I would like to know the difference between the engines. some are .15 and others are .25. What is the difference between the two because i heard that both go equally as fast.

Thanks

The .15/.21 designation is which size it is in cubic inches. So a .12 engine is an engine with a .12 cubic inch displacement. Simlarily, a 2.5 is displacement in cubic centimeters, also the same as .15.

the term "fast" that you used probably just applies to the rpm's the engine is able to reach. most of the time its between 30000 and 40000 rpm. the main difference is that you get more horsepower out of say a .25 than you do a .15. hope this helps!