Index of Performance/Thermal Efficiency (Merged)

[Henrik Pedersen]

Does anyone know how they calculted the "Index of proformance/Cubic" at Le Mans?
It was, from what I know, a formula that had mileage/cubic, but it changes over the years.

Would entrant to Spa 6-hours, that have an interest in a "Index of proformance" Trophy, come forward in this tread.

[Henrik Pedersen]

Can anybody tell me what the calculations and formula behind the "Index of proformance" and "Index of thermal efficiency" was at Le Mans?

[John Turner]

These were two entirely different awards; that for the Index of Performance a relatively simple calculation; that for Index of Thermal Efficiency a far more complicated affair that surely only the French could have come up with. Much perusing has found brief explanatory references in Hodges 1963 book, and Clausinger's from 1982.

The Index of Performance was an annual award that grew out of the Biennial Cup (itself a replacement for the once only awarded Triennial Cup - 1923-25). The IoP was a simple calculation where the distance (D) actually covered by the car was divided by the distance set (presumably!) by the ACO based on engine capacity (D1). Of course, the distance set, I guess was changed every year as performance in cars in general increased.

For the Index of Thermal Efficiency, I am going to have to quote verbatim from Hodges, since it is beyond my ability to put into easier form! Initially, it 'equated speed, weight and fuel consumption as follows

P+200 + V + (V -100) squared + (V-140) cubed
200 16 500 12000

All the above divided by E, where:-

P (Poids) = weight in kilograms with oil, water and spares but no fuel
V = the race average of the car in kph
E = energy in litres of fuel per 100km.

Got that! The formula was then modified in 1961, and then allegedly simplified 1962 and expressed as

Em
Er

where Er = consumption in litres per 100km, and

Em = 1.5 + (P + 1) V cubed
4 x 105

Where P = weight (now including fuel, but not tools) and
V = average speed.

As you can see the format of posting here does not allow me to place the 'divided by' figures directly under the 'divided by' lines (at least, I don't know how to do it) but I'm not sure it would make it much easier to understand, if I could!

[Henrik Pedersen]

Hi John

Thank you for a very specific answer.
I will try to find the books you refer to, and work my way back.

Regards
Henrik

[D-Type]

You could try writing to the ACO and asking them directly