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Sooooo...., actually = knowing little, but suspecting much:

I'd suspect that Mazda = designed the water pump and flow characteristics to provide adequate cooling for an = idling engine in Phoenix in the summertime. I'd also bet = that the majority of time, a car is sitting at idle, as opposed to cruising down = the freeway at 70 mph.

My suspicion is that the = water pump is designed for some arbitrary flow rate that will be adequate at idle - = and at normal cruise (with it's attendant increased airflow). I'd guess that the pump is = flowing a much greater volume of water at high RPM's than is really necessary, = because the pump impeller is not variable. The engine temperature does not = remain cold, because the thermostat closes, either completely or partially to = keep the block temperature at 180-deg.

Have ya-all ever = driven a car with a stuck-open thermostat...? The engine never warms up. = Runs OK, but builds-up sludge and carbon inside.

Jack;

The reason a car without a = thermostat never warms up is because it is typically running about 30-40% of rated hp. In = an airplane we are normally running about 70-90 %

I don’t disagree with your = flow assessment for the car; but consider this. My engineering analysis (all the = same stuff that our friend Bill Shertz has now plotted out for everyone) for designing a somewhat optimum cooling system for my 20B told me I needed = 40 to 50 gpm flow rate at 6000 rpm and 260 hp; that range depending on = variable factors. For a 13B that’ something like 22 to 33 gpm. When I measured the flow rate during the dyno tests I was pleased to find that = the pump produced 48 gpm, with no thermostat, and 37 gpm with a = thermostat. This was pumping thru the engine and through the dyno loop which I know has = less back pressure than my plane’s cooling loop, so my actual flow will = be less.

So I don’t see any excess = flow over what I need at high rpm/power, even running the engine without a thermostat. Yes, there are regimes where it will be pumping more = than I need – in the same regimes where I don’t care whether = I’m wasting 1 hp driving a bit more flow than needed. And, I could = also put in bigger (heavier) radiators and run a larger delta T and a bit less = flow.

(OK, I lied, get on with the = beheading – it’s only because I’m not feeling well and sitting around at = my computer instead of working on the airplane; honest)