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 axiom "lean is mean" seems to contradict common sense.  We need fuel to make power, right?  Well, you need to burn the fuel, but without oxygen the fuel in a spark-ignition engine will not ignite.  At peak torque, the ratio would be 12.7-12.9:1; at maximum horsepower, 12.9-13.2:1-- or about 13 parts of air (the constant) to 1 part of fuel.  During full power, the engine inducts a constant amount of air that is limited directly by piston displacement.  If the fuel flow is increased by changes in carburetor jetting or injector pulse width, the power output will increase due to the increased liberation of the chemical energy.  With the increase in fuel, output will rise until all of the oxygen in the cylinder is effectively utilized for the combustion event.  This represents the optimum release of chemical energy.  Since fuel flow can be increased and since airflow is fixed by design and engine displacement, it is the air rather than the fuel that imposes limits on power.