the 46 sf was available in ring or abc the fx from what i can dig up is only abc the fx has a larger squarer head and a little longer crank shaft. if you have a non ring run it more towards the leaner side they like to run more hotter then the ringed engines
but don't over do it. there is a problem if you run a abc too hot
the liner will peel off the cylinder wall .proberly a good guess is to run 30 - 40 degress hotter then the ring version a temperture
gauge is handy.if you want the extra power the abc is the way to go just need to be a little more carefull with them. i've had a lot of both and i've never had a abc go out yet have had to replace a few rings. i've run some of the abc motors with as much as 50%
nitro and they are still running.most of your engine life depends on how you operate and take care of your motor.
roy

fredericksburg,va.