Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

For some reason, my M90 seems to hit harder with more impulse torque with exhaust valve closing early about 25 degs Btdc with 0.012" lash on Coleman + 10% MMO fuel mix.

I am not stating fact here as certain knowledge.... just possible theory.
In the late 1970's, car engines were set up to recirculate some of the exhaust gases back through the intake along with fuel and air. This was supposed to assist with cleaner emissions. I am not certain I followed the logic but I do know that it was supposedly causing the engine to act as if it had somewhat higher compression by having exhaust gas take up some of the combustion chamber space. I wonder if closing the exhaust valve early leaves enough already inert gas in your chamber that there isn't enough room for a full charge of fuel/air mixture. It also might let it run a little hotter and these do run too cool to get the most complete combustion.