Exactly. Not only are the Jag cams short duration but they are also low lift. Stock springs will support a higher lift.
A small excerpt from Grass Roots Motorsports website. Much more info there but a pain to find and read through the non applicable stuff.
mguar wrote: The stock V12 has 3 basic versionsā¦ 1971-1980 was the flat head era, 1981-1992 was the HE era (still 5.3) and from 1992 through 1996 Jaguar made 6.0 HEāsā¦ 366 cubic inches instead of 326 in the 5.3 that added 40 cubic inches gives another 25 horsepowerā¦ The first roughly year and 1/2 of the 6.0 production the crankshafts were forged hardened steel. Later cranks were made from sintered Ironā¦ Stock the fuel injection will flow1200 CFM more than enough to support 450hp. The earlier carbs used prior to 1975 will only flow 800 CFM but that is mostly wasted due to how itās arrangedā¦ That is it enters down below the valve cover and is heated prior to it wandering itās way into the engineā¦ The MGBGT autocrosser guy cut off that goose neck and picked up a lot of power. There is even greater potential if you cut open the Fuel injection manifold and weld a plate across it to take 3 Strombergs per side. Thatās 1200 CFMā¦ (Thatās for us luddites who donāt like the complexity of Fuel rejection)ā¦ Webers while sexy and a whole lot of eye candy, require dyno work and complete rejetting to run rightā¦ Assume about $1000-$1200 for partsā¦ Power gain is right there with the 6 Stromberg modsā¦ Due to the too short intake stack to fit under the stock hood. Whereas almost any other mod requires a computer upgradeā¦ Headers: are wasted on the V12 (except all out designed to match the camshafts equal lengthheaders) Unlike a V8 the exhaust pulses are even and not in conflictā¦ Cam shafts: the stock cam is remarkably goodā¦ For all out racing it can be beaten but for street & autocrossing etcā¦ itās great right as it isā¦ Regrinds by ISKY and Crower will add power if combined with higher compression ratios. a gain of 100 horsepower is possible. Port work will add to those numbersā¦ The stock Valve springs: were reved by the factory to 7800 rpm before they bounced and since the power peak is at 5750 rpm not a restriction. If tired ISKY sells good valve springs cheaper than Iāve seen Jaguar valve springs priced atā¦ The pistons are typically 7.8-1 compression through 1980 (a handfull are 9.0-1 in the earliest XK-Es) Starting in 1981 compression jumped to 11.5-1 However those pistons cannot be used in the earlier flatheadsā¦ (1971-1980) What can be done to the later 6.0ās is use the earlier 7.8-1 compression pistons suitably relieved for valve clearance on a 6.0 and youāll be right around 13.0-1 compressionā¦ Stock the V12 has a 3.543 bore (both 5.3 & 6.0) max safe overbore is 3.681. which combined with the 6.0 crankshaft yields up 6.47 litres. All other things being equal overbore will yield an additional 25 hp. (plus whatever power improvement using higher compression ratio) In general itās safe to say V12ās start near 300 hp. and the 6.0 makes about 25 moreā¦ itās pretty safe to assume that with any mod the 6.0 will yield up another 25 hp over the 5.3 there is roughly 15 horsepower more from 9.0-1 compression to 7.8-1 compression another 10hp. at 10.0 & 10 more at each added one point of compressionā¦ (those are factory numbers) The cheapest power gain is using a stone stock 5.3 with a pair of turboās off junkyard salvaged Volvoās/ Saabs. (little T2ās)
Ride is subjective too, but by conventional definitions inevitably deteriorates when progressively less compliant tyres of the same rolling radius are put on a given suspension system (you did not say you were reengineering new suspension unsprung components or geometry).
It may be preferred that way, but less compliance with irregularities means more NVH passed into the same suspension and a proportion of that is passed into the body structure. Blowing your tires up to 59 psi will prove it.
I test new Jaguars for the Jaguar Journal and whilst they handle beautifully and ride very well, ride comfort in terms of controlled but smooth driving over poor roads is not emphasised as much as twenty years ago.
Jaguar are not alone of course - fashion is partly to blame. I went to a two-day Bentley press event when they launched the sports version of the Continental GT. Fabulous car and it had the worldās largest ceramic discs as I recall, but I wafted home more comfortably in my 17ā rim Daimler than the 20 & 21ā wheel GTs Iād been driving all day
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Iāve had a plan on the back burner for a long time now, though I have most of the parts already. My idea is to fit pre-HE heads to a 6.0 block using dished pistons similar to the pre-HE. I found pistons with the correct dish and compression height needed based on rough estimates. I anticipate an increase in compression ratio from the stock 7.8:1, due to the smaller dish and increased stroke. I also plan to treat the heads to some porting. This engine will be going into a Series 2 XJ12 with a 3.31 limited slip and 700R-4 overdrive automatic, induction system as yet undecided. Iām nowhere near starting this project, but in the meanwhile, the E-type is getting all my attention and funding.
A friend had a large American tank, and while we were all on the way to somewhere, he told us that the car mag reviewers had faulted this particular model for having ālarge, boat-like motionsā. Thatās why we were all feeling a bit green around the gills. IMHO, there is nothing "comfortable about a car that makes you seasick.
True. But then in those days Jaguar were almost unique in squaring that circle: smooth ride without going wobbly in the twisties like conventional luxo-barges.
The only car journey I ever threw up from was as a 14-year old passenger in the back of a Citroen DS21 driven for hours at night on twisty French rural roads with the heater on full blast. I got sea sick and the friend of my father had the choice of stopping to let me throw up outside or continuing to drive like Jacky Ickx and having to hose out his interior.
While car shopping, I once took a Renault LeCar for a test drive. That was a strange experience. The car would corner quite nimbly, but with no resistance to leaning ā itād lean waaaaay over in even a moderate level of Gās. But it wouldnāt scrub or feel like you were pushing it at all, it felt like it was supposed to lean over in turns. I found that a bit disconcerting, but I could see how some people would appreciate the resultant ride.
It also had a different idea in a sunroof. Rather than a little glass thing over the front seats that motored open or shut, it had a canvas panel that covered the entire roof. You could leave it shut, you could fold the front half back over the rear half to open the front half, or you could fold the front over the rear while flipping the canvas itself out the back to open the entire roof. That last position looked like a parachute to me, but the dealer said it works fine. We had to test it out, and he was right, it just laid there at speed, didnāt flop or flutter or anything.
The V12 was designed for FI, but by the time the engine was ready the FI wasnāt - so the first cars came with carbs - and the engineering may mirror this āaaahh, by the way, we wonāt get the FI in time, would you please put on some carbs instead, gentlemen ā¦ā approach.
Also, the V12 was originally designed for QOHC like in the XJ13. Production costs and noise made Jaguar go back to a more standard setup. Some years ago an XJ13 engine came up on ebay for like 20000 US. I was tempted, but as it came without any intake or fuel injection system I recognized that not only the price tag for the block and heads was beyond my reach.
Many say that the original design of production fuel injected V12s was the most sportive and straightforward; the efficiency gains of Michael Mayās HE head affected mostly fuel consumption of sedate city drivers - admittedly a high proportion of bank managers taken to their job were not blasting their cars through the B roads of southern Black Forest. That being said a ROW fuel injected 5.3 litre car in 1975 put out a whopping 287 DIN hp at a compression of 9:1. The mapped engine managment of the last V12s and their ignition system combined with an additional displacement would probably get you a seriously fast car, given that you already mated a decent gearbox.
As for suspension improvement, Iād go no further than the Jaguar building system yields parts: search for late V12 XJS with sportspack. They have bigger anti-roll bars, correct 16 x 7" tires and most importantly the ZF steering rack with a much more responsive feel.
If you cannot source the latter, poly bushings within the suspension and steering and the SIII front end combined with a bigger anti-roll bar front plus anti-roll bar rear will take you a long way. The Jag suspension was good enough for Aston Martins till the end 1990s.
Overly big rims with low-ratio tires will not only make the car look like the Queen on high heels and compromise ride, but inevitably increase wear on all other parts of the suspension. You can make a series Jag seriously fast, but not with chrome & flame parts such as big rims.
The reason why car designers have moved to large rims and low-ratio tires is the lost proportion of obese mid-size cars, SUVs and minivans. Remember that the first car in dire need of an electronic roadholding device was the A class Mercedes with its innovative sandwich floor. Your car doesnāt need that - it is four inches lower than a current Fiesta. My 18-year-old son with a fresh driverās license was more than surprised that it was easier for him to follow a local bend (higher speed, uneven radius, uneven surface) in my RHD Jaguar SII than with our current family hauler (on winter tires 17" wheels, 225/55 or summer tires 18" shod 235/45) with applauded all-alloy suspension. For details check out Fiboyās modification to an XJ12 coupĆ©.
IMHO - if you strive for more control and response, start with the driverās seat. Jaguarās preference for comfort started with plush foams and soft leather, ideal for long journeys but not so for spirited cornering. Later models might supply suitable seats more apt for that purpose. OTOH, a 13" MotoLita steering wheel in a full size sedan looks rather out of place. Again the last XJS wheels provide good feel without sacrificing on looks and use.
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
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All excellent advice, Jochen.
I think the fiboy website is down. But not with the wayback machine/ internet archive.
I respectfully beg to differ with regard to wheel size. As with all things, time has a way of marching on and so have the tire manufactures. Any suspension will only ever be as good as the rubber that you are able to put on it. High performance summer tires and the like no longer exist in the 15ā & 16ā tire sizes, and not only that but most of the big names like Michelin etc. no longer offer any selections in those small sizes at all.
I wish that I didnāt have to move up to an 18ā rim, but unfortunately thatās where todayās best tire selections start.
I dunno how bigger wheels with shorter sidewall tires can avoid giving a harsher ride. I will say, though, that itās amazing what the tire companies can do. Historically, every move to lower-profile tires was led by a desire for performance, and hence the first lower-profile tires always had stiffer sidewalls and aggressive performance. With time, though, tires with those same profiles came along with soft rides and long treadwear.
When I was in high school in the 1970ās, most tires were 78ās and the hot ticket for the car of your dreams was 70ās. By the time the XJ-S was introduced, 70ās were fitted to a luxury car. Then 60ās became the speed secret, again appearing first in high-performance versions. My early 2000ās Japanese econoboxes are fitted with 50-series tires, and yes, they are soft ride long treadwear tires. So, can you get 45-, 40-, or lower profile tires in smooth ride versions yet?
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Try 14"ā¦
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An XJR was my daily driver for several years; about 150k miles.
The low profile tires (255/45x17) were the worst design feature of the car.
-Tire wear. At the time all the tires in this size or close gave very poor tread life, as low as 10k miles (Pirelli P-Zero) and as high as maybe 14k miles (BFG/KDWS) . Such is life with āultra performanceā tires. Replacing 4 tires was a $750-$1000 proposition at the time. When you drive 1500-2000 miles a monthā¦wellā¦you can see where Iām goinā with this. I was buying two sets of tires per year.
-The short sidewalls are more susceptible to road hazard damage. I replace 2 or 3 as a result.
-The short sidewalls left the wheels more susceptible to damage. Three times I had to have wheels repaired, something Iāve never had to do or any car before or since
-Tramlining. Already oft discussed on these forums.
I eventually wised-up and went to some performance āorientedā 225/60x15 tires (V-rated) that lasted 40k miles. There was indeed a decrease in steering crispness, which I anticipated. Other than that I didnāt loose anything in terms of āperformanceā. I like fast driving but Iām not madman. The corners I took at twice (or more) the posted speed with the 45-series tires I negotiated just easily with the 60-series tires, never lacking for control, feel, or traction. My favorite twisty roads thru the mountains were traveled just as rapidly with the same degree of control. Personally, I couldnāt find any driving where I felt the 60-series tires were a detriment.
Iām sure some drivers would use the 45-series tires to maximum advantageā¦if they were on a skid pad or driving 9/10ths on a slalom course. At 8/10ths or less I canāt honestly say the 45-series tires were anything but a snare and delusion.
Just my 2-cents 
Cheers
DD
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**
This works both ways, pliant rubber gives a pleasant ride; as Kirbert says, the lower the profile the less tyre deflection. Tyre manufacturers cannot perform miracles - and on āourā Jaguars the fatter tyres is a vital part of the ride. An issue here is that the suspension itself is set up for this and lower profiles puts a high stress on the suspension parts - and, frankly, the car rides like an old fashioned dray cartā¦
On cars meant for lower profile tyres, the suspension is set up accordingly, but the ride is still harsher - some like it that way, I guess, equaling it with āsportyā. The roadholding is the domain of tyre compound - whether high or low profile is immaterial. However, low profile tyres do have lower slip angles, which does give better stability. And, of course, tread pattern is a factor in road noise - and indeed wet/snow performanceā¦
Aesthetically; the series cars were designed with ā70ā profile tyres, and neither looks nor rides ārightā with ā45ā - but that is, of course, a very subjective opinionā¦
Frank
xj6 85 Sov Europe (UK/NZ)
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I wonāt bother arguing about added suspension stresses or what profiles the XJS was originally intended for. In my view Iāve always likened the XJSās ride characteristics to that of Rolls Royce. Unabashedly, a performance oriented road handling ride, forgoing the originally intended smoothness, but without the unnecessary NVH of an all out track car is my personal goal.
As stated earlier, the world has moved on and I have moved on with it. Whatās interesting 
if not all out 
laughable is that the two XJSās V12, 4.0, XJ81, BMW 325i, Mercedes 380SL, and Porsche 996 were all purchased through the years for the sole purpose of being daily drivers. There is a strange thing that happens once a car gets pulled into the shop and then lovingly gone through. It is somehow no longer something that I want to pull out in the rain, snow, or with salt on the road (being caught out in inclement weather is a different story).
My point is, for all intents and purposes my 2008 F350 is my daily driver, and any of my other extra curricular activity cars, aside from long road and cross country trips, the concern of things like tire longevity, suspension wear and things of that nature become none events.
I would venture to guess that there arenāt many Classic Jaguar owners today that solely rely on their cars as the sole grocery getter for their households, as was more common the case 30-40 years ago.
As it happens, my Series III V12 is my daily driver (rain or shine) and my F250 is the āoccasional useā vehicle
Cheers
DD
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Ha, ha, ha 
I guess Iād consider you to be an outlier 
I guess you donāt get much snow or have to deal a lot of salt up in your neck of the woods? Iām out in the cornfields of Southern Indiana, but still in the eastern time zone, I am still a native New Yorker. The snow and salt here are not nearly as bad as in NY/NJ/PA
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