What is the purpose of a thermostat?

[JAM1775]

When I decided to drop my crate 351W thermostat from 180 to 160 I first called Ford Racing Tech line. They advised the 160 would be just fine and could improve performance, not inhibit performance. This with a carb, no computer and no heater.
I use synthetic oil and 160 engine temp is more than enough for our modern day oils to lubricate properly. The most motor wear is in the first few minutes not at operating temp. My 351W motor still runs in the 175-180 range in the summer and can even reach 185 range on hotter days but the thermostat is at 160. On coller days it levels out at 165-170 degrees. I have read artricles where carb motors have run better on the dyno at 160. Certainly EFI computers get confused with Air/Fuel ratio and a heater is warmer at 195 than at 160 degrees but each motor is different.
I agree if you pull your thermostat out you should NOT leave nothing but should cut out the center to slow down the coolant flow for proper radiator efficiency. I also learned that trick from Southern Automotive from a prior BB Cobra I owned. That 390BB seemed to run hot with ANY thermostat yet ran at 180 with the center punched out ! Cannot figure but it was the case! With a carb and a heater, if used, I would probably go to the 180 degree thermostat.

[chopper]

Renaissance man:

Lyle and dalola are correct. The thermostat is there to maintain a minimum temperature. Another important function is to aid in helping the engine come up to temperature quickly after startup. The thermostat does this by restricting flow of the coolant through the radiator until the coolant in the engine heats up. Once the coolant temperature reaches the thermostat's setting, the thermostat begins to open and allow flow through the radiator. As the fluid is cooled through the radiator, the thermostat partially closes and regulates the temperature by providing a partial restriction to the fluid. Once it's fully open, additional elevation of the temperature will have no further effect on the thermostat or the coolant flow through the radiator. The coolant flow is then regulated as a "controlled leak" through the thermostat to maintain coolant temperature.

The thermostat also acts as a flow restrictor, and provides some back pressure to prevent cavitation of the coolant as it runs through the engine, especially through the cylinder heads. This is the theory behind Southern Automotive's advice to remove the thermostat and install a restrictor plate. I have heard some racers swear by this and others say the theory is unsupported. Since I figure Mr. Parham knows more about his engine than I do, I took his advice and left his restrictor plate installed.

As far as coolant temps go, 190 on the road and 210 at idle is, IMHO, certainly acceptable. As others have pointed out, many late-model cars run as high as 230 for efficiency and emissions. Higher than that would cause me concern.

JAM1775:

I am interested in your comment about running the engine at lower coolant temperature for more power. For point of discussion, I have seen graphs in some automotive texts which show engine wear as a function of sustained coolant temperature, and most show a marked increase in wear (as a function of component clearances) when the coolant operating temperature is stabilized at temperatures below 180 degrees. I can certainly understand why cooler cylinder heads would aid in power production (denser fuel/air mixture into the cylinder) but do you have an opinion about the increased wear issue? Or is the 20 degree difference not significant?

[ohekk]

I was concerned with running too cool and too hot as I had
a NAPA 180 degree thermostat for about a year and watched my coolant temp fluctuate from 175 - 210.

I replaced it this year with a 180 degree Robert Shaw thermostat and it now holds a steady 180.

[JAM1775]

Chopper.....I recall reading an article in a hot rod mag and another at the Performance Unlimited website about running cooler 160 degree temps for power with no mention of longevity consequence. This past summer I installed the 160 stat, after speaking with a Ford Racing Tech. However, your comments caused me to again call Ford Racing. Your advice is very sound because the opinion expressed today differed from my last tech call and cited chance of increased engine wear.

Today I was advised the 160 degree temps make more power but ARE more likely to cause engine wear. Synthetic oils are better but I was advised there is more engine wear just the same. I would never expect a 100,000 mile life span from my Cobra motor (I do push it around a bit !) but I am swapping back to my 180 degree stat. Any excess wear factor is not worth the risk. Especially in the coming winter months when the motor will mostly run at thermostat temp. I also inquired with another race website I seek advice from since I cannot recall the mag article . No response yet but ...guess what...not worth the risk.
So, much thanks for your comments !

[chopper]

Sometimes I get lucky. Glad it worked out for you.

[renaissance man]

I ran about 40 miles yesterday in and out of traffic. Temp stayed right at 190 on the highway at 70 mph and the fan kept it between 200-210 at stop lights.

I guess I'll stop worring about the temp.

Thanks everyone for the input.

[mr0077]

Hey guys, my understanding of the T-stat is that it does two things: it helps the motor reach operating temperature quicker, (for longevity and efficiency in production cars, primarily), and it provides a restrictor to the coolant flow through the block, to promote pressure flow in the block and heads and to avoid localized hot spots or "steam pockets" in the motor. If your car wants to run at 210 with a 190 T-stat, changing to a 160 or taking the T-stat out will only make the engine take longer to warm up to 210; but, If the cooling system is working properly, the car will run at an acceptable temperature, or near T-stat temperature, whichever is higher. An interesting comment I once read was, if auto makers could figure out a way to get rid of the T-stat, there wouldn't be one on your production car...made sense to me. Hope this helps.

[concobra]

If sounds like you are asking one question but looking for an answer to another.

You question of what a thermostat does is: Controls the temperature point which the coolant is allowed to return to the radiator. I know this is an over-simplification but serves the purpose of keeping the focus on looking for the real problem. Which is why your cooling system is not controlling your engine temperature.

We need to define the parts associated with the engine cooling system and comment on their contribution to this process.

1. Radiator (better defined as a water to air heat exchanger)
2. Water pump. used to move coolant through the system.
3. Thermostat. Already defined.
4. Water pump drive (pulley's and ratio's)
5. Coolant (water and additives such as antifreeze)
6. Pressure cap.
7. Hoses
8. Waterpump to engine bypass. (The small hose usually running between the intake manifold and the water pump.)
9. Fan: move air through the radiator.


Possible reasons for over heating:

1. Radiator's core cross sectional size: Basic rule of thumb is 1 square inch of surface area to 1 cubic inch of displacement. Or as related to HP: 1 Hp to 1 square inch of surface area. If you have less than this area you may not have enough cooling area.
2. Radiator thickness: This has more to do with capacity than cooling ability. Too thick a radiator can be as much of a problem as too thin. REMEMBER a radiator is a water to air heat exchanger so if you cannot get air through the thick core you cannot cool the water. Good rule of thumb is no more than a two inch thick core for aluminum or 3 core for brass.
3. Water pump. Most stock pumps are more than up to the job. Especially Ford units. I would not think your problem is here. Be aware that some after market units perform well at upper RPM figures but actually flow poorly at normal street RPM. Therefore they can contribute to overheating at idle and low RPM running.
4. Thermostat: Not usually the cause of overheating. Unless stuck and this rarely ever happens and is quick to diagnose.
5. Running without a thermostat: Bad idea. Does not allow you to stabilize your running temperature. The engine does not get a consistant coolant temp. Can lead to overheating but rare. Oil sluging and excess ring wear. Besides, how do you control HP without temp control?
6. Water pump drive: Often ignored issue. You got to move water to cool the engine. Basic rule for performance engines is a 1:1 ratio crank pulley to water pump pulley.
7. Coolant volume: Air in the system. excess air (a common problem with low mounted radiators like Cobras) create hot spots. Fill your system to the very top then run engine and let the excess blow off through the 13lb. cap. The balance should be fine.
8. Fan air volume: VERY IMPORTANT. And I believe is your problem. Remember the radiator is a water to air heat exchanger. And without adequate air movement you cannot cool the coolant. I am going to assume you are using some kind of electric fan as most Cobra users do. Almost all electric fans offered that will fit a Cobra will not move enough air to cool a high performance engine at low speeds and idling. THE RULE OF THUMB IS 10 CFM PER CUBIC INCH OF DISPLACEMENT. In other words if you have 350 CID you need 3500 CFM. THe highest common fan rating is 2900 CFM.
9. There are other potential issues that can cause overheating but they do not usually contribute to a problem that allows the temperature to rise as fast as your question suggests. So don't concern yourself with these now.

Back to testing the fan issue.

Get a infrared temperature gun. (Usually available at most part stores and around $100.00) Start your engine and allow it to warm to thermostat opening temp. Use the temp gun and point it at the top of the radiator where the hose enters the tank. Take a reading against the tank as close to the hose as possible but not on the hose. Record the reading. ( Should be around 10 degrees cooler than thermostat opening point) Now let the motor to start to heat above the thermostat temp rating. Again measure at the top THEN take another reading where the lower hose meets the tank. ( Top should be very close to engine temp reading and lower hose shoud be aound the same) Record the readings. Turn on your fan and let the fan run for 2-3 minutes while allowing the motor to continue to heat up. Of course don't allow your motor to exceed 230 degrees. Measure the temp at the top and bottom hose points. IF THE TEMPERATURE DOES NOT DROP DOWN AT THE LOWER HOSE BY 30 DEGREES YOU ARE NOT PASSING ENOUGH AIR THROUGH THE RADIATOR. If you find that this is your problem there are ways to help solve this and you are welcome to contact me via e-mail or phone @ 541-447-1398.

Good luck.

[Dave Samson]

I've followed this thread with great interest having suffered overheating problems for a long time. I never did get to the root of the problem although by elimination I suspect that the cause was residual air somewhere in the block which I was unable to flush out.
My solution was rather drastic in the end, I eliminated the mechanical water pump and thermostat, replaced them with a staged motor electric pump whose pump rate is controlled by the voltage it receives which is determined by the dial setting on the control unit which gets its water temperature from a probe in the top radiator hose.
The bottom line is that I can dial in the temperature that I want it to begin pumping at 3V. then in temp. increments the pumping rate increases by additional voltage to 6V then 9V then 12+V for different situations depending on how I want the engine to perform.
No more overheating problems, no air locks.

But I don't think that this overheating problem should be seen in isolation. I would also be concerned about the engine oil temperature, a figure that is far more important than water temperature.

What oil temp. readings do you get when the water temp is 210F at standstill ? How do the oil temperatures move around relative to the water temp. ?

[JAM1775]

Air locks can be resolved by burping your motor or running it for 20-30 seconds then topping off the coolant. Once the rad cap is mounted the air remains trapped. When I refill a drained motor I remove the temp. swith or other intake plug . Air escapes from the block as you are filling with coolant at the rad cap. You can feel it escaping. Just add coolant slowly as the plug hole can quickly overflow since the rad cap is usually at a higher level.