Cam duration for slow running four cycle model engine

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Well Brian I think the problem might be that cams are not designed for "low rpm". They are designed to give high end or low end performance.

Comp cams had at one time, a catalog online with everything they made at the time. I'll send it to you if you can't find it.

Look for cams that give the best low end torque like rv's, tow trucks, and busses. Also look at other factors like flywheel, rotating mass has an effect. Also remember that cam specs are called out at .050 lift. Duration is from .050 open to .050 from closed. Actual duration from zero to zero is more. Most model engine builders put 220 -280 from zero to zero. They are likely way under what is used in full size automobile performance engines. That's why most cam lobes look like they have a sharp point.

I'm not going to try to steer you in any one direction but if you consider what I have typed and download the catalog I think you can make a pretty good decision.
The July/August 2007 issue of Model Engine Builder magazine has an article on valve timing by Dave Bowes ( inventor of the EVIC engine). “A Few Thoughts on Valve Timing”. it is an excellent break down on the operation of valves a cam’s for model engines and it has diagrams too!