MacGyvering my Zuma

June 20, 2010
I have been analyzing my stock zuma cylinder and came to some interesting conclusions.

I have measured the port time-areas as outlined in Jennings. My exhaust port is pretty close to stock at the piston and is about 400mm square. When I calculated the port time-area as he outlined it 40 years ago, I came up with enough exhaust breathing for 10,400-11,800 rpm.

Of course the real problem with the exhaust port is that the port is very retrograde. Instead of gently widening say 10% from the piston face it got dramatically narrower. It really hoses down the power and makes a pipe rather ineffective.

I have also checked the transfer port time area and it seems pretty good for the rpm noted above. At this point I don't plan on widening the transfers but am using my riffler files to smooth them.

I plan on widening the exhaust port further at the junction of the head pipe. Stock is something like 14 mm high. Raising the top of the port really puts the angle askew to a stock head pipe. I have opened the top and depending on how you measure it I am now about 21 high and 24 wide. I plan on making a stub exhaust flange and centering it at the port about 24 mm wide. The angle that the port opens to the face is quite an angle now so I'll have to compensate for it.

The stock head pipe is about 21 mm ID at the flange and gets smaller at the first bend. It does gradually widen again.

I figure that the stock piston should tolerate the 11000 rpm and maybe higher, if a 47mm bore engine can tolerate 10,000 rpm.

I'll report back once I get the engine back together again. Much to learn.