I found this item below somewhere ages ago: I don't know how true it is, but is sounds interesting and relevant to the discussion. If anyuone has had any experience of something like this, I would be very interested.

Regards

JohnH

"The very best thing I have found to avoid all the problems of low octane fuel is a trick I picked up from Nissan. When they increased the capacity of one their engines, rather than re-design the cylinder head to cope with the increased compression pressures, they just performed some simple surgery, and it goes like this!
Remove cylinder head. Have your friendly engine machine shop machine circles into the head, their position and diameter to match the location of the "fire ring" of the head gasket. I think this is called "fly cutting" in the US. Make each circle about 0.040" deep but no deeper due to the possibility of weakening the structure. Have them "cc" the combustion chambers to make sure they're all the same after they give the mating surface a very light skim. Re-fit head.
As near as I figure the theory is that while this lowers the compression ratio marginally (about half a point or so) more importantly it alters the burning characteristics of the flame front on the "power" stroke. The decreased degree of "squish" and lower level of turbulence results in a slower, more controlled burn. This is particularly effective in getting rid of "ping" or "knock" (detonation). The most expensive part of the exercise is the top-end gasket set.
The machining is remarkably easy (cheap) and it’s a permanent cure. I've done this a couple of times with nothing but positive results. My wife's '84 240 runs smoother, quieter, has marginally more power and uses less fuel on standard ignition timing.(even though after numerous head shaves in the hands of a previous owner it still has a about a 10.5:1 compression ratio). I guess if its good enough for Nissan, it'll do!"