Joined
·
12,626 Posts
Hot is Good
I’m going to try to explain, once and for all, why you want to run your engine at the highest temperature that is sustainable. Every year, there are a hundred threads about thermostats, fans and overheating.
There’s a lot of advice floating around the internet. Much of it is contradictory, much is based on the writer’s experience. I’m going to try to explain this scientifically. As with all my advice and suggestions, it’s your car, do what you want, I know mine will work tomorrow morning.
Heat Engine
The Otto cycle engine is one type of heat engine. A heat engine converts thermal energy into mechanical energy. It does this, adhering to the first law of thermodynamics, by exploiting the difference between high and low temperatures; the greater the difference, the greater the available power. Power = efficiency whether you use the power for the drag strip or fuel economy.
Since your heat engine exploits the difference between hot and cold, you get an increase in power when you increase the operating temperature of your heat engine. If your engine is very cold, you need a deliberate enrichment device to pump extra fuel into the heat engine. If you have fuel injection, the enrichment is a continuum, unlike a choke mechanism that shuts off, your ECU will guaranty that you’ll have the enrichment needed for the temperature you’ve chosen.
The heat transfer in your engine should be progressive. It is about 2000f in the combustion chamber 1500f at the back of the exhaust valve. 600f at the combustion chamber surface, 350f at the coolant passage in the head, 195f at the thermostat and 185f in the radiator. This assumes a normal 195f (89c) thermostat. A lower temperature thermostat will allow more coolant flow and suck more heat out of the combustion chamber. Note at this point that you can remove more heat from the combustion chamber (and buy the gas to do it) without increasing your car’s total ability to shed heat; you’re just choosing to shed heat, you’re not increasing cooling capacity. The limit of your ability to shed heat is in the surface area of your radiator and the airflow, not in your thermostat (coolant or fan).
MYTHS
Cooler is Better
Actually it’s worse, that’s why you warm up your car. The oil doesn’t start to work right until it’s 180f. Rich mixtures dilute the oil with excess gas. The pistons don’t expand to fit the bore until they’re hot.
Cooler Will Protect my Head Gasket
A 20 degree change in coolant temperature will not make your head gasket last forever. The things that cause head gasket failure are: corrosion, overheating, head warping, and high combustion chamber pressures (lugging, knock). Some say the number of heating and cooling cycles is a big factor.
Safety Margin
Some think that watching the gauge, and having more time to do so, will give greater warning of a problem, and more time to react. That’s true to an extent. If you lose your belts and the water pump stops turning, you may get an extra minute before the coolant boils if you run cold; but no more than a minute. Keep in mind that the boiling point of a 50/50 mix of Glycol and water, under 10 psi, is around 275f; a long way from 195f.
These are facts about how internal combustion engines work. There is a market for all kinds of things: 82c T-stats and fan switches, coolant additives, auxiliary fans, Water Wetter, hood scoops, ad infinitum. The fact is, if your cooling system is not broken, it will maintain your chosen temperature in arctic or desert conditions. Your cars would all do it when new, if yours won’t idle indefinitely, with the A/C on, in the desert or the tundra it needs to be fixed, not modified.
I’m going to try to explain, once and for all, why you want to run your engine at the highest temperature that is sustainable. Every year, there are a hundred threads about thermostats, fans and overheating.
There’s a lot of advice floating around the internet. Much of it is contradictory, much is based on the writer’s experience. I’m going to try to explain this scientifically. As with all my advice and suggestions, it’s your car, do what you want, I know mine will work tomorrow morning.
Heat Engine
The Otto cycle engine is one type of heat engine. A heat engine converts thermal energy into mechanical energy. It does this, adhering to the first law of thermodynamics, by exploiting the difference between high and low temperatures; the greater the difference, the greater the available power. Power = efficiency whether you use the power for the drag strip or fuel economy.
Since your heat engine exploits the difference between hot and cold, you get an increase in power when you increase the operating temperature of your heat engine. If your engine is very cold, you need a deliberate enrichment device to pump extra fuel into the heat engine. If you have fuel injection, the enrichment is a continuum, unlike a choke mechanism that shuts off, your ECU will guaranty that you’ll have the enrichment needed for the temperature you’ve chosen.
The heat transfer in your engine should be progressive. It is about 2000f in the combustion chamber 1500f at the back of the exhaust valve. 600f at the combustion chamber surface, 350f at the coolant passage in the head, 195f at the thermostat and 185f in the radiator. This assumes a normal 195f (89c) thermostat. A lower temperature thermostat will allow more coolant flow and suck more heat out of the combustion chamber. Note at this point that you can remove more heat from the combustion chamber (and buy the gas to do it) without increasing your car’s total ability to shed heat; you’re just choosing to shed heat, you’re not increasing cooling capacity. The limit of your ability to shed heat is in the surface area of your radiator and the airflow, not in your thermostat (coolant or fan).
MYTHS
Cooler is Better
Actually it’s worse, that’s why you warm up your car. The oil doesn’t start to work right until it’s 180f. Rich mixtures dilute the oil with excess gas. The pistons don’t expand to fit the bore until they’re hot.
Cooler Will Protect my Head Gasket
A 20 degree change in coolant temperature will not make your head gasket last forever. The things that cause head gasket failure are: corrosion, overheating, head warping, and high combustion chamber pressures (lugging, knock). Some say the number of heating and cooling cycles is a big factor.
Safety Margin
Some think that watching the gauge, and having more time to do so, will give greater warning of a problem, and more time to react. That’s true to an extent. If you lose your belts and the water pump stops turning, you may get an extra minute before the coolant boils if you run cold; but no more than a minute. Keep in mind that the boiling point of a 50/50 mix of Glycol and water, under 10 psi, is around 275f; a long way from 195f.
These are facts about how internal combustion engines work. There is a market for all kinds of things: 82c T-stats and fan switches, coolant additives, auxiliary fans, Water Wetter, hood scoops, ad infinitum. The fact is, if your cooling system is not broken, it will maintain your chosen temperature in arctic or desert conditions. Your cars would all do it when new, if yours won’t idle indefinitely, with the A/C on, in the desert or the tundra it needs to be fixed, not modified.