AirAg, there four letters used ..... I,G,S, and O as prefixes to the number, which is the cubic inch displacement of the engine.
An O540 is an 'opposed' cylinder, carbureted 540 inch engine.
IO means injected, opposed cylinder
ISO means injected, supercharged, opposed.
IGSO means injected, geared, supercharged, and opposed.
Radial engines use an R (for radial) prefix intead of an O, as in R985.
That would be a 985 cu. inch radial A/C engine.
K.