Was discussing XS650 heads earlier. The major limitation to developing horsepower in our beloved engines (developing hp is a different topic to actually reliably and effectively getting it to the rear wheel).
Design limitations are found in exhaust port shape and cross sectional area-too large. Inlet ports could be smaller and would be better inclined downwards. The ‘Yamaha Racing Tips’ manual produced in 73 also recommends pocket porting-careful removal of material around the valve seats.
The legendary dirt track battles between Yamaha and HD highlighted these limitations to the extent that for the ’76 season USYamaha brought in Tim Witham. The OW72 (I have seen these called OU72 and OW72-this would indicate 2 separate programmes-as far as I know this was a 1 off so I will, for continuity, use OW72) was developed. Didn’t stop the Harleys. Kenny Roberts lost the AMA Grand National Championship to Gary Scott. Time was short. Frames not properly set up. Shell Thuets eye for detail and reliability was missing too.
Still, from Roberts: ‘Best thing I ever rode’
Surprizingly Yamaha Japan had accepted Withams challenge to produce a new head. Probably as an R&D exercise for future 4 stroke engines. Kept King Kenny happy too. The head was cast totally different to the production ones. To accomodate the steeper valve angles ( racing: 56° production: 76° … inlet: OW72 – 43mm, prod’n – 41mm; exhaust: OW72 – 37mm, prod’n – 35mm) the heads were higher and the rockers shorter, enabling a lower rocker cover. Overall engine height was the same. Interchangeable with production motors.
The heads arrived from Japan in dribs and drabs. No ports, guides, valves, springs, cams. The engines had larger crankpins, improved pistons, rods, gearboxes, clutches and deeper sumps. I can imagine the stress. Modern business practice could learn a thing or two from this about networked local sourcing for local consumption.
Racing combustion chambers were hemispherical, aluminiun, with steel valve seat inserts. Production heads had a steel cap cast into the alu and valve seats were cut directly into this.
Cycle World wrote an article about the development of these motors in their Aug 76 issue.
Original consensus is that 25 of these were produced. AMA homolugation rules required 1 complete bike and 24 engines and transmissions.
‘In order to be approved, a motorcycle must be a standard catalogued production model, one complete motorcycle produced, race ready, and at least 24 identical engines and transmissions must be available for inspection and purchase within the United States.’
There seems to be more (?). There is some confusion about this. It seems that Don Vesco produced some tuned heads back in the 70’s based on production heads and TRD valves. 650 Central sells your production heads CNC machined to tuner Lillies’ racing specs (these are your remanufactured production heads NOT ‘Yamaha’ racing heads).
70 hp motors appear to be relatively reliable. 75 hp motors not so.
I have seen several other attempts to address this problem, including homemade cast heads and machined billet heads. Of course, head modifications increasing available hp will expose shortcomings in other parts of the drive train, namely crankcases, crankshafts, clutches……………..How deep are your pockets?