[alfa] RE: Engine swaps and Jaguar engines

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi All,

Slightly off-topic but since there has been a fair bit of discussion already I thought I
might add my 2c worth.

Amongst my collection of books is one I purchased many years ago written by Walter Hassan,
the Jaguar engine designer.  It covers the development of the V12 engine originally
intended for the Jaguar Le Mans sports prototype series.  Subsequently the management of
Jaguar decided to pull out of the Le Mans race and the engine was then redeveloped into
the road car version.

Hassan also designed the XK series with DOHC which saw duty in most of the Jaguar road
vehicles and was highly successful in the Le Mans race winning it in 1951, 1953, 1955,
1956 and 1957.  The XK engine was designed prior to 1947 when the British taxation
legislation made engine designers adopt small bore-long stroke designs.  In fact the
design dates back to before WWII having first been introduced in the late 1930's.  Hassan
acknowledges the pioneering work done by W.O Bentley with his 3 litres long stroke engine
and this formed the starting point for the XK design.  The XK engine was never intended
for racing, its long stroke small bore limited the maximum engine speed to about 6500 rpm
because of the third order vibration periods in its crankshaft.  However, it subsequently
became a useful engine when tuned with carefully modified components by Jaguar.

The reason Hassan chose the V12 configuration was to eliminate the torsional vibration
problem inherent with V8 engine designs and as the secondary goal was to sell large
quantities into the North American market, which was Jaguars major money maker.  It was
decided that the american purchaser desired a smoother engine than that possible with a V8
configuration with equal power and low down torque to that of a V8 to suit automatic
transmissions favoured by american purchasers.

Some of the information given is interesting, especially power outputs quoted for various
stages of engine tune.  The design goal of the V12 engine was "to equal or exceed in
production form, the XK 3.5 litre 6 cylinder engine output of 330bhp in racing form".
and "to fit into existing space of the XK engine without structural alterations to the
body".  Because the original design centered on the Le Mans engine the swept volume
was fixed at 5 litres, being the maximum allowed under the prototype sports car rules.
Later when the decision to abandon the Le Mans challenge came about the engine was
increased to 5.3 litres for road use.

The Mercedes M196 8 cylinder in-line engine fitted to their Le Mans cars (the W196)
developed at best 300bhp at 7450 rpm for the 1955 race.  The M196 had desmondric valve
operation and featured ball and roller bearings throughout the engine.  This was a
supercharged engine running on alcohol!

The racing version of the Jaguar XK was around 330bhp in Le Mans trim and could safely
be pushed to over 400bhp for shorter races such as the Sebring 8 hour.  The new DOHC
per bank V12 design in Le Mans tune was 502bhp and tests on the dyno saw as much as
800bhp for short durations at 7600 rpm.  In production tune for the Jaguar cars sold prior
to emissions controls the power output was 360bhp at 5500rpm.  Later emissions compliant
versions saw the output fall to around 260bhp when the CR was lowered to 7.6:1 from the
original 10.6:1 of pre-emission designs.  Later designs using fuel injection instead of
carburetters saw the power output rise to around 300bhp and discarding of the DOHC
heads in favour of the SOHC heads because of space constraints in the engine compartment.
The DOHC version was about 9" wider and caused a problem with turning circle in the
saloon car body because of the exhaust manifolds getting in the way of steering
components.

Now if you consider the Ford 302 Windsor engine of 5 litres capacity the best power output
was around 250bhp prior to emissions and about 110bhp at the peak of the emissions period,
later increasing to about 200bhp when fuel injection was fitted.  The Jaguar XK engine in
road tune with a capacity of 3.5 litres was about 260bhp prior to emissions and weighed a
lot less than the Ford engine, about 80lbs lighter.

John
Durban
South Africa
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