Not to piss on yer parade, but
M-18's are a flat head engine, (and a dam good old one.)
Once you go milling the head other than say .040 you begin to shroud the valves.
Even then I'd clay the head to see what the valve to head clearance is.
As far as 93 or more octane, it is a waste of money and a decrease in power on a low compression engine.
Now if you are wanting to build a competition engine, pop up pistons, relieving the cylinder, special design CNC head, larger valves/ special cam,special flywheel rated above 4000, rpm's special intake/exhaust etc. etc. to name a few things, are a start.
But a flat head engine is not an inherently power house design to start with.
They are made for work & torque.
If you are looking for factory specifications look in the service manual available from kohler free, or here on our site.
