As per my post above, the 143N-300A is the larger of the Aerodyne Series 53 family, obviously that is what you currently have. Even with the new Aerocharger Series 66 turbo's available on the market, they are hard to find used, and I believe they are around $2800 new. Used you will still pay $2000 if you watch the forums (SnoWest would be best to watch). If you want to go to a Series 66 upgrade, you will need to redesign your existing turbo Y-pipe to fit. On top of that you will need to also design and build a new turbo dump pipe for the Series 66; I'd recommend going to a 3" dump pipe diameter if you choose that route. Last you will need to go to an aftermarket fuel system upgrade which consist of a machining a new fuel log, new fuel regulator, fuel pump, and larger I.D. fuel lines to accommodate the benefits of using the larger Series 66 turbo; so its not a quick "swap out" job when upgrading a Vmax-4 from a Series 53 to Series 66 Turbo.
Oh yeah, last if you buy a used Aerodyne turbo regardless of it being a 53 or 66; make sure to ask the owner if they ever broke reed pedals under use... broken reed pedals will damage the turbine wheel and the adjustable vanes when they are hot from the exhaust heat. Its about $700 to have new vanes installed by Aerocharger or the Turbo Doc in Utah. Just some food for your thoughts!
Justin Durand