For a 40 size engine that will perform very well with out needing to fiddle with it all the time, spend the extra $$ and get the OS 46AX!! After you see how it runs a few times, you will be glad you spent the extra $$. For a 60 size engine the Tower Hobbies 75 engine is the best that I have found for great performance regardless of the low cost!! It does take about 5-6 tanks to get the engine to settle down and run smooth but after that, the performance and reliability is WOW!! Its the only engine that I trust on my 60 size warbirds including my favorite, a H9 Stuka 60!! The smaller TH engines such as the 40/46 size are more finicky than is desirable.