As JetMech and CGRetired have said, the biggest down size is that sizes. There are NO gassers for 40 sized planes. There are now two gassers for 60 sized planes, however there are trade offs there too. The Zenoah G20 is a 60 sized gasser, but because of it's size and shape it won't fit under the cowls of a lot of 60 sized planes. The other engine in this size is the new Saito FG20 4-stroke gas engine. This is great because it's the same size and shape as most 4-strokes so it will fit most planes. The down side?? The cost. The engine costs $529!!!!

As it stands now gassers really don't come into play until you start getting up above the 60 sized planes.

Ken