How much could I increase the displacement of a raptor 660 without losing too much reliability?

 

Probably the 686 or 690 kits are gonna be the most reliable.

 

The most reliable..IMO..would be a low compression 686/690...you can even go a low compression 727/730...very reliable also but that one would have to be done by a builder because it needs to be sleeved and you would need a new rod and balancing as well as new valves and a such...a good cryo coating on either motor would also help it outlast alot of home built stuff.

 

The 686cc kits work really well with the right cam,the 727cc makes lots of bottom end.You should set a budget to work with,and shop around.Relaibility should not be an issue with either build if done correctly.