Porting the heads on bikes and quads has become a huge and competitive market over the last several years. It is important, as a consumer, to know how to choose the shop to port your head. We have been made aware of several local shops who are charging top dollar to port quad heads and they do not have the equipment to machine valves and seats nor do they own a flow bench. Any person who understands how to manipulate horse power from a cylinder head knows that the first step is a proper valve job.
( For more info on valve jobs, click here.)
Also, anyone who as had any experience with a flow bench knows that every head is different, and without testing, there's no way of knowing whether your porting helped or hurt the head (other than on the dyno or the track when it's too late to change it).
The more air that can go through the head, the more horse power the engine will make. The flow bench is what measures this air flow. There is a formula published by Superflow to calculate the amount of horse power that a given head is theoretically capable of producing. The formula is: H.P. = cfm @ 10" x .43 x # of cylinders. On the multi-valve single cylinder heads such as the Honda 400EX, XR650R, and the Yamaha Raptor we've been seeing gains of around 20 cfm @ 28". This converts to 12 cfm @ 10" which multiplies out to a theoretical 5.1 H.P. increase. Based on the feed back we get from our customers, we feel this is relatively accurate.

32mm Manley oversized intake valves
for the Yamaha Raptor!