How Much Do Headway Performance Heads Flow?

The infamous question "How much do they flow?", has become a warning sign. Contrary to trade publications and advertising claims, a head that flows more will not necessarily make more power. Those who chase only big air flow numbers usually go home early wondering why they have an underperforming engine. While a flow bench is a useful tool in the development of racing cylinder heads, proceed with caution! A particular head that has more air flow isn't always the best selection. Large air flow itself it does not work.

 When evaluating a set of heads, air flow should not be the first concern. More important is the cross sectional area (which influences how much a port will flow - larger areas typically flow more air) as well as having an appropriate port shape for the application. If any of these are addressed incorrectly, the best flowing head on the bench will never make a competitive amount of power.

 If you are interested in making real power or being the best in the class, don't buy a cylinder head based purely on airflow numbers. Working with a professional racing head service will get you the power you are looking for.

What valve size and material should I use in my cylinder head?

Valve size is determined by the RPM that the engine is designed to operate in. Everyone knows that valves exposed to the bore equal RPM. The rule of thumb in a four valve operation is 29% to 35% of the bore in an inlet valve. Exhaust valve is 19% to 25% of the bore, depending on your piston speed.

How much of a horsepower gain does a cylinder head improve after a port job?

This question is difficult to answer. The reason why, is that this all depends on the inlet of the manifold heat transfer and inlet manifold “ram” effect. Inlet and exhaust flow friction must also be considered. Please keep in mind that 1 CFM is equivalent to 1.67 horsepower. CFM alone cannot determine horsepower. One must consider that a 4 stroke engine has 4 facets. Most people who flow test cylinder heads overlook this factor and focus on 2 of the strokes. I don’t want to over complicate the process but all strokes must be considered in order to achieve maximum performance from a cylinder head.

We learn that the additional 3 strokes can be located on the 4 facet engine. The first stroke is “Intake Pumping”. This begins after the exhaust valve is closed during overlap a few degrees after TDC. The second phase is “Intake Ramming”. This begins as the piston rounds BDC. When the intake valve begins to close, momentum causes the air fuel mixture to continue to ram past the intake valve and pour into the cylinder.

Careful attention should be applied to this process, because it is the one that will determine your volumetric efficiency. You may also exceed 100% efficiency! A typically normally aspirated race engine will have the volumetric efficiency peak near maximum torque with values between 110% and 130%.

The third stroke is “Compression”. This begins after the intake valve closes while the piston continues to move upward compressing the trapped air fuel mixture in the cylinder.

The fourth stroke is “Fuel Burning & Expansion”. This begins just after the spark plug ignites the charge as the piston continues to move forward to TDC. You must be very careful; your burn rate will change as a result of swirl or tumble. This will accelerate or decelerate the burn rate and affect how much timing you need to apply.

In a drag racing application, the idea is to eliminate any mixture motion possible. Mixture motion requires energy. In a road racing application some mixture motion needs to occur to create a broader torque band. The valve shape will have a direct influence on mixture motion.

The fifth phase is the “Exhaust Blow Down”. It begins after the exhaust valve is opened. There contained is an extremely high pressure inside the cylinder and the idea is to get rid of it as fast as you can. Remember that the first crank degree is supersonic flow. So your valve shape and valve size must be precise.

Super Tech also has a unique style and shape to its exhaust valve to help minimize the time that the port is in supersonic. Supersonic flow is the maximum flow that can travel through any orifice regardless of the amount of pressure built up behind it. Careful attention must be applied when choosing the right combination. The idea is to maintain the exhaust stroke on sonic as long as possible.

The sixth phase is “Exhaust Pumping”. It begins with BDC where the exhaust valve continues to be open fully. If you don’t have success by phase number 5, reducing the pressure inside the cylinder, the piston itself will do this for you. It takes horsepower to push out the remaining gases inside the cylinder. That is why the blow down stroke is very important.

Phase number seven is “Valve Overlap”. This begins just after the intake valve opens the exhaust valves are closing. In this process both are open at the same time. This will salvage the remaining exhaust gases and replace them with fresh air fuel mixture. It also initiates the direction of airflow to the intake.

How much CFM does my cylinder head flow?

This is a difficult question to answer, several factors must be considered that directly affect the results. Some of these facts are depression lift and valve size. Not all flow benches are created equal. This makes it difficult to make an accurate comparison.

The motorcycle industry favors testing cylinder heads at 10” of water. The early NASCAR guys favored the use of 25” to 28” of water. The pro stock favored the use of 28” to 65” of water. Formula 1 engines however demonstrate recorded tests of 28” to 100” of water. This becomes a problem because cylinder heads cannot be compared accurately unless we know how many inches of water are being used to test the head.

At Headway Performance we prefer to use the method of CFM per square inch. This is a more detailed version of measurement, and proves to be the most reliable source in determining how your cylinder head will perform.

Years ago Super Flow performed the following calculations to determine that the maximum “ideal” value for influx at 28” H2O was 146.00 CFM per square inch. This value is still published today in all Super Flow bench manuals. This factor will help determine what 100% flow at 28” of water is.

An example would be my Honda B-Series cylinder head, it flows @ .500 lift 281 CFM. This is equal to 133 CFM per square inch of the small cross section area. This would be the equivalent to saying it has .896 flow coefficient. Keep in mind that this would be equivalent to 89% efficiency (only 11% off from 100% the scale of perfection) according to Super Flow’s manual.

The best cylinder head in the racing industry, (pro-stock) would perform at 133-135 CFM per square inch in a single intake valve. For a 4 valve Formula 1 or Indy, it has been recorded to perform at 137-139 CFM per square inch. There really is not an easy way to answer this question. The answer is to be determined based on the current or desired application.

How many angles does my valve job have?

The valve job plays an important role on the performance of your cylinder head. This should not be overlooked! A well-developed valve job is time consuming and well thought out. There are several angles to mimic a radius. The width and the angles determine how well your flow will be.

A sample angle will be 30, 45, 60, and 80 degrees. This is standard in the industry. Each valve manufacturer requires its own particulars regarding valve jobs.

Super Tech has developed a valve with high standards. At Headway Performance we flow test all valves. Super Tech has an excellent management of quality control, good material, and excellent shape and design. This makes their valve shape unique as a result of research and development through the use of a flow bench and other mediums to maximize the performance of each cylinder head.

It is not uncommon to bolt on Super Tech valves on a cylinder head and see gains anywhere between 5 to 10 CFM. In some cases cylinder heads have improved more than others. For most racers this may not seem like too much, yet keep in mind that 1 CFM is equal to 1.67 horsepower (ex. 10 x 1.67 = 16.7 HP). It takes hours of work to pull 10 CFM out of the port. Super Tech offers a valve at full or 0.0003. Rule of thumb is @ 0.001 per square inch. Not bad for a valve that can be purchased off the shelf.

The rule of thumb for a valve seat profile is 30, 45, 60, and 80 degrees. This is not written in stone, but would be an excellent starting point of reference. The whole idea is to create a venturi effect. A good width for drag racing engines would be 0.040” to 0.060” at the seat. The bottom angle would be from 0.60” to 0.100”. The rule of 0.001 of the concentricity applies.

The use of small valve angles can cause valves to fail. The valve seats keep the valve from leaking compression and also dissipate the heat from the valve. The valve can over heat. In most cases customers blame this failure on the manufacturers. The fact is most of the time these situations occur when a valve job is applied incorrectly. We have not encountered any problems with Super Tech valves. They have a very wide valve seat. This helps to keep the valve cool.