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MG Midget and Sprite Technical - Reducing the grill opening

In Daniel Stapleton's book he suggests that internal flow drag can be reduced by reducing the size of the grill aperture without affecting engine cooling (using a blanking plate or two). He cites the Speedwell Monza front as an example of such a reduced aperture. I find this an appealing idea but am wondering what is the minimum aperture (possibly in sq inches) before there is an effect on cooling (or, more accurately, on overheating). Anyone done this?

My engine runs too cool anyway so I thought I might try 'killing two birds with one stone' to reduce drag and raise the operating temperature a bit.
Chris Hasluck

With out knowing what danial is specicflly referancing

Im making the assumption to close off all egresses around the front of the engine compartment so the olny air comming in to the engine bay is thur the radiator

In other words just a wall with the only access is the radiator

Agian, my assumption

Prop
1 Paper

Sounds like the old book because for the new book this was tested in the MIRA full scale wind tunnel so was proven. Costin calculated the exact size required for the Speedwell Monza but assumedly for a standard engine. I suggest you start off with a cardboard template and gradually increase it's size to the point where the engine can't run as cool as you want it to. Variables are the efficiency of the radiator, water pump pulley size, power the engine produces, if it has an oil cooler etc. However, I think you can start off with about 18 square inches and work upwards.
Daniel

"Costin calculated the exact size required for the Speedwell Monza but assumedly for a standard engine"

I presume that would have been a 948cc engine?
Dave O'Neill 2

Almost certainly but it was a public library copy of the book and so I can't be 100% certain.

Page 189 - one of Peter May's old cars is one size of reduced aperture.

Page 166 shows the blanking plate as tested - when I come to make one for the other side of the intake/grille I'll get an accurate Sq in measurement.

There is a blue (sponsored by 'King of Shaves'?) midget that has a very small aperture - I thought I had a photo of it but can't find it.
Daniel

It was mentioned in one of my books, maybe "The Sports Car" by Colin Campbell, that the bonnet opening only needed to be 1/3 or 1/4 the area of the radiator to allow sufficient cooling air in. A quick look in that book just now didn't find it but it's mentioned in my library somewhere.
David Billington

Took a couple of searches on the internet. First result found lots of cars and something random. Subsequent search found one of the cars I was searching for.


Daniel

Here's the rogue search photo - screen capture.


Daniel

Daniel - I'm not sure about 18 square inches (see above) as the aperture to start wuth. That's only 4 x 4.5 inches and that seems tiny. I accept that the process is likely to involve a lot of trial and error.
Chris Hasluck

Ok - removed the grille and made a grille sized template. Couldn't find my graph paper so ruled out inch squares on the template. The total number of squares including some half squares added together is approx 215. So the grille aperture is about 215 Sq In. A metric figure would be more accurate as would metric graph paper glued to my template and single mm squares added up (send your roll of metric graph paper to: Daniel Stapleton, etc etc).

The size of the current grille blanking plate which is also the one tested in the MIRA full scale wind tunnel is about 43 Sq In. I have aero data on that, so if I make and fit one on the opposite side of the car, the same size, (and the cooling is not affected) I can reasonably assume twice the aero benefit (reduction in CD and lift).

So 43 sq in works (20% reduction in aperture size), 86 sq in to be tested (40% reduction etc). I'm guessing 50% will also work (based on the width of the radiator.

All I need know is for Carl Bintcliffe to make some wing vents + a lot of money to get back in to the wind tunnel.


Daniel

Daniel
That information is very helpful - thanks.
Chris Hasluck

Chris
Is reducing drag really a priority ? - what tangible benefits are you anticipating ?
S G Macfarlane

The tangible benefits (you can't really touch them) are:

Reducing drag reduces fuel consumption.

Alternatively reducing drag increases top speed.

With the grille blanking device there is also a reduction in lift at both ends of the car so improves handling. However, the reason Chris originally posed the question was that his car ran on the cool side.
Daniel

Daniel, I have been very interested to read about the benefits and potential for improving performance by reducing the grille opening.

The car in your photo is clearly running as 'Modified Roadgoing'. For cars running in standard Roadgoing classes that need to retain their chrome grille, would the blanking piece need to be put in front of a normal grille or would it be just as effective behind it?

What 'side effects' does the reduction create in the engine bay? Is there any change in air pressure that affects the carburettors and in induction system?

Is there any affect on engine bay temperature, again, affecting induction or increasing the likelihood of fuel vaporisation?

Colin
Colin Mee

re: reducing drag - makes a discernible difference. I've added a spoiler (bigger than DS's one) and a front air dam (not as big as DS's one) and have found less throttle needed on the motorway, and overtaking things is easier. May be placebo, but there's definitely something going on. High speed stability increased, with a definite increase in available top speed.
Rob Armstrong

Rob, was that a boot spoiler, and are you using it with standard soft top erected, or a hardtop?
David Smith

Hi Colin, The grille blanking piece I tested in the wind tunnel was behind the grille. No tests were made with it in front of the grille though you'd expect it would work slightly better though it'd be as easy to simply use a larger one behind the grille to achieve the same thing.

The blanking piece I made (this evening) for the passenger side of the grille, is also designed to sit behind the grille. I suppose it's possible to simply make a flat plate and fasten it to the back of the grille.

I don't have any data on side effects in the engine bay. I'm guessing there is positive pressure in the engine bay and that this adversely affects engine bay temperatures so letting less air in isn't detrimental. One day I might build a seal cold ram-air feed to the carb. Interestingly there is a dyno roller in the HORIBA-MIRA wind tunnel so I'm guessing it would be possible to measure BHP changes in conjunction with aero changes after establish the aero change benefit. It's unlikely I'll ever have enough money to do so though!

Hi Rob, I'd be interested to see a photo and learn the dimensions and position of your boot spoiler. Note that while I tested 4 spoiler configurations in the wind tunnel it's still possible that if the spoiler had been extended in height following the 4th test further reductions in both drag and lift would have been seen.

Finally, for anyone whose seen the youtube clips of the wind tunnel testing, note that no measurements are taken for smoke visualisation testing. Rather the testing is done with just the test vehicle in the tunnel (no photographers, smoke wand man, etc) and at a higher wind speed. The measurements are taken from sensitive pressure pads under each of the car's wheel/tyres and it is those that provide the readings of lift reduction etc.
Daniel

Here's this missing photo.


Daniel

Hi Daniel et al. Pic attached. It's at a guess of a point, starts just behind the MG logo on the boot and extends backwards at a flat-ish angle. The spoiler itself is a cheap-y one from China (available on eBay for about 35 quid) and is formed extruded aerofoil section.

The bottom half of the foil is cut away to get it to sit "nicely" on the boot. There is a gap between the full foil shape and the edges of the boot and wings.

It mounts on the rear boot rack holes with 2 adjustable brackets.

Most road testing has been done (and above observations) are with no roof on. Less of an effect but still noticeable.



Rob Armstrong

And another one.

Interestingly, I've fitted a sort of grill blank - it sits on the driver's side and forms a cold air feed up to my air filter (K series inlet is on driver's side)


Rob Armstrong

Hi Rob,

You have a wing rather than a boot spoiler. A wing has airflow over both surfaces whereas a spoiler doesn't. A wing will generate drag whereas a spoiler doesn't necessarily do so. You might find that if you modify what you have so that it follows the curvature of the boot lid so it is a true spoiler it will be more effective. After that you might decide to add an extension section at the trailing edge to increase its height.

Daniel

I might respectfully disagree with you there Daniel - I think I have a spoiler, or at the least a mixture - it's joined to the bootlid in the middle over about a 10 inch width. I've tried to create a pocket of dead air even where the surface of the spoiler/sing is separate from the car body. It's not designed to add downforce, and appears to have reduced drag.

Although I'm just guessing without any wind tunnel testing. Did you try anything that was separate from the boot lid? I did a few back of the envelope CFD simulations that looked like the separate sections were of benefit, as there was little laminar flow over the bootlid that needed to be blocked at boot level.

However, the next time I'm not doing much else I'll extend the edges forwards (or ever rather brutally downwards) to the boot lid surface and see if there's a difference.
Rob Armstrong

Back on topic!

The cooling fin area of the rad is approx 13.5" x 10" = 135 in Sq so the 215" Sq of the grille aperture is massive in comparasion.

215 - 46 - 46 = 123" sq.

So assuming the rad can cope with 91% - being a long way from 1/3 or 1/4 suggested earlier, there is scope for further reduction.
Daniel

I didn't test any wings for lots of reasons and had I realised how well the spoiler would work I'd have had a longer extension (or 2 or 3) to test. I was advised by the MIRA aerodynamicsts (they come 'free' with the tunnel!) to extend the boot spoiler across to the rear wings and once I have the new boot and spoiler I'll be looking at how best to do this (additions to the wings with a slot for the boot lid to open through) as there is still scope to reduce lift at the rear. Each side amounts to about 1/8 of the total width so together = 1/4 so maybe another ten pounds of lift reduction. Not so sure on the Cd reduction but again I'd guess half a mile an hour on the top speed.

What data do you need to model this in CFD?
Daniel

This thread was discussed between 02/05/2017 and 08/05/2017

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