JM
Ah yes, the bottom line as you note is: Pump gas.
Union "76" gas, common before/after WWII had nothing to do with the American Revolution of 1776. It was the octane rateing that Union Oil was so proud of. In any case the compresson ratio on my 41 Harley was around 7 to 1 and that was higher than most auto engines. As I recall a Model T was about 4.5 to 1 because the really poor octane fuel available that far back.
Why is this mentioned? Because without reasonable fuel the low compression engines would be with us still. After about 1950 the situation changed and better fuel became available so it was then possible to mill the head on any auto engine with great results. I worked on and around a lot of Ford flatheads in the early fifties with a guy, Joe Reves, that raced them during the hard top days. The term that was then used to denote that the valves were unshrouded and the valve track opened up was that the block was "ported and releaved". My friends and I had a lot of heads milled, on flathead Fords, Chevy 6s, and Plymouth/Dodge 6s, and even a Pontiac straight 8 for street use. In every case the engines ran stronger from off idle to as fast as we dare rev them. This was without any other modifications other than maybe a "Two Place" manifold with a pair of Stromberg 97s.
Joe showed me how he did his porting and releaveing to Ford motors that had the static compression ratio substanually raised. However, over the years it has come to my attention that not too many people back then knew what they were doing. I believe that technology as it relates to flathead Fords has increased in the hands of a few crazy land speed racers. I recall that the fastest flathead Ford now runs 308mph in the 5 mile.
If the rev limit is NOT going to be raised there a only small reason to modify the carburation or exhaust tracks, and that includes the cam, on an engine that has been "Milled". If an overhead valve engine, again say an Intec, was subjected to a severe head milling there is no problem with restricting the breathing. Ya, the valve gear geometry will get a little bit wrong, unless the push rods are shortened, but that is not a show stopper. Some time back I discussed this flathead "milling/shrouding" issue with my engine builder. He agreed with the above but of course added that the next step in a performance engine would be to make it capable of consumeing more air every minute. That of course entails raising the rev limit and opening up the intake and exhaust tracks and providing valve timing events that agree with the greatly increased mixture and exhaust gas velocities. The latest engine built for my son and I has 16.1 to 1 ratio, red lines at >8,000rpm, has nearly 4.600 bore, and has 494 c.i. but it don't run on pump gas.
Here is my cut on the downside to milling the head on, say, a 5 HP Briggs. Heat. Every point the compression is raised will allow the engine to extract more power from the same amount of fuel. It also means that there will be a greater amount of heat generated and that will require closer attention to cooling. Less problem on a water cooled engine. On an air cooled engine it can be more of a problem. However, the noble Briggs is fan cooled and if not operated at full rated load and rpm for an extended period no problem should arise. I have trouble believeing anyone will often subject themselves and their mini to extended full throttle applications that result in that condition. The 5 HP Briggs being a typical flathead design has all the faults of that design. Namely the exhaust valve is next to the cylinder and makes cooling that area of the cylinder problamatical. The OHV Intec has no such problem and would, all other things equal, be less likely to suffer.
It's cheap. Mill the damned head and see what happens. How much to mill that 5 HP Briggs? A year or so ago, on two occations, I asked that question on this site. Answer? Basicly that I was dumb to ask because all sorts of high tech things also had to be done so it was a moot question. I also got the idea that there were some top secret things that affected National Security that were above my level to be informed of. I was born at night but not last night. So I suppose the only way I'll find out what an "optimum" mill would be [for a stock engine operating at 3,600/4,000] will be for me to do it myself. I've put to bed the Intec OHV question. 10 to 1 is fine on 92 octane, more might be better. Have a used 5 horse that can be experimented with but gotta finish the Caper Cycle first.
