temperature question

[oldboat1]

I have an old freshwater cooled 305 in my '77 inboard, and I've remained curious about the best running temperature. I go through this every year, but this year I did get overheating (over 200 and climbing), so shut down and replaced both the circulating pump and the raw water pump impeller. The problem was almost surely the raw water pump.<br /><br />Anyway, I now run between 160 and 180 with a 160 degree thermostat installed -- which I think is where I should be. The manual says I should run between 140 and 160, but I think in context the reference is to a raw water cooled system (raw water cooling was the standard system on this model Chris Craft -- freshwater cooling was an option, with descriptions and diagrams appearing later in the manual, but no additional discussion of t.stats and operating temps). <br /><br />I can put in a 140 degree thermostat and see what happens, but I can't quite figure out why the same engine could run well in either the lower range or the higher range. In a vehicle, it would be running in the 180 degree range. But if running at a lower temp might preserve the life of this old engine, maybe that is the way to go.<br /><br />Can anybody clear that up for me?

 

[ron7000]

Re: temperature question

the reason behind the 140F degree thermostat in raw water cooled engines (I/O applications) is so that the salt water doesn't crystalize in the coolant passages like if you ran a 180F - 195F thermostat. I remember hearing the minimum coolant temp, along the cylinder walls, for efficient combustion is 140F, hence the 140F thermostat I guess? But for efficient combustion and most power run as hot as you can or until the point of pre-ignition. You will not make more power by running cooler, unless you want to ignore physics (heat is energy) nor will you preserve the motor anymore.<br />With carbureted motors, they typically run rich so running it hot is good to keep carbon buildup down within the cylinders and on the valves. And with oil, you want to get it to 212F or close to that to burn off water vapor and combustion byproducts to keep the oil clean. If coolant temp is too cold, the oil temp will never get to its optimal temp. Combustion byproducts in oil, from cold startup or blowby, and water from condensation turn the oil acidic and will eat the metal inside the engine. Oil is formulated to neutralize this effect but only to a certain point (which is why you should always winterize the engine on fresh oil).<br />You really need an oil temp guage, but if you have an oil cooler I would think the motor would be real happy with a 180F thermo, and a good working heat exchanger. I wouldn't go lower than a 160F thermo though. As long as I can remember, especially on older fresh-water-cooled boat, they always ran a 160F. My 2002 bayliner, raw water cooled 3.0L mercruiser, even has a 160F stat not a 140F.

 

[oldboat1]

Re: temperature question

good stuff, Ron. Thanks. If I change the t.stat to anything, it should be higher temp rather than lower -- will remember that. I do have an oil cooler (part of the freshwater system), so could go to the higher temp t.stat. When running and fully warmed up, I'm holding at about 175 degrees or a little more. I'll go with that for a while before I start tinkering again.

 

[oldboat1]

Re: temperature question

I had that wrong. Actually, I have a transmission oil cooler (not engine oil).

 

[BobPhoenix]

This is an excerpt from my 1977 - 305, 307, & 350 G. K. KMB manual. It recommends an operating temperature of 145 - 165 degrees...