For my personal project, the CD90, I’m looking for a bit more performance. Currently the C90 engine (HA02, GB6) has a 52mm bore making 110cc on a standard C90 crankshaft (49.5mm). But the engine still has problems going faster than, say, 80 km/h. There can be 3 reasons for this. First is the cylinder head,

that’s a standard C90 cylinder head. Second the carburetor, it’s a 22mm Keihin which is quite small for 110cc. Finally, it is possible to fine-tune the front and rear sprocket a little more. Currently: 17 teeth in the front and 36 in the rear. The speed at the top is quite high.

Let’s focus on the cylinder head first. Honda designed this head for economical driving. These are small valves, but the ports are standard 22 mm. I was curious if an OT 70cc head would fit on a C90 configuration. Originally the OT 70cc cylinder head was designed for a 6v crankshaft (41.7mm length) and a high compression piston with dome. Since my 52mm piston is flat, it seems likely to fit the OT 70cc head without damaging the valves.

Looking closely at the C90 head, the combustion chamber is about 39mm wide. Actually quite small. The combustion chamber of the OT 70cc head is 47mm wide and the valves are slightly larger.

I thought it was an interesting experiment to see if the head fits and what it does in terms of performance. I didn’t expect miracles because the differences are minimal.

It turns out the 110cc gets a bit more punch! At high revs there is definitely more power available and I have no trouble getting 90 km/h now. I’m quite happy with the ‘upgrade’!