# Fan Clutch



## kjk990 (Sep 1, 2010)

I have a 67 GTO with a rebuilt 400. I has a hot cam, roller rockers, edlebrock intake and carb, headers, four core radiator, fan shroud with solid fan (no clutch) and 180 therm. My heater is not hooked up due to a leak in the radiator core.

Driving tem is around 185-200.
Stopped or in traffic 220-230 before I have to turn around or get of the freeway. 

I went into a radiator shop. He started off saying That I had too much flow. When the engine is at idle and the thermostat is open he showed me the flow through the radiator cap. He first suggested a three core dimpled low flow radiator.

After I talked to him a little more he had me run the engine faster than idle for a couple minutes. The temp stayed put, at around 200 (outdoor temp of around 85). So he changed his suggestion to putting on a fan and clutch. He said that a fan clutch makes the fan flow more at idle.

Is that correct? I thought that a fan clutch is designed to cut out if the temp was low enough and reduce strain on the engine.

I also asked him about an aluminum radiator. He said that the water flow would increase which would cause the same problem.

Either way I either need more air flow. Any suggestions?


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## BearGFR (Aug 25, 2008)

Hmmmmm..... Ok - purpose of a fan clutch is to keep the fan spinning at an rpm where it efficiently moves air. 'Too fast' and the tips of the fan blades can actually go super-sonic and the fan doesn't work as well. It can actually get to the point where it blocks air moving through the radiator instead of pulling it through.

Also true that coolant can flow through a radiator 'too fast'. It has to stay in there long enough to transfer heat to the metal and then to the air. If the coolant doesn't stay in the radiator long enough for that heat transfer to happen, then eventually the motor is going to heat up every ounce of coolant in the system way too much and bad things will happen.

Now lets look at your symptoms. You say it cools fine at highway speeds but overheats (or at least gets hotter) in stop and go traffic. At highway speeds the fan is essentially out of the picture, especially running no clutch like you are. The air moving through the radiator is doing so just because you're driving through it. Your engine is staying cool so that's working just fine. It also tells me you don't have a "too fast flow" problem. If anything the coolant is circulating faster at highway speed than it is in traffic, due to engine rpm. (Unless the pump is going into cavitation at rpm and "slowing it down" - but that's not likely). All the symptoms to me point to a problem of insufficient air flow through the radiator in traffic. You not getting enough heat transfer out because you're not getting enough air movement. I don't think a clutch by itself is going to fix that either, not at low rpm. You didn't say what kind/style of fan you have, but my suspicion is that it could be the problem - it just can't move enough air. It also might be positioned wrong in the shroud. To work the best it needs to be so that the fan blades are as close to 1/2 in and 1/2 out of the shroud as you can get them. That makes the fan work better because it helps prevent the air flow from "spilling" around the ends of the fan blades. That works very similar to the turned up winglet tips you see on the ends of the wings of some airplanes. They block the air from "cheating" and flowing around the ends of the wings. 

If'n it were mine, I'd make sure it had a good beefy 7-blade fan, with a clutch, installed 1/2 in and 1/2 out of the shroud then see what happened. (Either that or convert to a pair of good thermostatically-controlled electrics)

Bear


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## Rukee (Feb 8, 2007)

:agree put the correct clutch fan on and make sure it fits the fan shroud good.


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## geeteeohguy (Feb 2, 2008)

I agree too. Not moving enough air thru the radiator core at idle.


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## kjk990 (Sep 1, 2010)

I recently had a "new engine tune" done at the local muscle car shop. It runs so much better. But thats when the heat problems started. Before she was tuned she ran at about 190. One of the things they changes was lowering the idle. That would explain the less movement of air.

Do any of you have an electric fan? Do they work well? Is the buzz annoying?


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## BearGFR (Aug 25, 2008)

I've got twin electrics on my 69. I like that they're not tied to engine rpm so they can and do run at the most efficient speed for moving air, regardless of what the engine's doing. They're thermostatically controlled so they only run when they're needed, not constantly. My car's pretty "healthy" so unless you're really trying to listen for the sound of the fans, you don't notice them. 

Bear


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