David, I had better cooling when the horizontal flow radiator was full to the top like cars and light trucks. I replaced the original radiator cap with an automotive cap so it would allow fluid to flow back into the radiator when the engine cooled off. You have to watch the top radiator hose if it isnt rigid enough to withstand a slight vacuum that occurs during cool down. I never had a problem but it is something to be aware of. Jeremy, it would be interesting to know if one of your machines had a horizontal flow and the other had a vertical flow. A vertical flow radiator does not lose cooling capacity until the coolant drops below the top of the tubes. A horizontal flow loses capacity as soon as coolant drops a little bit because the tank is on the side and tubes are up to the top of the heat exchanger. Tom in MN