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More Cooling

    The car has never ran as cool as I thought it should; for instance, in mild springtime weather I would expect coolant temperatures to be reasonably close to the 88°C thermostat rating. However, even after the S54 oil cooler installation , and with the mechanical fan in place, I would consistently see 93-97°C coolant temps in 70°F weather, and well over 100°C with the A/C on in traffic; I found myself manually cycling the A/C in the summer to control coolant temperature, resulting in a lack of happy. I had bled the cooling system numerous times and was confident there was no air in the system. I suspected either an ungood thermostat, or low RPM flow issues with the race-oriented Stewart water pump, so it was time to swap in a new thermostat and try a different pump.

    Compared to an original BMW or OE replacement pump, the Stewart part is physically hefty, “looks the business”, and this flow chart offered for the Stewart BMW pump indicates a higher coolant flow rate than a “Stock” pump across the entire engine RPM range. However, I’m skeptical of the flow chart; while I’m by no means a water pump engineer, the Stewart pump appears to be designed to address high engine RPM pump cavitation, which from my experience would tend to impact lower RPM flow efficiency. Water pump performance is also typically plotted as pressure vs. flow rate; without knowing the test system pressure, I am of the opinion that the above-linked chart is more of a marketing tool than a definitive performance measurement.

    Stewart, BMW OE, and GRAF PA432A

    The GRAF pump on the far right is what I used to replace the Stewart part. The genuine BMW pump in the center is the pump I removed during my initial cooling system refresh .

    Stewart, BMW OE, and GRAF PA432A

    Next on the agenda was a thermostat test. I heated the 88°C thermostat I removed from the car, a high flow “Calorstat” 80°C thermostat made by Vernet, and a brand new a genuine BMW 88°C thermostat to 97°C so that they were all fully open. The Vernet thermostat is to the right in the photo below and obviously opens a lot farther than the other two; I really don’t know for sure if less thermostat flow restriction is helpful for an S52 cooling system, so we’ll see. There are a lot of opinions as to whether an 80°C thermostat is a better choice than the OEM 88°C thermostat for S52-powered street cars; since I have not yet went as far as going to an over-sized aftermarket aluminum radiator, and the car is a spring-to-early fall queen with no cold weather operation, I installed the 80°C Vernet part.

    Thermostat Testing

    I also installed a lower temperature 80°C (low speed) / 88°C (high speed) Auxiliary Fan switch, to trigger the factory front A/C condenser auxiliary pusher fan sooner; the OE M Roadster switch triggers at 91°C / 99°C.

    I have not reinstalled the mechanical clutch fan and the coolant now runs 87-93°C with the A/C off, even during long idle RPM periods in relatively hot 95°F weather, but in traffic with the A/C on, the coolant temperature creeps up into the 96-100°C range. Still, quite a bit cooler than previously with the Stewart pump and 88°C thermostat with the mechanical fan. I’m not sure if the the lower thermostat opening and auxiliary fan switch temperatures, less thermostat flow restriction, or the GRAF pump is responsible for the welcome drop in temperature. I suspect it’s a combination of all-of-the-above. I do know that the coolant temperature is now several degrees lower, even without the “hand grenade with the pin out” mechanical clutch fan in place.

    I’m debating whether to install a SPAL electric puller fan or just reinstall the mechanical fan and treat it as a scheduled maintenance item.