RE: Evolution engine, Need commend
In practice, the BHP number you see listed for some engines is useless. You can't use it to really compare engines, as the highest BHP numbers ususally come at overly high RPM with really small props, smaller than you would use in practice.
The timeing of the various ports in the engine can have more effect on performace than many other factors. A small increase in displacement or a slightly different bore/stroke ratio helps, but porting and compression ratios matter quite a bit.
The muffler design can also have a huge effect on engine performace. Just taking the baffle out of most OS mufflers can result in as much as 1k rpm increase.
So, the only way to really compare two engines is to get RPM numbers using the same fuel, same prop and simular air temp/pressure/humidity or do something to factor in the weather conditions. In general, there is nothing in the ads or specs that can tell you which engine is more powerful.
Oh, comparing engine weight is sometimes handy. Obviously, lighter is better, but it has no effect on gross power output.