If you go to the DOO website and click on past year models you can find what they did to boost the HO. I believe it was mild porting, new reed cage design, and some other stuff.
here is some more info
Engines with the H.O. (High Output) designation are the ultimate performing Rotax engines. The 600 H.O. is the class leader, winning grass drags in both single- and multi-pipe classes and the 800 H.O. set highmarks never before seen. For 2004, the 800 H.O. is now available in select MX Z and GSX packages.
600 H.O. Features:
Bigger engine block
8-petal reed valve
Nicasil cylinders
Benefits:
10% power increase *
Improved reliability
Better heat dissipation
800 H.O. Features:
Twin reed valve
Optimized exhaust porting
Increased compression ratio [Summit only]
Benefits:
5% power increase* [MX Z and GSX]
7% power increase* [Summits]
*Compared to
Rotax Series III engine
here is some more info
Engines with the H.O. (High Output) designation are the ultimate performing Rotax engines. The 600 H.O. is the class leader, winning grass drags in both single- and multi-pipe classes and the 800 H.O. set highmarks never before seen. For 2004, the 800 H.O. is now available in select MX Z and GSX packages.
600 H.O. Features:
Bigger engine block
8-petal reed valve
Nicasil cylinders
Benefits:
10% power increase *
Improved reliability
Better heat dissipation
800 H.O. Features:
Twin reed valve
Optimized exhaust porting
Increased compression ratio [Summit only]
Benefits:
5% power increase* [MX Z and GSX]
7% power increase* [Summits]
*Compared to
Rotax Series III engine
