@Doug

Interested to read your comments about the 3.31 ratio, AJ6 ECU and low restriction exhaust. These are all on my wish list.

My car accelerates effortlessly from 60 mph to speeds that will cause me to lose my license, but I find it sluggish between 0 and 30 mph, and uninspired between 30 and 60 mph. From what I understand, this is the result of conscious design choices that Jaguar made to elevate refinement and fuel economy over performance. I'm not looking to turn the car into a "hot rod" but think that it should have the spirited performance befitting its heritage and V12 powerplant. Prior to reading your post, my next project was going to be enlarging and re-routing the air intakes as suggested in Kirby's book. But I can put that on the back burner if there are better options to tackle first.

Assuming time and budget constraints prevent one from doing all 3 upgrades at once, in what order would you recommend doing them? What is going to have the biggest impact on everyday driving enjoyment?

Thanks in advance for your help.

 

Opening up the air intakes might help more with high RPM power but I doubt you'll feel any difference at low ROM. Sorta the same for the exhaust.

The ECU from AJ6 works as advertised. I had one on my XJS V12 years ago in addition to having one on my present day V12. Good product, IMO. But even it is more of a mid-range enhancement.

On my XJS I ran a 2500 stall torque converter and was please with the improvement in off-the-line acceleration.

Going from 2.88 to 3.31 gears made a large difference in the acceleration and responsiveness of the car.

Cheers
DD

 

After reading the book about the TWR race cars in the 80's and seeing their dyno tests when they were investigating the 6 litre, basically any engine modifications such as exhaust, air intakes etc will have no effect below 4000 rpm. Some, like larger intake manifolds actually caused a power drop below 4000. Over 6000 rpm is a different story.

Their conclusion was the only thing that helped engine power under 4000 rpm was increased displacement. They actually said that smaller intake manifold runners would have helped. If going to a 6.0 isn't an option, then the next best bet to get mid range performance increases is gearing. And possibly a 4 speed transmission conversion.

For piston engines, the point of minimum fuel consumption for a given load typically occurs at a mean piston speed of 7 m/s. This is true from lawnmowers to giant marine diesels in supertankers. Guess what piston speed 70 mph in an XJS with a 2.88 axle is? Right on 7 m/s! Jaguar made no secret that they were designing the HE for maximum fuel economy at motorway cruising speeds. I think if you can get a 3.54 axle combined with a 4 speed automatic overdrive you'll have the best of both worlds of performance and cruise fuel economy. My 6.0 with a 4 speed with typically get 3-4 mpg better than a 5.3 with a 3 speed, I think partially due to the 6.0's greater torque.

 

Jagboi says is true ,, more torque means less throttle for same MPH= better MPG!

 

After reading through several different threads on this topic, I think I have more work to do to tune up the car before I make any mechanical modifications to the driveline. I haven’t formally tested the 0-60 mph time yet, but my guess would be in excess of 9 seconds which seems a lot slower than what others get with a stock powertrain.

So far, I have done the following on this new-to-me car:
* Replaced spark plugs
* Replaced HT leads
* Refurbished injectors
* Replaced injector harness and moved out of the valley
* Replaced fuel lines
* Changed oil & filter
* Replaced trans fluid (but not filter)
* Cleaned PCV

I have not yet serviced the distributor, replaced the vacuum hoses, changed the air or fuel filters. Could one (or more) of these cause the sluggish performance at lower speeds? Is there something else I need to service?

I saw comments regarding an “HE Tune Up” write up by @Grant Francis but I have been unable to find the actual document. Does anyone have a copy they can post?

 

That Doco in now a STICKY at the top of the XJS Tech Section, as requested by yourself this morning our time.

My MAJOR findings with waaaaay too many V12's over as many years is the state of general tune of the whole bus. So many are "near enough", and most people have NEVER driven a V12 that is actually "on song", and therefore have little to compare it to.

Reliance on others to maintain a V12 these days is just plain scary. No dealer wants one anywhere near the place, and the Independents are picky, and unless the workforce is 50+ years of age, have very little real time with one, so a lot of "should be fine" is accepted by owners.

Owners that dont drive them as designed, and by that I mean the Redline is waaaaaay over there, so why are we changing at 1500 or so, this is an engine built to run, but owners are scared of them, no idea why, one of mine is now topping 650000K kms, and untouched, and spouse knows exactly where the Redline is.
OK., in fairness, a badly tuned, or cared for V12 is a scary sucker, and that is a fact, so spend the TIME, do all the forgotten things, set it up 100% (99% is crap), and enjoy the hell out of it.

USA, you have a brick wall with all the emissions junk you were forced to live with, and then you got the 11.5:1 comp ratio engine, where the rest of us got the 12.5:1 Comp engine, and basically zero emissions junk.

Mine had:

Large throttle discs, mine, not purchased.
16CU ECU
NO balance pipe or AAV.
Cold air intakes, picking up from down at the front spoiler.
Foam air filter, mine again.
Electric fans
Cooling system 100% spot on. Engine temp screws performance.
Trans cooler out the front, NOT in the radiator, and the trans 100% serviced, none of this near enough is good enough.
Compact A/C compressor, less drag than the Black Beauty
Tyres inflated properly.
Wheel bearings adjusted correctly.
Engine timing was set by "drive timing", and I have NO idea of what the deg BTDC is, as the timing plate is no longer there.
10W40 engine oil, Synthetic ATF (trans and steer).
2.88 Salisbury diff, I removed the Dana very early in ownership.

The list goes on, but it was quick, and the daily driver, and whatever else one uses a car for.

Once in the NT, no speed limits in those days, I held the loud pedal to the floor, and I chickened out at 5800rpm, but the damn thing was still pulling like a train.

Off the mark, who cares, its a 2 ton car, lets get real here, but once she gets into stride, look the hell out.

Whiskey time, see ya.

 

Many thanks Grant for your help and comments.

 

Yes, any and all of them could cause a loss of power.

Go to work!

Cheers
DD

 

As for the the 0-60 sprint, when is the transmission making the 1-2 shift at wide open throttle? I've driven some where it shifts wayyyy too soon, like 4500 rpm.....which is just when the engine is beginning to wake up.

Cheers
DD

 

Doug, I’ll have to check in a few weeks when I get back to Ohio. In the interim, I’m stuck in the UK with nothing to do but peruse various Jaguar parts websites! Hence all the questions as I am trying to figure out what parts I might need to buy while I am on this side of the pond. Cheers.

 

Re-gearing should include the transmission. The main design flaw regarding performance for the XJS is the transmission. The parasitic nature of the TH400 and the 4L80E and the close ratio spacing make for a numb engine that is being held back. It should not have been this way. GM had the TH350C Lockup 3 speed in 1979 and the the 700R4 available in 1982 but I dont know if they would sell them to other OEM's at that time. The 3.08 gears with a TH350C and it's looser but locking converter would have been a great setup that would have been viable until the mid 80's. The 3:31 gears with a TH700R4 would have been the next logical step. Both transmissions have better gearing for the broad torque curve of the V12 and would have offered FAR better fuel economy. Why a truck transmission was put in a performance GT is beyond rational and difficult to understand.

Case in point, I have two 6.0 XJS's one with a auto and one with a manual. On back to back drives there is a night and day difference. I've had three XJS owners comment, that they cant believe it's the same engine. Mind boggling change in engine character and waste heat. I had one chap insist I open the hood so he could have a look. Conversely I have 2 LS based cars, one auto, one manual. They're different but you'd never think the engine was changed.

 

I agree


Jaguar had a habit of sticking with something long past its expiration date.

Jaguar went from the old BW12 to the TH400 in 1978. At the time the BW was an antique by most any measure but the TH400 was still a viable choice. Not cutting edge but, still, an improvement. Besides being smoother and more refined it was (as I understand the story) is was one of few off-the-shelf choices that was built precisely enough to tolerate the 6500 RPM that the V12 was easily capable of.

I think the more salient question is why Jaguar kept using the old 400 until, what was it, 1992-93? Even then, the move to the 4L80E wasn't the wisest choice.

Was it dogged reluctance to change? Lack of financial resources for something more modern? Both?

Cheers
DD

 

Could a 700R4 or a 350C have been able to cope with the European version V12's 300hp? My understanding was the early 700 R4's were fairly fragile.

In the early 80's they were designing and testing the new AJ6 engine, and by 1983-84 the XJS sales were starting to pick up, oil prices were dropping and Jaguar was struggling to design the XJ40 and get it ready for sale in late 1986, so I suspect they simply didn't have the engineering resources to put into designing and verification of a new transmission option for the XJS. 3 speeds were typical in the market at the time, and the TH400 was reliable and tough.

They were very concerned with reliability and warranty costs then, and while it may be parasitic, the TH400 was tough. The XJS already had more power than a Ferrari 308 when the HE came out, so why worry about a few more HP, when the potential downsides were very large and costly for a company that couldn't afford it?

 

Many years ago I was 'this close' to using a 700 in my XJS. However, I couldn't find even one transmission expert who felt confident that a 700 would tolerate 6500 RPM without upgrades. Quite doable, but adding quite a bit to the overall cost of the conversion.

Since so many V12 Jags are driven quite gently this isn't a concern for most people. But I don't hesitate to pour on the coals

Cheers
DD

 

I think it was lack of resources. The XJ40 project stretched them well beyond what they should have been able to accomplish. They designed and launched a new car from basically a clean sheet of paper with around 350 Engineers. Toyota used 6500 to design the first Lexus LS400.

They did use the 400 is the Series III V12 until 1992, but that car and body was supposed to have died with the XJ6 in 1986. They didn't foresee the low fuel prices and surge in demand for the V12, so kept the old body in production for nearly another 6 years. It was never supposed to last that long, so why put money into a product that was supposed to be discontinued?

The early XJ40's did have a lot of problems, and their Engineering department was struggling to cope with that as it was the mainstream product. My feel is that as long as anything else worked, it wasn't being touched in a major way. An example was the Daimler DS420 limo received a facelift in 1988. It was mainly designed by the guys who built the cars, and In the official parts list, the old part numbers are crossed out and the new ones handwritten in the margin. There wasn't sufficient drafting resources to document the changes in a "pretty" way and the handwritten notes give sufficient information to order the right parts. It was a "mend and make do" sort of time for Jaguar.

Realistically, what were the other options for a modern 4 speed automatic in 1990 that could take the V12's power, preferably with electronic controls? There was the Ford AODE, but it couldn't take the power, the 4L60E didn't come out until 1992. The ZF4hp22/24 wasn't strong enough either. I think when they made the decision the 4L80e was about the only choice.

 

Fair points, And the XJ40 did use the ZF 4HP22 so Jag knew.

After some thought I'm sure Jag chose the TH400 and 4L80e for their shift quality too. Both dont use a band for normal (D) shifting so the shifts are very smooth and seemless. The 700R4 does use a band for the 1-2 and it can be abrupt in performance applications which was okay for the Camaro and Corvette and would have been Ok with me too If I would have bought and XJS at the time. Jag clearly placed all their chips on Luxury and refinement and gave up on performance. Hard for me to understand that too because in 1984 Popular Mechanic tested the XJS against other GT's of the day including the Porsche 928 and BMW 633 and the Jag fared very well considering the Corvette was an all new design on custom Good year gator back tires and the 928 was Germany's finest and had 2 extra gears and weighed 800Lbs less.

If anyone wants to read about the test,...here the link. I've mentioned it many time since I'm both pre-occupied with Limerock and the cars from this era.
https://rennlist.com/forums/928-foru...-limerock.html

I was shopping for a Jag in the mid 80's and cross shopped it with a BMW 7 series, The BMW did have a Electronic ZF 4 speed transmission with lockup and it was far more responsive. I bought the BMW, and that car needed and transmission too. 3 times while I owned it which didn't seem to harm BMW's future sales.

 

I like the stance on this car quite a bit. I've always thought the XJS benefited from a bit of a rear "lift", as it sets off the design lines in the front end of the car, particularly the headlight buttresses. It also get the tail up just enough that the exhaust pipes don't seem to end inside the rear bumper somewhere.

I wouldn't worry about giving it a "muscle-car look" unless you just got crazy with the amount of lift. This look isn't "muscle car," it's simply aggressive.

Jess

 

I'm going to find out firsthand, soon. I just switched from the GM400 to the 700R4 and the car should be ready to test within the next week or two.

I wanted to speak to cost of conversion: I bought a 700R4 for $50. The vehicles that ran these transmissions are now beginning to be parted out frequently. Mine came from a large Chevrolet SUV. Because of that, I also got a beefier case. Then I had it rebuilt with beefed-up internals by a transmission shop that does quite a bit of work on racing transmissions. I'll try to get a true build sheet at some point so someone can copy my build if they want. Point being, even though it added to the cost of my conversion to have the transmission upgraded, it's still a relatively cheap conversion.

Jess

 

What did you use for an adapter plate?

 

i used an adaptor from SUMMIT racing called BOP(Buick,oldsmobile,pontiac)!
at the time cost was aro

und $40. dollars!
had to drill some countersink holes to bolt to jag block(most important ailign ment dowels were perfect spot on center the crank to convertor)! 1978 up jag blocks!
added a trans performance kit, easy fix! and i use a light weight 10" convertor, for quick throttle response)