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MG MGB Technical - electric rad fan?

Why fit an electric rad fan? Is the stock engine mounted fan not sufficient or something? I notice its a popular modification but Im just curious as to why.


R Kelly

The biggest bonus of an electric fan is if you are having overheating problems at slow speeds. The electric fan always runs at full speed providing maximum air flow as opposed to the engine fan which is of course dependant on engine speed.
John H

The fan on the water pump pulley has to move a lot of air when the car's engine is idling, because the car is most likely to overheat when standing still in a traffic jam. This means the blades are a very coarse pitch, so that when the engine is whizzing round at 5000 rpm, the fan needs a lot of energy to keep it turning, even though once the car is moving it isn't necessary. It probably saps up to 5 bhp from the engine at max revs. So fitting an electric fan gives you a small power boost, and cools efficiently at idle speed, as John says.

Another valid reason for removing the engine fan is that the metal-bladed fans have been known to suffer metal fatigue with disastrous consequences. The broken bits commonly fly through the radiator or even up right through the bonnet lid. I don't think the plastic fans share this problem.
Mike Howlett

Hmmm. Something to consider then! Ill look into it. Maybe it will be something for further down the line though, all the same.
R Kelly

As Mike says, when you need the mechanical fan the most (idling in traffic), it is doing the least; conversely, when you need it the least (cruising at high speed) it is robbing the most hp from the engine. An electrical fan, properly installed, does nothing unless the water temperature justifies it and even then it is not robbing hp directly from the engine. I have found that my electric really only kicks in when doing stop-and-go driving or sitting idling.
David "not going back" Lieb
David Lieb

Im pretty sure I could knock together the parts I need at the local scrap yard. Its a free access scrap yard where you just take what you want using your own tools and pay for what you took at the exit. So let me see... Ill only need a fan with a cowling and a thermostat and whatever wiring Ill need I should already have. What size fan will I need and what sort of thermostat? Where would I fit the thermostat? Is there a bleed screw in the head that I could tap out for the 'stat or whats the story there? Also, is it better to have the fan on engine side sucking or on front side blowing?
R Kelly

I cannot address the bulk of your questions, but I CAN tell you that it is much better to have the fan on the engine side of the radiator. If you think about it, at speed on the front, it would be blocking a fair amount of airflow. On the back, the air would merely dissipate around it. Kinda like "how far away from the end of the vacuum cleaner hose can you detect air flow?" I have had the fan in my midget both ways and there is a significant difference.
David "Electric fan fan" Lieb
David Lieb

To add to all the above,
The electric fan is close coupled. That is the fan is placed as close a possible to the radiator and a cowl surrounds and ensures that air is drawn through the radiator, not in from the sides. This makes it very efficient. The engine driven fan can't be as close or so thoughly cowled becasue the engine's got to move around a little. A little while ago the cowl on mine came away from the radiator about one centimeter at the top and as a result the fan would stay on half again as long. Because of the heat gradient air does not "want" to go though the radiator.
Most after market fans come with their own plastic cowling and most one's you will find at a wrecker have their own cowling. However keep in mind that the after market fans are getting very cheap ie $50 and most come with fitting kits and relays.
these guys are a little more expensive, but very good quality.

You should set it up so that the fan is switched through a relay, and that relay is switched though the electric thermostat. The relay must have a fuse somewhere in the line. My Fan came with a fuses relay etc.
Electric thermostat.- The later MGB's had an electric fan so there may be a spot for a thermostat on your radiator, plugged up. Don't worry if it doesn't have a spot, I noticed at the auto shop last month that they now sell cheap anodized aluminum adapters that go on your radiator hose. These allow you to screw in a thermostat. You can see mine (a nylon one) on the left below. The thermostat doesn't have to be the MGB one, any thermostat will do, so long as it fits. The auto shop will have a book listing temperature range and thread etc. Perhaps a 92 degrees switch would do, check your haynes manual, or perhaps someone here knows exactly what it should be. It needs to switch about 5 degrees greater than the mechanical thermostat on the engine, otherwise it would be on all the time.
You can also get adjustable thermostats (I have one)

Peter Sherman

Nylon thermostat radiator hose adapter is on your right. Car's left/portside.
Peter Sherman


for a 1974 1/2 on car (as your's) i would put a large vent in front of the radiator, but be sure to pick one with the latest design of blades from your braker.

Although i agree with David (and have done this kind of installation on my V8) the later RB chassis has a large cut out before the radiator where a scope can be fitted to substitute the original cardboard .

With a scope and a vent from a Galaxy (with custom made light alloy frame to locate the vent and give it a shroud), it is capable to keep the temperature beneth 90 C -eavern in stop & go in summer time- on my stage tuned 1975 roadser with 100+ RWHP.
On my cars i use thermostatic switches that are offered by Conrad Electronics ( at 13. They are comparable to those offered from Kenlow but do not fail after 3 years of use. The temperature can be set from 40 to 120C with this units. On the B they work fine when set to ~85C, as i found out.
The rubber bit for fitting the sender to the upper water hose of the radioator is not offered as an MG part but you can order it from Moss as a Triumph spare part.

My installation has cost me 5 for a set of two vents fitted into a broken plastic frame, complete with two relais, 13 for the adjustable thermostatic switch and 3 for the rubber bit to seal the capilar of the sender to the top hose of the radiator. The frame and scope was fabricated from some pices of light alloy sheet metall and of length of rubber gasket (as used underneath the bonnet for sealing the rear half of the engine compartement from draining water) to seal the fabricated screen to the radiator. It took me two about 2 1/2 hours for fabrication and installation and has worked very satisying for several years up to now.


Electric fans do have also one big disadvantage, their demand for extra current is very high, between 9 and 14 amps, and that is for dynamo or standard alternator too much! At installing of fan you have to think also about bigger alternator or you will be soon faced with empty battery.

Toni makes a good point,
I once had a dumb ( and subsequently hearing impaired) friend who installed an incredibly powerful amplified sound system in his car, went for a drive (with eardrums bleeding) and when he stopped the battery was flat as a tack.
My alternator is a 100 amp one.
Check what amps your current (ho ho!) alternator or generator produces and match it to the spec' of the fan.
The Davies Craig guys give all this info'. For example the 14 inch draws 12.5 amps (which should not be a problem). You don't need a 16 inch, 14 is heaps.
If you are getting your fan off a wreck, check the alternator that is using. Find a fan that suits your car. maybe even salvage an alternator and fan both. More work, but bargains always are!
Just keep in mind that the fan is only on part of the time.

The very knowledgeable people on this site can tell you how many amps your car generates if you tell them the year. It may even be printed on your alternator/generator.
You could possibly even fit a two stage thermostat switch (VW's have them) and run two small fans to max' the efficiency. I'm thinking of doing just that and replacing my giant 16 inch fan with with smaller fans on opposite sides of the radiator. This gives you some redundancy as well.
Peter Sherman

Back to the original question, it shouldn't be necessary. These cars ran and still run in desert states with out problems. If you are having overheating problems then there is something wrong which should be fixed at source, and this is especially true in the UK and Ireland with our temperate climate. If you have a car with the 4-blade fan then the first step should be to fit a later 7-blade type, but again it should only be necessary in extreme conditions.

Any electric fan replacing a mechanical fan is likely to require an uprated alternator as well. The original alternators were rated at between 34 amps (16ACR) and 45 amps (18ACR), the latter were used on cars with electric fans from the factory and that is the *minimum* that should be fitted.

"If you think about it, at speed on the front, it would be blocking a fair amount of airflow" This reminds of the old airplane argument of whether the propeller should be at the front or the back. As far as air flow through the *blades* goes it doesn't make any difference of course, because an obstruction to the airflow coming in is the same as an obstruction to the outflow. A fan *is* more efficient as a puller than a pusher though, as when pushing air through the *radiator* a significant amount of air is flung off the tips and 'lost', whereas with a puller a far greater proportion is pulled through the radiator as the air flung off the tips has already done its job of cooling.
Paul Hunt 2

One other comment. The clearance between the nose of the water pump pulley (even with the fan off) and the back side of the radiator core for some years is insufficient to mount many of the electric fans on the market. They can be simply too deep. I was faced with this with my 68 GT and ended up having to use the less desirable pusher position to force air through the radiator.

Altho not as efficient as the puller positioning, in all the years of driving with only this fan in this position in 100F+ temps and across deserts in the USA, I have never had a problem with overheating, even on our long 6% grades here in the West. Its efficiency may not be as good, but apparently it is good enough when coupled with a decently maintained cooling system.

As for the temp switch location and mounting on a car that did not come with an electric fan, I would suggest something like what you will see in the attached photo. It requires a trip to the radiator shop to have them solder in a threaded bung to receive the temp switch of your choosing. I used the Wells TU-68.

More on Cooling? You might want to read this as well, . You may find that a cooling problem has less to do with the choice of fan than with other issues that are easier and less expensive to correct.

Bob Muenchausen

Lots of good advice. I took the metal fan off my '71 MGB and fitted a fan in front of the radiator, because there isn't clearance behind on the earlier cars. I used a fan from a wrecked Peugeot 205. It comes in a large plastic mounting which I was able to cut down until it fitted very neatly right up against the radiator, standing on its own feet and bolted through the lower panel in front of the rad. I bought a thermostatic switch from Kenlowe which fits inside the top hose. It worked efficiently and was never on for long, even on hot summer days in the south of England.

As for the electricity, my car had the standard alternator and in 10 years use I never had the slightest problem with flat batteries.
Mike Howlett

Another advantage of the electric fan is it makes poking about under the bonnet with the engine running a little bit safer. When checking timing and so on you don't have this giant spinning blade in the way!
Simon Jansen

Interestingly enough my car actually has an electric fan as I discovered when I picked it up. Its a small fan in front of the rad that looks like its actually stock for the car and there's a thermostat for it too, but interestingly enough theres a switch to turn it on and off under the dashboard too for some reason. Its questionable how much air this thing can actually move though but if its the stock fan Im sure its fine. Its bloody noisy though. It rattles like mad so I'm going to see if I can sort that out.
R Kelly

You haven't said the year of the car. The factory didn't fit electric fans until the 77 model year - one on UK cars and two on export (and all V8s). That should be enough (especially in Ireland and the UK), although the RBs do seem to run a little hotter in warm conditions, possibly because of the greater restriction around the intakes. 4-cylinder cars had the thermostatic switch in the radiator. Some people have fitted over-ride switches so they can have them on when they aren't needed, but that is only any aid to cooling if the thermostatic switch has failed, which on any one car is going to be a pretty rare event (although I did help a V8 with precisely that problem on the Snowdon Run last year when he erupted in a cloud of steam). The biggest problem is the perception of the user. People get paranoid seeing the temp gauge climb to mid-way between N and H even though that is exactly what it is supposed to do. For that reason many modern cars run the temp gauge via the ECU so normal variations such as that don't appear on the gauge. The standard fans get very noisy if the bearings are worn, which also reduces effectiveness.
Paul Hunt 2

Its a '78 car with the single fan and the thermostat in the top of the rad. It has chrome bumpers on it though so its not as restricted. I suppose at some stage Ill take a look and see what the rattling is. I reckon its just the metal grille that goes over the top of it. Its spins well anyway.
R Kelly

This thread was discussed between 10/03/2008 and 13/03/2008

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