Could differences between the two heads' porting be contributing to the difference? Ideally, you'd need to show the flow before/after seat cutting differences on the same port I'd guess.
Well that would be a perfect way to do it but actually the low lift flow is not going to be affected much by small differences that might exist between two rh10 heads so I think the results are pretty accurate.
But I am installing new seats in the head with the 15 degree top cut so I can do a comparison here later this week.
Comnoz addressed the question Tornado posed above, I agree with his response, and look forward to seeing his further airflow testing later this week. I happen to have some data congruent with Comnoz’ response that I thought readers might find interesting, where a large port and small port head were flowed over a range of valve lift. In this case the heads happen to be air-cooled vintage Porsche heads, but the same airflow trend would result with any similar head set.
Airflow tests on a flowbench are essentially controlled by valve lift, i.e., the further valve is opened the higher the airflow, up to a point. Valve lift is often specified in terms of the ratio of valve lift to valve dia, or the so-called L/D ratio, where lift is commonly increased in increments of 5%, i.e., 5, 10, 15% of valve dia, etc. Measuring airflow at such L/D ratios affords a means of easily comparing the discharge coefficient of various sized valves. It is generally accepted that airflow at the first 3 lift points (5, 10, 15%) is primarily controlled by the valve and seat, since airflow is restricted and relatively low velocity in the port at these lift points and the port is far from being the primary factor limiting airflow.
Below are images of the 2 port castings, where the small 34 mm port is shown on the left and the 41 mm port is shown on the right. Both heads have a 49 mm IN valve, identical throat size and valve job, and were fit identically (throttle body and velocity stack) for flow testing. Valve lift was increased in L/D increments of 5% as described above. The airflow results are presented graphically below and show that in spite of the heads having dramatically different sized ports, airflow at the first 3 lift points is identical, as one would expect in light of the above discussion, and thereafter as valve lift increases further and port velocity increases, the larger port head begins outflowing the smaller port head.
