HACKMUNCH said:4 valve does not make more power than 2 in most cases.
Kurlon said:A properly setup 4v will out power a 2v for the same bore. And it'll do it with better throttle response off the bottom. Problem we've got with our bikes is the one available 4v head is setup for a 52mm bore, and it doesn't even do a good job of taking advantage of that available space... If someone took the time to move the valve centers further apart... look the fsck out. : )
Oh, and ibmike, the general rule of thumb with intake valves, you take the combined area of the valve faces, and for dual intake valves, add 10%. So based on that, a pair of 19mm intake valves should flow as well as a single 28mm valve. To get that bonus, the twin valves need to have a reasonable distance between each other. (Otherwise they can actually impede their own flow.) Twin valve setups also expose more circumference when opening, giving them better initial flow than an equivalent sized single valve. They are a bit trickier setups to get right, but once you do, there is no comparison to a single intake valve setup.