Nov. - Dec. 1997 Lane Racing And Rodding Article By Jim Kaekel, Jr.
Cylinder heads are the key to a top performing engine. That's not exactly an earth-shattering statement, but without a good flowing set of cylinder heads, an otherwise top-of-the-line short block may not even make enough horsepower to drive over a speed bump. The finest set of forged pistons, top quality piston rings, the best camshaft for the application, and the most precise machine work are all for naught unless an efficient, good flowing set of heads is installed.
Efficient delivery of the air/fuel mixture and expulsion of the exhaust gases must take place before the engine can make substantial amounts of horsepower. Because air does not like to maneuver around obstructions or turn corners, the straighter an intake or exhaust port is, the better the air flow will be, and therefore, the larger and straighter the ports must be. It is possible to go too far and too large a port will hinder power, particularly at low and mid range RPM. Just as a camshaft is matched to a specific application and RPM, cylinder heads must be matched, through proper port and valve size, to the engine they will rest upon. These are all important considerations when selecting cylinder heads.
Three characteristics are used to evaluate the efficiency of a cylinder head port; volume, velocity and flow. Volume and velocity work together, and how well they work together is called "volumetric efficiency". To determine port volume, a plexiglass plate with a hole drilled in it is placed on the gasket flange (after the area around the port is coated with grease to seal the port). Fluid is then injected into the port via a buret calibrated in cubic centimeters. "CC" kits (Tavia #TAV08720), which include a calibrated buret, buret stand and a plexiglass plate, are readily available and can also be used later on for checking combustion chamber volume. Be sure that the heads are very clean prior to measuring volume, as build-up carbon and dirt deposits can vastly alter readings. NHRA Stock and Super Stock racers have relied on checking port volumes for years when sorting out pairs of factory heads with identical casting numbers and very different port volumes.
A specific velocity, or speed, is required for a maximum air and fuel charge to reach the cylinder. Port velocity relies on the same principle as header tube velocity, and has a dramatic effect on flow. The velocity of the mixture will be greater when traveling through a small port, however, because of it's size, a small port cannot carry the great volume of mixture needed to fulfill the engine's requirements at high RPM levels. Large ports will flow greater volume, but the larger size cuts down the velocity, hurting the low and mid range RPM torque levels.
The cylinder heads must be carefully selected with the engine displacement and desired RPM range in mind. Heads with intake port volumes of around 180 cc's are ideal for all around street driven vehicles. Port volumes between 180 and 195 cc's are well suited to dual purpose street/strip vehicles and offer the best compromise between maximum torque and peak RPM horsepower. Volumes of 200 cc's and larger are for race only engines which sacrifice low and mid range power to gain substantial top end horsepower.
Today's cylinder head technology allows flow to be precisely measured on a flow bench. Once the head is bolted to the flow bench, using an adapter which connects to the selected cylinder's combustion chamber, individual ports can be flowed one at a time. Flow is measured in CFM (cubic feet per minute) and once a baseline is achieved, the runners can be ported in desired areas and the head returned to the flow bench to verify the results. Experienced head porters will focus on a couple of areas; the valve bowl, located directly beneath the valve seats, and the port section, referred to as the "short side radius". The valve bowl area is often plagued with sharp edges, left behind from factory tooling, which are very disruptive to air flow and should be smoothed and blended. The "short side radius," where the port makes a turn from the valve bowl to the floor of the port is often restricted because of the tight turn the mixture must make in order to enter (intake) or exit (exhaust() the port.
Experienced head porters may study a port by using it as a mold and making a latex mold. A port is selected and liquid latex rubber is poured into the port, after it has been thoroughly sprayed with lubricant to act as a release agent. When the latex has cured, the port mold is carefully removed. The inside of the port is exactly duplicated and its strengths and weaknesses can be determined and then modified. Other than minor port matching of intake ports, where the intake ports are matched to intake gaskets, the serious porting work should be farmed out to an experienced professional, because the risk of ruining a rare set of heads by grinding through a water jacket or head bolt hole is too great. Keep in mind, when having exhaust ports modified, that they should not be port matched to the header gaskets because the sharp edge of the exhaust port provides a reversionary effect which prevents exhaust gases from re-entering the port and combustion chamber. By carefully selecting a set of cylinder heads for your powerplant, and then getting them correctly prepared, you will be able to "flow" your opponents' doors off!