I wasn't going to waste the bag of popcorn on this one so I stayed clear early . Now that inevitable thread drift has subsided I'll chime in .
I'm surprised no one mentioned Kevin
Now on to the why , as if it matters .
Before 6,000 RPM the 2 valve wins
After 8,000 RPM the 4 valve wins
These are estimates so Your Mileage May Vary (YMMV) . However the real point is there is no way the 4 valve can keep up with the 2 valve on the bottom and likewise the 2 valve can't compete with the 4 valve on top . That is already in the thread and correct .
Once again people , the first limiting weakness in the XS is the center crank pin . In 360 ,180, 270/90 , 277/83 , 284/76 I've seen carnage from all these combinations . It does seem to happen later in the rev range with the 277/83 and the 284/76 but it happens . The 180 degree crank failures were spectacular . More another time on the how or why but the weak point was and is the center crank pin . Welding the center pin and the crank throws only delay the inevitable .
The crank is reasonably well supported for nominal use below 8,000 RPM but the case itself begins to move if the RPM's are regularly pushed above 9,000 . It isn't so much the RPM's that kill the assembly as shock loading the crank at any RPM . It just happens more suddenly and explosively at elevated RPM's
Improving the quality of the case fasteners just transmits the forces back to the crankshaft . So we're back to the crankshaft center pin again .
Mrriggs has reinvented the XS yet again
Brett has found out what the tuners and builders of the past used to experiment with oddly twisted crankshafts . Once you are not limited by the splines anything is possible .
Even if the crankshaft is strengthened , lightened , and supported , just short of 10,000 RPM it all goes wrong . This is where the 4 valve head is already far outperforming the 2 valve head .
So for those of you not interested in why or uncaring of my personal experiences .
Before 6,000 RPM the 2 valve wins
After 8,000 RPM the 4 valve wins
The crankshaft must be welded , pin and throw.
The XS650 can't reliably turn 10,000 RPM
The cases are the next weakest point beyond the crankshaft
Strengthening the cases places the stresses back on the crank
Once the cases are strengthened crank failures are spectacular .
rephased 270,277,284, crankshafts only delay the inevitable .
Just thought I'd share
~kop