pistons. These changes reduced complexity, weight, size and noise, and were anticipated to help the engine meet future emissions standards. The revised head design by Hassan and Mundy also had longer, more restrictive inlet ports sacrificing top-end power but which—along with an increase in displacement to (90 mm bore x 70 mm stroke)—greatly improved performance at lower and mid-range engine speeds, which was more desirable in heavier luxury cars. The chain-driven SOHC heads and the softer valve springs fitted to reduce valve train noise resulted in the red line being lowered to 6,500 rpm from the 8,000 rpm of the original DOHC design. The engine was continuously refined with various carburettor and fuel injection arrangements before finally seeing production in the Series III E-Type in 1971. 5.3 Litre The production engine had an oversquare bore x stroke, producing to (depending on emission controls and compression ratio), and up to of torque in fuel-injected form. Right from the start of production in 1971 the V12 engine had Lucas OPUS (Oscillating Pick-Up System) electronic ignition. Initially the OPUS ignition amplifier unit was secured directly to the engine between the cylinder heads and had problems due to overheating. In later cars the ignition amplifier had been moved away from the engine where it could get air flow for cooling. Originally the V12 was supposed to use an advanced fuel injection system under development by AE Brico but this plan was cancelled at a late stage, possibly due to concerns that the design was too similar to Bosch products. The V12 as used in the Series 3 E-Types, Series 1 XJ12 and early Series 2 XJ12s (1973-April 1975) had four side draft Zenith-Stromberg carburettors. After April 1975, the V12 engine used in the Series 2 XJ12 and the new XJ-S had a licensed copy of the Bosch D-Jetronic system adapted by Lucas for use on the V12. This version was used in the following cars: 1971-1974 Jaguar E-Type 1975–1981 Jaguar XJS 1972–1981 Jaguar XJ12 (Series 1 and 2) 1973–1981 Daimler Double-Six (Series 1 and 2) 1972-1981 Panther J.72 1974-1985 Panther De Ville 5.3 Litre HE A "high-efficiency" (HE) version of the engine debuted in 1981, using special high-swirl design cylinder heads designed by Swiss racing driver Michael May. May's design consisted of a swirl chamber at the exhaust valve with a channel around the intake valve. The use of conventional flat-topped pistons in lieu