upgrade my engine
 

I am currently running a .15 Associated in my stock RC10 GT. I would like to buy a new or used engine. How big should i go? What engines will fit? What is the best engine?

depends what car you have, you dont want to go too big on a light car

I'm not very famalier with the GT but would assume it'd only accept small block engines, I.E. .12, .15, and .18 size engines. There is no best engine out there, just ones that suit each person better than another as far as the type of use they will use it for, price, and power/speed. A .12 I think is plenty of power for a truck but if just playing around I'd say a .15 would be the most you'd want, an .18 would be overkill and just break parts easier or sooner on your truck. I run Fantom engines, they're fast and don't cost a whole lot and tune as good as OS engines do.

Are you planning on racing with it or just bashing around?

Cheers

Trinitysirio evo 2,12 pixy rossi,O.S. 12 tr or cvr.

if the engine i have now is the stock .15...will .18 or .21 be better for racing? or will a OS .15 have more power?

A .12 will be best for racing IMO. Racing speeds are actually done at slower speeds than you would think, except the straights. No need for the power of a .15 or higher except to make it harder to not do spin outs. .12 motors are what is legal to use in racing. Most club races never check though.

But if you have a stock ae .15, the os cv-r .12 or tr .12 will probably beat that motor anyhow.