Back cutting valves is one of the best performance mods for a head.

You can also do a 3 angle cut to the valve once this is done.

What they do is remove all the metal from the back side of the valve and thus allows more flow when the valve is opening and opened fully.

It also removes weight from the valve train with allows for faster revs and less friction. It also helps in keeping heat down in the head and intake/exhaust tracts


Do you have any idea how much CFM your are flowing now with 5mm oversized and the cams?