Performance Cylinder Heads

If anyone said that ported cylinder heads can't make much of a difference in horse power they were either lying so you wouldn't have the upper edge or they were just plain ignorant. Cylinder heads are the most critical component for making horsepower (other than forms of forced induction, but ported and modified cylinder heads aid in making even more power) on a naturally aspirated (all motor) engine. Engines are bought, sold, and built by the amount of air/ fuel they can consume. The more air/ fuel in the combustion chamber, the bigger the bang (explosion or pressure of combustion). We want the biggest bang in each cylinder that we can possibly have, but there are many components of this big bang theory for ported and modified cylinder heads that are typically a mystery for most. In this article I will describe the processes and terminology used in porting and modifying cylinder heads to increase an engines volumetric efficiency (horse power making potential), we will be reviewing such terms as: Port matching, 3 and 5 angle valve jobs, full radius valve job, un-shrouding, porting, CC-ing ,and flow bench numbers.

**Port matching-- Is an operation done to a cylinder head that includes porting (grinding) material away from the inlets of the intake and exhaust ports. This is done by using machinist's layout dye (typically blue or red in color) by coloring the intake and exhaust ports that are to be ported. Then the intake or exhaust manifold gasket is laid in place over the surface of the cylinder head on the studs and a scribe is used to score the aluminum or iron making an outline of the shape of the gasket on the flat surfaces of the intake and exhaust cylinder head flanges; the gasket is removed and a trace outline of it is left behind on the cylinder head. A porting tool is then used with either an aluminum or steel cutting burr to cut away excess material so that when the gasket is placed back on the cylinder head there is a smooth transition from the gasket to the inlet or outlet of the port, this same procedure is also done to the intake manifold where it meets the head; however in most cases this procedure of port matching is not done on the exhaust side of the cylinder head, although the opening of the port may be slightly enlarged depending on the application it is not necessary to match it and many people that port cylinder heads prefer to leave this surface "stepped" for an exhaust scavenging effect.

**3 and 5 angle valve jobs--This is a procedure done to the inside of the combustion chamber on a cylinder head where the valve sits against the inside surface of the combustion chamber known as a valve seat. A machine is used to cut different angles into the seat that are not typically there from the factory. The degrees of cuts go in 15 degree increments beginning at 15 and go up to 75 degrees as in a 5 angle valve job (15, 30, 45, 60, 75 degrees). The reason for these cuts is to remove material from the valve seat and make a better transition to the bowl (an area behind the valve seat in the cylinder heads intake or exhaust port), less material and a better transition mean more air will flow into the cylinder heads combustion chamber.

**Full radius valve job--This is a valve job that is done after a 5 angle valve seat cut, the radius's are smoothed out by hand in order to remove slightly more material, but more importantly making a smooth transition for air flow to enter the cylinder heads combustion chamber. The smoother the transition, the faster the air will move. We want as much cfm (cubic feet per minute) moving into the cylinder heads combustion chamber as possible.

**Un-shrouding--Or "bowl blending" , this porting job is done by removing material in the intake and exhaust port directly above the valve seat and the surrounding areas that can also extend to the valve guide in some instances. Material is removed from this area ( sometimes including the lower portion of the valve guide itself) with a porting tool to ensure that the air coming through the port does not immediately crash into a flat wall, if the air crashes into a flat surface behind the valve before it passes the valve seat it will be considerably slowed down, the less air velocity in a cylinder head will mean we will have a greater loss in power. Air flow speed is everything in a performance cylinder head.

**Porting and polishing-- This is typically a term used in reference to reshaping the intake and exhaust ports on a cylinder head, such as we spoke about earlier in the un shrouding process (which is where most of your flow comes from), this porting (or re-shaping) can include almost anything involved with port shape to increase air flow speeds. In 4 and 5 valve per cylinder engines there is a dividing wall between the 2 or 3 intake ports, this dividing wall is like a fork in the road and it is usually cast widely from the factory. A cylinder head porter will knife edge these "forks in the road" in order to ensure that air coming into the port does not crash into them and create turbulence. Casting marks and all major rough transitions and surfaces inside the ports of the cylinder head are smoothed out to restrict turbulence in order to increase velocities. Polishing is an old school idea, thinking that a mirror like finish on the ports was important for velocity and flow; however, more recently it has been discovered that a rougher finish on the insides of the cylinder head port walls creates a boundary layer in which the velocities of the incoming air and fuel are sped up. Polishing is however a good idea to do on the inside of a cylinder head's combustion chamber as it will prevent an engine from detonating. It does this by ensuring that there are no burs of metal that are exposed in the chamber that could potentially glow red after combustion and be a cause of pre-ignition or detonation.

**CC-ing is a process where a liquid (normally water) is poured into a combustion chamber of a cylinder head while the valves are in to measure the cubic centimeters of volume the chamber has, this can be very important in measuring compression ratio's after any welding or reshaping is done to the cylinder heads chamber or milling is performed (milling is when the deck or the surface that sit on the block is shaved in a rotary broach type machine removing a few thousands of inches at a time creating either a flatter deck surface (in the case of a warped head) or a smaller CC combustion chamber for a performance oriented cylinder head.

Flow benches are fairly elaborate test platforms that can measure the overall amount of airflow through a head's intake or exhaust ports. Basically, flow benches measure the total amount of cubic-feet-per-minute of air moving through the ports, when a specific amount of vacuum is generated, usually in 28 inches of water. These measurements are taken with the valves in the cylinder head normally after porting and valve seat work on the cylinder head. The valve is normally put in a fixed position at .500 or .550 inches of lift or a lift that correlates to the actual distance the camshaft will push it into the chamber for a more accurate initial estimate of horse power on race motors.

Getting modifications done to your cylinder head can be quite costly and take a lot of down time because the porting and machine work can be extremely time consuming. So, if you are planning on getting port work done by a professional make sure to explain exactly what your intentions are for the motor and the kind of power you would like to make and let the professional tell you what they think needs to be done to your cylinder head instead of what you think needs to be done. This way you actually get the performance that you're looking for.