OVERHEATING

When I drove my 1969 Swinger 340, 4 speed, with a 3.55 rear end gear, on the expressway at 75 mph (3,500 rpm), it overheated at 220 degrees, in only 2 miles. Driving on back roads at about 45 mph, the temperature gauge always remained at about 180 degrees.

The 340 was rebuilt a couple of years ago and has 2,800 miles on it. It has the following stock engine parts:
heads, cam, (9.7:1 C/R), intake, AVS carburetor, rebuilt water pump (No. 2863067- 8 vane- for non-AC).I purchased a stock water pump pulley (6 1/2”) and a stock crank pulley (6 1/8”). It also has a fan shroud, clutch fan, 16 lb radiator cap and a 180 degree thermostat.

Then, I had the stock radiator (No. 2949066), re-cored (3 row) by Glen-Ray. It still overheated at 75 mph. Next, I replaced the 180 degree thermostat, with a Robert Shaw/Stewart High-Flow 180 degree thermostat and installed a coil spring in the lower radiator hose, in case it was collapsing at 3,500 rpm. The only improvement was that it took 10 miles to overheat instead of 2 miles.

My engine builder wondered if 3,500 RPM is too high to cruise on the expressway. But it seems that turning 3,500 rpm with a 3.55 would have been normal back in the 60s.

I read the following in FABO: “The other thing to consider is that Mopar and their testing came up with an idea that the eight blade might cavitate at really high RPM”.

Mopar Action (Rick Ehrenberg) stated “the enemy is not heat, but boiling, especially localized steam pockets in the cylinder heads…use a surfactant such as Red Line Water Wetter…”

Muscle Machines (Ask Ray) stated “providing a pulley ratio of at least 1.25:1 relative to crank shaft speed…a RobertShaw balanced -sleeve 160 degree F thermostat and, most important, Evans NPG+ coolant”.

Any thoughts about cavitation, pulley ratio, switching to the six vane AC water pump, Red Line Water Wetter, NPG+ coolant or a 160 degree thermostat?

Thank you for any comments.