Funny you should ask. I have one that I dropped in a Vegas for a while. Pretty easy swap. I actually used the mac header and pipe I had on the OS 18. The short manifold on the OPS won't clear the radio box. My modded vegas ran a consistent 42 with the OS 18 and about 45 with the OPS 21. I don't think that the side exhaust 21 is a big power house like their supermarine 21. Thus the big price difference.