The difficult first act.
The XJ-S’ first five years were undoubtedly troubled. Launched into a post oil-shock world, where 12 mpg would butter increasingly fewer people’s parsnips, while presenting a visual envelope which substituted the E-Type’s easily assimilated aesthetics for something more complex and dissonant, the Seventies Jaguar flagship would prove a cerebral, rather than a visceral choice. Also a far more expensive one, with an asking price more than double that of the last of line E-Types – but in mitigation, it was a far more sophisticated, more capable machine.
The car’s introduction also coincided with an increasingly bitter internal environment which saw Jaguar’s management (such as they were) engaged in a desperate battle for identity within a carmaking group which had become fundamentally ungovernable. As British Leyland’s flagship model, XJ-S would also help underline the national carmaker’s repeated ability to miss open goals.
Critics of the XJ-S routinely point to the first five years of its career as illustration of Jaguar’s error in abandoning a much loved, tried and true format. But the story is a good deal more nuanced than that.
1977 would be a year of contrasts. Amid the flag waving of the Queen’s Silver Jubilee, Britain’s parents looked on in mute horror as the punks on the street corners turned out to be their own little darlings. Cultures were fracturing. 1977 would also mark the point when BL’s volume car division would become a terminal-case – not that this was immediately apparent to newly appointed British Leyland CEO, (Sir) Michael Edwardes. With the discredited Ryder report now cast into the waste paper basket, Edwardes set to the task of mopping up what of the mould-ridden BL Cars that could be salvaged. Jaguar? Well down his to-do list.
Nevertheless, 1977 would also see a number of running changes to XJ-S, foremost being the replacement of the much-criticised Borg Warner Model 12 automatic transmission for the superior US-made General Motors Turbo Hydramatic 400 unit. Externally, the matt black finish was excised from the lower section of the bootlid, as was the chrome fillet trim from the B-pillar. Inside, the bright bezels surrounding the main instrument binnacles, criticised by the press for being cheap-looking were substituted for a matt-black affair. Despite this, the XJ-S’ cabin retained its somewhat cold ambience.
In 1979, manual transmission was removed from the (very short) options list. The manual, (Jaguar’s own four-speed unit) had not proven a popular choice amongst customers, despite its improved performance potential, economy and the remarkable flexibility it offered. Jaguar’s drivetrain chief Harry Mundy had overseen development of a five-speed manual capable of handling the V12’s torque outputs earlier in the decade[1], but the BL overlords refused to sanction it. A mere 352 manual XJ-S’ were built in total[2].
As levels of ambivalence towards the XJ-S’ appearance grew, opposition voices became more strident; critics now openly stating that Jaguar had abandoned its stylistic heritage, as epitomised by the E and D-Type models. While at the time engaged in speculative design work for Jaguar’s troubled saloon programme, a number of Italian design houses turned their attention to XJ-S.
Carrozzeria Bertone were first to show their hand in 1977 with the dramatic Ascot concept, Pininfarina showing their proposal a year later. Cambiano’s XJ-Spider garnered a lot more column inches, not to mention the attention of those few people at Browns Lane who mattered, and while neither concept elicited a commission, the Pininfarina design at least initiated an internal dialogue, which would lead to other things in the fullness of time.
With British Leyland by this stage focussed entirely upon the survival of the volume car division, and Jaguar’s energies (and meagre resources) channelled towards its crucial saloon range, there was little or no funding forthcoming for more meaningful changes to XJ-S, nor indeed much enthusiasm for a model which was not only viewed as a dinosaur, but the product of a subsidiary in virtually open revolt. Alongside the urgent manufacturing crisis facing the 1979 Series III XJ saloons, the stuttering XJ-S had become something of an afterthought.
But not entirely. The Jaguar V12 engine had originally been designed to run with a high (10:1) compression ratio. However, US market emissions mandates, not to mention the cancellation of the original AE Brico fuel injection system saw engineers forced to reduce this (and revise the combustion chamber design) to facilitate the much-delayed V12’s commercial introduction in 1971.
The advent however of the jointly-developed Lucas digital fuel injection in the spring of 1980 however remedied this handicap[3], restoring the 10:1 compression ratio, while boosting the power output of the V12 to an official 296 bhp[4], producing what many consider the most free-running and mechanically refined version of the factory V12 unit built by the factory. Lamentably, few were built, primarily because by the Spring of 1980, production of XJ-S had all but ceased.
Between 1975 and 1980, a total of 12,395 first series XJ-S’ were constructed, leading some to conclude that the early XJ-S was a commercial failure. But how accurate a reading is this? Initial BL projections for the model predicted annual sales in the region of 3000 cars per annum – the XJ-S programme having been carefully costed in order to ensure its business case. Allowing for the fact that production was still ramping-up in 1975, annual sales fulfilled (and in some cases exceeded) projections – only significantly falling short in 1979/80.
The commercial falloff during those latter years is not difficult to explain; stories becoming rife of BL middle managers being offered XJ-S’ as company cars only to turn them down, such was their reputation for poor build and reliability. Moreover, the Castle Bromwich paint plant debacle conjoined to catastrophically effect the supply of saleable cars, and with Browns Lane frantically rectifying every XJ saloon they could lay their hands upon, XJ-S simply didn’t get a look-in.
In the vital US market, the effects of UK government monetary policy adversely affected the price competitiveness of British exports, which combined with the debilitating 1979 US recession and the second fuel crisis with which it coincided would by the turn of the decade would lead to an almost perfect storm.
Furthermore, with more modern rivals like Porsche’s ultra-modern 928 offering a far more appealing and up-to-date package, the XJ-S was in dire need of a refresh, but BL product planning at Bickenhill had already condemned the car, convinced that both it and the V12 engine had no commercial future in the more austere 1980s. Even within Browns Lane, there appeared to be a resignation towards the inevitable.
Nevertheless, Jaguar’s engineering nerve centre remained undeterred, continuing to quietly develop the car. Newly appointed Managing Director, John Egan also took the model’s case on board – the wily Lancastrian quickly grasping[5] the importance of having more than one model line to offer, thus ensuring the XJ-S’ case was argued forcibly at the highest level. Survival would come. Not just on a hairsbreadth, but as it would turn out, on a veritable shoestring.
oo0O0oo
[1] Harry Mundy’s personal XJ12 was fitted with a five speed manual mated to a 6.4 litre experimental V12 engine. It was apparently a particularly well judged combination.
[2] The Jaguar four-speed manual was not a particularly pleasant device as fitted to the XJ-S – the change being rather slow and baulky and the clutch heavy. However, such was the V12’s uncanny flexibility, it hardly mattered, being possible to pull away from standstill in fourth gear without fuss. But the take-up of the automatic told its own story, suggesting that Jaguar might have been better off developing one of their own rather than a manual, for it appears that few outside of the UK motor press actually wanted a manual transmission XJ-S.
[3] According to Jim Randle, the success of the solid state injection system in the XJ-S convinced him of the value of digitalisation, applying it wholesale to XJ40, then in the first stages of programme development – as recounted to this author in June 2016.
[4] It’s believed that these last of the pre-‘HE’ V12 engines were the sweetest-running of the lot.
[5] (Sir) John Egan had prior experience of Jaguars as company cars, so when he was appointed in 1980, he stated that he already knew Jaguar had a saleable product in the XJ saloon. However, he was (initially at least), less convinced by the XJ-S. That would change however.
Sources: See part one
Driven To Write 
Good morning Eóin. This is an absolutely cracking tale, thank you. For anyone who hasn’t already read it, I would recommend also following the link to the ‘Castle Bromwich paint plant debacle’ which is an extraordinary story of ineptitude, ignorance and sheer bloody-mindedness. It is amazing that the XJ-S survived it all and went on to be a success. Looking forward to part four.
If I may suggest a correction – I think that should be ‘AE’ Brico fuel injection system, not ‘AC’.
Those promotional images are curious, aren’t they? Posing your expensive GT (as opposed to establishment-upsetting sports car) in an industrial landscape is a bit unusual, but not too far outside expectations. But then the models are neither clad as punk rebels to go with the environment,nor as glamorous potential mistresses, to go with the indulgence of an expensive Grand Tourer. What exactly were Jaguar trying to say about the car?
(My apologies if it looks as though I’m perpetuating stereotypes here; I suppose that IS what I’m doing, but that’s what advertising in general tends to do, and it’s the advertising, in the context of its time, that I’m trying to analyse!)
Hi Michael. Yes, those photos are interesting. The ‘V’ suffix on the registration plate marks them down as 1979/80, so the Thatcherite de-industrialisation of Britain hadn’t really started in earnest, but the women’s fashions are definitely 1980’s chique.
Has that woman in the white blouse got her skirt caught in the door? She appears to be a touch flustered and déshabillé, unlike her perfectly attired and coiffed friend, who is too busy striking a pose to notice. 😁
Hello Michael – yes, I think the artistic director thought, “I know, let’s do something edgy”. I recall that Austin Reed adverts were thought very daring when they were shot with industrial backgrounds.
Here’s a rather more genteel interpretation of the genre from Bentley.
It is very much in the context and currency of its time, following on the seminal “Welcome to the Machine” from “Wish You Were Here” (I urge anyone who doesn’t understand the artism of that photo shoot to listen to this track which practically paints this picture in sound) was 1977’s “Animals” (some including myself rate it as the band’s finest work) whose cover by the famous design house Hipgnosis depicts an inflatable pig flying over the (shuttered) Battersea power station.
Battersea has been preserved as a landmark, it still stands, and the pig still flys on occasion, as such appearing during the fantastic opening montage for London’s 2012 Olympics opening ceremony (it ends with a Union Jack Parachute, James Bond, and Elizabeth Windsor herself…do watch it again if you haven’t seen it). The photo in question here could hardly be more British, artistic, iconic, or hip and cool for its era.
Oh, I’ve overlooked that Eóin has titled this very series “Welcome to the Machine”. Credit goes where it is due, sir, as a token of my appreciation a rust-free Reliant Robin (body only, as the rest has long since disintegrated) has been shipped to DTW towers.
I’m glad you pointed that out, gooddog – I associate that track with the ‘Wish you were here’ album, so I didn’t make the connection (I’m a bit slow, today).
The photos don’t really work, for me, and I think I’ve worked out why – it looks like a photo-shoot for the clothes the women are wearing, not the car.
The fashion shots appear to have been a joint effort from Jaguar Rover Triumph (JRT) and Simpson of Jermyn Street for Vogue magazine, if memory recalls, in 1979, so are as much about the clothes, jewellery and accessories as they are about the cars. Interestingly, the promotional photos for the Series III XJ saloons (as seen in the original 1979 sales brochure) used a similarly brutalist industrial backdrop. It’s worth bearing in mind that Jaguar’s marketing and advertising were centralised within BL/JRT. It wasn’t until the the advent of John Egan that these functions were returned to Browns Lane.
Gooddog: see part one for further reflection upon Pink Floyd’s 1975 opus. You may have noticed that I’m fond of framing devices…
My thanks to Douglas – both for stopping by (welcome to DTW) and for his kind correction. Typo now amended.
The next instalment may be delayed, but it the series will be resumed as soon as is practicable.
Aren’t those pictures from the ‘extended expensive options’ price list?
I immediately suspected that the photos with the female models were from a fashion photo shoot and that the car had only been “borrowed” for decoration.
The lower photo with the two women is absolutely top class.
It reminds me a lot of my teenage room where I had such motifs as posters and framed photos. I loved that aesthetic – and still love it today.
(One day my mother showed my room to her friend – and my former kindergarten teacher. She looked around and her only comment was “Strange, the girls on the pictures are all wearing clothes, at my son’s room they are all naked”. Probably I was already “different from the others” at that time).
I would never have recognised the connection to Pink Floyd in my life. I don’t know the band, I didn’t like the people who listened to this band when I was young. (Yes, I know, I’m a barbarian with prejudices and I don’t even want to know what I missed out on as a result). I probably denied myself access by doing so. But later on The Flying Pig was “too much HA Schult” for me, anyway.
Oh, and in retrospect, one have to admit that the XJ-S was actually a pretty good-looking car.
Thanks, everyone, you are right of course, those images make perfect sense when one takes them as fashion shoot rather than car brochure artefacts. As Fred says, they are very attractive and appealing images in themselves, it was just the context I found curious!
As for Battersea power station, an Irish developer had bought it, and then been caught out in the 2008 crash. The Irish “bad bank”, NAMA, took over his assets. It was some years before they were able to unload the power station, so I guess I (in common with every other Irish citizen!) was a part-owner of it for a while in the 2010s…
I can recommend following Harry Metcalfe on a trip to Monaco in his (now sold) mustard yellow early XJ-S. Seeing him manually up- and downshift that car through the hairpin bends of continental Europe sort of resurrected the car in my eyes. Always a sport, always entertaining.
A big problem with the XJS was its fuel consumption or, rather, the perception of excessive fuel consumption (if you can afford one of these cars you can afford the fuel for it, surely). The fuel consumption was driven by the nature of the combustion chamber. It had started out as a Heron design (straight out of Coventry Climax experience, surprise, surprise). Nevertheless, when the engine went into production it was with a disc shaped combustion chamber with no squish whatsoever. This is not a Heron head, despite what mythology would have us believe. What it really happened to be was a bodge- cheap and primitive*. It is a surprise that the designers made the decision to go with a disc chamber as they were well aware of the lack of mixture motion in the Heron combustion chamber even with all its squish**. The disc was even worse. It had no squish at all, so there was nothing to get the mixture swirling or tumbling or rolling or turbulating. Fuel could drop out of the airstream and wet chamber surfaces. This fuel would not be available to combust and contribute to power. The disc chamber delivered high part-throttle fuel consumption, slow combustion, demanded a lot of spark advance and generated a great deal of heat in what was a tight engine bay***.
The issue of heat was to prove a recurring problem since there was little fresh air flow to the rear flanks and top of the engine. There was not much air flowing around the transmission either. The addition of air-conditioning did not help matters. The cooling system needed to be in tip-top condition or else….!
There was the potential for heat soak trouble as well. After the car had been parked and the engine shut off
high temperature air migrated to the top of the engine and ended up trapped under the bonnet. Unlike the E-type there were no louvres to let this air escape. Over time the shockingly low quality el-cheapo rubber hoses would bake off and eventually harden into a brittle plastic-like substance. Then they would fail, leading to all sorts of air and vacuum leaks****. In later cars the rubber hoses to the fuel injectors would fail and this inevitably meant a funeral pyre. There were more than a few cars in the USA to which this occurred. It was probably made more likely due to the extra heat generated by the catalysts.
Another heat related problem to which these cars succumbed was failure of the brake booster vacuum check valves on the inlet manifolds. The heat soak would eventually kill them and then allow fuel air mixture to migrate to the brake booster. The diaphragm would eventually rot out. The heat was troublesome, but the reality was that there were far too many marginal components and el cheapo short-cuts made on the ancillaries. Reliability was abysmal as a result. Terrible quality. Awful culture. It soon showed. It spoiled the car.
Still, despite all this, the disc had one benefit. It featured a low rate of pressure rise after charge ignition. It is suspected this helped make the engine as refined as it was. Now isn’t that interesting!
* modelled these and other alternatives in CFD. I sought to modify my car to be better.
** Keith Duckworth was scathing when he was shown the disc chamber and asked for his advice. He claimed the engine was utterly ruined because of it.
*** When Alan Scott was developing the Jaguar V12 for the Gp A TWR XJS he had to deal with the chamber shape, as well as the heat problems. He was successful. He, like Broadspeed and Bonner before him, completely altered the combustion chamber. Some years after he had developed the Gp C engines of some ~7 litres capacity, he was asked by one of Jaguar’s engine designers (name withheld to protect the guilty!) how much power he was achieving. He answered around 700bhp from the endurance version of the engine. The Jaguar man exclaimed, “Bullshit!” and, in a huff, departed the meeting. Fun times!
****Some of the breakdowns and intermittent faults due to this were quite difficult to diagnose for the uninitiated. The reputation of these cars plunged as the result. My approach was and is to replace ALL the hoses entirely with items of superior quality (rated to much higher temperatures). You must do the same for the brake vacuum valves IMMEDIATELY on purchasing one of these cars. If any gasoline has manager to get to the brake booster, out that must go as well.
Thanks J T for pin pointing the exact causes of XJ’S poor reliability.
The question that comes to everyone’s mind is: “OK, so why didn’t they fix it and instead kept reproducing the same mistakes?”.
I guess the answer is much more complex and part of it is allready written in Eóin’s interesting article.
I must say that extremely poor reliability springs to mind even to this day, whenever “Jaguar” brand is mentioned in our “about-cars” conversations among friends. Maybe XJ-S bad reputation has something to do with it.
Bad reliability wasn’t just a Jaguar bug but a feature of many British cars from a certain era – and Brit bikes from that time were even worse. Most of the flak goes into direction of Lucas and indeed they weren’t dubbed ‘prince of darkness’ or ‘inventor of the intermittent wiper (works, doesn’t, works)’ for nothing.
The background and reason of this has been described many times in mostly tragical documentations like Aabe Amidor’s ‘Shooting Star – the rise and fall of the British motorcycle industry’ or Bert Hopwood’s ‘Whatever happened to the British motorcycle industry?’ I’m sorry, but as an old motorcycle buff my bookshelf has books on Brit bikes but the story is the same for the car side of the business. It’s often blamed on management faults by peope like Edward Turner and their remoteness from customer needs and their workforce but things weren’t any better when workers managed their own business as the Triumph bikes made by the Meriden motorcycle co-op (Tony Benn’s experiment of a better world for free workers) weren’t any better (and often worse) than the ones made by the old Triumph company.
When I started entertaining the thought of Jaguar V12 ownership and hence talking to owners of those cars, the first recommendation I received was: ‘Hoses, you must take care of those hoses. Always have them checked and replace them at the first, faint sign of trouble.’
My XJ12 was a later example, featuring the hemispherical combustion chamber, Bosch-Lucas fuel injection et al, and it proved to be a reliable car. It overheated exactly once (courtesy of a blown fuse) over the course of almost a decade of ownership, which included a trip to Italy – including bumper-to-bumper traffic on the Milano Tangenziale in mediterranean heat – and back without the slightest of hiccups.
JT: I have read that story in the past, although the version I saw cited it as occurring during the Broadspeed era of the racing XJ-Cs during the 1970s, and was told against Harry Mundy, who was a man who (whatever anyone might otherwise say about him) knew his onions – especially when it came to that engine. When I spoke with Jim Randle a number of years ago, he was quite grimly amused by his recollections of the Broadspeed period and what went on. I believe (and Randle was of similar view) that stories often gain arms and legs in the telling – to say the very least.
There is no doubt however that heat build up was a huge bugbear with all the V12 XJ Jaguars, owing to the tightly packed nature of the installation. Bought in components were of notoriously poor quality, especially during the BL period, although this was tightened up considerably once Jaguar gained its autonomy and could set their own minimum standards. Matters improved further once they had the freedom to invest in a dedicated proving centre in Arizona – prior to that, the main hot weather proving site was Nardo in Italy.
As regards the Heron head controversary, I note that even dear Leonard Setright (who also knew a thing or two about onions – and other matters) referred to it as such. I remain on the fence on the matter. The V12 has its detractors (and they really don’t appreciate it) and its adherents (of which LJKS was one). I however wouldn’t want to be without it – if only for the sheer ambition of the thing – and its uncanny silence in operation.
Quite an interesting, illustrative story about suspension development from Jaguar Development Engineer, Norman Dewis. A bit of a daft situation to end up in.
Dear Norman – a man who never knowingly recounted an anecdote against himself.
The bit about the XJ’s steering is quite amusing. I was familiar with a wide variety of American, European, and Japanese cars when I was granted permission to take a Series 3 out for a brief but memorable drive on rustic New England back roads, replete with off camber turns and small hills. They surely overdid the power steering to a great extent, though it was quite like falling in love; nobody’s perfect, flaws become easy to absolve. Anyway the steering wheel and shifter were both impossibly delicate, also precise. She’s a very sensitive girl.
Regarding “Dear Norman”. Just imagine his inner pain after having completely wrecked the most beautiful car ever built.
Eoin
Sorry, I’m not clear on what story you are referring to. Can you clarify some?
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The XJ12 and the XJS are great cars. I prefer the XJ12 to the S. The refinement in either is so good that it is superior to many so-called luxury vehicles of today. That V-12 engine is a masterpiece even though the combustion chamber is a wreck!
Best not to talk about quality and Jaguar in the same breath.
The close packed engine room of the Jaguar was not the cause of the heat problem. It was a contributory factor, but it is one which other makers had previously addressed at one time or another. The heat issue was an excellent example of unintended consequence arising from convenient decisions.
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LJK Setright was a friend. We had extensive conversations about matters automotive amongst other things. I don’t think we discussed the Jaguar V-12 combustion chamber in any detail though. We may have, but I don’t recall that. We did discuss sleeve valves many times. Now surely those sure would have improved matters! Anyway, if he said Jaguar’s V-12 was a Heron, I can certainly see why he would.
The original combustion chamber design intended for the Jaguar V-12 was the Heron. In the Heron the cylinder head face (the one which mates to the fire-deck) is flat. The combustion chamber is entirely located in the piston crown, not in the cylinder head. The combustion chamber consists of a bowl in the piston surrounded by a squish band which, aside from valve relief cut-outs, goes all the way around the periphery of the combustion volume. This squish band all but touches the cylinder head at top dead centre. You are seeking to get as tight as mechanically possible here, so something around the 1mm mark would be reasonable. The purpose is to vigorously squeeze out any mixture located between the squish band and the cylinder head. This mixture is ejected straight into the bowl in the piston where it results in much turbulence (small scale turbulence, not bulk mixture motion). This helps speed up the combustion process dramatically. The overall layout is very similar to a direct injection diesel engine.
Note that this approach has shortcomings when the rpm gets high. It struggles in an engine which is intended to spend a great deal of its time above, say, 6,000rpm. It also has trouble where the engine is going to spend a lot of time at part throttle. The reason is that there is little or no bulk mixture motion in these situations, so combustion speed is slow. Low mixture motion also has the unfortunate effect of allowing some gasoline to drop out of the mixture and wet out on adjacent surfaces. This fuel is unavailable to do work. It ends up as UHC in the exhaust gas stream. Some of it does re-evaporate and does manage to get burned, but this happens well late in the cycle and contributes a lower amount of work than it ought. It leads to increased exhaust gas temperatures, which isn’t ideal really.
This is made even worse by the high approach angle of the V-12’s most excellent inlet port (something we normally want since it results in high gas flow capability into an engine). So a feature which is usually sought for good flow causes a problem in the case of the Heron. A much reduced angle would likely provide better bulk mixture motion, but would do so at the cost of gas flow and also of inlet manifold packaging (in the case of the Jaguar V-12).
Recapping. That slow combustion and late burning re-evaporated gasoline has the unfortunate result of an increased exhaust gas temperature. Also all that UHC is going to get burned off either in the exhaust manifolds (for export cars Jaguar deployed air injection into the exhaust ports to facilitate this) or in the catalysts. There is the number one source of our heat problem.
Now we come to a new issue. It is going to make everything even worse. It relates to oxides of nitrogen. These are created when the oxygen and nitrogen in the air are reacted together. The reaction is endothermic which means it needs the provision of energy to occur. Such a reaction does not occur spontaneously in the absence of energy being provided for it. In an engine, if the pressure and temperature are high enough it is possible for oxygen and nitrogen molecules from the air to be rearrange into various combinations of oxygen and nitrogen atoms joined together- hence NOx (for oxides of nitrogen where the x is an integer). The NOx are a pollutant which is regulated.
Recapping, NOx are produced in an engine when temperatures and pressures are high enough. NOx occur during the combustion phase of the engine. To get them down a reduction in pressure is a possible approach. This means reducing the compression ratio. Indeed, if this is done peak pressures and peak temperatures are reduced. Less NOx is a result (as is loss of thermal efficiency with higher fuel consumption and higher exhaust gas temperatures). Jaguar followed this approach. They reduced compression ratio and they also retarded the spark timing (for even less peak pressure and temperature near top dead centre).
To reduce the compression ratio Jaguar had some choices. They could have used a shorter con rod so the piston did not end up as high in the cylinder at top dead centre (not that this approach would have been very good, just sayin’ it was a possibility) or they could alter the piston crown shape. They chose to alter the piston crown. They did that by eliminating the squish band in its entirety. They dramatically increased the diameter of the bowl, almost to the edge of the piston (within 5mm). The periphery of the crown was also lowered and no longer approached the head closely at all. The clearance of the edge of the piston at top dead centre is some 5 to 6 mm and this is far too great to provide any squish at all. The compression ratio is lowered, reducing NOx as expected, but that comes at the cost of even further reduced turbulence and bulk mixture motion. Part throttle fuel consumption is even higher than with the original Heron and waste heat in the exhaust stream still higher yet. On the other hand, several machining processes were avoided for each piston- hence reducing manufacturing costs usefully.
From one point of view the majority of the combustion chamber still resides in the piston crown and the periphery approaches the head closer than the bottom of the bowl, even though it no longer creates any useful squish. Geometrically, you could say this is a variant of the Heron- bowl in piston gasoline engine, therefore Heron. To be fair the Jaguar system and the Heron do appear similar.
From this engineer’s view the Jaguar has departed way too far from Sam Heron’s design to properly be called a Heron. The Jaguar has no squish and providing the squish to agitate the mixture was THE central intent. Therefore, the Jaguar has a disc for its combustion chamber layout. It is bulged some, but it is of a disc type similar in many ways to that of the Merlin. Call it a variant of the Heron if you must, but it is not Heron.
However you call it, the thing they deployed was a bodge. It led to several serious knock-on problems which got worse with time. Still, they nearly got away with it and it seems it may have contributed to overall refinement.
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The Broadspeed, Bonner and TWR (Tarozzi) approaches to the Jaguar V-12 combustion chamber differ from each other. All sought to improve matters for performance and efficiency by increasing turbulence at top dead centre as well as bulk mixture motion. These are THE big quest in dealing with improving the Jaguar V-12 engine for performance, efficiency or longevity under high inlet manifold boost.
TWR used a Heron head designed by Paul Tarozzi. Alan Scott at TWR selected the Tarozzi cylinder head and then consulted with Cosworth to exploit their design knowledge and experience (including with CFD) to develop the V-12 for Gp C. Cosworth had worked with the Heron layout exhaustively during the Duckworth years with their SCA engine, immediately prior to the advent of the BDA and DFV engines.
Interestingly the final development of the TWR Heron cylinder heads were not flat across the fire deck face. They had a disc shaped recess for each cylinder. The top of the piston crown entered its respective recess in the head at top dead centre. Nevertheless the combustion chamber was in a bowl in the piston crown and there was circumferential squish.
None of the cylinder heads used by TWR in Gp C were Jaguar product or of Jaguar origin. The cylinder heads TWR used were designed by Paul Tarozzi (ex-Chrysler engineer, Hot Rodder extraordinaire) for Group 44. Originally the heads were all cast in California. Later the patterns for the early version were exported to the UK, for reasons of logistics, politics and so that TWR could have closer quality control over casting and machining processes (TWR had limited in-house machining capability, what was achieved by TWR Engines Division is so much more impressive when you realise how badly Allan Scott was restricted by this shortcoming). This move allowed for alterations to the patterns to be made reasonably quickly as and when needed. Meanwhile back in the USA Tarozzi redesigned the cylinder head for the Jaguar V-12. Brand new patterns were cut. This last of the Tarozzi Jaguar cylinder head designs was only ever made available to Group 44. It is identified by the D-shape exhaust ports and cavernous inlet ports. It was the best of all the Herons. Imagine if TWR had deployed that!
Alan Scott kept rpm moderate. He well understood all the limitations of the architecture of Jaguar’s V-12 engine*. He also understood its strengths. Note that he was big on mapping throttle percentages and developed his engines to suit their application.
Broadspeed developed its own castings for the V-12 cylinder head. This head had the valve centre-lines moved in order to unshroud the inlet valves for improved flow. It featured relocated combustion chambers, moved out from the piston and put into the head. Flat pistons were used. The combustion chambers in the Broadspeed head look almost like a bath-tub design. There is plenty of quench. It is all quite well thought through.
Bonner took Jaguar cylinder head castings straight from the factory, in un-machined form, but with an important alteration. They had the factory cast the heads with an extra thick face (the surface which would be installed facing the fire-deck). This allowed them to machine out their own special design of combustion chambers. Bonner Jaguar V-12 engines were highly boosted and marinised. They were used for off-shore powerboat racing. Typical output was over 800bhp continuous. It is notable that they achieved this while reducing exhaust gas, exhaust valve, piston and cylinder head temperatures dramatically.
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*Alan Scott designed four valve per cylinder dohc heads for the V-12 (as did another party whose name shall not be spoke). They developed extra power at high rpm exactly as expected but at the cost of higher fuel consumption throughout the rev range, greater engine weight as well as moving the engine’s centre of gravity higher. That last issue adversely affected the Gp C cars handling. It was soon realised that the two valve engine could more than recover the power deficit, do that at lower rpm and with less weight, merely by increasing swept volume. This is the path Alan took. Note that the main journal bearing diameter in the Jaguar V-12 is very large. As rpm exceeds 7,000 rpm, surface velocities start to get excessive. Power gains get eaten up if the revs are increased far beyond this limit. Alan understood his engine. He understood the regulations (it was a fuel efficiency formula). He played to the big V-12’s strengths- big torque and high fuel efficiency at high load and moderate rpm. A Heron can work reasonably well applied this way.
The Alan Scott TWR four valve per cylinder head (which is a completely different design from the one Jaguar themselves designed and prototyped) ended up being used in the quad cam V-12 powered 2+2 road car built for Tom Walkinshaw. That engine was considered for Aston Martin at one point. Ford demurred! They had a choice of two good V-12 designs (a 90-degree V-12 and a 60-degree V-12) they could offer! And that’s all a story for another day.
Hi Christopher
The XJ12 was not manufactured with a hemispherical combustion chamber. The valves were always directly in-line and parallel with the cylinder bore centre-line. The later V-12 engines featured the May head which had a combustion chamber predominantly located in a pocket under the exhaust valve. There was plenty of quench to help it develop lots of swirl and plenty of turbulence. It was a lean burn head and could run at incredibly lean mixtures. In the end that is what doomed the engine. When emissions regulations headed along the GM recommended path (catalysts and stoichiometric combustion) then lean burn died. Ford did not want to continue with the V-12 as they had a plant in Bridgend which had extra capacity. They did not want to modernise the Jaguar V-12. So we got a cloned Mustang engine instead!
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When you replace the hoses, don’t go like for like. Use hoses with higher temperature ratings. I use marine. Jaguar stuff is nonsense.
This discussion on combustion chamber design and engineering has been most educative for me. Thanks to all involved for the time and effort.
Same with me, very educative and inspiring conversation thank you all very much.
I enjoyed a brief journey, jumping and dancing and swirling into flames on top of 12 piston floors.
The small scale car companies also gained my admiration and respect. I hadn’t realised the amount of technological skepsis involved. I thought most of it as a straight forward procedure: standard engine + new cylinder head (i.e. more compression)=more hp.
Constantinos
You wrote, “I enjoyed a brief journey, jumping and dancing and swirling into flames on top of 12 piston floors.”
That comment reminded me of something. Below is a description of what goes on in an internal combustion engine given to us by the great Sir Harry Ricardo. He was delivering a lecture to the Royal Society of Arts on 23 November 1931 and this is a brief excerpt from it. When you read this, remember he is speaking to us from 1931, well before the types of technology we have now which allow us to look inside the engine and use ultra-high speed photography to actually see what is going on. He, as you, took an imaginary journey deep into the engine to see what is going on in there. His journey is into a compression ignition or diesel engine.
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I am going to take the rather unconventional course of asking you to accompany me, in imagination, inside the cylinder of a diesel engine. Let us imagine ourselves seated comfortably on the top of the piston, at or near the end of the compression stroke. We are in complete darkness, the atmosphere is a trifle oppressive, for the shade temperature is well over 500 Celsius – almost a dull red heat – and the density of the air is such that the contents of an average sitting-room would weigh about a ton; also it is very draughty, in fact, the draught is such that, in reality, we should be blown off our perch and hurled about like autumn leaves in a gale. Suddenly, above our heads, a valve opens and a rainstorm of fuel begins to descend. I have called it a rainstorm, but the velocity of droplets approaches much more nearly that of rifle bullets than of raindrops.
For a while nothing startling happens, the rain continues to fall, the darkness remains intense. Then suddenly, away to our right perhaps, a brilliant gleam of light appears, moving swiftly and purposefully; in an instant this is followed by a myriad others all around us, some large and some small, until on all sides of us the space is filled with a merry blaze of moving lights; from time to time the smaller lights wink and go out, while the larger ones develop fiery tails like comets; occasionally these strike the walls, but, being surrounded by an envelope of burning vapour, they merely bounce off like drops of water spilt on a red hot plate.
Right overhead all is darkness still, the rainstorm continues, and the heat is becoming intense; and now we shall notice that a change is taking place. Many of the smaller lights around us have gone out, but new ones are beginning to appear, more overhead, and to form themselves into definite streams shooting rapidly downwards or outwards from the direction of the injector nozzles.
Looking round again we see that the lights around are growing yellower; they no longer move in a definite direction, but appear to be drifting listlessly hither and thither; here and there they are crowding together in dense nebulae, and these are burning now with a sickly, smoky flame, half suffocated for want of oxygen. Now we are attracted by a dazzle, and looking up we see that what at first was cold rain falling through utter darkness, has given place to a cascade of fire as from a rocket. For a little while this continues, then ceases abruptly as the fuel valve closes.
Above and all around us are still some lingering fire balls, now trailing long tails of sparks and smoke and wandering aimlessly in search of the last dregs of oxygen which will consume them finally and set their souls at rest. If so, well and good; if not, some unromantic engineer outside will merely grumble that the exhaust is dirty and will set the fuel valve to close a trifle earlier.
So ends the scene, or rather my conception of the scene, and I will ask you to realise that what has taken me nearly five minutes to describe may all be enacted in one five hundredth of a second or even less.
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Awesome, isn’t it?
Sir Harry Ricardo was a great engineer, a great amongst a cohort of greats. More than anyone else he is responsible for the development of the modern internal combustion engine. Nearly every single automotive engine maker on the planet has elements of Ricardo in its engines (as do the manufactures of marine, off-highway and aviation internal combustion piston engines).
Ricardo’s biography, “Engines and Enterprise. The Life and Work of Sir Harry Ricardo.” is highly recommended. It was written by John Reynolds. This book includes the material from Sir Harry’s own autobiography and expands on it. It continues where the original work left off (beyond Sir Harry’s retirement from the company he founded, Ricardo plc).
Final point. Engines, especially the good ones, are an expression of art.
Chapeau, J T, that’s a wonderful, evocative description of the workings of a diesel engine. Who said engineers cannot truly appreciate art, and vise-versa?
J T
a book highly recommended is never to be missed out. I am much obliged, thank you.
Poetry in motion is sir Ricardo’s description. How excited he must have felt experimenting and visualising his outcomes in order to improve them. That must have led to such a deep understanding of the internal combustion procedure, I am certain today’s engineers, even with the use of modern technology, may find it hard to achieve.
How fascinating indeed!
Good Morning Daniel
Engineering is an art.
JT a thousand thanks for all your contributions to this page.
And how wonderful to be able to claim friendship with LJKS.