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MG MGB GT V8 Factory Originals Technical - The same old cooling problem

Once again the cooling problem. I have a rubber bumper MGBGT into which I have fitted a V8 salvaged from a rolled MGBV8. I have re-built the motor but I have not enhanced it in any way so it is a bog standard 3500 with twin SU carburettors, and it is running hot when driven at a constant 60MPH on the motorway.
The radiator is a four row standard “tropical” unit as fitted to the late model export MGBs, so it is about four inches longer than the radiators fitted to the chrome bumper models, and I believe that this is the radiator that was used for the V8 models.
I have fitted dual 10 inch “universal” fans purchased from a local aftermarket motor accessories shop which are connected via a relay directly to the output of the alternator which is a 65 amp unit.
My exhaust headers are original Costello units purchased from a local enthusiast who went for RV8 headers, but I have cut RV8 type apertures in the wings to allow hot air to escape. I have also lifted the rear of the bonnet by about two inches.
I have moved the number plate out of the way and fitted a scoop to divert air from the lower apertures through the oil cooler and lower radiator.
Bearing in mind that I live in Johannesburg which is around 6000 feet above sea level and the air density is lower with resultant reduction in cooling ability, my conclusion is that I simply am not getting enough air through the radiator.
The only way to remedy this is to either fit REALLY powerful fans, or to increase the size of the opening of the main grille aperture, which would mean cutting holes in the rubber bumper. Although this latter option would alter the appearance of the car it is the one that I favour as I do not fancy the idea of driving a car that is totally reliant on fans for cooling under all circumstances. Has anyone else tried this option?
Chris Barrow

In what way running hot? How high on the gauge? Have you confirmed the gauge reading with temperature measurements at the inlet manifold? What ambient temperatures do you have? What happens at idle on the temp gauge? Am I correct in thinking the fans are wired to run permanently? Have you confirmed the direction of blade turning and airflow (no insult intended but I must ask) is correct on the fans - the blades work better one way than the other and of course air-flow direction is critical? Are you able to monitor the rad inlet and outlet temperatures while travelling, i.e. with a digital probe taped to the ports?

To be honest at 60 in free air i.e. not following a truck the fans are unlikely to add much cooling to the ram effect. Whilst your conditions are fairly extreme, with what you have done with the scoop, wing vents and particularly raising the rear of the bonnet the problem certainly isn't going to be inadequate airflow, except that if you still have rubber bumpers fitted they are going to restrict air into the upper part of the rad more than chrome bumpers will.
Paul Hunt


2 things come to mind while I read your post.

How is the timing and are the fans spinning the right way.

With all you have done, it shouldn't be overheating.

I have also gone a fairly similar route. With my 69 Rover 3.5 into a 74.5 mgb roadster I have a 3000CFM puller fan and Ford falcon radiator with block hugger headers.
This was not enough to keep it cool.
Adding the front duct to force the air through the lower rad helped but it wasn't until I installed louvers high up on the inner fender to get the air out, was my cooling issues solved.
Now I can sit at a border crossing in 90 degree heat for an hour and 1/2 and not over heat.

Hope it helps


Bruce Mills

Chris:Have you taken the front bumper off to see if the improved air flow cures your problem, low cost,reversable,and if its still hot, you can put it back on and keep looking RIC

At 60mph, ram air should be sufficient to cool the engine without fans operational.
Start by checking the fundamentals as Paul suggests above. Might want to add a thermostat functional check to the list and ensure that your lower rad hose isn't collapsing under WP suction.
Graham Creswick


you did not tell about the year of your car. Earlier B's did not have cut outs in the front valance.
I am running a stock BGTV8 radiator with a ram air box in front of it and two puller fans behind it.
The later version of the 1978-1980 mod. 4-cyl. B seems to be not efficiant enough although it looks like a V8 one turned around by 180°.
My fans are set to start at 87°C. The t'stat is the 82°C version and there are no temperatures in excess of 92° on my gauge.
Before I built in the ram box, temperature sometimes became a problem.
As you mentioned, you have lifted the rear end of the hood. You should not do this as it is a high presure area and no not air will escape this way at speed. The openings in the inner fenders are ok, there is low presure in the wheel wells during driving.
An other problem often discussed is the interlock of air in the cooling system and there was a lot of discussion about that in the past,so you will find details and pics in the archives.
Finaly there was a report upon a modified coling system layout on the V8 register page last year and you should have a look at the workshop notes there.
Before you start to change anything, check you t-stat first. There is also a 78°C version acailable and it is a normed item so that those of many other cars can be used too.


Chris,would suggest you look at getting the hot air out of the engine compartment there could be a pressure build up in this area with the hot air having no proper exit channel.
t burnett

I thought that the area in front of the windsheild was a high prssure area at speed. If this is correct then the hood being raised in the back will be putting air into the engine bay not exiting hot air out at 60 mph.


I believe you are right Steve. The benefit of raising the rear edge of the bonnet (hood) is felt when the car is stopped and the fans are trying to create an air flow. Its the same with punching louvres in the hood. They need to be surprisingly far forward to have a good effect.
Mike Howlett

The engine bay is also a high pressure area - so, in this case with the rear of the hood raised, one shouldn't assume a specific flow direction. Chris could tape a few strands of cotton or wool along the back edge of his hood to see how airflow is affected at speed.
Graham Creswick

In addition to all the above...

Do you have a pretty good seal between the radiator and bonnet? (If you raised the back of the bonnet, you probably don't.) If airflow can easily bypass your radiator core altogether, it certainly will.

This article might be of some interest:

The article includes, for example, test results that demonstrate electric cooling fans are unproductive at 60mph and counterproductive at 70mph. If they really are running, you might want to figure out a way to turn them off. I don't think fans are causing your problem; I'm just certain bigger fans won't help you at road speed!

I like Graham's idea that the lower hose may be collapsing. There are other things to test along those lines. Could your radiator be blocked-up with crud?

It's interesting that you didn't mention any problem when parked or in low speed traffic. Is that right??? If so, that's a big clue that leads away from some of the usual suspects.


Thanks guys for all the input! In response to some of the questions raised:
My car is one of the last 200 MGBs manufactured. As the story goes, they put them on a boat and hawked them around the world. The only takers were the BLMC assembly plant in South Africa, who bought them for a song and put a sticker in the engine compartment stating that they were assembled in South Africa.
The temperature gauge is the electrical one. It has a small white band at the bottom and a small red band at the top. Everything in between is “normal”. I am using the same sensor as fitted to the 1800 engine, screwed into the V8 inlet manifold. Over a distance of about 10 Kilometers the needle slowly rises until it is bordering on the red band. At this point I usually stop and let things cool down, because on the one occasion that I allowed it to enter the red band, the pressure was so high that the top radiator hose felt like a rugby ball! By the way, all hoses are new, and the bottom one is a Kevlar reinforced genuine MG spare. The radiator was checked and modified by a local radiator shop prior to being re-fitted. I have also had the car back to this shop to check that I do not have head gasket problems.
Unfortunately I am only a hobbyist so I don’t have specialized equipment for measuring inlet and outlet temperatures.
The fans are switched on and off by a separate temperature switch located next to the radiator inlet. The switch-on temperature is 80deg C and switch off is 74deg C. The fans have been checked several times, and are definitely blowing through the radiator. I actually published a message on this forum whilst I was checking this as I found that one of the fans had a reversible blade.
With regard to the lifting of the bonnet rear, I did this in an attempt to stop the overheating. It has helped marginally, so I must assume that there is air flow.
Strangely, at standstill whilst idling the fans can handle the cooling, and there is actually some reduction in temperature. Although it is summer here the ambient temperature is quite cool at present, getting up to around 28 Deg C at midday.
Timing is set at 10 Deg BTDC at 1200 RPM. The thermostat is a new one, purchased from the local Rover agent.
The bit about air in the system is interesting. I drained the system this past weekend through the bottom radiator hose, and caught the water (and Glycol) in a bucket. It amounted to around 5.5 litres which seems low for a V8 engine. Possibly some stayed behind as I had the front of the car jacked up.
I have sat on the ground in front of the car and looked through the grille aperture. Two things are apparent: 1) The rubber bumper obscures around 50 % of the available opening and 2) The fans obscure around 25% of the area of exposed radiator. So there is not much radiator available for ram effect cooling. I must admit that I have also thought about removing the bumper and going for a drive, but I would risk being arrested if seen by an alert traffic cop.
Chris Barrow

I would guess that if she is OK idling and a problem at speed it would look like a problem getting air to the rad. What pressure rad cap are you using, it should be a 15lb one.

Don't you know a friend who has a chrome bumper you could try to see if you get better flow?
D M Tetlow

Quote: "I am using the same sensor as fitted to the 1800 engine, screwed into the V8 inlet manifold...."
An incompatible gauge/sensor combination can really throw things off - is the sensor the correct one for a V8 gauge?
Even 1800 engines had two different resistance senders.
If there is any doubt, borrow a compatible set and temporarily install in your car (even if the gauge is sitting on the console)
Maybe the gauge readings that you are seeing are nothing close to reality!
Graham Creswick

It doesn't take much air in a cooling system to really muck things up. A vent added high-up can be a big help when purging air. One example:

You mentioned that your fans are blocking 25% of your exposed radiator core. Ideally, electric fan motors should be spaced away from the radiator core so that they don't block core area at all. (There should, however, be a fan ring around the outer circumference of the blades protruding up to the core. Fan rings dramatically improve fan efficiency.) Anyhow, if you suspect airflow, you might want to actually test your car at speed with one or both fans temporarily removed.

One of the many things I strongly dislike about this message board software is it's inconvenient to include or view photos. However, it might be worth your time to paste some photos up so we can see if there are any glaring problems with your installation.

Most cars with marginal cooling systems run hotter on secondary roads than they do at highway speeds. If they don't, carburetor jetting should be included among the suspects. Has anyone goofed around with your fuel mixture? (It's a long shot, but there's also a small possibility your fuel system is crapping out and causing a lean-burn condition at speed.)

With regard to the accuracy of an original MG temperature gauge... If the external voltage stabilizer has failed, than your coolant temperature reading will vary based on input voltage, and thus on alternator output which is partly a function of RPM. Does your fuel level seem to slowly rise when you increase and then hold engine speed? This article includes "theory of operation" for that system:

What temp thermostat are you running?
I have to run a 195 F in my car or it will heat up too much.
It does overheat with a 160 or 180 F thrmostat in it.


Your fan switch is way too low, the factory cars were 90C, your 80C is slightly lower than the factory thermostat rating, and if the sensor is in the inlet manifold like the factory cars the fans are likely to be running all the time. However that is irrelevant to the hot running and until you actually measure the coolant temperature (carefully!!) when the gauge is reading high you are not going to know for sure whether you are running hot or not.

The first thing to remember is than unless it it losing coolant or steaming it *isn't* overheating, although it could be running too hot. Until you

It's actually *not* surprising the fans handle things better at idle as the engine should be generating much leass heat than running, and those fans should be shifting quite a lot of air through the rad.

28C is a quite hot summer day here in the UK so you shouldn't have any problems cooling, although I don't know the effects of your altitude.

A lot of coolant remains in the engine when the rad is drained as the block is tilted backwards even if the front isn't raised. There should be a drain tap at the rear on the left-hand side, however I couldn't get anything out of mine, and when I removed the heads the coolant was above the top of the block and ran into the bores of the rear cylinders. I've never had a problem with purging air on any car, just fill it up, keep an eye on the level over the next couple of heat/cool cycles, and top up as required. The later system with remote expansion tank is even easier.

As this is a conversion and the sensor is original I'd assume the gauge was as well hence they are matched. It also wouldn't explain why the temperature only *gradually* rises to H while running.

The hoses do get significantly harder when working in the normal temperture range, and that is a function of the radiator cap rating - should be 15psi. I did run 20psi for a while when diagnosing a cooling system problem and after a while the bottom hose (plain rubber) exploded in spectacular fashion a few minutes after switching off one day.

I don't understand Steve's comment - the thermostat only sets the *minimum* temperature. As such a 160 or 180 stat will open before a 195 and maintain a *lower* engine temperature.
Paul Hunt

We have had to use the higher thermostat on two different cars. Now we just use them as a matter of course.
The idea is, once a 160 thermostat opens the and the entire system reaches 160 the thermostat will stay open and water is passing through the rad too fast to be cooled. The 195 thermostat will close when the water from the rad reaches it as the water in the rad has been cooled. If the thermostat continues to open and then close, the water stays in the rad long enough to get cooled.
I had to do this on my BGT with 3.1 Chevy and AC.
Also on a 65 Mustang with a larger than stock engine.


Admittedly I'm not familiar with these Rover engines, but isn't 12 degrees spark advance at 1200 rpms wrong? Seems that with the vacuum advance disconnected and the engine at 700 (or so) rpms, the timing should be 6, 8, or 10 degrees BTDC. Could be that there's not enough timing at cruise.

Also, I know that if you want to cool a hot engine, you need puller fans (not pushers) and if electric ones can't move the air, put on an engine driven fan with a shroud. This I found out with a 79 Mustang V6 manual trans that I converted to a 351 Windsor V8 with C4 auto trans. Car did exactly as Chris explains; I messed with the timing, I messed with the carb jets, needles and springs, and only after putting a six-bladed stainless steel flex fan and shroud, did I cure it.

Wayne Pearson

Maybe your engine is running a bit lean due to the altitude. Pehaps someone who lives in the Denver area could comment on this aspect. Barrie E
Barrie Egerton

When I moved from North Carolina (~400') to Colorado (~5000'), I re-jetted my Edelbrock 4bbl carb but I don't recall exactly which jets I used before or currently. (Believe it or not, engine tuning bores me extremely.)

The thinner air here probably means that the engine has to work a little harder... However, I can truthfully say that my stock (non-"V8") MGB radiator core has had sufficient capacity to cool my 3.5L engine in all climate and altitude conditions so far. No oil cooler. 180 degre thermostat. My small "puller" fans (which are technically mounted closer than they should be to the radiator) are controlled by toggle switch, and I run with them "off" when I'm on highways. It gets plenty hot here in the summer. It was ~95F or so when I crossed the plains coming and going from BritishV8 2008 last July - I just about melted, but my engine was fine!


p.s. if you think it might help, you can review my cooling system here:
and some more thoughts on the subject here:

Again, many thanks for all of your input. I have spent some time digesting it all and although I must admit that I still am not sure what I should be doing next, I have reached some conclusions.
I have checked the radiator cap, it is rated at 14 PSI. I could get one that is a bit higher, but at this stage I doubt that it would help. My feeling is that hot is hot, regardless of the pressure and provided that the water is not converting to steam, the pressure rating is OK.
I do not have a works manual for the V8 MGB, but the Rover manual gives the ignition timing as 12 Deg BTDC at 1200 RPM. I can also state that the fuel mixture is not too lean. I am running with coastal needles in the carbs, and the application of any mixture enrichment even after only two or three minutes from starting causes the engine to virtually die.
I have taken the radiator out, reverse-flushed it and checked that there is no crud lodged between the cooling fins.
Unfortunately I do not have access to any temperature measuring devices for testing purposes. However, the fan temperature switch on my car is situated in the radiator right next to the water inlet. It switches the fans on when the temperature gauge is indicating just over half way, so I know that the thermostat has now opened and the halfway mark on the gauge is around 80Deg. C. I feel very uncomfortable about driving a car where the temperature gauge is hovering around the maximum mark most of the time, as I would never know if it was REALLY overheating.
Comparing it to my 1800 MGB, around 80Deg C (midway on the gauge) seems to be the “normal” running water temperature. Does anyone have any figures with regard to the ideal water temperature for the V8 engine?
Chris Barrow

Presumably the normal coolant temperature range should be between the stat temp and the fan switch temp, i.e. 82/180 to 90/194 for the faactory V8. But in practice going above that isn't in itself a problem, the hotter the coolant gets in relation to the ambient the greater the more heat the radiator will get rid of. I've seen mine in the red in the UK and it didn't lose any coolant. It did run a little rough and the oil pressure stuck in traffic was appallingly low so I gave it a bit of fast idle, which of course made it generate more heat! I agree that I wouldn't want to run in the red or close to it though, before I got my system working properly I was a bit paranoid, having the fan override switch on (not that the thermostatic switch was ever going to turn the fans off) and having the heater running at full blast for additional engine cooling. I installed an uprated rad with an additional row of tubes, but was disappointed in the results, a fourth row gives nowhere near a 30% increase in cooling due to the law of diminishing returns. In my case simply beefing up the power and earth connections to the factory fans was really all that was required, for the UK at any rate.

I strongly recommend you get a 'laser' temp gun to check the actual temperatures you are getting at rad inlet, outlet and across the core. The laser is only a pointing device, not the source of the measurement, which is done by a separate infra-red sensor. Some of these only have a 1:1 optics ratio which means the gun has to be virtually touching the thing you want to measure. Others are about 8:1 which are better, but still need to be pretty close to measure things like radiator inlet and outlet ports or you'll just get an average of the port and the surroundings. Have a look at this for both types
Paul Hunt

Something that you havn't mentioned-- Your car gets hot at 60mph but what happens when just driving in town in normal traffic is it ok or does it get hot then as well
Also at higher speed what happens. Willy

Hi Chris,
I've read your threads with sympathy, as I have experienced some of your problems here in Cape Town in summer.The following is the setup that we have used on our rubber bumper V8 conversion which is a fast road spec 3.5litre.
1) The radiator is the largest that we could fit in.It is a four core custom unit made by Silverton Radiators,with a core that extends down to the bottom of the cross member.It uses the expansion bottle as per the factory spec.Pressure cap is 15psi.The thermostat is 82 degrees C
2) Two factory pusher fans are used with heavy duty supply, individual relays and earth cabling to avoid any voltage drop.The fans are switched on at 92 degrees C.The mesh cover has a discreet shroud underneath it to eliminate air recirculation.
3)A Special Tuning air dam is used with enlarged air intake and the front valance panel is removed.The oil cooler is retained in the underslung position.Behind the air dam is a steel air duct sealing the air dam opening to the lower part of the radiator.This also carries a stone guard and is bolted to the chassis via existing tapped holes.
4)RV8 exhaust manifolds are used.This is very important as most of the hot air leaves the engine bay via the inner wing apertures into the low pressure wheel arch area .
5)It is also imperative to have the actual temperatures checked with a suitable probe or measurement gun.We found that the temperature sender and the gauge were initially mismatched and only read correctly when paired in the design arrangement.Normal running position for this car is halfway between cold and the middle.The fans kick in when the needle is just over midway between the two little dots on the gauge face.
6)We also used a MGBV8 PUMP pulley which is larger than the Rover SD1 pulley to reduce the pump speed and prevent cavitation at high revs.
7)Coolant is Castrol SF extended life coolant(orange pink)
8)The bonnet is standard with the rear seal removed.In summer we remove the stone guard in front of the fans with some improvement to the air flow.
The foregoing notwithstanding, there have been occasions when this setup has been taxed to the point of the gauge rising to between halfway and the red when stalled in traffic ( i.e approx 96 degrees C).I think the final solution has to lie with a 14" Kenlowe behind the radiator to deal with traffic jams in summer.I consider the twin OE pusher fans to be a marginal design at best. The cooling system is however more than adequate when on the move or during track days.I hope this information can be of assistance.



the temp. gauge is normaly feed with reduced power (10 Volts) from the voltage stabilizer mounted to the fire wall beneth the dash. If someone has made a connection to a 12 Volt direct feed, the instrument would read more than the normal temperature. Just an idea you might check. Another question concerns the radiator you use in your car. It seems to be the original late one for the 1800 MGB as it was used from late 1977 on. Have you had it recored to the specs of the V8?


Many thanks to all again for the input. I did some more mods over the weekend and took the car out for a test drive. It appeared that I was making some progress, but as I was in the process of reversing the car back into my garage, a seriously expensive screeching sound came from the gearbox/clutch area.
I have crawled under the car to investigate, and it appears that something has gone very wrong with the clutch release mechanism.
Since everything is new, I have no idea what the problem could be, but for sure the gearbox needs to be parted from the engine. In order to do this, I would have to remove the engine from the car
I don’t really know if I have enough enthusiasm left………………..
Chris Barrow


sorry to hear that, but let it alone until your enthusiasem has recovered. I can well understand your fustration and learned this lessons too in the early 80's when i did my conversion.
As long as you do not need this car daily, get some distance and you will find out an easy way to fix it in some weeks or some months. My 'partnership' with the V8 worked that way for more than 25 years (and 30 years meanwhile with it as a GT) and I don't want to miss all the experiance with it through this long period, although my Holley started making me angry again today.


This thread was discussed between 09/02/2009 and 17/02/2009

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