Heads make or break an engine. A head must pass the air/fuel mixture into the cylinders, accommodate spark plugs and reciprocating valve parts, contain combustion, pass the exhaust out of the cylinders and transfer heat away from the engine (and most people want them to look good too)........no small task. During the last few decades, more work has been focused on head development than any other aspect of the Harley engine. Why? Because that's where the greatest power limitations and potential exist. A reality, and limitation, of the V-Twin engine is that maximum horsepower is directly dependent upon the volume of air that flows through the engine (excluding the use of oxygen-bearing nitromethane and/or nitrous oxide). This fact means that, a 883cc engine that can flow a maximum of "X" amount of air, can theoretically make as much peak horsepower as a 1340cc engine that flows the same amount of air. These two hypothetical engines would have very different running characteristics, but they would share the same approximate power potential. Some people resist this concept but it is none-the-less true. Swift sleds have efficient heads. The main flow impediment to Evolution heads is valve "shrouding". When an Evo valve is lifted from it's seat into the combustion chamber, part of the area around the valve opening is restricted, or shrouded, by head material that interferes with air/fuel flow. This is because the combustion chamber shape is not large enough to unshroud the valves and the result is intake flow turbulence. Paradoxically, air/fuel turbulence inside the combustion chamber is desirable and promotes a good burn due to better vaporization of the fuel. Head-porting to the rescue.....! Porting is the reshaping of the intake and exhaust passages for improved flow. The art of porting is a skill possessed by few and sought by many. The success or failure of porting work is usually measured on a flow bench and flow results are expressed in C.F.M. (cubic feet per minute). Porting is not for the novice engine builder. Taking a die grinder in hand and arbitrarily enlarging the intake and exhaust ports rarely produces the desired power gains! Modifying a port for improved flow is a delicate balance of flow velocity and volume, the two traits being mutually opposed. The flow velocity must be as high as possible for responsive torque at low-to-mid rpm while the flow volume must be sufficient to supply the engine with all of the air/fuel mixture it needs to attain maximum high-rpm power. Attaining adequate laminar flow volume while maintaining high flow velocity across the rpm spectrum is the trick that results in a proper flow curve and broad power band. The issue of port polishing often stirs considerable debate among bikers. One school of thought maintains that fully polished ports present less resistance to the flow of air/fuel mixture and exhaust gasses. Another school of thought favors polishing the intake ports only to a satin finish and fully polishing the exhaust ports to a mirror finish. A side benefit of mirror-smooth exhaust ports is greater resistance to carbon deposits. The latter of the two, satin intake and polished exhaust, is preferred by many professionals. In reality, port contour is more important to flow than mirror finishes. The wizards of porting - Dave Mackie, Jerry Branch, Dan Fitzmaurice, Carl Morrow and others, offer ported heads that are profound improvements over mass-produced factory heads, and there are new, redesigned heads available from the likes of S&S, STD, Hemi-Designs, Pro-One, Patrick Racing, Edelbrock, etc. (sorry, I can't list everybody in the head business). Before you spend your money, just remember that even these advanced heads won't perform to their potential if they are not coupled with compatible cams, pistons, carburetors and other engine components. Each of these brands have serious investments in research and they know what works with their products and what doesn't. Talk to them and compare. Randomly mixing unproven combinations of engine parts universally causes unpredictable effects unless you really know what you're doing.