The 906 will see a benefit from installing a larger exhaust valve. It greatly improves the short side of the port coming off the valve. The Trick Flow has such a better exhaust port than the 906 it can use a smaller valve effectively. We did a dyno test on a 8:1 compression 440 where we all we did was install a larger exhaust valve in the factory iron head and got a significant increase in horse power. With 2.25/1.81 valves in 906 heads on a 451 we just missed 700HP on 91 octane pump gas by 6HP. For those math challenged the 451 made 694HP. Basically the same engine with ported 2.25/1.81 RPM heads made 787HP.
