Raising the hood and fuel consumption

[John Murch]

Yesterday I was on the M4 when we were brought to a halt by an accident. The heavens then opened so I hurriedly put up the hood and sat for 20 minutes.
I left the hood up for the next 50 miles in the hope that things would dry up. I could not detect any greater air resistance at 70mph compared with roof down.
In the 1925 LeMans the first 20 laps had to be done with roof up, apparently this caused a huge problem with fuel consumption.
Looking at photos of the event, the hoods look very high, why?

I’m sure my hood is far lower.

Was the hood really responsible for the 1925 fuel consumption miscalculation?

[Martin Webster]

I must confess John that I have often wondered if they had originally made a miscalculation with the fuel consumption figures.
The aerodynamic drag on a car without a hood erected would normally be greater than if the hood was raised due to the vortices being shed at the top and to a lesser extent the sides of the windscreen frame.
The energy required to form each vortex is consumed by the power of the engine moving the car through the air and this applies equally to aircraft as well. Therefore, for any given speed, the aerodynamic drag on the car should be higher with the hood down rather then when it is up and it follows that the fuel consumption would be higher as well. Even the rudimentary hood on your own car John ( apologies sir) will reduce the aerodynamic form drag by a small degree when up rather down.
I don’t know if the fuel consumption figures are still available for the race as they would be interesting to study today.

Martin

[John Murch]

Thanks for that Martin, your thoughts agree with my own, I have always suspected that the raised hood explanation for running out of fuel was suspect!

PS. My own hood is surprisingly effective, I have never fitted the sidescreens but stay completely dry regardless of how heavy the rain. (Also stay dry with the hood down, provided 45mph is maintained)

[Christopher Carnley]

The "extra" fuel consumption had a lot to do with faster speeds on the smooth race track.

[John Murch]

Had the track been resurfaced since the previous year? That would certainly be very relevant. One would have thought that that situation would have been known about.
Very interesting.

[Christopher Carnley]

Gentlemen,

See /keentechracing.co.uk Le Mans History, 1924 and1925.

For 1925, scroll down to "Race, The start", then read lines 9&10.

John,

Did you see your car as "The mystery car" about 3 mags past?

It was a photo with the first owner and his pal doing a recce for a rally in Australia. He and his brother had a furniture factory and possibly built most of the body.


He then fitted the saloon body for his wife's comfort.
Repatriated in 2004.

See also, "An Illustrated History,...." along with a DFP.

[John Murch]

Thanks Christopher, I did spot my car in the Review. I'm fortuneate to have quite a few photos from the 1920's and early 1930's of my car. Current body fitted circa 1962 in Australia.
The car came to the UK in 1972.
The link you gave was interesting.