Nobody? ;_;
Why is so difficult to find info on this one?
The closest thing to a drawing I could find is this 3D reconstruction:
Maxforums - Nakajima "Kotobuki" 9-cylinder aircraft engine - page 1 but the author admits several details are just a product of guess work.
Here's some data about the engine:
Nakajima Kotobuki:
9 cylinders air cooled radial engine.
Lenght: 1170mm
Diameter: 1280mm
Weight: 340Kg (Model 1,2), 376Kg (Model 2-Kai), 424Kg (Model 3)
Bore: 146mm
Stroke: 160mm
Capacity: 24.1 liters
Compression ratio: 5.3:1 (Model 1,2,2-Kai); 6:1 (Model 3)
Supercharged: yes, centrifugal blower (1:9.63 rpm to blower ratio)
Redution gear: No (Model 1,2,2-Kai); 0.6875x (Model 3)
Fuel: Gasoline (RON ???)
Performance data:
Model 2-Kai
Max continuous power at sea level: 460HP @ 2100 rpm; +40mm Hg boost
Max continuous power above sea level: 500 HP @ 2100, +40mm Hg Boost (2500 meters)
Take off power: 570HP @ 2300 rpm; +150mm Hg boost
Model 3 (Ha-1 Ko)
Max continuous power at sea level: 540HP @ 2300 rpm; +100mm Hg boost
Max continuous power above sea level: 690 HP @ 2300, +100mm Hg Boost (3500 meters)
Take off power: 610HP @ 2500 rpm; +150mm Hg boost
I hope it will help spark some interest into this one...