Cylinder Head Design - CHP How It Works
"A larger valve diameter will almost always produce higher 0.100- to 0.200-inch flow figures, which can be counterproductive to making power. /// For race engines we use larger valves, but are able to reduce the 0.100- to 0.200-inch flow even further with steeper seat angles. "
Why is low flow # counterproductive to power ?
Here's the whole statement
Al Noe: "One very important characteristic of airflow is not only peak cfm, or relative velocity, but where in the lift curve those events happen. Are you looking at flow numbers at 0.100- to 0.200-, 0.300- to 0.500-, or 0.600-inch lift and beyond? Are you building a street car, or an 8,000-rpm race engine? The head has to be tailored to what it's being used on, and if this is done properly, the higher-flowing head will almost always make more power. However, we need to clarify what we mean by higher flowing. At Trick Flow, our goal is to maximize the airflow from 0.300- to 0.500-inch lift, and for typical street cars we're not concerned with flow beyond 0.700-inch lift. That's because the majority of hot rodders use cams that peak in the 0.500- to 0.650-inch range. We also develop airflow with a different flow bench from most, and we feel this makes a difference as well.
"The next thing you must consider is the valve diameter. A larger valve diameter will almost always produce higher 0.100- to 0.200-inch flow figures, which can be counterproductive to making power. Larger valves also tend to be more shrouded and have trouble with the all-important mid-lift airflow from 0.300- to 0.500-inch, but then these larger valves tend to shine at the highest lift points simply due to shear volume. For race engines we use larger valves, but are able to reduce the 0.100- to 0.200-inch flow even further with steeper seat angles. We then try to maximize airflow from 0.400- to 0.600-inch. As rpm increase to 10,000 the intake and exhaust port shapes become very critical for power production. Lower-rpm engines don't seem to be as sensitive to the port shape as higher-rpm engines. Our customers are generally looking to make peak power by 8,000 rpm or less, so the importance of flow numbers are still very relevant in their search for power.
"The velocity around the circumference of the valve is actually more important than the velocity in the port. We feel having equal localized velocities around the circumference of the valve is far more important than having equal velocities in the port, and as these velocities are equalized at the important lift points, the port will flow more air at these same lift points as well. We can measure the velocity of the air around the circumference of the valve every 45 degrees, and having these velocities equalized during the all-important mid-lift flow area yields max airflow and best power.
"The better measure of head efficiency is computing the coefficient of discharge. This is simply the airflow at each lift point divided by the valve curtain area, which is valve circumference multiplied by lift. Let's say we have two heads that both flow 300 cfm at 0.400-inch lift. One has a 2.200-inch valve and the other has a 2.100 valve. If you multiply each valve by Pi to get the circumference, you can then multiply that figure by the lift to obtain 2.76- and 2.63-inches, respectfully. Now divide the 300 cfm by each valve circumference to obtain 108.7 and 114.0 cfm/inch, respectfully. I know that is an odd unit of measure, but it is the correct terminology. In this example, you can see the smaller valve clearly has more velocity, will be easier to cam, and will generally run faster. This is the best way for the average consumer to compare two heads to one another."
