The Accidental E-Type [Part One]

The E’s ‘pointless’ swansong.

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The more advanced students of Jaguar lore will by now have recognised that a good many of the most well-loved cars from Browns Lane were at best, incidental, if not wholly accidental in conception. Similarly, when it came to the subject of mid or late-life facelifts, not only were they predominantly of a reactive nature, but rare indeed was the aesthetic revision that amounted to a palpable improvement. But while it might be considered a little provocative to describe the Series 3 E-Type as being accidental, it would hardly be inaccurate to suggest that it was unplanned.

While Sir William Lyons ran Jaguar in his benignly autocratic style, product planning was also somewhat reactive in nature, largely informed by the ever-shifting vagaries of the US market, a case in point being the Autumn 1968 refresh of the E-Type, the series 2. Beyond this, the intention was to develop a new model to replace the E-Type entirely, one derived from the E’s basic substructure and suspension design. This model, dubbed XJ21, was projected to debut around early 1971 at the latest.

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A core component of XJ21 was to be Jaguar’s new, all-aluminium V12 engine, subject of a great deal of internal debate within Browns Lane[1]. Given the commercial importance of this new power unit, engineering director, Bill Heynes suggested developing a limited-run V12 version of the Series 2 E-Type as a means of carrying out a soft-launch for the powerplant, allowing not only valuable customer experience to be gained, but also for manufacturing to iron-out any residual issues before production was ramped up for the more significant XJ12 saloon[2].

In November 1968, following discussions with both Sir William and Deputy Chairman, Lofty England, Heynes assured both that changes to the V12 model would be kept to a minimum, allowing it to be introduced at least a year before XJ21 would enter the market. Designated XJ25, the bulk of the design work for the V12 E-Type would be carried out by Mike Kimberley and Malcolm Sayer[3], with oversight from Bob Knight.

That the wider (if lower) V12 engine fitted the E-Type engine bay at all was not only fortuitous, but a direct consequence of the E-Type’s ancestry, that of the Le Mans winning D-Type endurance racer. Had the E not employed a similar design of detachable spaceframe forward of the front bulkhead, the costs involved would have been prohibitive. As it was, the installation, entailing a redesigned subframe, proved a very tight fit, with a great deal of knife and fork engineering necessary to ensure that the first XJ25 prototype[4] worked at all. While the fixed-head bodyshell was largely carried over from the incumbent 2+2 model[5], a major advance was the adoption of the longer wheelbase floorpan (and doors) for the drophead model, allowing for a more commodious cabin[6]. Otherwise, visual changes were kept to a minimum.


Furthermore, the adoption of XJ4-style inclined front wishbones, provided an anti-dive geometry. This, coupled with low profile Dunlop radial tyres and ventilated disc brakes, made for more reassuring road manners. Power-assisted steering, was also offered for the first time[7], to help mitigate XJ25’s additional weight.

Visually, the most obvious change lay in the E-Type’s more planted stance. This was a consequence of wider track widths, front and rear, which also necessitated the wheelarches receiving a subtle flair[8]. For cost reasons, stylists investigated the adoption of ‘tacked-on’ wheelarch extensions, before abandoning the idea on aesthetic grounds[9]. For the first time, the E-Type was adorned with a chromed grille[10] which rather gilded the lily. Below it was mounted an additional air scoop to further aid cooling. Aft, apart from badging, only quadruple exhaust outlets suggested what lay beneath. Inside too, little of note was altered[11], although matters of cabin ventilation and air conditioning did receive attention.

By 1969, with XJ21 dead on the table, what had initially been intended as a short-term stop-gap, would now be forced to play a far more significant commercial role. As such, sales and marketing required the V12 E to assert its own identity, with the car becoming dubbed E-Type Series 3, or XK-E V12[12] in the United States.

The Jaguar V12 engine had been an open secret for some time and its introduction was greatly anticipated. But owing to delays with the V12 engine, the Series 3 made its belated debut in March 1971. The United States was the chosen venue for the official unveiling of the Series 3, presided over by Jaguar’s 71 year old founder, making his final visit to Jaguar’s most commercially significant market[13].

While the new V12 engine generated a great deal of interest and enthusiasm, there was an undercurrent of disappointment amidst the  press corps that the car in which it was housed was not as exciting – many expressing the belief that Jaguar really ought to have replaced the E-Type entirely by then. Because while the Series 3 looked distinctive, for many, it was not the breakthrough car they had hoped for.

BLMC’s Middle Class Ferrari” was how UK’s Car magazine previewed the car for their April 1971 issue, which also included road impressions gleaned from a pre-production fixed-head model. Car’s un-named “Jaguar-ophile of long standing” expended considerable column inches upon a technical description of the twelve cylinder unit, making much of the Formula One-inspired Lucas Opus electronic ignition system. This had been described within Jaguar’s press release as Pointless, a reference to the lack of traditional contact breakers within the distributor itself, promising more reliable ignition pulses, benefiting both emissions, and it was hoped, reliability.

Car’s correspondent was able to sample both manual and automatic versions, both of which impressed with the V12’s uncanny silence, outstanding flexibility and smooth running characteristics. Also discovered was what would become one the engine’s defining traits – the deceptive nature of its performance. Nevertheless, the character of the engine itself came under scrutiny, Car’s scribe lamenting that the V12 “wasn’t a fire-breathing hemi-headed, bellowing monster” … before conceding that “Common sense dictates that the emphasis on quietness and torque is the only way to go in these restrictive days.

Image: classiccarcatalogue

The official US press launch took place in the heat of Florida that year. The majority echoed Car’s praise for the power unit, if less so for the now somewhat dated E-Type concept. Given the E’s intended customer-base, the setting would prove apt, but it would also highlight an embarrassing oversight on the proving team’s part. While prototypes were thoroughly tested in the UK, on mainland Europe, and in the frigid wilds of Canada, little worthwhile mileage had taken place in the baking heat and humidity of North America.

As the press corps got under way, E-Types began cutting out left, right and centre, the much heralded Opus ignition modules failing in the heat. Jaguar technicians were forced to improvise on the spot, with overheated modules taking a step towards full immersion amid Palm Beach’s many swimming pools. But while it would make for an amusing after-dinner anecdote, it was hardly the auspicious debut Jaguar was hoping for.

Part Two will follow shortly

[1] The Jaguar V12 engine will be profiled in a separate article.

[2] The XJ12 saloon (dubbed XJ10 internally) was to have debuted in Autumn 1970. Owing to delays with the V12 engine, it was not introduced until July 1972.

[3] Confirmed to this author by Jim Randle in 2016.

[4] The first XJ25 prototype – a Sable fixed-head was built from an experimental bodyshell in December 1968. It was later raced with some success.

[5] This was insisted upon by Lofty England – who was one of life’s taller gentlemen.

[6] Calibrated to appeal to the tastes of American drivers, it was not met with universal acclaim.

[7] The 2+2 body-in-white had been substantially revised at Pressed Steel Fisher, with the number of body pressings reduced and simplified, making it cheaper to build.

[8] Dubbed ‘eyebrows‘ at Browns Lane.

[9] Jaguar insiders have suggested that retooling the wheelarches was possibly the most costly aspect of the XJ25 programme.

[10] The 1968 Series 2 E-Type had already been in receipt of considerable interior safety and convenience-related improvements.

[11] Before the original E-Type was launched, Sir William had experimented with a mesh grille arrangement, before adopting the elegantly simple horizontal bar. The Series 3 grille was styled to harmonise with that of the concurrent XJ saloon.

[12] E-Types were marketed in the United States as XK-E.

[13] The US market would account for over 75% of all E-Type production, and was, throughout the 1960s, the best-selling Jaguar model sold there.

 The 4.2 litre six-cylinder engine was also to have been offered on the Series 3, but apart from about three pre-production prototypes, none were built.

Fun fact: The Series 3’s bumper over-riders were shared with the Austin Maxi.

Sources: Jaguar E-Type – The definitive History: Philip Porter (Haynes)/ Norman Dewis of Jaguar – Developing the Legend: (PJ Publishing)/ Jaguar E-Type – Denis Jenkinson (Osprey)/ Car Magazine – April 1971.

Author: Eóin Doyle

Co-Founder. Editor. Content Provider.

37 thoughts on “ The Accidental E-Type [Part One]”

  1. I never liked the headlamp treatment. It was forced on Jaguar by govt. regulations. The earlier treatment was much better.

    The suspension was biased towards a more comfortable ride than previous iterations of the E. This cost the car much of its immediacy of response and directness. It became more like a GT than a sports car. The good news is that it is easy enough to retune the suspension in an E. They are a simple car.

    There is sufficient room in the E-type for even the quad-cam variant of the Jaguar V-12. Knife and fork engineering? Really?

  2. Good morning Eóin. More Jaguar goodness, thank you. The Series III revisions were a mixed bag. I like the wider tracks and flared wheel arches, but the front end treatment really is rather heavy-handed. The exposed headlamps are not a problem per se, but the heavy chrome bezels look all wrong. Couldn’t they have modified the bonnet so that these weren’t needed?

    Why did they feel the need to make the E-Type resemble the XJ saloon, a car completely different in character? The grille and chrome hubcaps look all wrong.

    Incidentally, was that prominent bulge in the bonnet really needed for the V12, or was it just carried over unchanged from the straight-six?

    1. Wasn’t it the other way round and the XJ started as a four-door version of the E with very similar looks.
      AROnline has pictures of early styling bucks looking very much like that.
      Then the re-unification would be somewhat logical, this time the other way round.
      I also remember a story that the XJ’s rear end was the E’s with some thirty centimetres cut off vertically to make the prototype fit some transport equipment.

    2. Hi Dave, I was referring specifically to the embellishments added to the Series III E-Type, the chrome, grille and hub caps.

    3. Never mind the hubcaps – the wheels themselves are just wrong for any Jaguar.

  3. Given enough money, all things are possible. Like ‘fixing’ the headlights, taking the capacity to 6.1 litres, putting in a five speed box, and bigger brakes.

  4. To clarify a couple of points raised:

    JT: My reference here was to the building of the first running prototype, which was overseen by Experimental Dept Foreman, and unsung Jaguar hero, George Mason, quoted below in Philip Porter’s Definitive History. “The parameters were that the car couldn’t physically be any wider and it had to have more lock, wider tyres and all that lot. To our amazement it all worked, but we had wheels and tyres tucking under engines and all sorts of things.” Another former insider stated that Mason and his team “were magicians to do that.” That the four-cam fits doesn’t altogether surprise me – once they got the single-cam version in, there probably wasn’t a lot more in it.

    Daniel: Once the E-Type was revised in 1968 – these raised and exposed headlamps were part of the Series 2 package – the E was not intended to remain on the market for very much longer. XJ21 was in hand. Hence, it was not felt that it was worth the cost of retooling. Similarly the bonnet bulge. It was not required for the V12, but the cost of retooling for what was only intended to be on the market for a year or so simply wasn’t justified. Also, by then, the bulge was part and parcel of the car’s visual appeal. Interestingly, Malcolm Sayer hated it. In just about every E-Type revision he drew, the bonnet centre was raised to eradicate the hump entirely. He may have felt it adversely affected the airflow, or perhaps it simply offended him aesthetically. I suspect the latter.

    Curiously, history repeated in reverse with XJ-S. I’ll come to that in good time.

    As to the XJ-aping grille. It was purely sales/marketing-driven, to provide a point of visual difference.

    Dave: I am afraid you are confusing your Jaguars. The rear of XJ4 was ‘cut‘ for purely visual reasons, on Sir William’s instructions. You’re thinking of XJ40 I suspect. A different car and a very different set of circumstances. You will find a very thorough exegesis on the XJ4 Series (XJ6) in the DTW archive, where all of this has been documented, if I may say, in considerable detail. Ditto XJ40.

    The Turbo Disc wheels with hubcaps debuted on the Series 2 E-Type as an option in 1969. Intended for the XJ12, the larger openings improved airflow to the brakes. They were eventually adopted throughout the XJ range during the mid-70s. Aesthetically, they are a matter of taste.

    1. Addendum: It appears that in November 1971, some months after launch, Lofty England issued a decree for an experimental bonnet to be made up with the centre hump removed. This was subsequently fitted to one of the XJ25 development cars for evaluation. No photos of this appear to have survived.

  5. i love those old jaguar hubcap wheels – every time i see an old XJ with them in real life i am amazed of just how right they look on those cars.

  6. How did this worked out, the lovely curved shapes got replaced by straight lines after 1970 in car forms. In my eyes, this was not the case of an evolution, it was two unconnected parts. It is a mystery to me.

  7. Quoting, “To our amazement it all worked, but we had wheels and tyres tucking under engines and all sorts of things.”

    This is good story telling, building a lovely mythology for Jaguar, but it isn’t accurate. They had a job to do and they went and did it. It was a straightforward project- very similar to one that many, many, many hot rodders have undertaken at one time or another- easier in many respects. Note that the wheels and tyres do not tuck under the engine. Go open the bonnet of an E, run the steering through from lock to lock and see for yourself.

    The quad-cam is quite a lot wider than the regular engine. It conveniently fits the E, but try fitting to an XJ…

    Those dreadful headlamps produce vortices which run halfway to the windscreen. They generate lift and produce lots of drag. Malcom Sayer was not impressed with them, but the car was not able to be sold with the original set up due to regulations.

  8. Can understand why the Series 3 E-Type was planned as a short-lived car before being replaced by XJ21, however as others have mentioned not a fan of the front end styling and feel more work was needed to improve things. Also would the Series 3 have benefited from carrying over the 4.2 XK6 engine? While a 3-litre XK6 was planned at one time for the E-Type, did Jaguar ever test the compromised 2.8-litre XK6 in an E-Type?

    Concerning the Jaguar V12, even though it is generally agreed it could have been much better than it was. Would it have not been better to have reached production as a 6-litre from the beginning instead of later, since that would have opened up the possibility of the XK6 being replaced by a more compact 3-litre V6. Aware Jaguar did look at a V6 and other ideas before starting what became the AJ6, though was of the understanding a V12 derived V6 was dismissed on the basis of it being small due to its lower displacement against the existing XK6 (more so if the V12 cut in half was the 5.3-litre).

  9. Hi Bob

    The Jaguar V-12 is a lazy engine. It was built to deliver unmatched refinement and that it surely did. Getting extra capacity out of it is easy enough on a one off-basis (there is enough room and structural integrity for over 10-litres). Jaguar never had the ability to mass produce a V-12 of more than 6-litres in house though. To get to more needed a new transfer line. Further, the cylinder head design is not ideal. You can get a lot more performance and efficiency with a different combustion chamber.

    Jaguar never had the money directed at further developing the V-12 unfortunately. Even when Ford took over they failed to give the twelve the updates it really needed. Instead Jaguar headed down a route where they built an inferior copy of the Ford Modular motor…

    V-6 engines are not well balanced. They are superior to a four, but they are not as good as an in-line six let alone a V-12. The XJ sedan really needed a small V-12 of, say, 4.2 litres and a bigger one of 6.4 litres (the Mundy option!). Jaguar also needed to get mass out of the pistons and hence also out of the crankshaft. That would have helped a lot. Alas, no money…

    1. Hello J T

      Was that down to Jaguar’s habit of acquiring used tooling from mainly Standard-Triumph to save money?

      Was under the impression the AJ-V8 drew upon work previously done by Jaguar on a modular engine family prior to the Ford takeover, ranging from a 2-litre 4-cylinder and 3-litre Six or V6 (whose work may or many not have been carried over to the Duratec V6 derived AJ-V6) up to a 6-litre V12?

      The V6 idea is mainly a way of indirectly salvaging work previously done on the blind alley that is the V12-based Jaguar V8 project, though with more wider applications (and an ideal capacity range of about 2.0-3.2-litres) as opposed to a duplicate V8 with an unfavourable angle that was always going to lose out against both the Rover and Triumph V8s.

    2. Bob: Jaguar did historically purchase a good deal of used plant, but this was largely for vehicle assembly and painting – both of which dated back to the pre-war era. Leyland I believe used to cast the XK cylinder blocks and didn’t always do a brilliant job of it (allegedly). However, for the V12, Jaguar invested over £3 million (which was a lot of money at the time) on three automatic Archdale transfer machines for the cylinder block and a 42 station Fuller transfer machine for the cylinder head line. There was also extensive investment at Radford for the production of smaller alloy components required – manifolds, sump cover, cam covers etc. This was good money spent on quality equipment. They were serious about the V12.

      Unfortunately, they lacked the foresight to provide for sufficient flexibility in tooling, meaning they were unable to expand the V12 to the 6.4 capacity which was its sweet spot, nor reduce the capacity to around 4 litres as JT suggested in a previous comment. (The latter is an assumption on my part, but JT might care to elaborate) Bob Knight later observed that this lack of foresight was a serious error on their part. This was another example of Jaguar investing heavily and being bitten hard for their trouble. By the time they realised their error, they had no hope of getting any further funding from BL. There wasn’t any money – and what was available was simply poured into the black hole at Longbridge.

      My understanding of the AJV8 is that it began as part of an experimental programme in Ralph Smith’s skunkworks in Whitley, but what it became by 1996 lies outside my scope of knowledge. Once Ford took over, heaven only knows what occurred. Oh yes, Clive Ennos. That’s what happened.

  10. Hi Bob

    Jaguar installed a new line at Radford. This was for the V-12. It was not used for XK engines. It wasn’t used for the AJ engines either. Unfortunately decisions made at the time of designing the line meant that it wasn’t as flexible as it may have been. One very limiting choice meant that the deck height of the block could not be raised. It needed to go up by ~8mm or so to accommodate the increased stroke of the Mundy 6.4 litre version. The 6.4 litre had the increased torque the engine so badly needed as well as being much more fuel efficient (even with the then standard flat faced cylinder heads it was better). There were two ways to get this increase. One was by installing new machining plant on the line. The second was to set up a separate line manufacturing deck plates. Either approach required an investment that was not made.

    The 90-degree V-8 which Jaguar eventually placed into production did indeed originate as a modular engine project. There were to be V-6, V-8 and V-12 versions. The idea was flawed from inception. Either the V-6 and V-12 had to be compromised with a 90-degree cylinder bank angle or the V-8 had to be compromised with cylinders on a 60-degree bank angle. This assumes the engines were to go down the same line. The problem is not as severe if separate lines are utilised for the different engine layouts. Even so, that leads to other compromises, some of which are of a fundamental nature. So, how far do you go fixing up flaws caused by the non-ideal geometric and dimensional compromises? Optimise for each engine type and you end up with an engine family which is not modular at all.

    In the end, the single engine architecture which did emerge from Jaguar was a bit of a mess. For example, the inlet ports were weird (in plan they are curved- it is as if the designers wanted to induce swirl when the pent-roof four valve set up they had naturally develops tumble- so why frustrate it?). The cam drive was problematic. Worst of all the cylinder bore pitch was far too small. They’d have been better off importing and fitting iterations of Ford’s Mod Motor rather than building an inferior version. Note that they were not allowed to do this nonsense again. The V-6 they used was all Ford. A “special Jaguar version”(!) with “Jaguar designed cylinder heads” (yeah, sure) of the Ford V-6 was what Jaguar fitted to its cars. This was an admission of what had become obvious.

    At one point there was talk of replacing the V-8 with a Ford derived V-12. This unit would have been a V-12 based on the Duratec V-6, just as in the Aston Martin cars. You can bet Aston acted very quickly to squash that idea!

    Note that Ford has turned out to design and manufacture some of the best engines available. Even the mighty Koenigsegg used Ford’s Mod Motors. They kept the architecture and have since developed from that basis.

    After Ford sold Jaguar to Tata an entirely new V-6 engine had to be developed in house. This was due to the supply deal with Ford expiring. Have you seen pictures of the V-6 engine Jaguar ended up manufacturing to replace the Ford? Take a look. Notice anything? What do you think? Hard to call it optimal!

  11. Expansion of the V-12 to 6-litres or even a little more was possible with the existing production machinery. At 6-litres all that was required was a revised crankshaft and pistons. To go a little further (~6.2-litres) would have required revised crankshaft, pistons, piston rings and liners. On the other hand the easiest way to go smaller would require crank and rods only. The crankshaft would have the classic 56mm stroke and could have been a cast affair if necessary. The rods would have to be longer*. Torque would be less but it would be a very smooth engine which would operate at a slightly higher rpm range.

    Alan Scott thought the ideal Jaguar V-12 would have been a 7.1-litre version. Larger versions than that were schemed out (up to 9-litres for road use) and several examples have been built and run. There is enough room in the crankcase to allow room for even bigger than 9-litres. That would be fun.

    * The production machinery did allow for a lower deck height for the cylinder block. The decision on what height to select would be determined by what the production machinery could reliably accomplish allied to attaining a good rod/stroke ratio and piston height. The designer would need to juggle parameters to come up with the best option. The lower the deck height was the shorter the rod could be and the lower the piston and/or compression height could be.

    1. J T
      Do you know the bore and stroke for alan Scott plans 7.1L and 9.0L V12?
      In addition, what do you think the cylinder head and combustion chamber are most ideal for the Jaguar V12

  12. Thanks for the enlightening me on the V12 and other Jaguar related aspects J T and Eóin.

    Was the experimental 6.4-litre V12 by Harry Mundy achieved with a 90mm bore and 84mm stroke (for 6413cc) or was it through another bore / stroke arrangement?

    The lack of flexibility in tooling for the V12 (and presumably any other prospective V12 based engines) was undoubtedly a big oversight on Jaguar’s part, whereas the Duratec V6 and related V12 appear to have had the potential for further flexibility via the 2.1-litre AJ-V6 and 7.3-litre V12 in AM One-77 (though what was the viable scope for the V6/V12 above 3-litres and 5.9-litres respectively).

    Was not aware experimental Jaguar modular engine project was flawed in that respect, though guess it was too soon for a Mercedes M276 V6 and M278 V8 solution to help resolve the compromises in angle (IIRC they were related and even built on the same production line).

    Were sub-4.6-litre versions of the Ford Modular V8 considered? Find it interesting that together with a Modular V10, even Sixes were investigated at one time.

  13. Oliver

    The 9-litre was the 98mm bore and 98mm stroke engine on second overbore. It used a deck plate.

    I don’t recall the bore and stroke for the 7.1-litre engine. I have to ask. I’ll get it for you as soon as I can. Note this engine was quite different from what had gone before. For example, it was a bedplate engine and not a Y-block. More on that later.

    The cylinder head question will take a bit of elaboration. I’ll think on it and respond soon.

  14. Bob

    You are right. Mundy’s 6.4-litre was 90mm bore and 84mm stroke. It used standard rods and pistons with standard compression height. A deck plate was used to allow this.

    The Ford Modular engine started at 4.6-litres. Larger versions were produced. A de-stroked 4-litre version was developed for racing, but did not enter production.

    Aside from the V-10 there was also a V-12 version of the Modular. Prototypes were fitted to the Ford GT90 (with turbo-chargers) and to a Crown Victoria (without turbo-chargers).

  15. Oliver

    Update: The ideal 7.1-litre was 96mm bore and 82mm stroke- like the 5.3 and the 6.4, a sweet spot for the Jaguar engine.

  16. Oliver

    To answer your question about the best combustion chamber for the V-12 I’ll go look at which would be the best cylinder head to use…

    If you are after all-out power, then you want Jaguar’s own four-valve head (developed in the 1970s) for the V-12. It was a narrow angle job with excellent breathing and a nice compact combustion chamber. There were only a limited number of these cast and machined. Years later they were delivered to TWR for their Group C efforts and also for prototype work up for proposed road cars (XJS and Aston Martin). These heads certainly made extra power, although higher in the rpm range than the Bob Tarozzi designed two-valve heads TWR raced in Group C. The four-valve heads had two disadvantages in Group C racing. The centre of mass of the assembled four-valve engine was higher and it also made less low rpm torque than the two-valve. There are a few sets of these heads still around and a company in the UK even has a set of raw castings ready for machining. Most of the four-valve heads are gone though. After their experimentation programme, TWR scrapped the heads they’d received from Jaguar. Yes, they went in the skip! Can you believe it? So, if you find any laying about, they’re worth a lot- unicorn tears. You’ll also need the cam drives and front covers and manifolding….

    The TWR Group C Jaguars raced with a Bob Tarozzi designed two-valve cylinder head. The head was developed for Group 44 in the USA. Originally it was cast and machined in the USA (Bob Tarozzi was a Chrysler engineer). Subsequently a set of improved Tarozzi patterns were purchased by TWR and imported to the UK so they could be used to make heads locally to TWR (thus solving certain logistics problems and risks). All variants of the Tarozzi cylinder heads had similar architecture to the standard Jaguar V-12 cylinder head, but many of the known problems were designed out. Tarozzi’s heads have thicker port walls and also a thicker deck. The water jackets are better. Valve centre-lines have been altered so that bigger valves can be fitted. There is plenty of detail improvement, but the guts of it is that his heads were designed to allow cylinder head developers (porters), as well as engine developers (such as Alan Scott), plenty of latitude. If you can get a set of these you are onto a very, very good thing! People who have them know what they have got though. Recommended, although likely difficult to locate and very expensive. Alternatively, the patterns still exist. There is at least one set in UK and several in California.

    Next choice would be to use two AJ6 type four-valve heads and adapt them to the V-12 cylinder block. The heads are easy enough to find but this sure is a demanding conversion. You’d need to be good with machining and welding and heat treating aluminium. It would only work if the V-12 you were intending to build were large- more than 7-litres. Over 8-litres is better. The AJ6 style cylinder-head has largish valve angle and this results in a big combustion chamber which will not give reasonable compression ratio with a flat piston in a smaller (sub 7-litre) engine. Also the port volumes are too big for smaller engine sizes. Go for more than 8-litres and most of the problems go away. Hint: absolutely use a deck plate for this conversion. Note that the centre of mass will be higher and the engine will end up wider. OK for an XJ or even an E (easier to fit). Have fun with the exhaust manifolding or, if you enjoy a real challenge, extractors.

    This leads us to factory heads which are also easy to find. The first type is the standard “flat head” and the other is the May “Fireball” head a.k.a. the “HE” head. For all out power and also refinement the early “flat head” is often preferred. It flows reasonably well (never quite as well as a Tarozzi though). Build your engine at 6.4-litres, which is ideal. Rather than port the heads out too far (and risk breaking into water) the best strategy is to fit slightly larger valves, carefully do a decent valve-seat job and give it a mild turbocharge or supercharge. The operative word here is “mild”. It is easy to get over 600 bhp, but not as a continuous rating. Still most road use (or track use for that matter) calls for power on an intermittent basis. 600 to 650 bhp is adequate, even in a 4000lb sedan. Warning: you’ll need to make absolutely ruthlessly certain of your management of under-bonnet heat and engine cooling. Spend some quality time planning ahead on that topic. It’ll definitely pay. Another warning: the rear suspension will need a lot of attention.

    The HE head is an interesting compromise. While the compression is much higher and part throttle efficiency is improved over the “flat head” (combustion chamber is in the head and not down the cylinder) the exhaust valve is shrouded so flow is restricted here. It is also not quite as refined as the “flat head” either (runs a little rougher). It gets quite difficult to make 450 bhp or more. Getting to 400 bhp isn’t easy either and can be a struggle. Even supercharged there are limitations which mitigate against this head, unless…..

    Recommendation for the HE heads would be to have them reworked for superior combustion chamber geometry by a certain company in the UK called Bonner Engineering. These guys really know what they are doing. Get your HE heads prepared by Bonner and then apply the turbocharger therapy. An engine prepared like this will go and go and go. How much power you want? And you could even have it continuous! Such reliable goodness!

    Note that for both the flattie and the HE, another reason supercharging/turbocharging is a good way to go is that it means you’ll not have to use high lift cams. The trouble with those is that to do the job right you need a larger base circle for your cams and that in turn demands you fabricate a new pair of cam carriers. OK if you have access to a CNC mill etc., but even then it takes up a fair bit of time. It isn’t a basic job.

    Final choice are heads which are presently available brand new! They’re a little pricey and only have two valves for each cylinder, but for a beautiful looking engine- a classic Jaguar look which would be brilliant in an XJ – you’d likely not do better than Building the Legend’s XJ13 cylinder heads. These have double overhead cams per head with down-draught inlet ports between the cams. They are of the classic hemi geometry. They’ll handle up to 96mm bore (possibly more). They work well, make good power and look absolutely perfect. OK, I have not tried these heads myself, but they have very competent people behind them and there is no doubt at all that they work extremely well- even better than the originals (after all the Building the Legend heads are of much improved design with decades of more recent knowledge and technology invested in them). Look them up.

    Now you asked which chamber is best. From the perspective of chossing heads for the Jaguar V-12 engine, that’s hard to decide. It depends on context. What sort of car do you want to build?

    Let’s make matters easier by discarding heads which are difficult to get presently, super rare or impossibly expensive (likely one-offs from a specialist). That eliminates the Jaguar four-valve heads. Sadly it also removes any of the Tarozzi heads from contention. I don’t think the AJ6 heads are contenders either, not unless you have access to a quality machine shop and are prepared to undertake a lot of fabrication (else have a lot of spending cash floating around at your place presently). I don’t feel up to doing this project yet again (at least not at the moment- maybe one more time a few years from now but just not today). So we have the Jaguar early “flat head” or the HE or the Legend XJ13. If it were me and I was after serious performance it’d have to be Bonner heads (modified HE) fitted to an enlarged V-12 (6.4 minimum but more is preferable) supercharged (challenging) or turbocharged (much easier). For a classic XJ sedan, well that’s easy, Legend XJ-13 heads on a Mundy 6.4-litre in a series-2 XJ sedan (make it a Daimler Double-Six). Hooray!

    1. That’s a lot of info JT. I’d probably have an XJC rather than a series XJ saloon.

    2. Thank you J T, I just want to design an alternative history, not to work on metal
      You mentioned “You can get a lot more performance and efficiency with a different combustion chamber”
      In the past, you said that V12 was damaged by Heron’s head
      So I just want to know what the Jaguar V12 could look like when it was born with more wisdom and abundant funds
      DOHC head is very good, but I think SOHC would more suitable for road car at that time

  17. Gaaaaah! I forgot to mention the Warrior Automotive* five-valve heads. These are exceedingly rare. I don’t know who has the patterns, or even if they still exist. In fact, it is not generally known whether any sets of these heads survive after all this time. So, as interesting as they may be, extreme rarity disqualifies them from our consideration today! So very annoying as years ago, shortly after the company was liquidated I got so very very close to getting some.

    Cheers & hope you have a great weekend.

    * they were originally manufactured by Kearns Richards Group

  18. Freerk

    Ah yes, of course. The perfectly beautiful XJC. You are certainly a gentleman and scholar of refined taste, good sir!

    1. Thank you, J T. I’m impressed by your knowledge of the V 12 engine.

      I have never driven a V12 Jaguar or Daimler before. One thing I really notice with older cars is the automatic gearbox. There has been a lot of progress with automatics in the last couple of generations. I wonder if an upgraded V12 would work well with a more modern gearbox. I don’t mind shifting my own gears, but maybe this is not the right car for that.

    2. Freerk: The four-speed manual transmission as fitted to production V12 Jaguars (Series III E-Types and some early XJ-S’) rendered most gear ratios somewhat redundant in normal road use. From contemporary reports, one could start the car and move off from standstill in fourth, without tremor or protest from either engine or driveline. Harry Mundy’s drivetrain team devised a five-speed manual which had been intended for the V12, but it was (allegedly) rather big and heavy (it probably had to be) and not only was there no sanction forthcoming to productionise it, there was little demand from customers for a manual option (it was never offered on the XJ12 saloon at all), so when the Jaguar four-seed manual was discontinued around 1978, so too was the manual XJ-S. (Only a few hundred manual XJ-S’ were officially built).

      Throughout most of its production lifespan, the V12 drivetrain was let down by the limitations of its automatic transmission. Initially offered with the Borg Warner Model 8 (if memory serves), this was heavily criticised, especially by the US press. In 1977, the General Motors 400 three-speed unit was adapted for Jaguar’s use – a much improved unit, albeit one hampered by its gear ratios (it was said), owing to Swiss noise regulations, which necessitated a detent, preventing engagement of first while on the move. It was also felt that an overdrive top gear would have helped, although Jaguar did develop a two-speed axle for much the same reason, but couldn’t refine it sufficiently.

      When the factory 6.0 litre V12 was introduced, a more up to date four-speed unit was fitted, but by then, the V12 was on borrowed time for the second time in its career, this time however, for real.

      From a personal preference, I would nonetheless advocate the automatic, since it better suited the electric motor characteristics of the engine.

    3. Thanks, Eóin. I forgot about the 4 speed automatic on the 6.0 versions. I was only familiar with the GM400 units.

  19. The pre ’77 Jaguar V12 automatics used the Borg-Warner Model 12, a heavier duty version of the Model 8, both of the “Flashomatic” family first produced in 1956, originally for use by Studebaker, Packard, and Rambler.

    According to Jeff Daniels’ ‘Jaguar – The Engineering Story’, the XJ-S was intended to have the far better GM THM400 gearbox at launch in September 1975, but the installation wasn’t ready for production until 1977. Jaguar were embarrassed by the inefficiency of the old B-W transmission, and its effect on performance and fuel consumption, and therefore only provided manual cars to the media for performance testing until the GM transmission was available.

    The four speed automatic, a GM 4L80-E, arrived much later, in February 93 for theV12 XJ40, and in May of the same year for the XJS.

    Daniels also mentions Mundy’s five speed manual gearbox, which reached an advanced enough state of development to have five prototype units running in test cars. The reason it never went into production was perfectly rational; most customers were specifying automatics, and the small number of high-capacity manual gearboxes required would never recoup the cost of tooling for an in-house gearbox. I read somewhere, many years ago, that despite its 500Nm torque capacity, the Mundy five-speeder was lighter and more compact than the later LT77 gearbox developed by Triumph engineers for the Rover SD1. It’s possibly just Leyland folklore, but just possibly true as the iron-cased LT77 was never an outstanding example of the gearbox designer’s art, despite finding many applications in its long production life.

    1. Thanks Robertas for the correction regarding the Borg Warner model number. Further to the Mundy five-speed, Philip Porter, in his Definitive History quoted the Swiss owner of a Series III E-Type which somehow escaped Browns Lane with one of these gearboxes in situ. The owner mentioned when he required a clutch change how large and bulky the transmission was and Jaguar’s reaction when he enquired to the availability of a replacement. He did however point out that the five-speed utterly transformed the car.

      Thanks also to JT for your highly detailed observations. Interesting to hear about the GM400’s adaptability. I recall reading that Jaguar engineers thought rather highly of it. But the enemy at Browns Lane was always money.

  20. Freerk

    Yes indeed, the selection of a transmission for the upgraded V-12 is key. Fortunately there are many choices. My first choice would be to go for a manual. There are several excellent US manufactured 5-speeds you could consider. Even the standard engine really wakes up performance wise with a manual*. On the other hand an automatic is much more in keeping with the luxury aspect of the XJ.

    The best of the standard transmissions is the GM TH400. GM built a version specifically for Jaguar. This features a cast single-piece housing. The reason Jaguar insisted on that was to eliminate a bending vibration which would otherwise be present (although not intrusive and a little hard to detect under most conditions). The guts of this transmission are standard GM fare. The ratios are standard and as such are not ideally suited to the V-12 unless cubic capacity is added. The standard ratios of the TH400 are intended for the big-block V-8s and similar. So we are talking about engines which range in size from ~6.6-litres and up (most of them 7-litres to 7.4-litres). These engines feature a lot more low end torque than the V-12. In the Jaguar XJ application the TH400 really should have a shorter second gear. First gear could be shortened some as well, but this is not as important as getting second gear right. For cost reasons Jaguar did not specify a revised ratio set from GM. However, there are vendors in the USA which do supply the ideal TH400 gear sets for Jaguar’s V-12. These can really improve your car.

    One most excellent and successful approach is to retain the TH400 as is and fit a numerically higher rear axle ratio to let the V-12 rev out some. This will markedly improve performance at the cost of loss of top speed and an increase in fuel consumption (which in the standard car is already getting up there). Further there is some loss of refinement when cruising on the motorway since you will be carrying more rpm. It’s noticeable 60mph and above (especially if you have experience with the standard car). So then you fit the Gear Vendors overdrive unit. The Gear Vendors unit is an epicyclic two speed transmission which is installed on the tail of the TH400. You’ll need to do some revision of the driveshaft/s and a modification to one of the mounts among other things. Gear Vendors overdrives are very strong and reliable. They are most commonly found in US muscle cars where they allow the use of dragstrip oriented final drive ratios while retaining the ability to cruise on the interstates (no way anyone really enjoys 4,500 rpm at 70 mph- the Gear Vendors overdrive avoid this while keeping or improving the accelerative performance the big V-8 cars are known for). I’ve never had trouble with them whether installed behind a big-block, Hemi or anything else (Jaguars included). This is the most robust set up available and it is well supported by the aftermarket. Most highly recommended.

    You have another alternative available. You can also fit the GM 4L60 or 4L65 automatic transmission. These are the four-speed GM automatic transmissions which came along after the TH400. In essence they are a TH400 with an extra gear and they fit in the same space. They are not quite as strong as the TH400 and require some electronic work to make everything operate properly. There is also the option of retaining a lock-up torque converter with them. This conversion all works out nicely indeed. Good results.

    If you want to have a more challenging project, try swapping in one of the more recent 6 or 8 speed GM auto transmissions (6L80, 6L90, 8L90). I’ve not done this conversion, although I am aware of some that have. They work, but be aware you delving into the realm of electronic control systems and emulation and the like. The mechanical work in fabbing up brackets and adapting everything to fit in properly is the least of the problems. There is a 10-speed automatic available from GM and Ford (they share the same transmissions these days). The 10L90 is the GM variant and the Ford is 10R80. Again, beware electronics!

    Which to select for the Jaguar V-12….

    For a standard engine or one which has been enlarged and modified for much more go power use TH400 and Gear Vendors. For a standard engine or an engine with moderate improvement you have three choices- TH400 & Gear Vendors, 4L or TH400 with revised gear set for better ratios.

    * E-type convertible with V-12 and Jaguar 4-speed manual runs 0-60mph in 5.6 seconds . Compare that to the automatic version at 7 or even 8 seconds… That’s a difference you can not only see but really feel!

  21. Hi Eion

    It’s no surprise Jaguar engineers liked the GM TH400. They were in good company. Not only did Jaguar use these transmissions but Rolls Royce did as well.

    The TH400 in an excellent trans. It is extremely robust. It is a smooth shifter- very refined. There is a lot of aftermarket support for it and a great deal of expertise available to set it up however is needed. It is capable of handling the shock of a dragstrip launch from a big-block even one fitted with a power adder or two! 1,000bhp is no trouble for it.

    Ford and Chrysler had similar three-speed auto transmissions to GM’s TH400 and lighter TH350. Ford had C4 and C6. Chrysler had the 727. They are all very robust and easy to build to cope with major torque/power/shock. They are also refined. There is very good aftermarket support. Build any of them tough and stick a Gear Vendors overdrive on the back!

    An aside about someone else approving of GM transmissions.

    There was a transverse four-speed hydramatic GM developed for front wheel drive cars. The 4T65HD was mounted in the Pontiac Bonneville. It took the power from a V-8 (the 5.3-litre LS4) to the front wheels. Meanwhile for Cadillac the 4T80 was used for the Northstar V-8. It also drove the front wheels. It was a heavier transmission than the 4T65, probably because the Cadillacs were so much heavier than the Pontiacs. Both the LS4 and Northstar V-8 engines made ~300bhp, although the LS4 had more bottom end torque.

    Honda’s factory supported drag cars started using automatic transmissions quite early on (for faster elapsed times than with a manual). The poor Honda auto transmission was in no way capable of handling the severe duty it was subject to in the unforgiving environment of drag racing. Some of those early front wheel drive Honda racers were getting 475bhp out of the B-series four cylinder engine (and they were still a streetable proposition- drive to the track and drive home after). The transmissions of choice to swap in to the Hondas was the GM 4T65 and 4T80. Power levels continued to improve as important lessons were learned and as the racing got more competitive and more serious. The use of 4T65 and 4T80 allowed much larger outputs, much more than 475bhp. No wonder Honda though highly of GM transmissions.

  22. Oooops. I may have given a false impression when I wrote, “The poor Honda auto transmission…” It isn’t that it is a bad transmission. It is good. It’s just that when it was designed no-one on the design team could have foreseen the need for it to be able to cope with an ultra-powerful B-series during the ultra-violence of a competition drag-strip launch. When I used the term “poor” I meant it as in feeling sympathy.

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