The stock 750 SF1/2/3 head is very inefficient, the 750SFC Eletronica head only slightly better. If a free-flowing port with no restrictions were to be rated at 100%, the stock 750 would be around 35%, the SFC head would yield 45... In comparison, the triple head would rate about 60%, sort of ok, but still not very good. Typically, 2-valve jap sports bikes of the era would show around 55-65%.
After a lot of work on the flowbench, a '75 SFC head gave something around 65-70%, representing just about all that was possible within the stock casting without reverting to welding and re-shaping, resulting in 85 healthy ponies compared to the 70-odd hp the dyno showed before the porting was performed.
In my experience, flowbench work is worth every minute. If a 5% gain can be achieved at the bench, this will invariably show on the dyno in the same proportion. Don't need to simulate the piston going up and down, a dummy cylinder is all that is needed to simulate the conditions within the combustion chamber. Very often, no great changes are needed to achieve giant leaps. Of the 30-odd hp gain I achieved with the Moto Witt racing H-D Sportsters, 20 were found at the flowbench, simply with careful re-profiling of the valve seats and carb intake choice. We were limited to using a variety of stock H-D parts, no metal was allowed to be removed! Just about the only parts allowed to be modified were the valve seats, reading the rulebook thoroughly was to prove invaluable.
Once the principles of flow dynamics are grasped, it isn't too dificult to achieve decent results without a flowbench, but having one to verify any changes is more than helpfull!
piet
After a lot of work on the flowbench, a '75 SFC head gave something around 65-70%, representing just about all that was possible within the stock casting without reverting to welding and re-shaping, resulting in 85 healthy ponies compared to the 70-odd hp the dyno showed before the porting was performed.
In my experience, flowbench work is worth every minute. If a 5% gain can be achieved at the bench, this will invariably show on the dyno in the same proportion. Don't need to simulate the piston going up and down, a dummy cylinder is all that is needed to simulate the conditions within the combustion chamber. Very often, no great changes are needed to achieve giant leaps. Of the 30-odd hp gain I achieved with the Moto Witt racing H-D Sportsters, 20 were found at the flowbench, simply with careful re-profiling of the valve seats and carb intake choice. We were limited to using a variety of stock H-D parts, no metal was allowed to be removed! Just about the only parts allowed to be modified were the valve seats, reading the rulebook thoroughly was to prove invaluable.
Once the principles of flow dynamics are grasped, it isn't too dificult to achieve decent results without a flowbench, but having one to verify any changes is more than helpfull!
piet
