Hi Dave,

I've read some of your other posts, you've played with a few engines! Your numbers do not make sense though, do you mean your blower is 132 degrees duration...? 66 deg BTDC and 130 total duration do not add up.

As standard my engine had 122 degrees, but the key numbers are the port time area and the port angle area, both of which were woefully low, in the end its a mass production 6000rpm trimmer engine not intended for turning a 16" prop at 9000rpm[:-]

To get it to those revs it needs a load more transfer, a bit more exhuast and a mid amount more inlet. Sorry for the SI units, but as standard the engine runs a PTA of 0.000063 sec-cm^2/cm^3 but given Jennings data for 8000rpm it wants to be 0.00014 to 0.00016 IE its half what it should be. The port angle area as standard is 3.01 and it should be 6.7 to 7.7, again half. Well I cut the port as high as I could without it exposing the inlet to the cylinder over the top of the piston at BDC, but that only gave 126 deg duration, PTA of 0.0001 and PAA of 4.5 so the next step was to cut the piston skirt to increase the figures, so now it runs 156 deg duration, 0.000138 PTA and 6.6 PAA. Its on the low side still, next step will be to widen the port to get the figures up a bit more but seeing as a reed valve motor runs 180 deg inlet timing the size of notch in the piston is not so critical, the thing that is important is the port is open long enough to let the engine take as much as it wants to.

Of course there is more than one way to skin a cat, or even tune an engine, this was just my take on this engine and I'll probably port the next one a little different, ie more area and less duration. The disadvantage of large area ports is obviously the lower air velocity which in itself can cause problems with fuel falling out of suspension etc etc.

Cheers,
Rich