market, although Weber carburettors were only available from the dealer or on the aftermarket, and not supplied by the factory. The exception to this was 2.4 litre cars up to the introduction of the 240, these earlier cars having twin Solex downdraught carburettors. Some D-Type sports racing cars received fuel injection, but it did not become available on the road cars until 1978, beginning on certain US market models in order to comply with government fuel-efficiency standards. It was eventually extended across the range and to all markets. Power rating During the 1950s and 1960s Jaguar released SAE gross horsepower figures, using the measurement system preferred by US manufacturers, as otherwise its cars would have appeared under-powered in comparison with US cars. This practice was superseded by reliance on the more realistic SAE net horsepower system around 1972, although Jaguar power ratings of that era may also be expressed using the very similar DIN net rating system. Since the conditions upon which the SAE gross figures were determined were not recorded, an objective comparison of the power outputs of the various models of XK6 powered Jaguar over time is impossible. The 1965 4.2 XK fitted to the carburetted E-Type had a listed output of SAE gross; the 1986 fuel injected 4.2 XK fitted to the series 3 XJ6 was listed at DIN net. Variables such as compression ratio, cam lifts and durations, and fuel consumption rate for each engine during testing would, at a minimum, be additionally required to develop any estimated comparisons. Individual displacements 3.4-litre The first production use of the XK straight-6 was in the 1948 Jaguar XK120, which used the version with a bore x stroke of . It had an iron block without cylinder liners and aluminium cylinder head. It had a wider gap between cylinders 3 and 4 than between the other cylinders. The 3.4 was first rated at SAE gross at a compression ratio of 8:1, rising to SAE gross with the C-type cylinder head (confusingly not the head from the C-Type as raced at Le Mans) all the way to SAE gross with the "straight port" head at a compression ratio of 9:1 as fitted to the XK150SE. Almost as soon as the new compact Jaguar 2.4-litre was released, there was pressure on Jaguar to fit the 3.4-litre engine to it. This was duly done in February 1957, the car being known as