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MG MGA - To louvre or not to louvre....

Hi everyone,

Setting aside the questions around looks and originality, what does everyone think about the merits of putting in louvres in the bonnet ?

I am not an engineer of physicist, but I would assume that the air flow over the bonnet when driving would tend to suck out the hot air that would builds up underneath the bonnet. It would I think certainly impact the air flow and temperature but not entirely sure to what extent.

I know that the bonnet louvres on the Jaguar MK2 (Coombes) led to problems with water ingress, with those people that do have them adding small trays under the bonnet to prevent this becoming an issue.

I have recently met someone with the ability to press louvres in any number/width and my mind began to consider whether this would be of any great advantage (in addition to the usual heat proofing the cockpit and installing a heat shield above the exhaust pipe).

Thank you


Aleks
Aleks Stojanovic

The louvers would work but they would need to be installed well forward of the windscreen. There is a lot of pressure that builds up at the windscreen and actually forces air back into the engine compartment through the cowl grilles as you are driving along. Having those grills too close to the windshield would be counter productive. I've attached a study from 1963 that discussed the airflow of an MGA roadster and you can read through the findings.
Nick Kopernik

Nick, that study is fascinating. I wonder if the recessed grill of the Mk.II helps any with the flow of air to the carbs and heater box.

Anybody with hair longer than an inch knows well about the reverse airflow behind the windscreen

My old ('1952) Plymouth and my dad's old Studebaker both had side vents that allowed air to flow into the footwells. I wish my MGs had that feature.

Jud
J K Chapin

Hi Nick,


I agree with Jud - that is a very interesting document and has certainly given me food for thought.

Whilst I take on board that any louvres would need to be well ahead of the windscreen/shield, how far forward would seem to me to be a matter of guesswork.

Also, what would happen if there was a strip of louvres, some of which were forward of the line where the pressure build up starts and some behind it. Would some louvres extract air and some permit air into the engine - creating a vortex or circular movement of
air ?

I should have concentrated more in my physics lessons at school !


Aleks
Aleks Stojanovic

According to Barney Gaylord's site, you should keep the louvers "forward of the windscreen about 1-1/2 times the height of the windscreen". I've attached a link to his site for more explanation:

https://mgaguru.com/mgtech/body/bd310.htm

Nick Kopernik

I tend to save photos of MGA design concepts that interest me and this one is one owners crack at reducing heat and air pressure under the hood. Essentially it requires cutting a hole through the inner fender panel area into the engine compartment; it looks like the top of the hole would be just below the level of the heater shelf. A sheet metal "tunnel" is then fabricated and attached, and it would run down between the inner panel and outer fender. At speed this would serve as an escape path for heat and help depressurize the engine compartment with the force of forward motion pushing escaping air down and under the car. I do not remember who did this modification and whether or not it would actually help, but it does seem to make sense in a way and what I like about the idea is that its not seen from the outside and from the inside the hole would barely be noticeable. If, and its a very big if, I ever get around to redoing the car, I would definitely go with one of these on each side of the car.



Nick Kopernik

Aleks

Metal Mickey does MGA louvring. He can likely advise on the optimum positioning.

As an aside, I have a 13" sports windscreen with more raked (Le Mans) stanchions that Bob West sold me. It greatly altered the airflow over the bonnet, almost totally removing the stagnant boundary layer air immediately in front of it. I stuck loads of cotton threads in the area to confirm my suspicions. If memory serves me correct I also ended up with air blowing outwards through the MGA vents at most speeds. It was inwards with the standard screen. Although that might have been due to the more powerful 7 - blade asymmetric fan I fitted. So long ago now, struggling with memory on detail.

Steve
Steve Gyles

Hi Steve,


Thanks for your note - that is useful to know.

I will speak to Metal Mickey and take a view.


Thank you



Aleks
Aleks Stojanovic

Since most overheating is in city traffic, with lengthy stationery periods, the pressure build up may not be relevant.
P R Macwhirter

Hi Aleks
the louvres on my MGA were cut towards the rear of the bonnet as you can see in the attached picture.

I think they look fantastic on the car in that position but I don't think they work all that well where they are.

When the car is stationary, you can actually see the hot air rising out of the louvres, so they are helping to keep the engine bay cooler.

But, the theory is that the upright design of the windscreen against the airflow causes increased air pressure in this zone and so this actually forces air inwards through the louvres when the car is moving forwards.

In previous discussions on this subject, the consensus is that bonnet louvres would work better if they were situated nearer to the front of the bonnet where there is less positive pressure over the bonnet.

I have often considered doing an experiment with my car which could test this theory. I thought that I could do back to back runs up a long hill, the first with the louvres as they are.
Then I would repeat the run after I had applied masking tape over the louvres to close them.

It would be interesting to see if there was any difference in engine temperatures between the two runs. It would show if louvres at the rear of the bonnet actually negatively affect the airflow through the radiator.

If I get the chance to try this, I will post the results oh here for you.

Cheers
Colyn







Colyn Firth

Thanks Colyn,

Interesting that in stationary traffic you can see the hot air rising out of the louvres.

When travelling, I think most people don't have issues with overheating, it is really when in slow moving or stationary traffic. So as PRM (Paul ?) from Victoria mentioned earlier in this thread the louvre positioning may not be as important as first thought.

I take on board what you say about the louvres possibly needing to be a more forward of the windscreen than yours are, which seem to finish just in front of the air vents.

If at speed, air is forced into the louvres that are sat in this area of positive pressure nearest to the windscreen, surely any louvres that are outside of this area, i.e further forward along the bonnet, will permit air to escape - creating a circular air motion. Cooler air being forced into the louvres by the windscreen and hotter air being forced out of the louvres further down the bonnet by air pressure.

I have dropped Metal Mickey an email but he has yet to come back to me.

Thank you



Aleks
Aleks Stojanovic

Just thought I would throw a little caution in here on the subject. I added three rows of louvers for the main reason I just liked the looks of it. I had a well known panel beater do the work and the bonnet was stretched out of shape so bad that, as they say it was a back breaking fiddle and many hours to get it to fit.
L Frisch

As has been mentioned here right from there beginning, when louvres are added, they are are almost always placed towards the rear of the bonnet, where they are, at best, impaired in their function, and often actually counterproductive.
In this regard bonnet louvres are yet another example of a poorly executed "improvement" formulated by ill-conceived amateur "experts".
Such folk will be inclined to deride the efforts of the original designers of these MG cars, forgetting that these same engineers were actually at the top of their game, and, what's more, had the facilities of a factory to test their ideas.

I drove a 1973 Jensen Interceptor for 16 or 17 years. It had a huge 7.2 litre Chrysler V8 crammed into the engine bay.
Heat management was a major issue with the Interceptors, and in the later part of their production the factory placed louvres into the bonnet (too far rearward, except for the SP model). The (massive) radiator incidentally had a shroud fitted, right from the beginning.
What always surprised me about these louvres, when placing my hand over them after a run, was how little heat actually came through them!! At the same time huge heat waves could easily be seen rising up from under the sides of the car!
What the louvres however certainly did achieve in the Interceptor was a propensity for the bonnets to rust, due to water ingress through these louvres onto the the channel shaped structural bonnet struts that lay under them.
(MGA owners may not often drive in the rain these days, but they do wash them, sometimes often!)

On the other hand, if you care to try "the hand test", put your hand over the MGA's oval shroud vents after a run. A surprising amount of heat rises up out of them, and palpably much more from the side nearest the exhaust manifold. (As we now know, their function reverses at speed).

If your MGA tends to run hot in traffic, and you still have the engine driven fan behind the radiator, a shroud will contribute hugely to air flow through the radiator by ensuring that virtually all the air drawn by the (circular) fan will pass through the (rectangular) radiator. A shroud will however confer no benefit once out of heavy traffic.

If your MGA runs hot at speed, and is mechanically in good condition and tune, your radiator (or your water pump) will be sub-optimal and needs attention, or replacement, with an original style V-cell ("cell core") radiator or a quality aluminium radiator.

http://mgaguru.com/mgtech/cooling/cool_140.htm
T Aczel

Great information from Tom Aczel as usual, I always take notice of his advice on the subject of keeping the MGA cool.

His comments about the bonnet reminded me of a couple of negative things I have noticed about the effect of cutting louvres towards the rear of it that I didn't mention earlier.

The first is that I think whoever cut them, also removed a re-enforcing strut that ran beneath the bonnet in that position. I suppose this was done to make access to that area easier.

This has reduced the stiffness of the panel and the bonnet tends to flex more when it it held up by the bonnet prop.

The second point to mention is that of water ingress to the engine.

I have never had a problem with excess rain water entering through the louvres, either when the car is in motion or stationary.

However I was caught-out the very first time I washed the car, I used a hosepipe to rinse the car without giving any thought to the water getting through the louvres.

Naturally, the car wouldn't start afterwards and this was because I had flooded the fuse box area with water.

It was fine once I had dried it all off though, but from then on, I always cover the engine and electrics around the fusebox with a large old cloth curtain and this has prevented any reccurence of this problem.

And yes, I have forgotten to remove the curtain a couple of times, but fortunately the cloth is placed only over the rear of the engine and the engine fan has always blown the cloth backwards over the heater unit, well away from any moving parts, when the engine starts and the car is moving.

Cheers
Colyn
Colyn Firth

In addition to my love of MGAs I also have an admiration for Morgans. As Colyn once pointed out to me, MorGAn has a bit of MGA in its name!!

Every Morgan has bonnet louvers, and II have driven one many miles in rain with no problems.

More about that trip here:

https://www.mga-mk2.com/50thAnniversaryTrip.html

- Ken

PS: that page also references a wonderful article written by Colyn that appeared in an issue of MGA! magazine
KR Doris

Here’s the lnk to Colyn’s article: https://www.mga-mk2.com/HostingTheYanks.pdf
KR Doris

Thanks for inviting us to join your "Two for the Road" tour of the UK Ken, that was a fantastic weekend, one of the best MGA trips that we have ever done.

I also enjoyed writing about it from our point of view, just one thing I would like to clear up, "Did you really eat the Cornish Pasty?", or did it actually end up thrown into a hedge bottom" :^)

Colyn
Colyn Firth

Colyn - we did indeed eat and thoroughly enjoy the Cornish Pasty!

So much so that we also ate similar Pasties on our return trips to England in 2022 and just last month!

- Ken
KR Doris

Aleks
Just to add to the louvre question
I saw a pic. of what I think is the best looking set of louvres I've seen the other day. They were on a T type bonnet and instead of poking up facing back, they were pressed down facing forward--still would work the same but the top of the bonnet was still flat--looked very special---All I have to do is remember where the pic was-I'll post it here shortly --when I find it
willy
William Revit

That was fun - found it
First pic 2 cars one with louvres up and the closer car with them down--still flush on top
I believe they are the same louvres just pressed from the other side as shown in pic 2

I quite like the pressed down /flush top look-
What do you guys think----?





William Revit

Hi Willy,


Yes I do like them alot - must admit that I had not seen or noticed them like that before !

The blue car looks very good with them...




Aleks
Aleks Stojanovic

Morning all. Been reading this thread for a while biting my tongue, but can’t now resist putting forward a purely personal view on louvred bonnets. Please excuse me but I think they look awful and spoil that wonderful smooth curve of bonnet running all the way from radiator grille to windscreen. Such a shame to cut it up and spoil that sleek look. If the engine and cooling system are in good order louvres shouldn’t be necessary from a cooling point of view. Then it’s purely your view on looks, not cooling. You know mine! Bruce.
Bruce Mayo

Interesting thread. These cars did not overheat when they were new. They don't have to overheat now ; an electric fan works well in an MGA as it does in our 3.9 GTV8 in London traffic at 100+F, So I am with Bruce; these cars are amongst the best looking ever built in UK. There are lots of changes that you can and probably should make to the mechanics and the electrics but there is nothing one can do to improve their appearance.

Sorry Aleks, this really isn't a good move. If you're moving to India with your car, you might try a vent in the port inner wing plus the fan and an alternator
Roger Walker

I have no real view on this subject but will make one point as my ownership goes back to 1969. The cooling requirement on MGAs now is NOT the same as when cars were new as the chemical mix of petrol has changed dramatically. For instance in my early 30k miles a year time I never heard of fuel vaporisation problems but I have suffered them occasionally these days. Also of course engines may be more demanding for instance I have an 1800 now whereas as it was 1500 in early days. My only change from standard is a small fan to keep carbs cool in traffic jams although I rarely get in jams.

Enjoys your As whatever you do, you own them.
Paul Dean

Hi Bruce,
sorry you are not a fan of louvres, the louvres on my car were courtesy of the DPO.

I have attached a picture of what I will be wearing from now on, so the next time you see an MGA with louvres in the bonnet Bruce, you won't realise that it is me! :^)

I still like how they look on an MGA though, but if I had an original bonnet, I would probably leave it alone because I am fairly sure that they only help with the cooling by letting some heat escape when the car is stopped.

The previous owner of my MGA had also lowered the suspension all round, fitted negative camber front suspension arms and a adjustable rear telescopic shock absorbers.

I absolutely loved how the car looked but it caused the ride to be rock hard, the steering was ridiculously sensitive and the exhaust would ground over the sightest bump. In fact, the vibration was so bad over a slightly rough road service that I had to drive the car much slower than a standard MGA could be driven.

I loved the lowered look of the car so much that I put up with the downsides for over 10 years, but finally, I had had enough of the horrible ride and handling and decided to return the suspension back to it original height and original type shock-absorbers.
The ride and handling was transformed and I love how it drives now.

So, I suppose this proves to me that modifying the MGA for mostly cosmetic reasons is just counter productive and not worth the effort because MG pretty much got the car right from the start.

But, I still love how my car looks without the bumpers and I am not planning to change it!
(See 2nd Pic)

Sorry Bruce! :^)

Colyn








Colyn Firth

This thread was discussed between 19/07/2024 and 09/08/2024

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