Chris Prata wrote: > so I wonder has anyone run a dyno on the various valve sizes to see power developed at aircraft RPM's?<
Dyno time costs a lot of money, and I doubt it's worth it to anybody but the dyno owner to do that test, and if he's going to buy both sets of heads, he'd likely just buy the big ones and call it done! But DesktopDyno2000 (software) says a 2180cc VW with the right cam, stock size valves, 8.73:1 compression ratio, and open headers will put out 67hp at 2500 rpm, 82hp at 3000 rpm. And with 35.5 x 40mm valves and all else equal, it will put out 68hp at 2500rpm, 84hp at 3000 rpm, and 99hp at 3500 rpm. So at typical cruise speeds of 3100 rpm or so, you might be leaving 2hp on the table, which is maybe 2.5% power (or efficiency) drop. Not big enough to worry about, considering the crack between the seats. Before anybody questions the accuracy of DesktopDyno, keep in mind that it may not necessarily be accurate, but the difference between the two setups is probably pretty close. At these RPMs, there's just not a huge difference in power. I did the same comparison on the Corvair head years ago, and the big-valve 140 hp heads showed lower power below 3600 rpm than the 110 or 95hp heads. But there's a much larger difference in the size of the 140hp valve diameters than the other two. Mark Langford ML at N56ML.com http://www.n56ml.com
