Re: Crazy Speeds
The red Schwinn on my profile had 5 stock 66cc engines in it's lifetime (3x Dax, 2x GT5). I blew my first Dax engine at 300 miles trying to hit 30MPH with the 44T rear sprocket. Con rod gave out. I put a GT5 in there and laser-cut an adapter to take 130mm BCD 5-bolt BMX chainrings, then slapped a 39T on the adapter. 30MPH was then cruiseable, but still revved too high for my taste - indeed, that GT5 failed after 1200 miles (con rod), and I replaced it with another and switched to smallest sprocket my adapter would take - 34T.
With the 34T, 30MPH was a pleasant cruise and topped out ~42MPH (load limit). By my estimation, that's 3HP or slightly less, and the tag on the engine said 2.85HP. Some sites do claim 5-6HP, and maybe these engines can make near that with porting, tuning, and an expansion chamber. I watched Baird do over 50 a couple times, and his engine had to be making at least 4HP to hit that.
Of course it seems no 2 Chinagirls are truly the same, not to mention the builds themselves, so mileage is going to vary. Yet it's easy to call BS on a lot of these speed claims with 44T sprockets.