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Port to port airflow variation

By John Cailey
Posted: 03/26/2003

There are many articles written about head porting with resulting improved airflow numbers, but never have I seen an article relating to port-to-port airflow variation and how to improve it. Recently I did some porting work on a pair of small block”915” casting Chrysler heads (including installing larger intake valves – from 1.88” to 2.02”). This is the assembly line technique I used to improve an out of the box air flow variation of 3.6% from port to port down to 1.9 %.

I started by fabricating two sets of head stands (one pair for each head). The stands were basically flat plates that bolted to existing holes on the ends of the heads. I made the plates tall enough so that I could rotate the heads infinitely through 360 degrees (like a spit) while they sat on my workbench. After deciding which ports I was going to work on (intakes or exhausts), I divided each port in to two main sections (the bowl section - from the valve seat to the valve guide and the port section – from the guide to the intake manifold mating surface. I than took these two main sections and subdivided them in to four sections (floor, ceiling, Long, or push rod wall, and short or other wall). I now had a port with eight distinct sections to work on.

I lined both heads up side-by-side on my workbench and angled them equally for the particular section of the port that I wanted to start grinding on. I started with the floor section of the bowl area, not spending more than 10 minutes on this area before I moved on to the same area on the adjacent port, trying to duplicate the area I had just done. I did this for all eight intake ports. I than went back to the first port I started with and started working on the area next to the one I had just done, again going through all eight ports. I did the entire porting job in this manor.

The obvious key is repeatability of your work. By working on small sections and not spending too much time on any one area (even if you had to go back over that spot multiple times to get it right) I was able to replicate my work substantially better than the factory could, and this included some major metal removal on the intake ports as I was installing larger valves. Since I had flowed these heads prior to doing any work on them, I had a good base line for comparison. Beside to making some major improvements in airflow, I was surprised to see that I had cut the port-to-port airflow variation in half, which I attribute to the assembly line technique I used.