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MG MGA - Drag Coefficient Revisited

Presently working out in Cairo on rotation from London, I have had a little time to spend on researching the modifications to my MGA coupe which will later this year have its 240HP engine transplant. Don’t bother with all the comments about why am I doing it, I have heard them all.

What interested me was the theoretical top speed of my coupe with this sort of power so I did a bit of searching in the archives, having remembered the previous thread on this subject. In the most comprehensive thread the base data referred to was from Barney’s site relating to a MGA electric conversion.

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=6&subjectar=6&thread=2006103009022825981


Here it is claimed that the Cd (drag coefficient) of the MGA was 0.7. This seemed very high, as with a little internet searching you find that a truck has a Cd of about 0.6. Other items on this base data also made me suspicious, because although the conversion was on a 1500cc MGA it claimed it had 88 hp, which is 20 hp more than BMC claimed.
To cut a long story short, it struck me there is actually a lot of data relating to the different MGA types, with contemporary road test data showing top speed. It therefore should be possible to back calculate the Cd value for the car. The best site I found to do these sorts of calculations is:

http://www.pumaracing.co.uk/TOPSPEED.htm

Now if you take the equations given there and solve for Cd you get the following formula:

Cd = (375x0.83P – Wx0.0167xV)/fAx0.00256xV3 (that is cubed velocity)
Where P=Horse Power, W=Weight in pounds, V=Speed in MPH, fA=frontal area in ft2 (square feet)
Notice the 0.83 used of P is due to 17% of losses in the transmission etc. Also in this equation the rolling resistance factor of 0.0167 has been used from the MGA electric data as it was assumed they measured this on the car they converted. This number is higher than stated on the Puma racing site but may be due to the type of bearings in the MGA wheels etc. Also the frontal area was taken from the MGA electric conversion site as being in Cairo I can’t measure the front of my car.

Using this formula and the information as above from the electric MGA conversion for the following MGA horse powers and old test data from Road and Track, Autocar etc. you get the following Cd figures. Note these are for the roadster.
1500cc MGA 68HP top speed 95mph – Cd = 0.54
1600cc MGA 80HP top speed 103mph – Cd = 0.513
Twin cam MGA 108HP top speed 113mph – Cd = 0.535
All reasonably consistent and the average Cd = 0.529

A lot lower than the 0.7 quoted but still quite high.
So if the Cd of the roadster is 0.529 what is the Cd of the coupe?

It so happens that Road and Track did a test of not only the 1500 roadster but of the 1500 coupe as well where they achieved a top speed of 101mph versus the 95mph of the roadster. If the power to the wheels is the same and you just compare the cars using the top speed the ratio of the Cdc (coupe) versus the Cdr (roadster) is 95squared /101squared which is 0.89. The Cd of the coupe is therefore 0.471.

So the final question is from my point of view is what speed will a MGA coupe do with about 240HP. The answer is theoretically just over 160mph.

Just as a matter of interest the calculation for Cd was carried out for an MGB using 94hp and a top speed of 103mph and a weight of 2300 pounds. The Cd worked out to be 0.609, so where the quoted numbers on websites of 0.4 for a MGB come from I have no idea. However the result I obtained is consistent with the MGA results which show that with less power (see the 1600cc results) the MGA is as fast as the MGB.

These Cd numbers are quoted to 3 figures but they are not that accurate. They are probably reasonably indicative of the actual numbers.

Any comments/corrections are welcome.
R J Myers

Using published power figures to back calculate CD is a waste of time. The hp figures from that time are gross, without accessories like exhaust, air filters, fans, and sometimes even water pumps, and are significantly higher than real world available power. And remember that they are fundamentally advertising hype, not engineering numbers. You would be closer if you subtracted 25%-30% from advertised hp figures. Since about '75, hp is quoted as "net", a more realistic configuration of the engine as installed, but it's still advertising. For example, the oft mentioned drop in power of the late B is about half from emissions and that wretched manifold, and half from the change in rating method - if it really had as big a power drop as it appears, the late cars wouldn't go nearly as well as they do.
If you want to try this, use actual rear wheel numbers, from dyno tests, corrected to standard conditions.
From looking at many sources over the years, I would put the CD of MGA in the 0.3-0.4 range, the coupes being better, like .035, and the open car near 0.4. That matches the B numbers.
Remember that these cars were developed after the speed record runs, and are much better than average for the time; the aero qualities are very evident in the shape of the nose and the coupe windscreen especially - in direct comparison to modern designs.

FRM
FR Millmore

FRM

Thank you for your valuable comments and your estimated Cd figures for the MGA. I was well aware that the power figures quoted may be old SAE gross numbers with the engine not fully dressed and I almost put a sentence in about it. However on the websites I looked at it did not state that it was gross SAE HP and I did not know how BMC tested their engines. Of course in different circumstances I could have rung my brother up who worked at MGs for many years in the 60s and 70s.

What I was trying to show was that the previous Cd figure in the last thread of 0.7 was wrong and there is a way of back calculating it. This is probably more accurate than many sources over the years particularly where one shows a Cd of 0.7 and previous calculations on this board seemed to confirm the number.

Does anyone have some rolling road data for standard MGAs showing the rear wheel horsepower?

Rod
Rod Myers

I think Twist/Heideman have done chassis dyno tests; I figure about 60hp at the wheels for a good A. That matches Spridget figures at around 50 for a 1275 rated (advertised) at around 70. Virtually any published hp numbers pre-75 are gross of one sort or another - some manufacturers cheated!
I don't recall if I entered the last discussion; if so, I would have said the same thing, but I think I despaired of getting anywhere. You are correct in theory, but your CD is still way high, due to the power factor.

FRM
FR Millmore

A bit off subject, but only a little....

Does anyone have the drag coefficient of an MGC GTS? I've been wondering what the estimated top speed of my GTS clone would be with 200-210 HP at the wheels (estimated 280 HP at flywheel).
Steve Simmons

FRM - if the 60's lotus elite has a drag coefficient of 0.3 its difficult to see how even the sleeky mga coupe can come close to this. Just guessing I would have thought the coupe Cd is closer to 0.4
J H Cole

Just a reminder about airflow flow effects over the MGA roadster. Scroll just over half way down the following page:

http://rutgerbooy.nl/MGA_specials_page_2.htm

Steve
Steve Gyles

Steve,

Don't know about the drag factor of a MGC GTS with those wide arches etc. but you might get some idea of top speed if you use the factors for an MGB which are located on this site, assuming their drag factor is correct.

http://www.mayfco.com/mgb.htm

If you then plug these numbers in the puma racing site given before you'll at least get a number to start with.

Obviously this mayfco site gives a drag factor for a MGB of 0.4. How this was derived I don't know although some of the difference between my calculation which gives a drag factor of 0.6 is definitely due to my use of the SAE gross rating of horsepower.

Rod
Rod Myers

I'm assuming the GTS, even with the wide arches, is more aerodynamic than a standard MGB GT. The lower ride height and lack of bumpers surely help greatly. I wonder if the hood bulge helps or hinders overall air flow.

Thanks for the information, and I'll stop stealing the thread now! :)
Steve Simmons

I just found this site which shows measured rear wheel horsepower for the MGA 1500 and the MGA 1600.

http://www.mgexperience.net/article/performance.html

If you use these numbers in the previous equation in my first posting, using a car weight of 2,250 lbs you get the following Cds

For a 1500 MGA roadster 45HP at rear wheels top speed 95mph Cd = 0.408

For a 1600 MGA roadster 55HP at rear wheels top speed 103mph Cd = 0.403

Pretty close figures. The average Cd is 0.406

Using this number, from my first post, the Cd of the coupe is 0.89 of this value = 0.361

If someone feels like checking the numbers that would be good.

Rod
R J Myers

RJ,

I would like to be in your Coupe when it does 160mph.

I'm serious. Please.
Mark

Hi Guys

My rolling test on my MGA 1600 with standard bore, bearings etc was 51.5 hp at the rear wheels (52000mls since new). I would be happy to get 55hp - but would have to clean up the head, I would suggest.

Regards, Dave
David Godwin

Of course you can remove the windscreen on the roadster, making it much slipperier than the coupe.....
Bill Spohn

David,

In the interest of science,please (if you have not done so already) go and thrash the living daylight out of your car and tell us all how fast it goes! (using satnav of course)

If it's the sporty little number on the members forum, then you might like to refit the bumpers and missing grill slats.

You can report with hood up/down and oh yes why not do a test without the windscreen (we'll even let you take the bumpers back off for that one!)

Seriously though, if you do have an idea of top speed it would be nice to make the correlation.

I'm sure Rod will pay your speeding tickets!

Neil
Neil McGurk

One thing concerns me about going that fast in an MGA, lift. My coupe is good for about 120mph and feels safe (maybe more if I didn't chicken out when I attempted it) but I read somewhere that the Le Mans MGB's started to lift when going at speeds well over the ton (somebody correct me if I am wrong) It acheived a top speed of 139 mph with a modified front. The race boys fit a spoiler to 'B's help with the high speed stuff. May be the coupe is safer at these speeds than the flat fronted 'B'. I'm quite sure your MGA will go that fast with that power but will it be safe? I hope you can tell us all the answere.
Re - the roadster without a screen, I had a chat with somebody who had cut the roof off of his coupe race car to save weight and thought it would be faster with just an aero screen, it was slower!

The problem with crashing an old car is that you are very unlikely to get out in one piece, seat belt or not.

On that happy note, all the best

Terry

P.S Make you fit some decent tyres and brakes
Terry Drinkwater

So, by Rod's latest calcs, my guesstimates of 0.35/0.40 are looking pretty good, likewise the 60rwhp for a GOOD A. When back figuring the rdst/coupe numbers, note that the coupe has more frontal area, so the Cd is actually a bit less than Rod got.

The Lotus Elite is to be sure one of the very best production cars, but a good bit of that is that it is just so clean - remove the bumpers, mirrors,etc from the A coupe and you'd approach the Lotus. I had an Elite, and in the winter, with the right snow conditions and steady wind from the front, most grossly similar shaped cars (MGA, B, midget, Spitfire) will come out shaped just like the Elite - must be just about perfect. I was a bit perplexed when I looked at my driveway one winter's day, and saw a half-dozen "Elites" out there!

Terry's concern re front lift is very important. This is a constant problem with most cars of the era. The rounded lower valance at the front acts to compress air under the car, giving a lot of lift, and drag too. It always surprised me that it took until the LE for MG to get around to paying some attention to this.
Shelby found this out with the Cobras, with a very similar lower valance - at around 140 they would simply leave the ground. I have an AC Aceca with Cobra power, and I can tell you that it gets pretty spooky at high speed, to the point that I never tried to see how fast it actually is.
I wouldn't even consider trying for more than 120 at most without a serious thought to an air dam/spoiler on the front of any car with a rolled under front valance.

FRM
FR Millmore

I should have added that a rear tailgate spoiler is available for BGT's, this also helps the stability and drag.
My P.S. should have read - Make sure you fit decent tyres and brakes. I always read through my messages and still manage to omit somethin
Terry Drinkwater

This thread was discussed between 11/06/2007 and 14/06/2007

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