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  #16 (permalink)  
Old 11-14-2003, 08:25 AM
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I got it! It dawned on me what that guy was saying in his 'Myth' article. Of course there will be no reduction in heat removal by the radiator, regardless of the water flow. This assumes that the engine keeps running and doesn't melt down.

It must be true that the system reaches equilibrium - heat rejected = heat generated - otherwise heat would build up in the engine until it melted something, stopped and quit generating heat. BTUs in MUST = BTUs out.

However, when the temperature of the water is high, the temperature in the engine must be higher to generate the temperature difference (pressure/voltage) that forces heat from the engine into the water. That higher temperature resulting from high water flow/low radiator efficiency is the problem. Heat is being rejected but at the expense of higher operating temperature which is the bane of the hot rod engine.

Here is a great link I just picked off another thread which goes into great detail on a lot of cooling topics.

http://www.grapeaperacing.com/GrapeA...oolingmods.cfm

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Last edited by willys36@aol.com; 11-14-2003 at 11:09 AM.
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  #17 (permalink)  
Old 11-14-2003, 04:29 PM
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Willys,
I'm still not sure that is quite right. As I think about it, higher flow coolant through the radiator decreases temp coolant change through the radiator, but conversely it also decreases coolant temperature change through the engine.

Using that fact that there is no reduction of heat removal by the radiator, regardless of water flow, than the assumption must also be that the engine is generating the same amount of BTU's, regardless of the water flow.

So....The engine must be generating a certain amount of BTU's, and the radiator is shedding a certain amount of BTU's, regardless of water flow.

So as long as coolant flow is adequate, higher coolant flows probably won't help nor hurt.

By the way that is a good link and they also said "contrary to popular belief, you cannot make the coolant flow too fast" and that slow water flow, even stock water pump flow will result in unequal head cooling causing #3&5 to run hotter than the other cylinders. Makes me want a better water pump

Last edited by Wings; 11-14-2003 at 04:46 PM.
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  #18 (permalink)  
Old 11-14-2003, 04:46 PM
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As the flow rate increases, the residence time of the water in the radiator decreases. Since the water-to-air heat exchange process is very inefficient, this reduced residence time results in the water leaving the radiator at a higher temperature. It thus necessarily circulates through the engine at a higher temperature and enters the radiator at a higher temperature. This is absolutely necessary if it is to move the same amount of heat at the different flow rates. It is also necessary that the engine operate at a higher temperature if it is to transfer the same amount of heat into the water. The water in the radiator will reject the same amount of heat but must do it at the higher temperature. It is this higher temperature process (shows up on your temperature gauge) that causes all the consternation and attempts to get the engine to run cooler.

As I mentioned in my first post, a log mean temperature calculation across the heat exchanger will demonstrate this phenomenon.

Don't confuse temperature with heat. They are different things.
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Old 06-08-2008, 06:45 PM
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Aluminum heads

I have a question on this topic, I have tfr aluminum heads, I run a rpm air gap manifold, I do not use a cross over hose to connect the heads at the back of the manifold, am I loosing any cooling advantage?
I am runiing a 404 sbc, Afco aluminum radiator, stewart aluminum water pump, and a flex a lite balck magic extreme electric fan, yet at hot august nights in Reno at cruising speeds my temperture will start to creep up to the point I shut the car down at about 250. It does not, and has not boiled over.
So i wonder if adding more flow from head to head would help? Keith Bowen
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Old 06-09-2008, 07:42 AM
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here is a copy and paste from a tech paper about T-stats.


What some people will say in public. This is absolutely false, of course. Try to conceive of blowing on something hot, say a spoonful of soup, first slowly and then more rapidly. Which cools faster? For extra credit, try imagining the same spoon with first slow and then fast water moving across the bottom. Sheesh!

The issue with running without a thermostat is two-fold. The first part is that the thermostat provides drag on the water flow. This drag increases the backpressure the water pump and all of the enginesees. This additional pressure, over and above the nominal 15 psistatic pressure the radiator cap sets, raises the boiling point of the coolant. The reason this is important is that it suppresses localized film boiling at hot spots such as around the exhaust port. The transition from nucleatic boiling (bubbles of steam originating from irregularities on the surface) to film boiling (where the hot surface is coated with a film of steam) is called Departure from Nucleatic Boiling or DNB. DNB is very bad, for steam is a very good insulator compared to water. Once DNB occurs, the area under the steam gets hotter because the steam doesn't remove very much heat, adjacent metal which is still wetted heats from conduction. DNB happens there. The process spreads until substantially all the coolant-wetted surfaces are insulated by a film of steam. The engine overheats. In addition, the buildup in steam pressure forces the radiator cap open, bleeding coolant, therefore making the situation worse.

The second issue is that of water pump cavitation and surge. If the pump is operated at high RPM with insufficient head pressure(provided by the frictional losses in the coolant passages and the thermostat), there is a great likelihood that the pump will either cavitate (localized boiling and/or degassing on the impeller) or surge (an unstable flow regime). Either phenomena is destructive. Cavitation's collapsing bubbles act like little sand blaster, eroding away impeller material. Surge can do the same thing and in addition, can vibration stress the impeller enough to break it. Many times what looks like corrosion damage to the impeller, especially when the housing is damage-free, is actually cavitation damage.

The myth of velocity originated among those unschooled in physics or thermodynamics, I suppose, because a common racer "solution" is to press a fixed restriction into the thermostat housing neck when no thermostat is desired. The conventional (but wrong) wisdom is that the restriction "slows the water" as stated by the previous poster. In reality, all it does is provide some more dynamic pressure in the block by restricting the flow. The exact same result could be accomplished (assuming the water pump doesn't surge or cavitate) with a higher static pressure (cap pressure), assuming the system could withstand it.
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  #21 (permalink)  
Old 06-09-2008, 10:46 AM
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cooling

very good post, cavitation is a large factor, rad restriction will cause cavitation especially when the thermostat opens. industrial engines us a shunt reservoir so the water pump does not cavitate when thrm opens, re flow chang from by pass cycle to rad cycle. these resevoirs must be8 to 10 in above water pump.res is kept full with bleed line from eng side of stat.so a radiator with not enough flow will cause cavitation re a rad from a low hp engine ,lower volume water pump will usually cause problems when higher hp engine is used along with larger water pump. running with no thermestat ruins the intended flow threw the engine-re the bypass circuit
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Old 06-09-2008, 11:37 AM
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Quote:
Originally Posted by 404camaro
I have a question on this topic, I have tfr aluminum heads, I run a rpm air gap manifold, I do not use a cross over hose to connect the heads at the back of the manifold, am I loosing any cooling advantage?
I am runiing a 404 sbc, Afco aluminum radiator, stewart aluminum water pump, and a flex a lite balck magic extreme electric fan, yet at hot august nights in Reno at cruising speeds my temperture will start to creep up to the point I shut the car down at about 250. It does not, and has not boiled over.
So i wonder if adding more flow from head to head would help? Keith Bowen
You need a thermostat or restrictor to develop dynamic pressure in the block/heads to prevent small steam pockets from exploding next to hot spots inside the head. The water pump can add several pounds of pressure inside the heads. These steam pockets can occur even if your temp gauge reads 170*. The SBC the front cylinders run the hottest and always have steam pockets. Usually inconsequential.

You need a pressure cap that actually is holding at least 15 pounds (yours does or it would be blowing out into the puke tank), and there is nothing wrong with 20 if your heater core/system will take it.

Your water pump should be over driven at least 10 %. Never under driven.

Also you need to consider if your Stewart pump has a bypass. All street engines need a bypass of some sort. No bypass is designed for non-thermostat systems.

If you have a full flow heater core system, the water flowing through your core is not being cooled by the radiator. Rig a shut off for hot weather. If you do get into high temp situations, turn you heater on full blast to help cool your engine. It is uncomfortable, but it might stop the overheating. Most full flow systems have heater restrictors of about 1/4 inch diameter to reduce flow to the heater cores for cooling purposes, and also to prevent high rpm from blowing up the heater cores.

Since you are overheating "at cruising speeds" your next problem is airflow.
If your definition of "cruising speeds" means, "cruising" in slow traffic and that is where you get hot, then you need a better fan, bigger radiator, and better airflow into and OUT OF the engine compartment. We have all seen these guys "cruising" with their hood up to let heat out.

You don't need any fan above 35 mph, so let's look into airflow through the radiator. With enough airflow even a too small radiator will cool the engine. Look into air dams as was suggested, and seal the radiator to the core support and hood so all ram air entering the grille must go through the radiator. Consider that at very slow speeds hot engine compartment air can be sucked under the bumper and back into the front of the radiator.

If airflow is OK, then the radiator is too small cooling capacity for the high ambient (outside air) temperature.

Always remember that a cooling system is a closed system. It is full of water, and what goes in and out moves at the same speed.
If you slow down the flow through the radiator to allow more time for the water to cool, you also slow down the water through the engine so it has more time to get hotter. Slowing the coolant speed is a false solution to overheating problems. Over drive that pump for dynamic pressure.

Just my thoughts. Former cooling system engineer for 12 years.

Last edited by ScoTFrenzel; 06-09-2008 at 11:46 AM.
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  #23 (permalink)  
Old 06-09-2008, 12:20 PM
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Quote:
Originally Posted by jo5hi3
i have a 77 camaro with a 350 out of a 77 camaro i run a 160 thermostat in it and the temp guage when i am on the highway goes up to 195 to 200. i have 3.73 gears so i am turning about 2500-3000 rpms is there anyway i can get the car to run cooler? could i just take the thermostat out? thanx
What is the radiator, how old is it, has it been rebuilt or replaced?

Cooling systems for the typical user are pretty simple. You don't need to worry about flow velocities and all that. The thermostat contols the temperature by controlling the amount of flow thru the cooling system; there's no need to turn this into a science project

Assuming the engine doesn't have timing and mixture issues that cause it to run hot then the problem lies in the cooling system's ability to transfer heat as measured in BTUs from the coolant to the surrounding air. If you have a 160 degree thermostat and the the engine is running 190-200, it's telling you there's a problem at cruise. Typically engine temp will hang about 10 to 15 degrees above the thermostat's stated temp. Any higher than that indicates a flow obstruction which is usually (in older vehicles) is crud (for lack of a better technical term) that has reduced coolant flow thru the radiator's tubes. What this looks like is that at moderate speeds where the flow obstruction is not so great, the engine cools normally. As speed is picked up the increased flow cannot get thru the tubes fast enough to supply the engine's requirement for X amount of gallons of sufficiently cooled coolant per minute. Pressure builds on the engine side of the pump since the coolant isn't getting thru the radiator core fast enough and the "trapped" slow moving coolant gets hotter and hotter. This is also symptomatic of a radiator that is to small for the engine, in this case there just isn't sufficient BTU transfer capability between the inlet and outlet sides of the core to allow enough reduction in temp. This can also be an air flow issue where there is sufficient dirt and debris in the cooling fins to reduce the radiator's BTU transfer from the coolant to the atmosphere.

Typically, idle and low speed overheating is an airflow issue thru the fins. This is related to insufficient fan capacity or a missing shroud.

For the average enthusiast this is about all you need to concern yourself with, most cooling systems are well engineered from the factory with plenty of reserve capacity, but they don't last forever, a 1977 model now being 30 some years old.

If you're using an alumimum replacement radiator all I can say is you're pretty much on your own. I find the cooling abilites of these things all over the map, especailly when you're using those less expensive generic models.



Bogie
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  #24 (permalink)  
Old 06-09-2008, 12:32 PM
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Wow, this is one out of the vault, original post date; 11-12-2003, 02:37 AM

Reckon he´s cured the problem ?
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Old 06-09-2008, 03:16 PM
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Maybe a look at the fan would be a good idea?? If its a clutch fan and the clutch is worn out (that happens) and the fan isnt turning fast enuf at lower
road speeds then the airflow -wont- be great enuf.

How about the radiator? all the cores open? Large number of fins closed or clogged with dirt?
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