Editing Jaguar V12 engine (section)

=={{anchor|5.3HE}} 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 (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 of the original design's dished type allowed [[Squish (piston engine)|squish]] from the compression stroke to push the air through the channel around the intake valve to the chamber below the exhaust valve, causing turbulent swirling flow around the spark plug (which had been relocated near the exhaust valve at the top of the chamber). This design created a [[Stratified charge engine|stratified charge]], allowing the engine to run at an unusually high compression ratio for the time (10.5:1 to 12.5:1, depending on market and year) while running a relatively lean fuel mixture. In any given market power levels remained similar to the previous model, but fuel economy was improved by nearly 50%.{{cn|date=March 2020}} A new fuel injection system called "Digital P" featuring a [[Digital electronics|digital]] [[Engine control unit|ECU]] with integrated manifold air pressure transducer was installed, replacing the older [[Analogue electronics|analogue]] control unit and remote pressure sensor from Bosch's original D-Jetronic design.<ref>{{cite web|url=http://www.jagweb.com/aj6eng/lucas_efi.php |title=LUCAS EFI |publisher=AJ6 Engineering |access-date=2011-03-11}}</ref> (However, cars sold in Australia, Sweden and Switzerland continued to use the D-Jetronic system until at least 1985.<ref>{{cite book |author=<!--Staff writer(s); no by-line.--> |title=Series III Service Manual, AKM 9006 |edition=5th |publisher=Jaguar Cars Limited |date=1988 |page=05—4}}</ref>)

The OPUS ignition was replaced by Lucas's Constant Energy Ignition (CEI) in 1982, to deliver a more reliable spark. Series 3 XJ12 and Daimler Double Six cars used the CEI system until the end of their production in 1992, but it was superseded in the XJ-S in mid-1989 by another from [[Magneti Marelli]]. The Marelli ignition system was used until the end of XJ-S production, and on the {{convert|5993|cc|L|1|order=flip|abbr=on}} version used in the XJ81 four-door saloons made in 1993 and 1994.

The 5.3 HE was used in the following applications:
* 1981–1992 Jaguar XJ12 (Series 3)
* 1981–1992 Jaguar XJ-S
* 1981–1992 Daimler Double-Six (Series 3)