Hi all,
My underbonnet (4.2/ 3xSUs) gets stinking hot (as in phenomenal - its like an oven)
The gauge reads OK - I can see when the thermostat opens and the gauge water temp is kinda normal.
I am an engineer, have had ~40 cars and have never seen such a teeny fan (although it seems to keep engine coolant at temp).
I know that auxilliary fans may mask underlying cooling faults, but I cant see how such a rudimentary fan has any hope of getting rid of the underbonnet heat.
Maybe I am barking up the wrong path.
Has anyone seen a fan like this?
Cheers,
Michael



Cheers,
M

 

The car should have a shroud around the fan to ensure it sucks air through the radiator rather than circulating it around itself. And high temperatures under the bonnet (or at least the impression that they are high - I've not measured temps) is fairly typical.

 

That fan is custom. Ditto Peter.

 

Yes, your fan is not OE in that it does not look like mine, not that my OE looks as though much fluid mechanics went into its design. The radius of my fan (centre to blade tip) is 8 inches.

 

Taking a photo of the fan in situ is impossible but this photo of my engine being installed is the best I have of an original fan. Note it has twelve blades which are made of metal but not sure of the diameter. The shroud is important as stated as it forces the air to be drawn through the radiator and not from the sides.
My only advice when fitting the fan is make sure all the blades are well away from the back of the radiator. I had an engine mount go which allowed the engine to twist slightly and the front of the fan destroyed the back of the radiator. Over £500 for a recore.



This is the Original radiator cowling which is made of fibre glass and has to go on over the fan and left to dangle before the radiator is lowered into place as there is no room between the front of the fan and the back of the radiator for it to be slide between them. The cowling is then bolted to the back of the radiator.

 

I have had 6 MK1 and 6 MK2 cars and I only remember the fan shroud being intact on two of them.
Consequently, I don't really think it is absolutely necessary but nice to have.
There is no doubt that the engine bay does get very hot and I wonder if this was the reason for putting a liner in the engine bonnet.
That 12 blade fan as originally fitted does a very good job. I fitted one to a 2.4 which originally had a pathetic alloy 4 blade fan and that stopped the overheating in traffic.
By the sound of it you are not having overheating problems, so I suggest you leave it be for the moment and sort out the rest of the car.

Do your windscreen wipers work OK? That is one hell of a bend in the operating cable (180degrees?) compared to the typical 90 degrees in the original location.

 

that looks like a Flex Fan. used by hot rodders to improve horsepower at full throttle, because the blades flex into a flat orientation the more speed the fan goes. But during traffic, they don't do much cooling.

I agree with Cass and Bill about the 12 steel blade fan, it does cool. My S type has the original fiberglass shroud and yes it is a pain to fit it, takes a lot of contortions to get it right, but it works in tandem with the 12 blade fan, and I never experienced overheating of the likes being described by the poster.

the bonnet insulation is for heat and sound Insulation, no brainer.

on a totally out of subject comment, I am painting my 12 blade fan Fire truck Red.

 

Series 2 and 3 XJ-6 have a lovely auxiliary cooling fan operated by a radiator-mounted Thermal switch which turns on the fan automatically after a certain pre-set temperature.

It fits nicely behind the grille in MK-1, MK-2, S type, and 420.

In the XJ-6, this fan is mounted under the front bumper, in front of the right side of the radiator, small but powerful, it quickly reduces the temperature and shuts off when temp is lowered. it also keeps running after engine shutoff and turns off automatically.

Can be fitted to small saloons with On-Off switch and a Relay, no need for a thermal device. Just two wires and two fabricated brackets are needed.

​​​​​​

 

Thanks Peter

 

Thx - your are really helpful!! Its a great group

 

Thanks Peter - again!! I am a Mechanical Engineer, and there doesnt seem to be much fluid mechanics indee!! Have ordered an electric fan to see what happens. At least it might blow some extra air out of the engine bay

 

Thats great Cass - I was wondering about the lack of cowl - it seems like my current system has nothing going for it at all. Except the engine doesnt boil so thats good!!

 

Thanks Bill.. Miraculously, the wipers and washers work. I have even got the clock to work (a bigger miracle!!) . Busted indicator switch so epoxied and tomorrows task is hooking up a two pin relay as a work around (hopefully works with wiring accordingly)

 

I seem to remember a Forum member showing pictures of an engine compartment heat vent modificaton he undertook that basically opened up a big piece of the sheet metal on the exhaust side of lower engine compartment ? This would have been exiting hot air under the wing. Has anyone else made a similiar modification. ?
Schmitty

 

Primaz car both sides with his V8.Triangular cut out.

 

Then there was Ian Callum's car with external vents.

 

A cowl is very important to a fans performance. I read Society of Petroleum Engineers paper that measured the airflow on a natural gas compressor cooling fan and found that the maximum gap between the fan blade tip and the shroud couldn't be more than 0.75", or there was serious degradation in the amount of air the fan could move - this was on a fan 15 feet in diameter!

 

Regardless of increasing your fan that is separate from the engine bay issue of these cars. An electric fan or any fan to improve airflow across the radiator will be really for just dissipating the heat from the fluids that flow thru the engine; a more powerful combination of radiator or fans will depend on what the engine needs. A separate issue to me is that these cars engine bay has a hard time getting airflow out.

If your engine is operating at the proper temperature that is all that matters. If the issue is heat build up of the engine bay that is totally a different issue with different needs.

 

The question here relates to high under bonnet temperatures rather than coolant temperature. However, some of the considerations apply to both. Here’s my opinion and emphasise that it’s only opinion!

There are three situations to consider:
1. The car is moving at reasonable speed;
2. It’s crawling or stationary with the engine running;
3. It’s stationary with the engine recently switched off.

The first, we don’t know about with any certainty unless we have a sensor in the engine bay and a display in the car. In fact, for most cars in most situations, the air flow at speed is sufficient to keep temperature at a reasonable level, some way below the radiator temperature. If it isn’t, there’s not likely to be enough air going through the radiator to cool the engine. There are exceptions to this where flow through the radiator isn’t directed in to the engine bay or where peculiarities in the aerodynamics produce higher air pressure under the bonnet than in front of the radiator.

At low speed or stationary, the forward motion of the car isn’t working for us and we are relying on the fan(s) to generate a flow through the radiator. This is the situation where extra holes to let air out might help. Many have added holes in the honest belief that they are resolving situation 1 above, but the holes are totally in the wrong place. However, they are helping when the car isn’t moving at speed. Again though, if the engine isn’t getting hot, most likely the under bonnet air temperature isn’t excessive either.

Situation 3 is what we’ll most likely experience, because it’s what we see when we open the bonnet after a run. It’s also when under bonnet temperatures are likely to be highest. There isn’t much space under the hood of our cars; they aren’t For Capri 1100 with space for two men and a dog to walk around the engine. There’s an enormous heat store of iron, aluminium and water at 70°C or a bit higher (and the exhaust manifold at much higher temperature). There’s a small volume of air with (relatively) very little heat capacity. Obviously, it’s going to feel hot when you lean over it. Is it going to do any harm? Most likely not to most of the original components, possibly for later additions. The biggest hazard in this heat soak phase is the development of a local hotspot inside the engine or the transfer of a lot of heat to water inside the engine that expands and causes an overflow. Neither will be influenced by anything that simply cools the air under the bonnet.

In spite of all that, a lower under bonnet temperature remains desirable. Anything done to increase air flow should take the air predominantly through the radiator. If it doesn’t, it may increase under bonnet pressure, reduce flow through the radiator and make engine overheating more likely. Fans that run at low engine speed or after the ignition is switched off are helpful. Holes for air to exit will help, but think about the aesthetics and don’t put them where they will have the opposite effect at speed. Exhaust manifold coatings also help as they will slow the transfer of heat into the air.

 

Nice write up and I commend you on your opinions, but having put these cars to more extreme power I think my experiences do shed more light to explain things further.

The problem with these Jags is that they do not have a good design for introducing enough air and having it escape. I say that as to me the problem is more your #2 but with some aspects of 3 you do not realize. I did what most everyone here has thought would be the right steps but after my exhaustive trials I can tell you that it is that all of the shielding, wrapping, upgrading to the most powerful pusher & puller fans did keep the engine cool both at speed and at your condition #2. What the problem is that the stock Jaguar design is not providing sufficient ways to get the hot air out at both #1 and #2. Prior to adding the front air scoop that pulled additional air from under the car and then direct that air around both sides of the engine, the car's engine did not overheat at all but the engine bay throughout was excessively hot. How do I know that I had plastic parts at multiple areas such as the coil packs, starter, spark plug wires, etc. actually melt and cause failure. The engine remained at good temperature both driving and stop and go.

After I installed the lower ram air scoops with it directing the air around the engine the temperature was cooler at all conditions and no engine parts were failing due to residual heat trapped in the engine compartment due to the air was not trapped anymore. In real world testing that was probably they number 1 thing that did the most for all 1-3 and second place would be the openings at the lower inside fender wells in my opinion. These cars just do not have a large enough air opening in the front and just trap the air so the heat keeps building up even when the engine is at good temperatures.

#3 is normal, all cars will increase in temperature when you shut it off and often will go higher than normal operating temp but that like you said is a non issue. But without additional ways for the air to escape the engine bay when the car is in stop and go even with powerful fans, the hot air does not really escape.