What happens in your XJ350 is differant than what happens with
the HE engine… Your XJ 350’s computer was designed to provide
correct fueling at all times. It does it not by starting out ultra
lean like the HE is but putting the correct amount of fuel for the
That’s the flaw in the HE engine… they are trying to make an
engine run extremely lean in a vain attempt to improve mileage…
But is that true ?
The ideal A/F ratio for maximum economy is around 1.05 lambda, that is
5% excess air.
In cruise the ECU fitted to 5.3L HE engines looks at the response of
narrow band lambda sensors, which only indicate the transition around
Hence the ECU is continually adjusting the A/F a few percent above and
below 1.0 lambda to keep an average of 1.0.
In fact with a bit more software it could err on the side of lean and
average it out 2 or 3 % above 1.0 but that might not make a really
significant difference to economy.
Any ECU made in the last 30 years or so could also do that.
The pre HE system did not have the lambda sensor feedback.
There is no flaw in the HE, Jaguar went for a high CR to improve
economy since in theory the higher the CR the more power you extract
from a given amount of heat energy.
However there are a number of drawbacks to high CR. It is not my area
of expertise, but it seems that with high CR the spread of the flame
front after a spark ignition event is more complicated.
It becomes even more complicated if the mixture is not on the rich
side of 1.0, and this makes pre ignition more likely.
The May head design enabled high CRs, 11.5 and 12.5, to be tolerated
in cruise mode with a mixture around 1.0.
You still need fuel with a decent octane to make it work.
This improved economy, especially with a 2.88 diff, and did not put a
big dent in maximum power or acceleration. The result was a much
better proposition for selling V12 cars in an era when high fuel
consumption is frowned upon or legislated out of existence.
The HE does not run extremely lean, but only around stoichiometric ( 1.0. ) in cruise.
Probably no different to the majority of cars with EFI systems made in the last 30 years.
I seriously doubt the HE fuelling is anywhere near lean when the
driver gets out of cruise mode.
Since then engine management technology has moved on a lot, driven to
a large extent by electronic hardware and software.
Consider: My XJ350 has a much better way of pumping fuel for the engine
demand. There is no return line, the ECU calculates exactly how much
fuel is needed from various driver input and feedback signals, and
delivers it to the fuel rails. That is software intensive.
This engine runs a CR of 11.0 and will tolerate fuel with lower
octane because it has sensors and management to detect pre ignition
and back off to prevent damage.
With only 3.5L, an alloy body and a 6 speed box it is quite lively and economical.
It does not stand out from the crowd like an XJ-S of course, or even like the XJ6 & XJ12.
In principle the XJ350 has the same technique as the HE for fuelling.
The HE does not start out by running ultra lean, it starts with a fuel map
same as the XJ350 but perhaps more on the rich side for safety. In
cruise it will let lambda feedback override the fuel map to achieve 1.0.
The XJ350 can run closer to maximum economy over a much wider range of
engine operating points since it has more sensors and much more software.
It also has to meet stringent emission limits which can mean operating
away from maximum economy over parts of the operating range - even more software.
Richard Dowling, Melbourne, Australia.
1979 XJ-S coupe + HE V12 + 5 speed, 1988 XJ-S V12 convertible, 2003 XJ350
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