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MG MGB GT V8 Factory Originals Technical - Why thermal runaway???

Why does this thing I've heard you guys call "thermal runaway" occur? I just can't figure this out: my car has one puller electric fan and two pushers. In hot weather, it's absolutely vital that I turn them all on before leaving the driveway to avoid the car overheating further along. And it won't overheat -- never. But God forbid I should forget to turn one of them on, because half an hour later it will go up to H and at that point even if I turn on all three of them, the car's temperature won't come down much if at all. (I don't keep them all on in milder temps because they are loud and undoubtedly they really increase the alternator load.)
Terrence

It’s obvious that the radiator will only do its job of transferring heat from the coolant to the air if the air is cooler than the coolant. You know that even on a pleasant day the air under the hood (bonnet) is very hot. Picture a hot air balloon, it’s open at the bottom and the heat stays in because heat rises. If there were a hole in the side of the balloon the heat would escape through it. Now picture a cross section of your MGB where the highest opening is the grill. When you are moving forward, air passes through the radiator and out the bottom of the car and the car continues to leave that heated air behind. At idle the air that is heated as it passes through the radiator is mixed with air that is even hotter from heat radiated from the exhaust and although some will pass out the sides and back, some will pass back through the radiator, some will pass under the radiator and in both cases will be drawn by the fans more easily than the fans can draw in fresh air beyond.

The only thing I can think of in your case is that if the car stays fairly cool from the start, the under hood temperatures are not high enough to be a factor, but if the car gets hot…
George Champion

hey george,
how are you doing? i will be back in town soon. i look to seeing you and that v8 burning up the roads of yuma. sorry to interupt just wanted to say hi.

andy heston

Nice to hear from you again Andy. When you return, you’ll need to take my V8 for a spin now that it’s once again burning up the streets of Yuma (and the streets of Yuma are burning me up). I won’t be happy until I am personally responsible for someone swapping the motor in his MGB.
George Champion

Terrence, what pressure is your radiator cap?
If you radiator can take a 16 to 18 lb pressure then change it, this will help to avoid boil overs.
Bill Guzman

One reason for thermal runaway is that the cooling system never has more capacity for heat transfer than it does upon first starting out, with the system full. As the coolant is warmed the thermal expansion will force a small amount of fluid out past the radiator cap into the overflow bottle, thus incrementally decreasing the system's capacity, and this decrease will not be regained until the system cools down. There's more to it but I'm no thermodynamicyst. The design of the car has a number of design features that compromise cooling, this is certainly one which contributes to runaway overheating.

Jim
Jim Blackwood

Thermal runaway is usually a term used in electronics - it is when a component resistance decreases as it gets hotter, so more current passes, so it gets even hotter and this goes on till the component burns up.
I have found with my MGB that putting on the fan once it starts to overheat does not work, its too late by then.
I suspect that the mechanism is not so complicated, it is just that by the time the total water content is too hot, the airflow through the radiator (and the water flow!) is insufficient to allow it be cooled down.
T Green

Terrence
You need a radiator with more core surface area. (not thicker, that even requires more air flow) One that is longer (thats the only place you can go larger in a MGB) that drops down to about the same level as the crossmember. The air that passes though the radiator down there, exits under the car, out the fender wells, and some goes under the hood. As long as you do not go below the level of the crossmember you should not have to worry about bottoming out on the radiator and damaging it. I have never bottomed mine, it does not hang below the level of the air dam, so you dont see it when looking at the front of the car except though the air dam vent. If you are using the stock front valance, a small air defector can be installed at the bottom of the radiator to push more air though the radiator. Similar to what you see on most new cars ie. Camaro, mustang, etc. I would make the air deflector out of flexable plastic or rubber so if you do hit something it dosn't damage anything.

HTH Bill
bill jacobson

By careful about increasing pressure over 15lb. for various reasons I have been running with a 20lb cap for a few weeks and five miles after returning from a 1000 trip to Le Mans and back, and just after I had pulled onto my drive, the bottom hose exploded. For a given ambient the hotter the coolant the faster the fans will remove heat from it, but I suspect that problems are only occurring with high ambients which will reduce the ability of the fans to remove heat from the coolant. On a day when the fans are able to maintain the temp at normal, if you disable the fans and let the temperature get higher than normal, then reconnect the fans, they should eventually get the temp back down to normal again, and that is with the bonnet shut and car stationary. When the fans are sucking ambient air in from the front it will displace the very hot air in the engine compartment from under the chassis rails and out of the wheel arches. But of course it all depends on what your ambient is.
Paul Hunt

The problem is most likely to occur when you are at tickover and the car is stationary. In these circumstances, there is not much water flow due to low engine speed but there could be a lot of heat in the system. The rad could be cooling the water OK but there just not enough going through. So what about a smaller pulley on the pump? Or an electric pump?
T Green

This thread was discussed between 01/07/2002 and 02/07/2002

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