Okay, so I think this is on topic, and if not ill be happy to see it go poof, but - It is considered common knowledge around the hotrod community that running without a thermostat may cause overheating. The reason quoted for this is that the coolant moves through the radiator too fast to properly cool. Now, a little background on me. I have a Mechanical Engineering degree with an emphasis in thermal systems. My day job consists primarily of designing and testing cooling systems for liquid rocket engines. I consider myself to be fairly well versed in the fundamentals of heat transfer and thermodyamics. And I say that this "common knowledge" is just flat wrong. I cannot think of a single justification for this claim, and I've spent a good amount of time thinking about it. I frequently get into arguements on boards with some guy who's grandad's cousin's brother had a SBC that ran hotter without the thermostat, therefore the aforementioned must be true. What I want to know is, without this turning into a flame war, can someone give me a good reason why they think this is true? In my own experience, it is emperically false as well as analytically, so I feel pretty confident in my statement. But this seriously bugs the shit out of me. At work we say that if you are calculating velocity and you get 2.5c, don't just assume you've invented warp drive - You've probably done something wrong. So it seems all of the hotrod community is convinced I am doing something wrong, I just can't figure out what it is.
In some case's couldn't the coolent be travling though the rad at a rate that wouldn't cool the coolent enough, and that the engine would run hotter?
Just like you can't burn your hand if you run it thru a flame fast enough, you can't cool down water if you run it thru a radiator too quickly.
The purpose of the thermostat is to maintain the the engine at a theoretical optimum operating temperature for performance or economy, in our purposes performance.
I understand that is what most people say, but that is just wrong. If you think of the system as a closed system (Which it is) then your heat input vs time will be the same, so it won't really matter how fast it is flowing. If it is flowing at a speed that won't allow that much heat out in the radiator, similarly it is flowing at a speed that won't allow much heat in in the engine. The equation looks something like this Q (heat flux into or out of coolant) = dT (change in temperature from inlet to outlet ) * Cp (specific heat of the coolant) * mdot (Mass Flow) Since the water is a constant density fluid for most purposes, the mass flow is analagous to volume flow, which is analgous to velocity. So when you set the engine side = to the radiator side (which must be equal for the engine to be at a constant temperature, heat in = heat out) then you get dT_engine *Cp(water) *mdot(water) = dT_radiator * Cp(water) * mdot(water) Anyone who has taken jr highschool algerbra can see that the mdot term and the Cp of water cancel out, so the mdot has no effect... (once you acheive a steady temperature, of course, having too low of an mdot will affect your ability to ever reach a steady temperatyre I.E too low of flow, Q_engine > Q_radiator = boil over)
From what I know of the subject, a restrictor (thermostat or a washer in the thermostat housing) is advantageous if the radiator has vertical tubes. If the radiator is a cross-flow with horizontal tubes, then there is no advantage to having a restrictor. I guess the reason for this is that the flow by gravity through the vertical radiator can sometimes be too fast for the radiator to effectively transfer heat out of the coolant.
Makes sense. Doesn't a given material(engine coolant) have to spend a sufficient amount of time in the heat exchanger(radiator) to exchange the heat to the atmosphere?
Its been my experience that typically a motor won't run hot when at speed on the highway due to the increased airflow across the radiator's cooling surface. However, when in stop and go traffic, stock mechanical type fans typically won't pull enough air to keep the engine cool, therefore the steady increase in engine temperature. I've also known guys who are traveling type distributors/salesman who would completely remove the stock mechanical fans to decrease the drag on an engine and increase fuel mileage. They say this works great until they get stuck in traffic. My personal truck - an '89 Chevy 1/2 ton with a 350/5spd is what I'd consider a bit of an enigma in itself. It runs great, with no real cooling issues. However, as soon as it warms up, it runs between 200-210 degrees consistantly, regardless of load, ambient temperature, or anything. It has a new radiator and the thermostat has been recently replaced when I changed the motor awhile back. It does have a lowly 2 core equivalent radiator in it, and possibly this summer I'll be changing to a 4 core. The point being is that once it reaches 195 degrees, the thermostat stays open on it continuously. So to answer your question: Yes and no, it does/doesn't make a difference.
That seems intuitively correct, but I assure you that is wrong. It is more analagous to if you put your hand in for 10 seconds or if you put your hand in 10 times for 1 second each - the amount of heat transferred to your hand will be the same. There is a constant flow of water.
This may sound like the answer of someone who is a blind follower, but,"The engineers must have put it in there for SOME reason,"
"If it is flowing at a speed that won't allow that much heat out in the radiator, similarly it is flowing at a speed that won't allow much heat in in the engine." I think this may be where your reasoning is flawed. If the coolant speed is too fast to dissipate heat properly in the radiator AND it is too fast to absorb heat from the engine that would exceed the radiator's capicity to cool at a certain flow rate...then the engine is going to continue to get hotter and hotter. The hotter engine components will then transfer additional heat to the coolant which is continuing to flow at the same rate through the same capacity radiator. Then the temperature gauge climbs and the coolant boils.
Even if allowed to cool for 10 seconds, 10 minuites, an hour between exposure to the heat? Just like cooling in the radiator with the thermostat close for a while, before going back into the hot engine block? I appreciate your education and background, but this probably can't be explained in layman's terms, will require nerd/techy/labcoat $20 words.
Here's my take. For this conversation, let's say that without the thermostat the water travels through the cooling system twice as fast as with. The water picks up only HALF the heat from the engine, and dispells only HALF the heat while in the radiator. But, it does it TWICE as often. Therefore, i believe it's a "push" (even results either way). Also, in my shop i've had several heating problems over the years and NEVER, i repeat, NEVER has the addition of a thermostat been the cure. NEVER. I also believe the tubes in the radiator are a natural restriction for the cooling system. I also believe this "wive's tale" originated with a 57 Chevy type radiator, where the upper hose that feeds water back to the radiator feeds water to the middle of the upper tank right below the radiator cap. Perhaps without a thermostat, the water could have enough flow to unseat an old cap, pushing water out the overflow creating the "illusion" of an overheated engine. You want cooler? Get the coolant out of the system! It's a well known fact coolant runs way hotter than water. Run water with an anti corrosive
Yes, they did. To bring the engine up to operating temp quickly for efficient wear, heater operation, and economy. NOTHING more
tell me more about running water with an anti corrosive... wont water turn to a gas mush faster than the chemical coolant and pressurise the system much faster and at a lower temp, causing it to boil over??
I always thought the thermostate was just for warming and engine up. and that once it was warm enought it just let things flow freely. Shows how much I know.
I had the exact opposite problem in a Charger I had, it would not heat up, I thought the thermostat was stuck open and when I went to pull it out there was no thermostat at all, put in a new thermostat and it ran right in the middle of the temp guage after that and I had heat. I would also think it depends on the surface area and speed the coolant is traveling, after all a radiator is just a heat exchanger.
Isn't the radiator a restrictor by itself? It is designed to slow down flow at a particular rate to keep a given engine at temperature. I experimented with different radiators and thermostats and coolant mixes, etc. The hot so-cal summer months and long freeways are perfect testing grounds. I found that straight water is the most efficient coolant, however it eats metal without additives. If an engine runs hot, it doesn't matter if you use a thermostat or none. The only thing that helps cooling then, is a larger radiator or cooler outside air. Or in standing traffic, more airflow from an electrical fan that runs at high rpm, regardless of engine rpm. Thermostats are minimum temp valves. I use the high-flow type with a bigger opening and less restriction, so the radiator can flow at the rate it was desgined for.
Couldn't agree more. Stick to rocket science. I have seen many a post on "why does my car run hot?" and how to fix it. I say if it works for you, then you fixed the problem.
Years ago someone at Dow Chemical figured a way to sell snow to Eskimos(Anti-Freeze to people in hotter climates) by calling it COOLANT. They say it'll keep your car from overheating by raising the boiling point. Every pound of pressure in the cooling system raises the boiling point of straight water 3 degrees. So, let's say we run a 15 pound cap. This raises the boiling point of water 45 degrees, for a total of 257 degrees. Hell, how much higher ya wanna go? At 257 you're fucked anyway. Add to the water an anti-corrosive and you're done in warmer climates. If you live where it freezes, you will need the protection of anti-freeze. But, if you live in THAT climate, overheating is NOT an issue
I ran my 350 in my ElCamino without a thermostate, cause it was overheating all the time, when the car was standing. The reason for that after all was a little rubber ring in the heat-exchange radiator inside my ElCamino, where the system sucked air. In mechanic-school we learnt, that the thermostate is for keeping the temperature constant and for getting the engine faster on working temperature. I ran my Opel straight six engine without thermostate and had no problems with it. But I will put it in again, because when its overheating, there must be a reason for that.....
Actually, overheating is more of an issue.....I can't explain it, but you'd see more cars on the side of the road, overheated, during the winter.....(i grew up in AK)
I have to see this BORT's way. I have heard the same thing before about the water going through so fast that it doesn't cool. Doesn't make sense to me. The efficiency of the radiator doesn't change. The Delta between the air flowing through and the water flowing through increases if the water temp goes up thereby extracting more heat from the water. The engine produces no more heat with or without a thermostat. It should hit an equilibrium between the amount of heat the engine produces and the amount of heat the radiator can remove and that would depend on the size of the radiator. But it should not overheat. Educate me. Scrump
Probably a bunch of stuck thermostats. LOL. I couldn't resist. On "average", an older car like what we drive will run 100 degrees hotter than the air temp outside given a healthy cooling system. This ain't carved in stone, but it's close. Check it out
Well, our cars weren't running at 60* (100 + 40 BELOW zero)hahahaha!!!! That's ALOT of stuck thermostats.......we got a WARRANTY problem..haha!
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