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MG MGB GT V8 Factory Originals Technical - Would the cooling fans work better if they pulled instead of pushing ?

Whilst running my engine up to temperature on the driveway with the bonnet open, I noticed that as the cooling fans started up the heat emitted from the engine bay increased markedly. This is not surprising given that the fans push cold air through the hot radiator core and send the heated air into the (normally enclosed) engine bay. As the fans sole purpose is to increase the air flow through the radiator core and this is normally only required because the car is stationary, wouldn't they be more effective pulling the air out of the engine bay ? Initially the supply of cooling air would be poor but once the air was flowing freely would the cooling be more effective or does the heating effect on the cooling air by the engine/exhaust manifolds make things worse ?
Charles Bryan

So what happens when you transition from stationary to moving with the fan on? The fan bucks what should be the incoming air and stays on because the heat continues to climb. And, yes, nobody wants to run preheated air through the radiator.
George B.

Yes, if you mean pulling the air into the engine bay, a puller is more efficient than a pusher pushing it in. But as George says, what about those times where the car is moving forward relatively slowly and the fans are on? You'd cook it.

PS, if you want to get the temp up on the driveway keep the bonnet closed, it heats up much faster.

PaulH.
Paul Hunt

One source I read stated that pulling air through a radiator results in a 20% improvement over pushing it through. I don’t know if that meant 20% more cubic feet per minute or what.

You need the air to move front to back whether it’s pushed or pulled. The cooler the air that passes through the radiator the more heat will be removed. As hot as it gets here (124° as the record, 110°-115° most summers) it’s not as hot as the air under the hood. I measured the temperature under the hood of my other cars and they were about 130°, but the temperature device failed at 150° when I checked the MGB V8! I’ve had plastic melt in there so I don’t want that air passing through my radiator.

However the air is moved through the radiator, it has to go someplace and it’s got to be hard for a fan to push the hot air down around the motor. While you’re moving the heat is being left behind, but not at idle. Using tissue paper, I found the air leaving the radiator would cycle back under the radiator and again through the radiator so I replaced the duct that goes between the radiator and the valance and found an improvement. Next I want to seal above the radiator and through the hose openings to prevent recycling air. Still, I expect hot air to cycle through the radiator next to fan when at idle.
George Champion

Wheither you are moving air from the radiator through the engine bay or in reverse the air flow rate is still determined by the resistance. When the bonnet is raised the resistance is greatly reduced and you will feel more air flow than with it closed. You definitely don't want the fan trying to force air forward through the grill opening. When the car was moving this would effectively cause no air flow through the radiator at times which would really cause problems. If you want greater air flow find ways to reduce the restrictions in the engine compartment. You might louver the bonnet or inner panels.
Bill Young

I've not seen figures for the different efficiencies of pullers and pushers but I do know that there is a huge amount of 'pushed' air that just circulates around the fans, partly through the centrifugal effect of getting thrown off the tips of the fans instead of being thrown forwards, and partly because the LH fan in the factory car sticks way past that end of the rad. By contrast a puller will throw just as much air off its tips, but it has to come from somewhere, which is the centre of the fan, i.e. it has to come through the rad. The RV8, either by moving the rad even further forward or having a different water pump or both, had a single, thin, puller which does give far better cooling of what is a bigger engine.

PaulH.
Paul Hunt

I tried to measure the airflow of the stock fans by taking a small fan and testing the voltage produced when it was turned like a pinwheel by the big fans. Unfortunately, the junk fan I used had too much resistance so I just had to use a much more subjective feel method.

The stock fans as used on the MGBGT V8 and ’77-’80 four cylinder MGBs through much more air outward than push through the radiator for the two reasons listed by Paul H. As much as it pained me to deviate from the stock design, I replaced them with a single 14” pusher fan with a double shroud. One shroud is part of the mounting system and the other is part of the blades to limit the centrifugal losses. The new fan doesn’t extend beyond the edges of the core allowing the fan output to take the path of least resistance beside the radiator. Overall very little air blows away from the fan in the front and what I feel through the radiator is much more than before, yet much less than I feel from other puller fans on other cars. When I can spare the car I will try to find room to move the radiator forward enough to allow rear mounting that won’t foul the roll bar, but first I want to take another try at the wind tester.

George
George Champion

I have found that placing the fan in front of the radiator caused the car to run hotter when moving down the road at highway speeds. When the fan was placed to the rear (and configured for a puller) it worked much better and did not heat up on the highway.

My $.02 - with the fan in front it blocks air flow to the radiator. At higher speeds, the blades will free spin (no EMF when using a std thermostat) and present a large spinning disk to for the incoming air to go around. This is the same reason why a prop plane slows down so fast when pulled to idle. Unless the prop is locked and feathered, it is a large amount of drag.

With the fan behind the radiator, the cool incoming air will emerge through the radiator at a reduced speed (compared to air speed on the front of the rad), thus keeping the fan from spinning so fast and blocking flow.

Almost every auto maker uses electric fans for the main cooling system and I'd say that 95% of them are on the back of the rad pulling the air. They must have some valid reasons.

Sealing up all air leaks around the rad will help a bunch.
Brian Kraus

The reason the fans free-wheel when moving forward is because the air is pushing the blades out of the way and therefore hardly having to 'go round' them at all, I suggest, since the air only has to overcome the friction in the bearings - the fan is doing no other 'work' and hence absorbs no energy. Front fans *will* cause more of a restriction than rears though, by dint of the cross-sectional area of each motor giving two significantly sized 'dead' areas in front of the rad. I suppose one could get comparative figures by running with motors with blades vs motors without blades vs no motors at all.

PaulH.
Paul Hunt

I always thought that a fixed-wheel(propeller) would provide more drag than a free-wheeler, but research on boats has shown otherwise. The analogy most often used is that of a helicopter at height with a dead engine. With a fixed-wheel, the helicopter will crash at a terminal velocity approaching that of a rotorless 'copter, but with a free-wheeling rotor it will autogyro at a much lower speed; thus proving that there is more drag in the free-wheeling configuration. I didn't really believe this argument, but did discover that my auxiliary cutter(sail boat)does sail somewhat faster with the prop shaft fixed. Apparently the rotating blade blocks off an area larger than its size.
George B.

This thread was discussed between 29/01/2001 and 01/02/2001

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