LETTERS from READEARS

Capacity Discrepancies

Sir,

In a letter published in the October issue of MOTOR SPORT Mr. Ronald R. Cann raises the question of the engine size of the 38/250 h.p. Mercedes-Benz. The correct figure. using the usual formula, is 7,071 c.c., although Buyers’ guides sometimes gave 7,069 c.c. or 7,100 c.c. The figure of 7,020 c.c. shown on a plate under the bonnet is the cubic capacity for German taxatum purposes.

From April 1st 1928 German cars were taxed on their cubic capacity, but the “tax capacity” was calculated by a formula different from that used for the effective capacity. The whole matter is explained in an article entitled “The Legendary Mercedes S-Types” by Hans-Otto Neubauer, published in the January 1970 issue of the late lamented Veteran and Vintage Magwr.ine.

I have recently found a similar problem when delving into the history of the Maybach Zeppelin V12 models. The type-DS7 with a bore and stroke of 86 mm. x 100 mm. in stated by contemporary reports to have a dinplacement of 6,922 c.c. However, by the usual formula tpt o bore squared divided by 4, x stroke o number of cylinders, the correct capacity is 6,973 c.c. The difference between the two figures is the same proportionately as that between the two figures for the Mercedes-Benz quoted above.

In your article entitled “The Eternal (American, Triangle” published in the August 1981 issue of MOTOR spoRT you mention that the V12 KB Lincoln is sometimes credited with a displacement of 7.2 litres and sometimes with 7.4 litres. Undoubtedly this discrepancy arises from the fact that American engineers work in inches, while their European counterparts employ’ millimetres. The KB Lincoln engine had a bore and stroke uf Wu” x 4,”, giving a capacity of 448 cu.in. This capacity, converted into metric l’orm, is 7,341 c.c. The figures quoted in road tests published in The Ammar and The Motor of 7,230 c.c. and 7,402 c.c. respectively arise out of an approximate conversion into millimetres of the’ inches in the bore and stroke, followed by use of the usual formula. I am not an engineer or a mathematician, but it seems to me that conversion of the cubic capacity from cubic inches to cubic centimetres is the more accurate method.

This conversion has always presented a problem. When the J Duesenberg made its appearance at Olympia in 1929. the Show Report of The Autocar gave its capacity as 5,675 c.c. ‘the Show Report of The Motor dectded on 7,495 c.c.

The true capacity was 420 cu, in. or 6,882 c.c. The VI6 Marmon was shown in Buyers’ guides as having 9.1 litres under its bvinnet, whereas it was an 8.1-litre engine.

I am sorry to write such a lengthy letter on a mathematical subject, especially as I am not handicapped by any knowledge of the subject! Norwich CLIFFORD S. PENNY