Ah sorry, I didn't see the part about using them as jump planes. Not sure about that, the ones I've seen are usually turbines. Plenty of twin-pistons used for transport though.
I should have said "high-compression or turbo-charged". The C182 after serial number 65176 uses the O470-U high-compression (8.6:1) engine which does not have an auto-fuel certification. C182s prior to that number use the less efficient, lower compression (7.0:1) O470-R engine, which can run physically unmodified on ethanol-free auto fuel.
(Note that what aviation considers high-compression is laughably low-compression in modern automobiles.)
IIRC all else being equal, larger cylinders tend to need lower compression. I don't remember why, maybe larger random fluctuations in air-fuel mixture making knocking statistically more likely with bigger cylinders?
(For airplanes vs. car engines, probably other effects are more significant, like higher head temps, poorer mixture due to carburetors, fixed ignition and whatnot.)
The limits on cylinder size at a given compression ratio are dictated by the engine speed, and spread of the flame front in the cylinder. You need the fuel to burn relatively quickly in a gasoline engine, and the fire only spreads at a certain speed (determined by compression ratio and turbulence in the cylinder). If you increase the compression, you decrease the speed of the flame front in the cylinder, and eventually cause incomplete or inefficient combustion.
Note, for folks familiar with auto engines (that are overwhelmingly one spark plug per cylinder), avgas piston engines are overwhelmingly two plugs per cylinder, so there are two flame fronts to cover the wildly larger radius pistons.
An IO-550 is a very common 6-cylinder aircraft engine. 550 cubic inches (just over 9 liters) of displacement from 6 cylinders (5.25” bore x 4.25” stroke, redline typically 2700 RPM).
Having two flame fronts does help a little, but not as much as you'd think, though it depends on the head design. My understanding is that the primary reason for dual plugs in aircraft is to allow for fully-redundant, parallel ignition systems.
That makes sense, thanks for the explanation. It didn't sound right to me that there would be so many turbo-charged piston aircraft, but if we add the high-compression engines then the fuel usage would definitely follow the Pareto principle.
