I really appreciate all the information. But at what point do Norton parts begin to fail. Are the head and cylinder that good a design that if I run 12:1, head gasket and stud breakage doesn't happen?? If I supercharge what's going to fail? What's the weakest link. This is going to be a short run bike not built for the long haul.
This is my take on the question of parts failure in the Commando engine, based on years of racing them. The biggest cause of engine failures from mechanical breakage isn't so much the compression ratio or horsepower as it is rpm. The usual mechanical failures on a Commando race bike from regular high rpm usage are crankshafts, crankcases, and connecting rods breaking. In essence, the crankshaft flexes and eventually cracks, the cases crack through the drive side main bearing bore, and the rods fatigue, crack, and break from lots of force cycles as the piston changes direction. The most significant forces causing all that are proprtional to the square of the rpm, so really get nasty much above normal red-line rpm. These failures used to be fairly common on race Commandos, but the availability of aftermarket parts really helped in that area. I had all the above problems with stock parts, but never broke a billet crankshaft, Carrillo rod, or Maney crankcase. They also occur sometimes on street bikes, but not nearly as often. I've always assumed those were cases of folks who regularly over-revved them, but some might also be a result of poor quality control at the factory. Hard to know for sure.
Higher compression does increase the loads on top end fasteners, and probably increases the risk of head gasket failure, but has nothing like the impact of over-revving. The real risk of damage from high CR, as many have already pointed out, is from detonation if your fuel quality is not up to the engine's demands. I've run as high as 12.3:1 in a 750, standard stroke, race Commando with no issues from the higher CR, but that was with race cam and high octane race gas. I have run 10:1 on the street, but it can be iffy. You do have to be careful in your fuel choice, as well as ignition timing and carb jetting. Still, it seems to work for quite a few Norton owners. I've settled on 9.5:1 as my target for normal street Commando. Even that can be risky with poor fuel on a hot desert day at freeway speeds. or up a long hill.
On the other hand, if you're only running avgas, and don't encounter the extreme environmental conditions, you can probably live with a higher CR, say 11:1. Above that, it just gets mechanically more difficult to raise the CR in a 750, unless you choose to use a high dome piston, which isn't a great choice for the Commando head design.
Anyhow, it's just my humble opinion. Take it for what it's worth.
Ken
