Think of your engine as a pump. It is physically capable of “flowing” a given volume of air (cubic feet per minute, or CFM). The more air if flows, the more power it makes. Imperfections in the passages into and out of the combustion chambers— the intake and exhaust Manifolds and/or Turbo —will restrict your engine’s air flow, or volumetric efficiency.
Volumetric efficiency is the ratio between actual and theoretical flow of air through your engine’s combustion chambers. All mass-produced engines have less than optimal volumetric efficiency, because the manufacturers cannot afford the skilled labor required to clean up the ports of each individual engine. most of us are not behind the wheel of Hand Built Skyline engines.
This cleaning-up process, porting, is the enlarging and refinishing of intake runners and exhaust manifold ports to maximize its volumetric efficiency: its ability to “pump air.”
Because of its complexity and the physical dexterity required by the operator, porting is the black art of engine tuning. It can not be done by a machine, only by a person. If your engine’s porter is skilled, he can improve its volumetric efficiency—and power— by 20 percent or more. If the porter is inexperienced or incompetent, he can decrease your engine’s volumetric efficiency by at least the same amount, and bring on overheating and rough-running to boot.
Proper porting gets “free” horsepower out of any engine, from dead stock to fully race prepared. The improvement in “breathing” increases power all the way across the rev range, and adds smoother running and greater reliability, all without adding parts or using more fuel. Apart from the cost of the service, there is no downside. It’s beautiful!
So what exactly does an engine porter do?
Think of a porter as a sculptor, who incorporates specialized tools instead of hammer and chisel, and whose marble is your Manifold, Turbo, or even Throttle Body.
The porter clamps your item to his porting bench, and proceeds to sand, shave, grind and polish in all the right places, to remove the inconsistencies of mass production, as well as known deficiencies in your Manifolds cast design.