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Old 02-12-2012, 03:20 PM   #1
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radiator cooling theory

Some basic for radiator cooling.

Using the situation of a radiator that has a frontal area of 24"x18". That gives a gross area of 3 sqft. However, due to the tubes and fins, and subsequent resistance to air flow, the effective open area of the radiator will be in the area of 1 sq ft (1/3).

At 20 mph a vehicle is moving at approximately 1760 ft/min. That would result in approximately 1760 CFM flowing through the fan just due to the vehicle moving through the air.

I am not familiar with the actual capacity of the engine driven fan. It would be a function of engine speed certain.

An electric fan the produces 1760 CFM would be equivalent to driving 20 mph in calm air. Driving with a 20 mph tail wind would result in no air flow through the radiator due to the vehicle moving.

If a person can get the fan curve for the engine driven fan, and can determine the frontal area of the radiator, along with restriction, it can be readily calculated what electric fan requirement is necessary to meet the capacity of the engine driven fan.

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Old 02-12-2012, 05:29 PM   #2
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Old 02-12-2012, 06:01 PM   #3
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Thought I'ld add from other thread

Virtually all engines are designed to run more efficiently at 195. It is my experience that the 165 thermostat will, in no way, reduce over heating. It just determines the temperature at which the cooling starts. If it is going to overheat with a 195, it IS going to overheat with a 165. It is just that the engine won't operate as efficiently. The computer will think it is not up to temperature and will enrich the mixture.

Regarding the radiator. If you are planning on doing some off road when it is hot, and especially if high humidity, then you definitely want to bite the bullet now and get at least a 2 core unit.

The cooling capacity of the radiator is a function of its area. The frontal area is determined by the length and width. That determines how much air can pass through at a given velocity. If you want more cooling with that given surface, then you have to increase the velocity. Unless you go to an electric fan and get exotic, there isn't much you can do there.

The main way to increase the cooling capacity is to increase the thickness of the radiator. If you double the thickness you immediately increase the cooling capacity almost double.

The cooling is actually accomplished by the fins between the tubes drawing the heat out of the tube and the air passing over the fins. If the fins get plugged, that reduces the cooling.

If you increase the area by doubling or tripling the core (dual or 3 core) you will increase the cooling but not actually doubling or tripling. The air first passes over the first tube/fin and it is heated. The amount of cooling is based on the air flow and the "delta T" or differential temperature between the fin and the air.

When the air gets to the 2nd tube/fin it is warmed so the "delta T" is less and the cooling will be less. Same with the 3rd core.

If you aren't anticipating any really slow, heavy pulling, then the original set up should be more than adequate. If you are anticipating slow, heavy, pulling (typical of off roading and can be no more than very slowly driving up a steep incline) then you want to upgrade to at least one more core than present.

If you have an automatic, you also have the tranny cooling built into the radiator and probalby already have a dual core. If you have A/C you have another radiator in front of the main radiator for the refrigerant. That heats the air and creates a lower "delta T" across the tube/fins of the radiator.

Since heat is the worst enemy of an auto tranny, it is even more important to increase the size of the radiator, and even to install a separate, transmission cooler.f

If you are in a situation where you are seeing it overheat because it is going very slow, then it is time to bring in the gearing. Get down into a lower gear where you can get the engine speed up to 2500 or 3,000 and get the vehicle can pulling the air through.
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Old 02-12-2012, 09:11 PM   #4
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That's some good information....

Whats the Stock size of the Factory yj rad?
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Old 02-13-2012, 02:46 PM   #5
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Stock size

The stock usable area is approximately 17"Hx20W or 2.4 sq ft. Then one has to subtract out the restrictions (tubes and fins) and take into account the skin resistance to get the actual free area. I would estimate about 1/3 or 0.8 sq ft.
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Old 02-13-2012, 03:28 PM   #6
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Very good info. Thanks!!!!
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Old 02-13-2012, 06:59 PM   #7
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some added fan info

I see most automotive electric fans specified as 2700 cfm, etc. However, I don't see anything relating to static pressure. Every fan's output is dependent upon what kind of static pressure it is working in. There is both an inlet (return) and outlet (supply) static pressure. Typically, in an engine compartment the outlet (supply) static pressure would be low and could be ignored. However, I remember in the Jeep Super Wagoneer (1414X) I had, the air flow through the engine compartment was highly restricted. This was of significance and created cooling issues. I believe that the 6cyl exhibits some of the same.

On a puller type electric fan (mounted on engine side of radiator) there is a significant inlet static pressure. To really know what the fan will move it is necessary to know what static pressure it is working against and what the fan curve is, relating to static pressure.

Typically, the electric fan SHOULD be rated based on imperical testing of what the fan will do installed on a typical radiator. However, if a large number of the fins are bent over, or if the fins are plugged with dirt, then the fan's output will deteriorate rapidly. If there is something mounted in front of the grill (like a winch) this can cause obstruction. It can also cause cavitation which can drastically increase the static pressure on the inlet of the fan.

A good radiator has the openings between the fins sized appropriately such that the ratio of open space to fins is such that the air will increase in velocity throught the fan if there is more than 1 core. When the air passes over one core you want the air to be as hot as possible after passing through. However, when you pass the air over the first core on a multicore radiator, you want the air to not be as hot so it picks up more heat from the 2nd and 3rd core. The net effect is more air, all reaching that same high temperature as on the single core, thereby, extracting more heat.

So, if increasing the core thickness, we also have to consider increasing the air volume through the radiator, considering that it is going to have a higher resistance.

Thankfully, it is not an exacting situation, so we get by just by doing the obvious of adding the cores and upping the capacity of the fan.
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Old 07-16-2012, 07:09 PM   #8
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OK.. I like to restart old Threads rather then start new ones..... SO here is my issue... I have an 88YJ that has a new radiator and an AMC 360 V8. she does fine and restarts when warm unless the outside temps are 90+ and I have driven her on the highway or for an extended period, 30 mins or so. I end up with the fuel shrinking back into the pump or lines like a turtle pulling its head back in the shell. Cooling the fuel pump lets her fill back up the filter and she will start. I am thinking about putting an Electric fan, thermostaticlly wired to compensate for the heat buildup. Anybody have an opinion or ideas about this???
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Old 07-16-2012, 08:57 PM   #9
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Old 07-16-2012, 09:01 PM   #10
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leak?

It sounds like you might have a leak in the fuel system? It shouldn't be shrinking back.

I don't think a fan is going to help unless the engine is overheating. The thermostat is going to work against you to keep the temp up.

What I did on my older, carb'ed, engines was to install an electric fuel pump and make sure the fuel lines were kept as cool as possible. Route them away from the engine and try to get some insulation between the fuel line and the engine heat. Don't just wrap the fuel line in insulation as that will keep the heat in and will let the line heat up when there isn't fuel flowing through it.

kinda sounds like it might be vapor locking?
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Old 07-17-2012, 06:14 AM   #11
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OK, our first thoughts were that there might be a leak in the fuel system. I have gas fumes when I first fill her up also when I open the gas cap there isn't the normal woosh of air escaping. We are going to drop the gas tank after I burn off the fuel in it. Any hints on places to look for the possible leak? First thought would be gas cap but I just got it when I got the jeep in November. Second thought would be the vent tube. All gas lines are off of the engine and I did put DEI's "cool tubes" on the lines from the pump to the carb. SHe is not overheating so I guess the fan won't serve much purpose. My goal is not to put a bandaid on it but rather FIX what is causing this. As to Vapor Lock... What exactly is that and what causes it.
Oh and I forgot, she did have an electric fuel pump when I got her that was mounted in the driver side wheel well. It clicked really loud when you first turned the key. WHen we found the huge vacuum leak at the carb and got her running right I decided that the electric fuel pump was not needed. It was Winter at that time. Should I put one back on? Should I put it on a switch and just use it when it's hot? Just curious....
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Old 07-17-2012, 07:35 AM   #12
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possibles

If it is being caused by a leak, the leak would be on the engine side of the filter. Something is letting/causing the gas to flow back towards the tank and be replaced by air. It can't get that air in unless it comes from the opposite side from where the gas is going, which is back to the tank. If there is a visible filter, it could be a leak there.

One way to check is to install the electric fuel pump and turn it on with the engine off. That pressurizes the fuel delivery system up to the carb. See if any fuel is escaping anyplace. It could be that the electric was put on because of the problem.

Vapor lock is when the fuel in the lines overheats and starts to vaporize. The fuel pump can't push gas because it is getting vapor instead of liquid. The resolution is to get the fuel lines away from that heat source and keep the fuel from vaporizing. Usualy manifests itself when pulling heavy loads in hot weather and slow (extreme heat buildup in engine compartment) or after you shut off the engine and the heat builds up.
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Old 07-17-2012, 09:04 AM   #13
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Quote:
Originally Posted by ballistx View Post
If it is being caused by a leak, the leak would be on the engine side of the filter. Something is letting/causing the gas to flow back towards the tank and be replaced by air. It can't get that air in unless it comes from the opposite side from where the gas is going, which is back to the tank. If there is a visible filter, it could be a leak there.

One way to check is to install the electric fuel pump and turn it on with the engine off. That pressurizes the fuel delivery system up to the carb. See if any fuel is escaping anyplace. It could be that the electric was put on because of the problem.

Vapor lock is when the fuel in the lines overheats and starts to vaporize. The fuel pump can't push gas because it is getting vapor instead of liquid. The resolution is to get the fuel lines away from that heat source and keep the fuel from vaporizing. Usualy manifests itself when pulling heavy loads in hot weather and slow (extreme heat buildup in engine compartment) or after you shut off the engine and the heat builds up.
Ok... I forgot to say that the carb is an Edelbrock 600 and I do have a clear filter between the mechanical pump and the carb. I will check the clamps at the carb and at the filter. We figured out yesterday there wasn't one back there. Also, I hate to just buy a $100+ electric pump to then figure out there is another issue. Right now I see no fuel leaking from the Mech. pump to the carb. Nor do I smell any gas under the hood. Sooooo.....

1.Should I do away with this clear filter and use a cartridge one that should be back on the frame rail?

2.Could the carb have a way of leaking air back into the line?

3.If I install an electrical pump will it in anyway hurt the mechanical pump?

Thanks for the explanation on VL. I thought that is what it was but I had only seen it in the SU carbs on my old 260z.

Thanks again....
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Old 07-17-2012, 10:36 PM   #14
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Was going to reroute fuel lines today and when I tried to remove the fuel line the entire top of the mechanical fuel pump moved counter-clockwise. Sooo we replaced the pump and all lines around the engine. Routing them to the firewall and around the back of the engine compartment. Let you know how it works tomorrow.

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