I'm wondering how much performance difference is there between the .12 and .15 range of engines. I'm talking about Magnun / ASP / Super Custom engines here. They are all basically the same engine and produced in the same factory. What I want to know is how much performance does the extra 0.03 cid give? Will a model designed to fly with a .15 fly with a .12? Most people overpower their models, but I am looking at underpowering. I have a couple of .12 engines, and the LHS has some nice .15 ARFs. The models are Extra/Cap/Edge types.

The .15 planes will generally fly just as well with a .12

Having an ASP 12 I agree with this one little difference between a 12 and 15 ditto for 21/vs/25 have ASP 21 also, it comes into play with a little bigger engine example a 40 vs a 45 or 46 martin

There is always an exception my enyna CX11D is quite a bit stronger than my. OS10 diesel conversion it is BB engine (the enya)