As others have mentioned, there is a transition area around the 1 to 2 cubic inch area where glow engines fade out and gas engines start in.

Gas engines have extra “stuff” with them, such as batteries & ignition modules, or a magneto. This weight penalty is a lot on small displacement engines. However, as engines increase in size, the battery and ignition module weight becomes less significant as they represent a lower % of the total weight. Gas engines also tend to have much lower power to weight ratings.

Glow engines draw a lot of fuel. A ~1.5 cu.in. can draw around 3/oz a minute. A 6 cu.in. gas engine has about the same fuel draw. This is a triple whammy. Not only is glow fuel much more expensive than gas, but the glow engine is more hungry by a factor of about 5X or more… that’s the double whammy. The triple whammy part is that a 6 cu.in. glow engine may weight less than gas, but the required tank size and fuel weight for a 10 minute flight would be large, like a gallon or more, which actually makes the gas engine setup lighter and explains why there aren’t 6 cu.in. glow engines as you’d need to carry 10lbs of fuel for a 10 minute flight.

It is these factors combined, and I’m sure other factors, that explain the switch from glow to gas around the 1-2 cu in area. I am of the opinion that you are better off with glow until the high side of that transition, i.e. I’d use glow up to 2cu in, and wouldn’t go smaller than 40-50cc for gas. But there are some good running gas engines in the 25cc area (~1.5cu in). They are cheap to run, but they don’t have near the power of a similar sized glow and they tend to weight more.

Cheers