No easy answer here. In some cases, we can give a honest approximation where we have done a similar job, however, even in those situations, dyno results vary due to test procedures, dyno cells, and different correction factors. At the very best, it’s an approximation.
It has much to do with the cubic inch of the engine, and it seems that larger cubic inch engines pick up more. Increasing cubic inches should increase your gain in percentages, and fewer cubes would be less. On 500 C.I., 9000 rpm Pro Stock engines, we have seen 8%. Assuming that you have everything else optimized, I don’t believe one can quote hard exact numbers due to varying test procedures and the infinite amount of engine combinations.
If they were originally ported by CFE, the answer is no. Otherwise, we prefer that the manifold be built on your heads directly, or at the very least allow us to accurately measure port locations for an exact match of a model to build your manifold on. We take pride in the fact that no additional labor is needed on our manifolds in order to run them short of gluing on a manifold gasket. Having the actual heads in our possession eliminates error and reduces cost.
After nearly three decades of working on racing cylinder heads, 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 airflow numbers usually go home early, wondering why they have an under-performing 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 airflow isn’t always the best selection. Large airflow itself does not work!
When evaluating a set of heads, airflow 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 the appropriate port shape for the application. If any of these are addressed incorrectly, the best flowing head on the bench will never make competitive power.
If you’re interested in making real power or being the best in 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’re looking for.