Ya reckon so?
It is about Engineering and marketing STANDARDS.
The STANDARDS for stating displacement is Bore and stroke - not minus anything else.
Funny thing is - if the company involved atated the actual displacement based on those same standards, they would probably sell more enignes becaus "everybody" knows people will buy an 18cc over another brand of 15cc because it is bigger for the same sized package.
But then maybe ALL the other 2 stroke manufacturers in the world don't know what they are talking about. Honda, Kawasaki, Yamaha, DA, 3W, MVVS, Detroit Diesel, and the list goes on and on.
Those companies use the full stroke and bore to state cubic capacity. By using the same basis for stating it on an RCGF / GRPro / SV product it really shows up a low power to displacement. Eg - the 15cc is really 18cc using the STANDARD method of statement, the 20cc is really 23cc using the same method of statement - now use those and compare power outputs against the opposition products.
That is all I am saying on the subject - back to the normal channel now - buy and fly what you like.
Oh, I almost forgot - "Nice plane"
(seems to be a pretty common answer for "Darned if I know the answer to the question posted" around some parts of the forums these days)