At the last annual, we put REM38E spark plugs in my 140 (O-320-D2A). These replaced REM40E spark plugs. I wanted to try the cooler plugs to see if there was any difference in CHTs, especially in climb. Can't notice any difference there...however, I have noticed a definite difference (lower) top end speed (about 5 knots) at altitude. Obviously, lower top end also means lower RPMS at full throttle. The plug changeout was the only change made during the annual. Could the cooler spark plugs be the culprit?
That would be hard to explain, as far as the lower performance. The absolute only difference in the two plugs is the heat range which is simply how far up the electrode is. The only thing you should get is an easier fouling of the plugs. See, your engine is a low compression 7:1 and needs the tip out more to stay hotter and thus cleaner. Higher compression engines can run at hotter temps, cylinder pressures, etc., and need a cooler plug which actually runs at the same temp under those conditions as your engine would with the 40s. I'd suggest using the correct plugs for your engine. The spark plug is not going to affect CHT any. It is just that the correct heat range plug was designed for the conditions your engine produces. A too cool plug will foul, a too hot plug will erode quickly as a result of running too hot. ...and when I say too hot or cold, I do not mean CHT, but merely the actual tip and insulator of your plug. Proper leaning and good baffles and baffle seals will reduce CHTs... :)
Lycoming approves the REM38E and well as the REM40E for the O-320 series. Additionally, you can use the REM37BY. So Gary is using the correct plug for his engine.
That said, I really don't see how changing the plug could have affected you top speed.
