[Tony Wheeler]

Very interesting post Phil. I had similar issues with my F15A when I first put it on the road. It was mid summer and the radiator kept boiling. I tried four different radiators before I found one that worked satisfactorily. Some of them worked on the flat but they boiled uphill. There's a long hill near my place where I could test them, using a digital thermometer probe in the top tank. The one that worked best has been recored at some stage, with the usual thin core seen on recored Ford CMP radiators. The others had the original "tropical" core, which is much thicker, having an extra row of tubes across the front.

Naturally I examined each radiator before fitting them. The tubes appeared to be clear, insofar as I could see them through the filler cap hole. I tested them for leaks by plugging the bottom outlets (using small lemons off my tree) and filling with water, and when I removed the plugs the water gushed out under gravity, which would seem to indicate no major blockage.

In the course of this radiator swapping I decided to test the flow through the system, by running the motor with the top hoses disconnected and the radiator level maintained via a 3/4" garden hose from the tap (there are no thermostats in this motor). I don't know how much it's supposed to flow but it sure pumped out a lot of water at around 2000 rpm. It gushed out in front of the truck through both hoses and the garden tap was flat out keeping up. The water pumps are brand new and have the improved impeller design, ie. helical vanes instead of the original straight vanes.

Based on these tests I'm satisfied that the radiators are all clogged to varying degrees, despite appearing to drain quite rapidly under gravity. Your IR gun may pick this up, although if the tubes are randomly blocked I suspect it may not, as there are multiple rows of tubes which may average things out from side to side. Also if they're all partially obstructed, or most of them, the effect would be the same. I guess that could result from small particles of rust lodged in the tubes, or perhaps a coating of residue internally, although chemical flushing should remove that if done properly.

One thing I've noticed is that the recored radiator has much larger tubes than the original ones, which are flat and thin. Obviously these would be far more prone to blockage/obstruction, and would also flow much slower. On the other hand they allow more airflow, which is also a factor in cooling efficiency. However if you look at modern radiators you'll see they have large diameter tubes, so I imagine these are an improvement.

That said, we know these CMP cooling systems worked in hot climates during the war, often in heavily loaded trucks grinding up long hills at full throttle. I'm told Fords had a reputation for boiling sometimes and Chevs were preferred in this respect, so you shouldn't haven't any boiling problems in New Hampshire with the stock Chev radiator, even on peak summer days. Which brings us back to your present problem!

Let's consider the physics. Boiling is caused by one thing and one thing only - insufficient heat loss through the radiator. Heat input is determined solely by the rate at which fuel is being burned. It can't be increased by blocked water jackets, because they're not generating heat energy. Only combustion gases can introduce additional heat energy to the coolant, via a leaky head gasket or cracked head. Blocked water jackets may cause overheating of the surrounding metal, but they can't add heat energy to the system. All very obvious of course but worth covering nonetheless, as it helps us to focus on the real problem, namely heat loss rather than heat gain.

Heat loss occurs through the radiator and is a function of coolant temp and flow rate, cooling air temp and flow rate, and heat exchange surface area. We're not concerned with cooling air temp and flow rate, and we've fixed the coolant temp at say 170F, which leaves only coolant flow rate and heat exchange surface area. If all the radiator tubes are clear, then the heat exchange surface area is maximum, so the problem has to be insufficient coolant flow rate. Obviously if the radiator tubes are clear it has to be in the block! It seems unlikely that all the water jackets could be so badly blocked as to restrict coolant flow rate sufficiently to boil, but if the acorn was restricting the flow out of the water pump it could conceivably be the problem on its own. At that point in the system the effect on flow rate could be similar to a partially closed thermostat. Not having seen its position I'm unable to judge.

Alternatively, if some of the radiator tubes are blocked, then obviously the heat exchange area is reduced proportionately, and also the flow rate. That's the usual cause of overheating, however there's usually no acorn in the block to consider! Of course, it could also be a combination of these factors - acorn, partially blocked radiator, and badly blocked water jackets. If that's the case, then the radiator may be adequate with the new motor after flushing, and it won't be subject to subsequent blockage from crap in the block. Whatever the case I'd persist with the Chev radiator myself, even if it needs recoring.

I look forward to future instalments. My blitz mate down the road is putting a 235 in his C15A project so I'll post some pics when he gets to that stage.

Cheers,
Tony