? for randy hall + others about compresion+fuel

From recollection, a M has a C/R in the range of 6:1 to 7:1. The 85 octane at the pump is optimum for the 8:1 cars running around today. Actually, an M will want something around 70 octane, which you could get by mixing white gas or Coleman fuel in the tank.

The downside of it is that max performance makes more heat in the combustion chamber than pump gas. The M is around 50+ years old, and probably has straight cast heads, no hardened exh. valve seats. So more heat causes more valve reversion.

All in all, I'd use normal pump gas for convenience and add some two stroke oil, run it a bit rich, or some guys add ATF fluid; instead of adding expensive octane booster (when you really don't need more octane -- you're just trying to keep the exhaust valves cool).

Also, if I remember correctly from chemistry classes and flight school -- the heat ducting shields help prevent the heavier kerosene molecule (with 10 to 12 carbon atoms) from falling out of suspension; as it's heavier than the octane gas molecule with 8 carbon atoms. It was all academic in chemistry, but in flight school the benefits carb heat and the inverse of augering in, or "bought the farm" were heavily emphasized. You know.. the old pilots and the bold pilots, but rarely do you find an old, bold pilot.

In aircraft, the purpose of carb heating is to prevent ice from forming within the carburetor- carb heat is used mainly in conditions within a certain range of humidity and tempetures to prevent ice, and lower efficiency results, which is why you don't really use it at altitude, just when you're coming up and down in most (not all) circumstances. Damn, that aviation ground school I took all those years ago finally got put to use. Ice can litteraly choke off the carburetor (it can also happen in cars and tractors, but is rarer).

The use of carb heating in a kerosene/distillate engine is only part of the story. Radiator shutters are used as well to bring the engine tempeture up quickly to promote clean, efficient burning, and to prevent crankcase oil dilution. The whole purpose is to raise the tempeture of the fuel/air misture to vaporize/evaporate the fuel, which takes place at a much higher tempeture than gasolene- believe the measurement is vapor pressure, rather than octane.

Carb heat is used at altitude. Promise you! You can fall out of the sky at 10,000 ft as easily as 500 ft on final. Been there, done that (65F ambient, but with high humidity - got carb ice - nearly bought the farm). The trick with icing is the 20# between temp and dew point. If fuel or liquids like water falls out of suspension with air in a carb or manifold, it creates ice when temp and dew point are <=20 of one another. If it falls out of suspension at higher temps or lower dew points, you still have a fuel separated from the air and it simply makes lousy combustion at high temp/ low humidity instead of ice at low temp / high humidity. Same coin, just two different sides of it -- liquid or solid. What you want from a carb is a mist - air and fuel mixed -- not air and a buncha dribbles.

They call it the Joule-Thomson effect - which basically means that temp drops when fuel meets air through a venturi. Sure, cold air is more dense, but in relation to what? Heavier fuel (kerosene) still drops out of suspension in cold or hot air faster than lighter fuel -- gasoline --if all other things are equal. Therefore, the need to reverse that cooling effect is greater with kerosene than it is with gas. Carb heat - manifold heat - is simply a way to counter the Joule-Thomson effect.

Common sensical proof of all the above is that I bet you can find a lotta guys here who have seen frost form on their tractor carbs (gas or kero) on a dew laden summer morn. It's not rare.

They say the Continental engine does worse than a Lycoming in that regard. Don't know; but as to all the other who-ha, see the above diatribe.