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Engine basics => Engine set up tips => Topic started by: Joe Messinger on March 27, 2007, 09:37:31 AM
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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
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H^^ I have an OS 40 from back inthe early 70's that we hemi shaped the head and it run's in tnat familiar 4-2-4 mode. I haven't run it in a while but I'd bet it'll still run right. The fuel that I use is Powermaster 5-10% with 22% oil half & half and it works good here in South Louisiana..
Good luck with yours, Bootlegger
South Louisiana, where "Spice" is a food group.....
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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.
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As mentioned above, I'm sure just one modification by it'self won't change the engine run dramatically unless you go overboard with whatever you change. Doing that can be a disaster.
I don't know how much Bob opened up the heads on my engines but I got to fly one engine (1970's OS Max-H) today. The results of Bob's modification were noticeable and desireable. Good 2/4 break, plenty of power and a smooth run. The engine wanted to run at a higher rpm and was inconsistant before the chamber work. I plan to try out the OS FP that Bob worked on this week end. Hopefully the results will be good also.
Joe