We decided to flow the intake and exhaust runners of several popular factory cylinder heads and then port an intake and exhaust runner to compare the difference. We’ll be performing our testing on the Superflow flow bench at Auto Performance Engines, so the numbers you see are real-world flow numbers for popular Mopar big-block cylinder heads. For comparison, we’ll also flow two popular aftermarket heads, the 440 Source Stealth and Edelbrock Performer RPM, then port the intake and exhaust runners of the Edelbrock head and measure the difference on the flow bench.
Remember that we’re not out to gain every last cfm of airflow through these heads, we simply want to show what spending a little time on the ports will produce in terms of flow numbers. To keep our testing legitimate we will only spend one hour porting and polishing each runner, which will only allow us to hit the above mentioned beneficial areas and clean up the casting flashing of the runners, bowls, and areas under the valve seat. We also stayed with the factory valve sizes for both the factory and aftermarket cylinder heads as increasing valve size and performing trick, multi-angle valve jobs is a whole other area in terms of cylinder head work.
While it is tempting to get crazy with the grinder while porting heads, removing too much material from the ports not only risks damaging the heads, but significantly increases the volume of the runner which is not so good for low and mid-range torque. So rather than whittling away a considerable portion of the runners for maximum high-end flow, we’re removing only a reasonable amount of material, and cleaning up key areas that should improve power throughout the normal rpm range of a street engine.