Cylinder Head Evolution.
Debatable are the merits of the so-called "straight port head" with which
the XK ended its days. In the original cylinder head design the inlet ports
had quite noticeable curvature to impart swirl to the income charge so that
it would rotate around the cylinder axis (fig 4). Such swirl is considered
useful as an aid to efficient combustion and was very much the right thing
to do in the days before the greater benefits of horizontal swirl, a
feature of modern short-stroke 4 valve engines, was recognised - this
taking more of the mixture through the vicinity of the spark plug to
achieve better flame propagation. There was of course an uprated version of
the curved port head derived from the C Type racer but confusingly known as
the B type head.
Unconcerned as it must be with part-load operation a racing engine requires
maximum flow, this in itself providing ample charge turbulence (arguably
more effective than swirl) under full throttle conditions, and so straight
port heads were soon introduced for racing and then subsequently appeared
on the higher performance road cars. Gas velocities are higher with larger
engine sizes so as the XK was stretched to 3.8 and finally 4.2 litres so
the advantage swung more in favour of the straight port head and it made
some sense to rationalise on the one type. The XK cylinder head, in its
various forms, always possessed above average gas flow properties but the
valve included angle was much wider than would now be thought desirable and
the resultant deeply hemispherical combustion chamber was, with hindsight,
less than ideal for good combustion. Although no comparison was ever made
it is quite probable that the original curved port head would have provided
better mid-range torque, better part-throttle economy, and lower exhaust
emissions. The larger bore of the 3.8 and 4.2 engines created a peripheral
squish band to push the outer layers of mixture back into the combustion
chamber but this was not of much consequence.
With regard to the deficiencies of part throttle combustion it is
interesting to consider how far back the catalyst was placed (almost behind
the transmission) in the carburetter emission cars, all of which had
straight port heads. This indicates that a fair amount of burning took
place downstream following air injection into the exhaust ports and the
catalyst was moved back to keep its temperature within bounds.
Of course, the ultimate racing head was the so called wide-angle 35/40
version (fig 5) used on the later D Types and the light weight E Types with
various sizes of valves and in single and (rare) twin plug versions, the
latter showing no measurable benefit. The numbers relate to the valve
angles away from vertical and in this case the exhaust valves were shifted
outwards by 5 degrees to permit fitment of larger (2" then 2 3/32") inlet
valves. At the same time the tappets were increased in diameter to allow
higher lift cams (7/16") to be used. In the continuing pursuit of power the
3 twin choke Weber carburetters of the early D types gave way to Lucas fuel
injection with carefully developed ram induction tracts and slide throttles
(fig 6). Technically this was a great leap forward so much so that the same
basic injection system survived in Formula 1 racing into the 1980s before
it was finally ousted by electronic engine management.
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