Pat 12 gets a nose job

[Tony Wheeler]

Originally Posted by Phil Waterman

Results with the two water pumps came out almost identically so it probably the thermostat throat that controls the flow rate. The at 500RPM flow is 2-10 gallons per minute at 1000RPM flow rate is 8-20 gallons per minutes.

After two hour long test cycles to day with the 235 I'm getting strange results. I have two water temperature sensors one in the normal location at the rear left of the block and the other into the water pump housing. At 500RPM the engine seems to stablize with the rear reading 170F and the return reading 150F good 20 degree delta for the radiator. But when you bring the engine up to 1500RPM rear is reading 170 and the return is reading 170.

Phil, you won't detect any delta once the motor speeds up with the thermostat open, because the water passes through the radiator far too quickly to cool down measurably. At normal flow rate it's only in the radiator tubes for an instant, you'd need quite sensitive equipment to measure the temperature drop from top to bottom. The cooling system relies on the huge volume circulating every minute, so it only needs to cool down a few degrees through the radiator.

We can do a rough calculation based on your 8-20 gallons per minute result at 1000 rpm, choosing say 16 gallons per minute, which is 16 x 3.8 = 60 litres per minute = 1 litre per second. I don't know what the total volume of all those skinny tubes would be, but if we estimate it to be 1 litre, then the water passes through in 1 second. It just won't cool down much in that time, esp. stationary with modest fan rpm. Different story at idle when the thermostat is closed and you're only getting 2 gallons per minute flow. That's 8 times slower, which means the water spends 8 times longer in the radiator, ie. 8 seconds by this calculation. That would account for your 20F delta at idle.

Of course, the delta will also be greater at speed when you're getting maximum airflow. We can do a rough calculation based on a CMP at cruising speed, let's say 50 HP engine output. Petrol engine efficiency is about 25%, which means there's another 150 HP being lost in heat. About half goes out the exhaust and half into the cooling system, which means the radiator is operating at 75 HP (x 0.746) = 56 kW. That would heat your workshop nicely Phil, and it also explains why the engine cover gets so bloody hot!

56 kilowatts = 56 kilojoules per second, and since the specific heat of water is 4.2 kilojoules/kilogram/degree Celsius, and a litre of water = 1 kilogram, the delta will be 56/4.2 = 13 degrees Celsius. However...that's based on our 1 litre per second figure, ie. 16 gallons per minute, which applies to 1000 rpm. If the flow rate is 3 times higher at 3000 rpm, the delta will be one third, ie. 13/3 = 4 degrees Celsius. Also, if the volume of all those skinny tubes turns out to be 1/2 litre, the delta will be halved, ie. 2 degrees Celsius.

It's a fascinating experiment you're running Phil, I don't think I've ever seen it done before, and it gives a lot of insight into how cooling systems work, esp. the thermostat. We tend to think they're only for cold start up, but in fact they're constantly varying the flow rate. It would be interesting to set one up in a perspex tube instead of the housing, so it could be seen operating. It would also be interesting to have a delta gauge on the dashboard, I'm not sure what earthly use it would be but the same could be said of vacuum gauges!