Hello,

  I saw Bob Zambelli at Brodak's a few weeks ago and I mentioned that I have an OS .40 and an OS Max-H (both converted RC engines) that needed "tamed" down some.  Rather than add several head gaskets to reduce compression, Bob suggested removing some metal from the dome (hemi) in the head.  He mentioned that he had made a jig for that purpose to use on his lathe and kindly offered to do that for me if I would send the heads to him.  I did so and just got them back yesterday.

As usual, Bob did a stellar job and although I haven't flown the engines as yet, I did try them on my test stand.  Both engines seemed much happier at a lower rpm than before the modification and demonstrated a clean 2/4 break (which they wouldn't do before) when I tilted the stand/engine up.  I'm anxious to mount the engines on planes and see how they do.  I think it will be a big improvement.

My thanks to Bob. 

Joe

Some engines and some props on some planes benefit from lowering the compression.  Increasing the combustion chamber size or even making it a double bubble style can help if you want to swing a larger prop.  Lower compression helps match the ignition to a lower rpm.

On the other hand, I went the route from stock to huge back to stock on the LA 40.  The best run ended up with the stock head, opening up the hole on the stock muffler to the max, and adjusting the venturi to get 10,800 on a 3.5 in. pitch 3 bladed prop, cut down to 9.625in.