TD .09 with MVVS throttle and custom made muffler

On 25% Omega fuel and a Cox 7 X 3.5, this throttled TD .09 does 4K up to 14K with good acceleration.

For the longest time, conventional wisdom had it that small engines run too cool based on the percentage of fin area to displacement as compared to larger engines, as in .40s and 60s. As Cox engines like it that way and as Norvels have massive cooling fins and they run very cool in comparison and do just fine, this is a case where figures can lie.

No doubt the math is correct but I always had a feeling that air cooled engines, while they CAN run hot and this can hurt performance, running cool, too cool or cooler does no harm and can even help.
Note the engine sagging a bit till the ice cube is applied. As the ice cube makes it's way down to the cylinder fins, the rpms pick up and the engine sounds smoother. When the ice is taken away, the engine starts to sag again.

Yes, a muffler causes an engine to run hotter but if this engine ran too cool in the first place, the muffler supposedly should have helped.

It's been argued that I have a loose cylinder to piston fit and the extra cooling on the cylinder tightens this up giving more/smoother power. In fact, the opposite is the case here. The cylinder was obtained from Cox with the ports cut but without final honing to size. A homemade lap was used to create a very tight, tapered fit such that it mimics what you find with an ABC engine. As the engine wears in, power does increase as expected. This was common in the old days of getting a tight fit that you broke in to a just right, perfect running fit.

Yes, it's actually running kinda crappy but this one is still a work in progress. Will try lapping the piston/cylinder in place using coarse rubbing compound to take out some of the tightness. The idea is to sneak up on that perfect running fit.

My conclusion is that small engines are generally designed correctly and DO NOT run too cool. As evidenced by Cox's RC .05 and the RC .09 and my own version here, adding a muffler makes them run hot. Without the muffler, they're just about right.

No doubt that's why Norvels run great even with those massive cooling fins.

BTW, considerable testing in the air with a variety of props has proven to me that Cox props are THE most efficient props ever designed. None can match and only one comes close. Wish they or someone would come up with the sizes we need all the way up to .90s and beyond.

Duration : 0:0:48