I have a buggy with a RB S7. I love the thing. The reason why I am asking everybody for their opinion is because I'm seeing that 50% of the guys that race truggy use a .28 . I see many .21s' out there and they do fly!! One the other hand, at my track, they spray and the traction goes up 100%! The guys that run .21's burn up plugs as they are pulling hard. I've been running a 28 for almost two years. The only problem that I have is getting the top end. Mind you I was running a RB TM728 originaly designed for a Savage/MGT.
A couple of friends have the C6BB and that thing is a beast!! As with all of us here, we want an edge over the next guy; hence the 928 or the MR Ninja .28.
I chose these two companies because I am a on road guy that just happens to love my truggy! I run Novarossi and RB in my on-road cars. I know they are expensive, but in my opinion nothing runs as strong as the NR or Novarossi based engine. I know that OS are beast!! God knows that they are out there putting it down, but I am a Novarossi driver.