Not trying to go crazy here but it just bothers me that the Euro model has more power, and not to mention the realized potential of the 4.2 liter in other Jags. I'm sure the catalytic has something to do with it but besides that I read somewhere about "removing the throttle plates (and plugging shaft holes) inside the water heater over the intake ports". I'm a Jag newbie and new owner of an '85 XJ6 and not all that familiar with the car yet. Can someone explain this further?

 

Doug is your man for this, and as it's Christmas Eve, you may not hear from him for a couple of days or even more. I think US cars had a lower compression ratio too, as the fuel in the US can be of a lower octane rating. Then there is all the plumbing for Californian smog regulations that takes away power, but I don't know what is involved in that, as my car was a UK car so had untrammelled power ! Certainly the torque of the engine did give me that "Master of the Universe" feeling for the first time.

 

A little history. About 1975 the US based EPA set forth a schedule to drastically reduce tail pipe emmissioms. See this... https://www.epa.gov/air-pollution-tr...d-climate#1970

In order to meet these new emmissions and CAFE standards using existing technology, Jaguar made a few key changes to their vehicles.

The first and most significant was to increase differential ratios to reduce rpm and fuel consumption on the EPA test. Jag went from 3.31 to 2.88 diff ratios and the torque converter tightened up. This made the car a slug. Other manufactures like Mercedes GM and others responded with 4 speed automatics to reduce cruising RPM. GM was able to maintain performance and effiency on their existing 3 speed autos in 1979 with the novel idea of a locking torque converter on th350. This was something Mercedes didn't do until the late 80s. Jag didn't use a 4 speed lockup until 1988

The second and most significant change was the addition of the catalytic converters and reduction in compression ratios in about 1975. Two requirements European cars didn't have. These required leaner mixtures and other emmisions regulations required reduced ignition timing to cut back on NOx. These changes were not as signifigant but they really shaved off the performance edge of most engines.

The 3rd change was the adoption of EFI and electronic carburators to allow for closed loop emissions and on vehicle real time control of timing and fuel delivery. The early systems were not up to the job and didn't have enough processing power to deliver emmissions control and performance. It wasn't until 1985 when Bosche rolled out the hot wire air flow sensors that performance returned. This system was used on Porsches and the Corvette. Lesser cars still soldered on with a physical air flow meters that lacked resolution until the early 90s and they were a major impediment to performance in most cases.

So if you want to wake up that 85... Install a 3.31 diff, 700r4 transmission, modern high flow catalytic converter and modern MAP based fuel injection and ignition control.

 

Thank you icsamerica. So I suppose my '85 has the hot wire airflow sensor anyway. But what about that "Removing the throttle plates (and plugging shaft holes) inside the water heater over the intake ports"? Which sounds kind of odd but is pretty much a direct quote from wherever I found it.

 

The carbs were replaced with FI in 1978 MY Sedans for the U.S. The secondary throttle butterflies were eliminated when that happened.

You are mixing fuel delivery systems. Your car has Bosch 'L'Jetronic FI.

bob

 

Here is a link to an article by Roger Bywater of AJ6 Engineering that best explains the fuel injection system used on the 4.2 litre XK big valve engines:

Fuel injection and the Jaguar XJ6 4.2 Series 3 / AJ6 Engineering

 

I have no idea what throttle pates and shaft holes are on an 85.

BTW...Your 85 Jag has L-jetroinic...it has the rather restructve and driviabiliry dulling vane sensor to measure air flow and this system was mass produced to solve 1 problem...emmissions control.

I have owned a late XJ6 and and an early carburatored version. The early carb'ed cars when properly tuned are a joy to drive. The 3:31 diff and slightly richer carb mixture make for a good driving experience.

The first car I know of with LH-Jetronic (hot wire MAF) was the Euro only 1984 Porsche 928s. Followed by the 1985, Porsche 928 and 1985 Chevrolet Corvette. The rapid hot wire measureing device and a more powerful processor allowed for emmissions control and performance. Jag didnt use this system in 85.

I was planning and I did start collecting parts for augmenting an early XJ with a retofit of a modern GM SD 1227730 ECU, and a 700R4. Simulation software sugested it would have made 240HP, 280 with some head work. Modern ignition should have allowed it to rev clean to 5500 RPM but the vehicle ended haveing a 1 dead cylinder so I abandoned it for a AJ16 conversion.

 

Ok, sounds like that was a carb tweaking comment. Actually this is the first FI "project" vehicle I have owned. I've done a lot of toying with carbs. This is also the most modern "project" vehicle I have owned and I think about the limit of modernity and complexity for my tastes and abilities.
Question for icsamerica: the retrofit of the more modern ECU that you planned and abandoned, would it have use the same airflow meter? Can you reference me a write up of that upgrade?
I think if I was to swap in a different motor I'd also look at another Jag 6 cylinder with better performance, 4 valve per cylinder etc.

 

For a more modern ECU would be "easyiest" to fit a GM 1227730. It's late 80's speed density ECM. It's relately a powerful and customizeable density ECM with lots of memory. I've seen them used on everything from 4 cyl tractors to 10 second race cars. It uses an engine MAP instead of an air flow meter. It supports 4cyl, 6 cyl and 8 cyl applications and can be setup for distributor or distrubitor-less applictions and even a turbo if you want to get fancy. It also can control lockup on the 700R4 which is a common 4 speed upgrade for the Xk. This is what I used on the red coupe with DIS and it even has launch control which I dont use but probably should

I dont have a write up becasue there is no direct path and I have not actually done it yet but it's certainly possible. It's a good path for someone that wants classic XK looks with modern performance and driveability. Want it done? send it in, but there is a another idea.

The best upgrade is a Aj16 / Zf 4hp24 from a late XJS. For now there are enough donors as the best XJS is also the one of least desired Jag for some reason. A Cosmetically clapped out XJS that runs and drive great can be found for a great price. To do it well and at reasonable cost you need the whole XJS. In the end the AJ / ZF upgrade would cost about the same as an ECM / 700R4 upgrade and would have late 90's drivability instead of late 80's driveavbility.

Check this out. It's running now.
https://www.jaguarforums.com/forum/x...2017-a-184891/

 

FWIW I'm now understanding more clearly why some (many?) Americans view the XK 4.2 as an under-powered unit. I've always struggled to relate since my experience with this engine is totally different i.e. strong, powerful, plenty of torque and acceleration, even exhilarating when in good fettle. These engines in our country are UK/Euro spec, therefore unencumbered by the apparent de-tuning and power-killing plumbing added to US spec. models. I sympathize that in the US you don't get to enjoy the potential of these engines and I can even see why lumping is a popular option.

 

Just a point on lock up transmissions, my ‘65 and 66 ‘S’ typs had/have lock up on top in the DG box. Different car I know but just not wanting to malign Jag for being slow on the take up

 

Yes, Tony!
I was not aware until I read this thread that Mercan Jags were different from ROW Jags.

Those engines are/were So unpopular in USA (and especially California) that the very first question a stranger to Nix (who knows anything at all about Jags of her age) asks me, "Does it still have the Jag engine or has it been converted to Chevy?" When they hear/see the 350 they smile and nod approvingly.
(';')

 

To be fair to the car, it is really only by todays standards that it seems to lack in acceleration. I think in 1985 the XJ6 compared quite favorably with its competition.

 

start by replacing the Air Filter with a K & N Air filter. You will get a tiny bit more power out of the engine. Nothing to celebrate but the engine will breathe better and you will never have to replace the air filter again.

Otherwise the 4.2 liter engine develops as much power as it will in stock form.

 

Wow facinating...i just read up on the DG or "Detroit Gear" Borg Warner unit and it did have lockup. But it was only a 2 speed trans with 3rd being locked up direct drive.
I read on this page that American Manufacrues stoped using this trans in 1956 and with that "lock up" wouldnt return for 20 years.

https://ateupwithmotor.com/terms-tec...up-converters/

 

It was a true 3 speed transmission as the first two gears are lower ratioe’d than top (1:1)
Another point is that the ‘S’ could be tow started as it has a rear pump on the trans, this meant that the car had to be towed with either the rear wheels off the deck on a dolly with the steering wheel locked in place with ropes through the front windows or with the drive shaft disconnected.

 

If you want a better pick up then advancing the inlet cam by 4deg helps with low down torque.

 

Good point.

The ROW version of the Series III 4.2 was rated at 205 HP versus about 170 for the USA version. That’s significant. Objectively that means a couple seconds faster to 60 mph: 11-12 seconds versus 9-10 seconds.

But nowadays even 9-10 seconds is tepid at best.

Cheers
DD

 

Yup! And not difficult to do.

Plus the cylinder head can be milled almost a hundred thou....which would probably bring the compression ratio up to ROW spec.

Cheers
DD

 

Thanks for all the great info! First thing for me will be getting rid of that catalytic converter and maybe a silencer or two. I'm looking into a header from The Driven Man - hopefully with a bung for the O2 sensor. I won't mind if I can hear the motor a bit! Milling the head is a possiblilty as well but aren't there concerns as to valves and timing even? The US/Euro compression difference is slight. 8.1/1 vs 8.7/1
ROW = Rest of World?