AH/MG Power Requirements - Analysis by Colin Ham

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AH/MG Power Requirements - Analysis by Colin Ham

Post by HealeyBN7 » Thu Feb 27, 2014 8:30 am

With Colin Ham's permission I thought it would be of interest to share an analysis he completed while working at BMC/MG.

Here is a bit of our dialog to help explain the chart.

Colin: I came across this in my archives from my time at BMC/MG – thought you might be interested.

Dean: Is it from wind tunnel testing or road testing? It is interesting that the MGC GT has a lower power requirement then a regular MGB GT. Do you think that is from the bulge in the hood which could act to deflect air over the windshield or do those cars sit lower?

Colin: I don't know where the data was obtained I suspect it was from wind tunnel testing. It was given to me by Eddie Maher, head engine designer at Morris Engines where I worked in Engine Development. It was part of Nuffield/BMC at the time (prior to British Leyland) and manufactured MGB, (B series) AH (C series) engines as well as Mini Cooper S. I was just out of apprenticeship and my responsibility was to calculate engine power output from the engine test beds. We did not have any computers and most engine brakes were water dynamometers not electric, I had to hand calculate all results using a slide rule.
We needed to know some idea of the effect the engine modifications we were testing would have on the car's top speed. You will note that we quoted flywheel horsepower as that is where we measured it. No road roller dynos. Very few MGC engines produced 150 HP as quoted and most were around 140.
The MGC figures were probably influenced by the hood bulge and also possibly by the cleaner underside of the car (Torsion bar frt suspension, lack of frt engine cross member etc,. The car also used 15 inch vs 14 inch for the BGT.)

Dean
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Re: AH/MG Power Requirements - Analysis by Colin Ham

Post by Larry Kluss » Thu Feb 27, 2014 1:28 pm

Neat stuff, Dean. If I'm reading this right, with the 92(?) stock horsepower in the MGB and GT, the GT should go 2 MPH faster top speed.

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Re: AH/MG Power Requirements - Analysis by Colin Ham

Post by HealeyBN7 » Thu Feb 27, 2014 1:51 pm

Larry Kluss wrote:Neat stuff, Dean. If I'm reading this right, with the 92(?) stock horsepower in the MGB and GT, the GT should go 2 MPH faster top speed.
Yes. That's how I read it as well. With the same HP the GT has a higher top speed. Another way to look at it, is it takes more HP to push the Healey to 100 MPH then any of the other cars listed.

If you want to keep up with Steve in his GT at 100 MPH you will need to find an additional 3-4 HP - all other factors being equal, which we know they are not:)

I am not exactly sure what a water dynamometers is all about, but it probably gave birth to the viscus coupling that I had to replace in the Range Rover transfer case...

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Re: AH/MG Power Requirements - Analysis by Colin Ham

Post by Steve Simmons » Thu Feb 27, 2014 2:40 pm

I was talking with Dean about the data. The MGB and MGB GT drag coefficients are accurate based on past research I've done on the subject. I had never seen data for MGC before, so best I could figure was the bulge on the hood of the MGC lowered the high pressure area at the windscreen. The relatively vertical (by modern standards) position of the glass makes that the least aerodynamic part of the car, and a big source of wind noise.

The MGC does not sit lower. If anything maybe 1/2" higher due to larger wheels but the same rear springs. Front is of course adjustable for height.

Regarding top speed of MGB versus MGB GT, the GT was rated for a higher top speed by the factory simply due to better aerodynamics of the coupe body. I believe it was 108 for MGB and 110 or 112 for the GT version. These are early (pre-68) figures. Later cars were a bit slower.

As an aside, a few years back I calculated the top speed of my upgraded MGC based on horsepower, torque, gearbox and rear end gearing and aerodynamics. I had to estimate the drag coefficient based on MGB GT data but looking at this chart I may have been too high (slower). I did not account for tire size or drag, nor do I think it would be a large influence. With the tires I have there may be more drag than stock, but possibly better aerodynamics. Anyway, the theoretical top speed based on estimated power output (as stated by the previous engine builder) was 152 MPH. I do not plan to test that in the real world.

I do admit I was surprised how inefficient the Austin Healey was. It must be mostly due to the front end and that large vertical grill area. Probably five times better than my MG TC though!

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Re: AH/MG Power Requirements - Analysis by Colin Ham

Post by Larry Kluss » Thu Feb 27, 2014 5:56 pm

I'm Sure Steve knows this, but aerodynamics were the main reason the factory always raced the MGB with a hardtop, before switching to the -GT after it was introduced.

At Le Mans in '63-'65, they raced the MGB with a different "droop-snout" front end to improve the aerodynamics. Not really sure how they got away with that in the GT class.
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Re: AH/MG Power Requirements - Analysis by Colin Ham

Post by Steve Simmons » Thu Feb 27, 2014 6:08 pm

Plus they actually had two versions, a long nose and a short nose. I wonder what those extensions would do for a GT's aerodynamics.

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Re: AH/MG Power Requirements - Analysis by Colin Ham

Post by HealeyBN7 » Thu Feb 27, 2014 6:11 pm

Larry Kluss wrote:I'm Sure Steve knows this, but aerodynamics were the main reason the factory always raced the MGB with a hardtop, before switching to the -GT after it was introduced.

At Le Mans in '63-'65, they raced the MGB with a different "droop-snout" front end to improve the aerodynamics. Not really sure how they got away with that in the GT class.
Healey must have figured out that they were at some disadvantage with the up front nose and started stretching the front and back into "Streamliners". Here is a photo of three on the grid at Sebring.

Image

More photos and info here http://www.healeysix.net/Streamliners.htm

Dean

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Re: AH/MG Power Requirements - Analysis by Colin Ham

Post by marankie » Sat Mar 01, 2014 8:45 am

An engine dynamometer is simply an impler pump with a known length reaction arm and a scale under the end of that arm. A shaft from the engine flywheel drives the pump shaft. All this is used to measure reaction torque when the testing engine drives the dyno. The amount of water introduced into the dyno (pump) reduces or increases the load on the motor, so the dyno technician will vary the amount of water introduced (hand water valve) or allowed to flow out of the dyno until maximum engine power output revs are reached. At this time he takes note of the reaction torque load reading on the scale. This process is iterated a few time to find maximum power out put revs, maximum torque (and corresponding revs) an usually torque output at various set revs, such as 3, 4, 5, 6K revs. This allows power curves to be graphed. There is a simple mathematical formula that converts dyno torque into horsepower. Torque is a measurement of force, power (horsepower, kilowatts) is a measurement of work (force times time)
Electrical dynos came along later, but in principle work the Sam on that output is measured torque.
Martin

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Re: AH/MG Power Requirements - Analysis by Colin Ham

Post by Larry Kluss » Sat Mar 01, 2014 9:52 am

Nice explanation, Martin.

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