Ken,

Efficiency is a complex matter. Gasoline engines are more than 15% efficient. Recent Honda engines are ~33% if I remember correctly. The largest diesels in cargo ships are about 50% efficient. Compression does have an effect on efficiency but there is a point of reduced return. Injected diesels need 18:1 to allow cold starting. Spark ignition gasoline engines are limited to ~13:1 for the best designs due to fuel characteristics.

http://en.wikipedia.org/wiki/Specific_fuel_consumption
A Prius BSFC according to one link I found is 225g/kWh.

I estimate BSFC of my FS-48 conversion to be 360-380g/kWh on kerosene.

Andy, if Kelly was talking about the difference in carbs due to fuel, the real question is how much air does the fuel displace? With a 10% difference in volume comparing diesel and gasoline, this is trivial compared to methanol which requires twice as much volume and mass to equal gasoline. Since the density of methanol and kerosene are close we could say it's just the fuel air ratio. Straight methanol is 6.5:1 but I'm sure we run rich at ~5.5:1 and Kerosene should be 14.5:1, but I think we run rich near 11:1. We should be able to reduce the area of our carbs by 10% without ill effect. This doesn't consider the cooling effect of fuel evaporation. Restrictors in my opinion go back to the idea of mass flow reduction associated with the reduced RPM we run diesels at. The biggest change in carburetion is jetting changes(needle setting). Because we use needles instead of fixed jets this is a no brainer, the engine won't run unless the needle is close. The main thing is that you want o keep a certain velocity thru the venturi so that there is enough pressure differential to have good fuel draw.

I have tried restrictors in my four strokes and have not come to a definite conclusion at this point. So far I have just tried them but not evaluated the science behind it. The Enya 41-4CD has a substantial restrictor.

I was thinking the same about comparing viscosities to known lubricants. The probelm is keeping the lube at engine temperature to get a meaningful comparison.