.15 or .12 relates to the american Cubic inch, so it`s .15 cubic inch motor,
so a .21 Ci is about 3.5cc why we use this i dont know .
you`ll probably find more torque in the .15 certainly bottom end pull away but may not rev as highly or produce as much power top end , but it does depend on the engine manufacturer and what the engine was designed for. the engines supplied in kits are really designed to be reliable and easy starting BEFORE performance really comes in to the equation.
which is why you`ll find more specialised engines like teh sirio .18 or some of the tuned os engines abit more fiddly to set up and run and also more succeptable to climatic changes

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