ORIGINAL: combatpilot
Wow im suprrised you got that much of a gain. I didnt get anything like that in fact it didnt seem to make a difference at all. I dont think i raised the exhaust roof anywhere near the .110. It just didnt take that much to get it from 140 deg to 150 degree. I didnt do the eliptical port but stayed mainly with the original port shape. Did you remove the center bar? Its hard to tell as im having such trouble with my test equipment and am not gona make any more tests till my new tach gets here. I have some more cylinders so i should be able to do side by side performance comparisons. Thats a good idea on taking the average of three testing points. One thing that seems to be common on these engines is they lose a couple hundred rpm after running full speed a while and get hot. after cooling at idle and run back up it will produse the same results time and time over. I try to take my readings after it gets hot and the numbers stabilize.
My motor's stock exhaust port timing was 128deg. The exhaust port of the older and newer style heads are very different so it might follow that the timing is different too. The stock port had no center bar either and was one continuous open port.
I noticed too that the engine sagged a bit for sustained WOT. On my runs I did let the motor stabilize but the time from pushing the throttle up to WOT and when I took data was probably only about 20 to 30 seconds. This is not a terribly unrealistic time to take data as the amount of time WOT is used in actual flight is only about that much too.
To get head volume I filled a small medical style cup that has cc graduations, with total capacity of only 10cc, with water and pored it into the cylinder, with spark plug installed of course, until it was just at the level of the deck. Then I looked to see how much was left in the cup. A real syringe would be even better. For the deck height I put a piece of bailing wire bent into and L shape that would reach to the side of the cylinder into the spark plug hole of the assembled motor and rotated it through TDC and adjusted the thickness of the wire until it would just barely rotate through with out binding up. The volume then is the area of the cylinder bore times the deck height. The total volume at TDC is then the head volume plus the deck volume, of course.
diceco