Combustion chamber differences also. Basically it looks the same and that's the end of useful similarities. Can't use an A head on a manual and can't use a manual head on an A. Yes, it'll bolt up and you'll be able to get it to run but there's a big loss of power.
Whats the cause of the power loss? Is it due to having lower compression? Just curios...This stems from me asking about the 450 jugs and heads swapping into a CB400A and you had said to use the 450A stuff due to power loss with the non A parts and it got me to wondering the difference in them.
The difference is in the velocity of the intake, smaller ports equal higher velocity for better filling of the chamber. Combined with the A model flat top pistons it makes a difference in torque which is what the A model needs. Manuals work better from HP vs. torque since there's more gears
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