders.  The ideal manifold would offer a straight path to the intake port of the cylinder head and cause small flow losses brought on by turns in the runners.  Whenever gas is required to make a turn, flow losses are created.  A conflicting goal arises when it is considered that the longer the intake-manifold runner is, the more torque the engine will make at lower rpm.  Since we buy horsepower but drive torque, this is a desirable characteristic.

    Dual-plane intake manifolds add runner length--the path the air must travel increases by virtue of a 180-degree turn, with the intake-manifold runner usually feeding a bore on the opposite side of the engine.  Subsequently, the dual-plane design makes more torque and provides greater velocity, but the distance the air must travel limits rpm.

   Single-plane intakes provide a straighter shot into the head but have limited runner length and require higher rpm to work effectively.  The job of an intake-manifold designer is to make trade-offs: flow for velocity, velocity for distribution.  Next time you look at that lowly piece of aluminum, realize that there is more engineering time devoted to it than to the wing of an airplane.