Hi all,
This question is purely out of interest as I don't own an XJ6.
A recent post made the comment on how thirsty an XJ6 can be.
I was just wondering from a technical point of view why this would be so in comparison to other large six cylinder engines pushing around reasonably heavy cars.
I understand the engines ancient origins and obviously one wouldn't expect to match a very modern design, nevertheless they are high performance which must involve considerable efficiency, have fuel injection and the car itself is reasonably aerodynamic, even by today's standards, I presume.
Is it cam design or perhaps head design that allows fuel to end up in the exhaust.
regards
Al

 

I don't have the knowledge to explain it but certainly head and camshaft specs have a big influence.....'engine breathing'. And, fuel management.

Three speed automatics (versus modern 4-5-6 speed) certainly play a role as well

I had a Series III Xj6 as as daily driver for many years. Now I use a Series III V12 as a daily driver. The fuel economy is virtually the same with either car.....although the V12, in USA trim, enjoys an additional 100 horsepower. This suggests the the much ballyhooed HE "High Efficiency" cylinder head design was something more than mere marketing flap

Cheers
DD

 

I don't think there is any technical reason why the XJ6 is considered "thirsty" In relation to other cars of their time they were very heavy. Take for example a Ford Cortina 1600 of 1970's vintage. They weighed around 900kg (1980lbs) and the best fuel consumption was perhaps 32mpg. Now from the same era, my series 1 4.2 auto does 23mpg (I can do better if I really try) but it weighs 1680kg (3700lbs) so the ton/mpg is better than the Cortina. Later series XJ6 weighed 1860kg (4100lb) - we are talking very heavy cars here so it is not surprising they have high fuel consumption.

 

I agree.

Most, near all, 4.2 and V12 cars that have landed in my shed over the years were so neglected, and "fiddled" with, I was surprised they ran at all.

Once the "wand" was waved, and things returned to how the beast was made, reliability, economy (such as John has mentioned) returned, as did the pleasure.

My 4.2 cars, S2, were regular mid 20's MPG, and even better on longer jaunts.

The V12 were roughly the same, with the HE returning 25mpg regularly, and the PreHE around the 20mpg, if care was taken (that PreHE was a thirsty sucker), but why have a V12 to take care??

 

Doug's comment about the three speed transmission definitely comes into play. A 4-speed automatic gives you better acceleration and a bit of overdrive for better mileage.

I always wondered about the phrase "the XJ6 is a heavy car". I just figured it out - I think people are comparing to other comon cars of the era in Europe. Here in America they were definitely mid-sized. A few years ago I bought a beautiful 1972 Cadillac sedan de Ville. It weighed 4900 pounds, had a 7.7 litre engine. As for gas milage, on the open highway at constant speed I got 13 mpg if there was a tail wind. We drove it across America and back and had a blast. So I guess "heavy" and "poor gas milage" are relative.

Jeff

 

Beautiful Cadillac. Those were marvelous cars, along with the many other big American 'land yachts'. People often speak of them disparagingly because the suspension was so loosey-goosey. True enough, but they're a dream to drive. Soft, comfy, quiet. And room to s-t-r-e-t-c-h out! And back in those days the Cadillacs, Lincolns, etc really were a cut above ordinary cars in terms of quality of materials, workmanship, etc. These biggies deserve their place in automotive history.

Agreed on the 'very heavy' Jaguars. It's all relative. Here in the 'States a 4000 pound car is merely 'heavy-ish'

And when talking about miles per gallon remember that, while American cars are big, American gallons are small...compared to the Imperial measurement used in other parts of the world. That 13 mpg from the Caddy would be about 15.6 mpg in the UK or down under.

Cheers
DD

 

Here are the why’s. The Long stroke cast iron 4.2 six cylinder has a head designed to make maximum power rather than clean emissions. To clean the engine to meet smog rules. Less than optimum tuning was used. In addition newer engine designs control fuel much better than older carbs/ fuel injection. While ignition timing is much more accurate using ELECTRONICS THAN THE OLD DISTRIBUTOR. ( slop in the timing chain/gear drives for the distributor / and advance and retard weights/ springs all require additional fuel to safely operate).
The transmission behind the 4.2 engine is a Borg Warner which was originally designed in the late 1940’s as a 3 speed non overdrive. With a built in slip factor of 4% as compared to later transmissions which have 2% or total lock up. 🤒Newest transmission have many more gears and some may be overdrive. The slower the engine goes the the less fuel it uses.
Finally the final drive ratio is likely 3:31-1 whereas later final drives were 2:88-1 ( again the slower the engine turns the better fuel mileage is obtained).

 

According to my trip computer, I consistently achieve 22-23MPG at 120 KPH highway (75MPH). Measuring the tank usage seems to show these numbers to be relatively correct. I consider that very good for a unit of this age and weight.

 

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