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Discussion Starter #1
What was the temperature rating on an OEM 1964 390 thermostat? It looks like the options are 160, 180, or 195. I'm inclined to believe it's 195. Thanks gang.
 

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That is kind of a tricky question. At that time, permanent anti-freeze was coming into being and an alcohol based coolant was used. It required annual draining and most people had the thermostat replaced at the same time, 160 for summer (water) and 180 for winter (anti-freeze).

Now modern multi-stage oils require a minimum 180 to operate properly. 195 may be a bit too much as they came online with later emission control(s).
 

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What was the temperature rating on an OEM 1964 390 thermostat? It looks like the options are 160, 180, or 195. I'm inclined to believe it's 195. Thanks gang.
The thermostat controls the MINIMUM operating temperature of the engine.

The cooling system capacity - radiator size and airflow through it - controls the MAXIMUM operating temperature.

In a perfect world they're not far apart.

I generally use 180ish. There's no benefit to going any colder. 195 would be okay, but once again the real question is making sure you've got enough cooling system capacity to keep the temperature from going much above the thermostat number.

Most US production cooling systems back in the day were not built to cool an engine run at high load for prolonged periods, so the cooler thermostat provides a little bit more margin during incremental heavy-load operation. If you've got enough water pump, radiator, fan, and airflow to keep the coolant temps at 200degF at sustained high load you won't have a problem using a hotter thermostat.

Use of a cold thermostat in EFI cars will often fool the control system into staying in cold-start mode, running rich and dumping too much fuel into the engine, so if you really want to run that 160deg thermostat you need to double-check the temperature-mixture mapping.
 

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What was the temperature rating on an OEM 1964 390 thermostat? It looks like the options are 160, 180, or 195. I'm inclined to believe it's 195. Thanks gang.
My listings (take with a grain of salt) show 195°. However, as the others have stated, I prefer to use the temp that benefits the intended use. While the rated temp is the 'cracking' temperature for the thermostat and sets minimum temperature, typical operating temps are 5°F to 15°F above that. In no case should the system exceed 20° above or (as JEM said) you have cooling system or related failures. Note here that the factory had their reasons for the temperature they chose, but our reasons may be different. To that, I will take a moment to throw my 2-cents in with JEM's good stuff to show why I might choose one rating over another:
The thermostat controls the MINIMUM operating temperature of the engine.

The cooling system capacity - radiator size and airflow through it - controls the MAXIMUM operating temperature.

In a perfect world they're not far apart.
:tup: This is important to remember if diagnosing an under or over-temperature issue, as well as setting your own intended operating range.
I generally use 180ish. There's no benefit to going any colder. 195 would be okay, but once again the real question is making sure you've got enough cooling system capacity to keep the temperature from going much above the thermostat number.
Here I counter with lower temperatures making more power. There are different reasons for different temperatures, and compromises for how each is used in the particular application. So, colder means more power. This is why engine dyno operators looking for big numbers will often run the engine at 140°F or colder. Conversely, engine wear is less as you get colder, with the curve bottoming between 160°F and 175°F. OK, so that would indicate maybe running a little warmer than a max-power application.

But another contradiction is moisture and acid buildup in the oil from combustion residues. While oil temperatures are typically 20-50° higher than coolant temperatures, the engine must run long enough to reach that steaming temperature and stay there to get the bad stuff out.. In days past, this was not a huge concern, as everyone applied the 3000-mile change rule (or tried to). If they didn't, they received a sludged engine in return. So, the idea is to get the engine hotter right away to remove the bad stuff even on short grocery trips. Hotter still.

Add to that - if the temperatures are higher, the fuel is vaporized better for more complete burning. This is great for emissions reductions. However, with the loss in power from the heat causing lower air density, it hurts performance, including mileage. Some part-throttle mileage can be recovered with careful ignition tuning to compensate, giving nearly zero loss in mileage, but it has to actually be done to get it.

So, my takeaway from this is to lean colder if performance is primary (custom T-stats or temperature regulators). To the mid-range for an all-around car with a balance of power and mileage with emissions, and good maintenance with regular oil changes (160-175°F rated). Then to the hot side if emissions are important, or it's a short-trip car that doesn't get hot long enough to burn-off residues at lower temps (185°F rated or higher). So, there are the decision guidelines I use.
Most US production cooling systems back in the day were not built to cool an engine run at high load for prolonged periods, so the cooler thermostat provides a little bit more margin during incremental heavy-load operation. If you've got enough water pump, radiator, fan, and airflow to keep the coolant temps at 200degF at sustained high load you won't have a problem using a hotter thermostat.
Agreed. There's that 20°F range again, and the choice of rating may depend on the whole system's ability to cool - e.g., typical running temps. Adjust from there if cooler, of upgrade the system if hotter. So, if your system is so good it runs all day at 165°F on a 160°F thermostat, you might want to raise the rated temperature if you need a higher average running temperature.
Use of a cold thermostat in EFI cars will often fool the control system into staying in cold-start mode, running rich and dumping too much fuel into the engine, so if you really want to run that 160deg thermostat you need to double-check the temperature-mixture mapping.
True enough for many older factory EFI systems that use 'step' warm-up control. One advantage to newer and aftermarket systems is the full range control, or lack of steps. In this case, the AFR is maintained at the correct ratio no matter the actual engine temperature.

However, in the case of carburetors, as the air density changes with temperature after the carburetor, the carb cannot self adjust like EFI can. This just means that a change in the average operating temperatures of a carbureted engine may require re-tuning to take advantage of the change, or reduce losses. This final point makes it clear that if you are making no other changes to the car, and anticipate no re-tuning or other changes, the stock thermostat rating or possibly one step cooler is probably the best choice.

David
 

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Discussion Starter #5
Those were some excellent replies. Thank you so much guys.

Advanced Auto Parts charges $7 for the 160F thermostat and $5 for the 195F thermostat, and were out of stock of the 180F. Since there's no rush, I'm going to wait for the 180 to come back in.

The 390 is a fresh build to OEM specs, but given the consideration at the factory vs. the consideration of my application, 180F seems like the most reasonable compromise.

Thanks again!
 

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BTW, the thermostat is one place I always buy the best one I can get. Most brands use something like "ultrastat" or other premium designation. These versions are high-flow, and usually have an air-purge feature, such as a V-notch, jiggle valve, bypass, etc. I'm cheap about some stuff, but never thermostats.
:tup:
David
 

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Here I counter with lower temperatures making more power. There are different reasons for different temperatures, and compromises for how each is used in the particular application.
Yes, in a controlled situation...I remember a tour of Steve Dinan's facility some years ago when he was explaining that if you wanted the best dyno numbers you'd run the car until it was very hot - get the engine oil, the transmission oil, the rearend, wheel bearings very hot so that the lube would be as thin as possible - then you'd introduce cold coolant into the engine as quickly as you could without breaking something and do your runs and take your number.

Most modern engines have much less coolant in the block - the lower half of the bore won't even have a water jacket - and particularly in the case of a aluminum block they will heat up - and cool down - much faster than an old FE.
 

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Yeah, that's with a chassis dyno, where they are juggling the benefits between a hot chassis and cool engine. I was talking engine dyno where the chassis did not need heating, but the goal is the same. In either case, you want the overall engine cool for best power, but it's not worth sacrificing too much in the other ways mentioned - at least in the typical street engine. This isn't splitting hairs like racers cold-soaking with bags of ice, or running 140° or less on endurance racers to balance power and mileage to save fuel weight.

David

PS: It's funny, watching shops race dynos for big numbers, to me anyway. My boss when I was young would threaten to fire anyone that mentioned only peak HP (by itself a useless number), or ran testing at other than the conditions the engine would be used (controlled-condition data irrelevant to the use). He was in the business of building winning engines - not advertising numbers. :tup: If someone thermal-shocked an engine, he probably would have beaten them to death with a crankshaft right there. :D
 
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