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The V12 was designed for high rpm operation that is why it can cruise at such high speeds. Well over 100 mph. There was an article in Octane magazine a few years back where an XJS was driven to Monte Carlo. The author describes how the engine really doesn't start to come to life until it's going over 80 mph. With twelve smaller oversquare cylinders as opposed to the typical V8 it needs to spin to make the power and produce speed. The speed comes from opening the throttle, not from a bunch of transmission gears. Since the majority of XJS cars are equipped with automatics they rarely hit higher revs unless they are run at high speeds.
I eliminated this problem by ditching the 2.88 final drive and installing a 3.31
As one would expect this really perks things up in terms of response and acceleration. At 60-80 mph, in particular, the engine is in a sweet spot of the torque band where just a little squeeze on the throttle gives quite an impressive burst of acceleration. Plus, the off-the-line acceleration from a dead stop is considerably improved.
. What's worse is that many like myself are afraid to over stress their old motors and are afraid to run them hard.
Give her the boot !
Respect and kindness towards old machinery is a nice thing. But after over 20 years of being on Jag forums, and owning two V12s, I've yet to experience or hear of any engine damage related to high RPM. In fact I've never heard of any failure of the reciprocating assembly on a Jag V12 nor any (RPM related) failure of the valve train. Nor any failure of the oiling system.
Small confession:
Truth be told, and despite my ballyhooing, I seldom go right to the 6500 redline. I find that 5500-6000 is plenty; little if any additional power is developed after 6000 RPM so there's really no point in going higher....except for the fun of doing so from time to time
Is changing the Final Drive from from 2.88 to 3.31 more effective in Terms of Acceleration and or Fuel Economy than fitting larger Throttle Bodies or am I completely on the wrong track there?
Is changing the Final Drive from from 2.88 to 3.31 more effective in Terms of Acceleration and or Fuel Economy than fitting larger Throttle Bodies or am I completely on the wrong track there?
It reduces fuel consumption, gives better acceleration ( but how often do you floor it in first Alex?). A UK spec XJS in decent tune will do a sub 7 secs 0 to 60 MPH no bother, if you hold it in first on the stick. Once rolling, if kickdown is working, or you use the stick, if you floor it at anything from 40 MPH upwards in second, the thing will hit 120 MPH before even thinking about a gear change. The fact is the car is pretty fast from the factory, as long as you thrash it!
I am willing to bet that very very few owners make the car do what, ex-factory, it will do in the acceleration stakes. To find out just how impressive the take off in first gear is, warm her up, find a nice stretch of tarmac, put the stick in first and floor it. I guarantee you that your attention will be grabbed! The box will change up from 1st to second on its own at about 5,500 rpm (62 mph) even though 1st is still selected. It then stays in second for ever until you move the stick to top. By the time you hit the red line in second you will be pretty impressed with what the car can do, how fast it does it, and well into 12 points territory!
Now a Golf GTI will out accelerate it from 0 to 60, but above 110 MPH it is a very different story; our problem is that the Grand Tourer is legislated against by modern speed limits!
The AJ6 Engineering throttle bodies and ECU mods do make a very big difference to the car, and I am a huge fan. In my exprience, and I have done this mod on three cars, it is an absolute must and makes a very great difference at legal speeds. It is easily the best mod from all aspects anyone can buy for the XJS; but I have never done the throttle bodies only.
Last edited by Greg in France; Dec 6, 2021 at 12:11 PM.
I eliminated this problem by ditching the 2.88 final drive and installing a 3.31
As one would expect this really perks things up in terms of response and acceleration. At 60-80 mph, in particular, the engine is in a sweet spot of the torque band where just a little squeeze on the throttle gives quite an impressive burst of acceleration. Plus, the off-the-line acceleration from a dead stop is considerably improved.
Give her the boot !
Respect and kindness towards old machinery is a nice thing. But after over 20 years of being on Jag forums, and owning two V12s, I've yet to experience or hear of any engine damage related to high RPM. In fact I've never heard of any failure of the reciprocating assembly on a Jag V12 nor any (RPM related) failure of the valve train. Nor any failure of the oiling system.
Small confession:
Truth be told, and despite my ballyhooing, I seldom go right to the 6500 redline. I find that 5500-6000 is plenty; little if any additional power is developed after 6000 RPM so there's really no point in going higher....except for the fun of doing so from time to time
Cheers
DD
You are very much correct. The power does fall off above 5500 RPM due to the camshaft used by the factory. They traded low RPM grunt for more power.
Knowing the typical buyer of their car liked a relaxed cruiser over a high power screamer that trade off makes sense.
Is changing the Final Drive from from 2.88 to 3.31 more effective in Terms of Acceleration and or Fuel Economy than fitting larger Throttle Bodies or am I completely on the wrong track there?
Never tried the larger throttle bodies...but they're on my wish list
I do, though, have the Superenhanced ECU..... which works as advertised.
As for acceleration, one beauty of the 3.31 diff is that you feel an improvement at all normal road speeds and throttle openings. Even if just gently moving away from a stop the acceleration is more brisk and the engine feels more responsive. If you really plant your foot the difference is even more impressive
Paraphrasing poorly from the XJS owners manual: after prolonged urban use or slow speeds, upon reaching highway speeds or utilizing lockdown, mild detonation and a puff of smoke maybe be observed. This is carbon deposits being burned away and is no cause for concern.
Would it be very difficult to Change the Final Drive over or would I need to try and find another Axle from somewhere
Never mind the Throttle Bodies!
I wan't one of those!
The 6 cylinder cars have lower ratio diffs (numerically higher numbers) and all XJS s have LSD diffs, so one from a 6 cylinder car is a straight swap. I do not know the exact ratios in the 6 s, but someone here will. You do need an LSD diff though, not an open diff.
Or just buy a 6.0 car. Has the 3.54 ratio for better acceleration and overdrive for motorway economy.
I have not owned an XJS with the 5.3/2.88 combination, but I have owned a V12 sedan with that combination. On my usual trip to the west coast of Canada the 5.3 would average around 12.2-12.4 L/100km and the 6.0 XJS is about 11.5 L/100 km. Makes a big difference to economy if the top is up or down though. The 6.0 is nicer to drive, as the power is there for overtaking in a way the 5.3 doesn't have.
I do not know the exact ratios in the 6 s, but someone here will. You do need an LSD diff though, not an open diff.
It changed through the years depending on market and model. For North America after 1982(ish) the ratio was 2.88. All USA cars were open diffs, in Canada the base XJ6 was open, and the Sovereign model was LSD. Early Series III cars were often 3.08. Series 2 cars were a mix of 3.31 and 3.08 and Series 1 could be 3.31 or 3.54. The earlier the car, the more likely it is to have 3.54. I can't speak to the UK market cars, except that again the post 82 cars were 2.88. Not sure about earlier cars.
When you look at the case, assuming it's original, it will either have a PL tag under one of the cover bolts to indicate Power Lok, and another tag showing wither the actual ratio (ie 3.31) or the tooth counts ie 43/13. Later diffs have a colour coded sticker to show ratio, and if there is a black X across the colours that indicated LSD.
ICS... Maaaaan, 50hrs + per week, a Lady and Brooklyn, 3 Jaguars and a Porsche, classes - have all conspired to keep me away from you and your garage. I know we'd talked about some stuff and I apologize. Jus this city can be consuming. I'll reach out soon,,, that is unless YO MAD at me, lol...?
Mad at you, lol, Not at all. Reach out when you can. The weather here has been better than ususal so next time I'm headed to Sunday Morning cars and coffee at Great Kills Park, I'll txt you.
Last edited by icsamerica; Dec 7, 2021 at 08:18 AM.
But ethanol does not affect EFI fuel line. (Which isn’t rubber)..
You really need to stop posting false information. It's border line dangerous at this point. The only fuel-injection line that is ethanol compatible are modern barrier style hoses. These were not used when the XJS was manufactured. I have seen what even modest amount of ethanol does to XJS fuel lines. The later XJS are lucky enough to have some rubber in the tank and this rubber quickly turns to black mush.
Below is what an 1995 XJS fuel tank looks like after a steady diet of ethanol. There was a time here in NY when more than 15% ethanol illicitly made its way into fuel. I worked at a dealership at the time and cars would routinely come in with fuel system problems and it was routine to test the fuel. Most of the time the fuel in the tank was way more than 15% ethanol. Notice the milky color of the fuel and completely deteriorated rubber line inside the tank.
You really need to stop posting false information. It's border line dangerous at this point. The only fuel-injection line that is ethanol compatible are modern barrier style hoses. These were not used when the XJS was manufactured. I have seen what even modest amount of ethanol does to XJS fuel lines. The later XJS are lucky enough to have some rubber in the tank and this rubber quickly turns to black mush.
Below is what an 1995 XJS fuel tank looks like after a steady diet of ethanol. There was a time here in NY when more than 15% ethanol illicitly made its way into fuel. I worked at a dealership at the time and cars would routinely come in with fuel system problems and it was routine to test the fuel. Most of the time the fuel in the tank was way more than 15% ethanol. Notice the milky color of the fuel and completely deteriorated rubber line inside the tank.
ethanol has been completely tested. Both by the US government and the agency which checks import cars. It’s actually a violation of import laws to use fuel lines that don’t meet those standards.
I can understand if the alcohol added was methanol which does attack fuel lines. And will do damage. It’s also cheaper than ethanol. ( hence the motivation to use) Selling the uncontaminated ethanol for Booze is a second line of profit.
But ethanol which is what is supposed to be used is benign. Ethanol is in Beer, wine, and booze. Considering how delicate the human throat is compared to fuel line.
Since ethanol doesn’t kill you like Methanol will, I believe what happened in that case was someone used methanol instead of ethanol and the trouble you displayed happened. Install the cheaper methanol ( which damages stuff) and sell the uncontaminated ethanol. Offers more than $5 a gallon potential reward. Times the hundreds of thousands of gallons in a tank! Big money!
Most of the time the fuel in the tank was way more than 15% ethanol.
It's been obvious to me for a long time that E10 experiences are all over the board...to such an extent that I've concluded that other factors must be coming into play. You've just pointed out one of them and it's something I've long suspected.
Typically the gas pumps says "Contain up to 10% ethanol". It's reasonable to assume that it may be 3-5-8% in some cases. And I've suspected that in some cases in can be a lot more, as you've just said: "Way more than 15%"
Together with other variables it is easier to understand some people have no problem with ethanol and others hate it !
It changed through the years depending on market and model. For North America after 1982(ish) the ratio was 2.88. All USA cars were open diffs, in Canada the base XJ6 was open, and the Sovereign model was LSD. Early Series III cars were often 3.08. Series 2 cars were a mix of 3.31 and 3.08 and Series 1 could be 3.31 or 3.54. The earlier the car, the more likely it is to have 3.54. I can't speak to the UK market cars, except that again the post 82 cars were 2.88. Not sure about earlier cars.
When you look at the case, assuming it's original, it will either have a PL tag under one of the cover bolts to indicate Power Lok, and another tag showing wither the actual ratio (ie 3.31) or the tooth counts ie 43/13. Later diffs have a colour coded sticker to show ratio, and if there is a black X across the colours that indicated LSD.
Here in the states the XJS w/ 6 cylinder engines had a 3.54 and positraction. Some sedans with 6 cylinders came with both the 3.54 & positraction as did some V12’s after 1992 However they switch from 4 shocks to 2. Check the tag or raise the car and spin a tire. If both tires turn in the same direction that’s positraction. Count the revelations of the drive shaft to figure ratio. 3 is either A 2.88 or a 3.08. 3&1/3 is a 3.31 and 3&1/2 is a 3.54
Mad at you, lol, Not at all. Reach out when you can. The weather here has been better than ususal so next time I'm headed to Sunday Morning cars and coffee at Great Kills Park, I'll txt you.
Cool and,,,.
Yeah, the weather has been nice.... strangely nice
Please do!
Ok. So, just because all of what I have heard (but don't completely understand) a out ethanol, to the best of my ability I have avoided it.
I have a 30+ yo car. I have replaced some, not all, of the soft fuel lines in the car. Are folks saying that for rubber old school non barricade hose, today's ethanol ladened gas will weaken and eventually melt the older lines? And, if so, what to do? Some of my lines in the trunk, the more factory-ish ones right off the tank say, what are THEY made of?
Also,,,, This past Friday I got on the highway at 10.30pm. For us here in NYC finding nice looooong stretches of road, ANYWHERE, is not easy. BUT, as soon as one enters into 278 and the BQE at that time of night chances for ya are GOOD to have some open road...
Anyways, when I got on I did what I never do. I FLOORED it... She stayed in second for longer than I was comfortable with, over 4 - 4.5k rpms... When she shifted, I swear the car JUMPED oft the road accelerating (I pooped a lil) and then, there was a whole NEW spirit. Little puff of smoke out of the back, both sides, and THEN it was time for MORE fun. A dude in an Infinity noticed this and pick it up. He shifted lanes and came up my left hand side... I leaned into it a little more with nearly the entire length of the Verrazano bridge ahead. I KNOW I surprised him! We stayed right together up thru 85 90 where ol cherry pie started WINNING, lol. I'll be damned. I let off. Satisfied, pleased and completely surprised...
Brothers!!! Y'all have never lied to me about this ol car!!!!
