IO VS O

How much HP do they make with the helicopter pistons, something like 9.7:1 compression IIRC?

The 10:1 compression ratio pistons make 225 HP in the angle valve... but the STC limits Cardinals and Mooneys to reduced manifold pressure to flat-rate the engine at 200 HP... some pilots forget to observe the MP red line, however...
 
The 10:1 compression ratio pistons make 225 HP in the angle valve... but the STC limits Cardinals and Mooneys to reduced manifold pressure to flat-rate the engine at 200 HP... some pilots forget to observe the MP red line, however...
FWIW I have a Thunderbolt 10-1 IO-390 and the data tag says 220hp, up from 210hp with 8.9-1. Add electronic ignition, auto plugs, cold air induction, API 200A servo.... zippity doo dah. :)
 
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