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Engine Vs. Radiator Temp



My '75 coupe (L-48, Turbo 400) runs warm at freeway speeds. I know, I know, most C-3s seem to run a little warmer than momma's Taurus. I have read on several occasions that one should check to make sure that the temperature sending unit is reading correctly, before assuming that the engine is indeed running warm. This makes sense, especially if the resistance Vs. temperature rise does not change on a linear scale. I would assume that it should be linear, however this could be incorrect. There is more to say concerning that, but maybe another time.

I have read on several posts that one should remove the radiator cap and place a thermometer (or pyrometer) of known accuracy directly into the coolant in order to determine actual coolant temperature. So far so good - except that, because of the direction of coolant flow, there should be an appreciable temperature drop between the coolant running past the sensor in the cylinder head and the coolant temperature at the fill spout.

Before I commit myself to buying my wife a new meat thermometer (although I think I'm gonna get stuck with that anyway . . . she wants one of the new fangled read it from outside the oven jobbies), would anyone have empirical knowledge of what that drop typically would be? I have tried to roughly calculate what the drop might be at various temperatures. However, thermo dynamics was never my strong suit, especially with so many variables.

Any help would be appreciated, not only by me but, I'm sure, a number of others. Thank you in advance.

Its nice to see another M.E. on the forum.

I will be graduating in May of this year.

Did you actually try and calculate the temp?

I am not to sure the path the water takes as it travels through the engine or the distance from the temp sensor in the valves to the exit spout. I am sure it is not that much more that it would go up more than 10 deg or so, as it is not that much more distance. I am not to sure velocity of the water through the system.

I would assume the water is at its tale end of the cycle and that the increase in temperature between the sensor and the spout would not be that significant. The velocity of the fluid through the engine should be significant because i am currently getting the radiatory flushed out because the water is not "shooting out" of the cores.

those are my $0.02
Actually, there should be a temperature DROP between the sensor and the spout. The coolant travels from the sensor, through thermostat, into the top left side of the radiator and the left to right, where the fill spout is located. The velocity of course, is relevant to engine RPM. The temperature drop would probably also vary depending upon whether or not the car was moving, which would increase air flow and thereby cooling capacity.

See what I mean by all of the variables? And that's only scratching the surface. I'm sure it could be calculated if one truly wanted to take the time. Not me!!

So anyway that's why I'm searching for hard empirical data. Maybe someone with a sensor that is known to be good will be kind enough to take some readings and pass along the info.

BTW - an early congrats on your graduation - what is your specific discipline?

An exercise in thermal mapping.
1) Go ahead and purchase the new fangled read from outside the oven temperature thingy, if it is what I think it is. I would guess it is an infrared pyrometer.
2) Map out your block and exhaust at the same point on each cylinder.
3) At the water pump suction.
4) Take a set of readings,
1. Cold (ambient conditions)
2. Warmed up and at idle. Provides a constant for factoring the load on the water pump. There should be a curve available.
3. After a formidable turn around the block to replicate as close as possible to the conditions you are looking to remedy or understand.

One constant to pay close attention to is the rating on the thermostat. That car is probably a 190 degree. That should be the expected temperature at the thermostat housing plus the loss through the housing itself.

I would if I thought I could get away with it, buy a Pyrometer from the local parts store. You can pick one up for about $75.00 and be confident in getting good information and the wife can borrow it when needed……….Surrrrrrrrrrre…..we all know it’d wind up in the kitchen.

If all is found to be fairly uniform I would look at losing some of the coolant. Coolant is not really a fair term for the green stuff, in fact if you’re running a 50/50 blend you’re not rejecting heat at the radiator as efficiently as can be done.
Water in the summer is the best for heat rejection but you will need to add a corrosion inhibitor. You also need to watch the pH. It is a simple maintenance procedure, but make sure you protect with the green stuff in the colder climes.
I shouldn't even try to post because I really don't know for sure what the heck you are talking about:confused But, I assume you are wanting to know if your temputure guage is or is not properly functioning. Am I correct, so far? If so, what you said about testing the guage is right, however, just go down to the local parts store, get you a mechanical guage, install it per directions where the temp sending unit is or the other head and compare it to the original guage. The problem with the meat thermometer is that, if the car is hot and you open the radiator cap, your Corvette is not going to be the only thing hot................The coolant will come out and it will come out fast and hot, all over you, your engine, fender, etc. Boiling point of water is 212 degrees, with antifreeze it is a little higher and under pressure it is even higher, therfore if you open the cap and relieve the pressure, boiling occurs, and the coolant hits the fan, so to speak. If, you put the thermometer in and then start the car, the coolant will begin to come out of the radiator and by the time the car gets hot enough to tell what it is doing, it will be empty and again it will be all over the car......For some reason, most temp meters that I have dealt with seem to read fine, up to a point, then they go waco. Here again, the meat thermometer would not work, even if you only tested it until the coolant began to rise. What ever you do, be careful, this stuff is hot, even with a car that is not overheating the coolant is still around 190 degrees, so please be careful with your testing............I'm no Mechanical Engineer,but I did sleep in a Holiday Inn last night.:gap I have been burnt a few times and I would hate for anyone to have to go through that pain.......Steve
OH i misunderstood your question.

To cut down the variables the best would be at idle and at cruising rpms, due to the fact thats where you will be most of the time. Also go with the worst case scenario 110 degree day at idle and cruising rpms.That cuts it down to 2 cases. I am not to sure what the fan pulls and the flow of the fluid across the radiator. I have not actually looked for that kind of data.

I would do the calculations if i had those numbers. Also assuming an ineffiency in the flow across the radiator do to the fact that the air is not completely going across the whole radiator.

I took heat transfer last spring so the stuff is still fresh in my mind.

I am doing the general M.E. nothing specific but i am going to stay for my masters and maybe do my thesis on the current research i am doing. Which is Particle Image Velocimetry (PIV) measurements in anatomical models and cross checking them with CFD. So i guess, to answer your question, specialize in the thermo fluids aspect.
temp check

Heck I would just take the sending unit out of the block, boil a pot o water on the stove and dunk the sending unit in it. Turn on the ignition and see if the gauge looks about right for 212 degrees.
Re: temp check

resto75 said:
Heck I would just take the sending unit out of the block, boil a pot o water on the stove and dunk the sending unit in it. Turn on the ignition and see if the gauge looks about right for 212 degrees.

And if I were you, I would make sure it isn't your wife's favorite pot that you use while attempting this test :L.

]Originally posted by ssvett
The problem with the meat thermometer is that, if the car is hot and you open the radiator cap, your Corvette is not going to be the only thing hot................The coolant will come out and it will come out fast and hot, all over you, your engine, fender, etc.

That's what I was thinking too, Steve, glad to know I wasn't the only one...

Barb :w
Once you determine the accuracy of the gauge and sender and find their accuracy acceptable. I'd check the following:
1) make sure the spark timing is not retarded
2) make sure the engine is not running lean at part throttle
3) make sure the stock front air dam is in place and fully-functional
4) make sure the radiator and other cooling system parts are working properly
Thank you everybody for your comments and suggestions. As for burning myself . . . I did that with a '73 Toyota Celica back in 1977 or so. And that from a guy who cut his teeth on connecting rods. My old man was a motor head . . . those of you who are truly familiar with that term know what I mean. I rebuilt my first engine when I was 14 . . . a Chevy straight six.

So, for anyone listening to these posts, take special heed to the safety aspects of this little experiment . . . if it can happen to an old motor head, it can happen to anyone.

Okay, now for the good stuff. In response to the question of the "outside the oven thermometer", I wasn't really refering to an infra red pyrometer. Rather, momma has her eyeballs on one with a standard probe (actually it is probably an RTD) connected by wire to a gauge. In this way, you can string the wire to the outside of the oven. I tell you this, because I am now stuck buying one!

Yep, I stuck the ol' meat thermometer into the fill spout, with the engine dead cold. Started her up and waited for the thermostat to open. BTW, it is a 180 degree thermostat. After allowing some time for temperature stabilization, it appears that - A.) The temperature gauge reads about 10 degrees too warm, B.) There is a temperature drop at idle between the sending unit and the fill spout of 10 degrees, C.) A combination of the above.

I guess what I'm saying in a long winded way is that I have proven nothing. That is why I needed empirical data. So, has anyone done this? If so, what were your readings?

One more thing. Hib mentioned the timing being retarded. This is a California car and the spec calls for 6 degrees BTDC. Even at that, the engine sounds labored. What would be your recommendation for total advance?



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