I will say one thing about Matt's cooling air flow chart. The relationship between amount of air flow is directly proportional to engine RPM. However, engine load, fuel consumption, cylinder pressure & temerature are ANYTHING but. The engine speed control lever only controls the governor, not the throttle valve in the carb. I think you'd all be surprised how seldom an engine in a CC is actually "Wide Open". In the case of a K181/M8 in a CC running 3600 rpm with the mower engaged, tractor in neutral not moving therefore not mowing any grass I suspect the throttle blade would "probably barely" allow a 1/4" dia. pin to pass between the throttle blade and sidewalls of the venturi. Too many variables on that claim to list but if someone wants to pull their air cleaner off their CC 70 or ???? with a deck mounted and let us know what they find it would be interesting, not sure you could really tell just by watching the throttle linkage on top of the carb. Not sure how that amount of throttle opening would relate to percentage of actual HP the engine is producing.
SON & I actually started making a carb spacer for my K321 that I was going to install a vacuum port in to monitor theoretical percentage of load for tuning purposes based off vacuum. Problem is it's 3/4" thk steel roughed out with the plasma cutter. LOTS of finishing work left and I'm not really sure it would allow the governor to control engine speed properly.
MATT - Your comment about fuel burn rates.... Best comparison I can offer is on a 903 CID V-8 Cummins diesel. At full rated RPM & Load, 320 HP, the engine would burn up to 20 GALLONS of fuel per hour, maybe a touch more. At an 800 RPM idle letting the engine run all night to stay warm it would burn about three quarts of fuel per hour as near as I could figure, and it was a VERY inefficient burn, it would puke out a gray-white smoke after less than an hour that made people think it was blown-up.
The problem comparing a gas engine to a diesel is a gas engine has to run at a relatively constant air/fuel ratio, anyhwere from 12:1 to 14:1. A diesel burns fuel in direct relationship to the amount of torque the engine is producing and that can range from over 100:1 (very lean, actually detonating which is the way diesels run ALL the time) to somewhere in the single digits-to-one in the case of pulling engines.
I know, Another one of "Denny's l-o-n-g boring posts about engines". I'm actually trying to break Harry's record for longest post. And make people "Think". Chances are pretty good that you will not "over-heat" your engine mowing wide open, 3600 rpm. But with tuning or maintenance problems it's still possible. And it's also entirely possible to run much less than 3600 for an extended time without problems depending on the situation.