ORIGINAL: opjose
There are many other factors at play when going from Glow to Gas...
I've found that to obtain similiar performance as a 1.00 - 1.20 glow engine I need to go up to a 26cc gas engine.
The Gas engine is heavier and requires an ignition pack. This adds to the plane's weight and increases the power requirements.
On a power per displacement basis, glow does better than gas, but gas is much cheap to run... so you normally have to increase the displacement to get power equivalence when moving to gas.
You also must adjust prop sizes and pitches accordingly too.
Agreed. A .91 size model will work with a 26cc gas engine if you can shoe-horn it in. Gas engines weigh a lot more than glow per Hp of output . . . but there comes a point it works out. If you're flying a magneto that adds weight for the flywheel and magnet/sensor, and the alternative is electroinc ignition which eliminates the flywheel and magneto but adds the weight of another 4.8v battery pack (in addition to the receiver pack, that is). But the gas carbs have their own internal fuel pump and that solves some fuel feed issues. Right around 8 or 9 pounds of model (without engine) gas comes into its own.
Gas engines turn larger props but slower. The noise is less, or at least in a lower register so it sounds better.