Camshafts, tappets etc.

Jaguar camshafts for the XK twin cam, V12 and AJ6/16 engines were made from cast iron and chill cast to produce very hard, wear resistant, surfaces on the cams. Tappets were made in a similar way, with a chill cast top surface, changing to cold formed steel extruded tappets for the XJ40 in 1986 to reduce reciprocating masses. Squeeze cast (liquid metal forged) steel tappets came later, mainly for improved refinement from further weight reduction.

These components have been shown to have outstanding longevity compared to those of many other manufacturers and have displayed minimal wear even over huge mileages, assuming the provision of an adequate supply of lubricant and freedom from abrasive debris.

Production cam profiles.

The basic cam profile used for the V12 continued throughout its life and can be traced back to the 16,56;56,16 timing used in the pushrod 2 1/2 and 3 1/2 litre cars dating from the 1930s. The XK cams were quoted as 15,57;57,15 (252 degrees duration) with 0.375” lift, being of simple geometric design, originally with clearances of 0.004” inlet, 0.006” exhaust, then evolving to a more modern cam profile based on parabolic acceleration curves in the late 1960s with constant velocity ramps allowing the clearances to be opened up to 0.012 – 0.014”. The same basic profile carried over to the V12 initially, quoted as 17,59;59,17, but tappet noise became a concern even before reaching production. The constant velocity ramps ensure that the rate at which clearance is taken up is the same regardless of the setting so other factors must be involved in noise generation.

Essentially the side clearance between the tappet and bore was found to be important and if excessive, could allow tappet movement similar to piston slap to occur. The combination of clearance above and around the tappet seems to be critical. Because the tappets of the V12 run in aluminium carrier blocks rather than the cast iron guide sleeves that had been used on the XK engine there was a differential expansion condition which had not applied to the earlier engine and consequently the side clearance would increase as the engine warmed up.

On the other hand too little side clearance could cause seizure in extreme sub-zero conditions so this consideration determined the minimum clearance possible within the component tolerances. Tolerance build up in the other direction could produce enough side clearance to allow the tappet to shift around under the wiping action of the cam. When combined with excessive clearance against the cam, tappet movement could create noise but the behaviour of lubricant entrapped in the gap, influenced also by tappet rotation, made the condition very unpredictable. Other discrepancies such as minute deviation of the tappet axis from the valve axis added further variables.

Manufacturing tolerances of the V12 were probably as tight as in any production engine of the period so there is no question of these engines being made to poor standards. The variability mentioned above involved only very small dimensional differences.

It was known that exhaust valve clearance remained more or less constant with engine temperature due to compensating expansion but inlet valve clearance opened up by about 0.005” as the engine reached the fully warm state. This was another contributor to the noise problem.

It was observed that a tappet that does not rotate is likely to be excessively noisy but the lack of rotation usually indicates the presence of some other problem such as deformation or scuffing around the side. Tappet top surfaces are machined dead flat (two can be wrung together like gauge blocks) and the cams are ground with no taper because Jaguar engineers reasoned correctly that it would be impossible for there not to be some sort of offset loading that would cause rotation however precisely the parts were made. Slightly tapered cams were tried at one stage but they did not offer any benefits regarding either tappet rotation or noise level.

Investigation included taking sound level readings at various points around and above the tappet block and experiments took place with various modifications to tappets and materials, including copper plating, without much effect. Baffles and sound damping materials were also tried but without practical success.

Perversely, at a quite early stage of production it was found that a very significant reduction of HC emissions could be achieved by opening up the valve clearances to 0.014 – 0.016” mainly due to shortening the overlap period. At one stage engines for markets where emissions were more critical were manufactured to this standard, with normal tighter clearances used for other markets. The possibility of a higher level of tappet noise was accepted as unavoidable. Modifications were made to the cam form to achieve the beneficial overlap condition whilst reverting to normal clearances.

Cam part numbers C42176 and C42177 were introduced sometime around 1973 and remained standard fitment in all subsequent 5.3 engines over the next 20 years. The dynamic features of this cam are shown and to be able to define the acceleration and jerk curves in that way requires very accurate measurement, typically to within 0.000001” (0.024 microns). Jerk is an indicator of either a high compressive loading or a tendency for component separation.

These cams have been quoted as 15,57;57,15 and 13,55;55,13 at different times but it all depends on the lift point from which timing is measured. However measured, it was never far removed from the 16,56;56,16 of those pre-war pushrod engines.

Tappet noise was sometimes marginal throughout the life of the 5.3 and the writer was involved in further investigations during the late 1970s. Slight ‘barrelling’ of the tappet or a relief around the mid-section both seemed to improve the noise situation but added undesirable manufacturing complication and as reliability was never in question there was no great incentive to change anything. Tappet noise always seemed to be more of a problem for the critical ears of engine testers than for customers.

A further refinement of the cam form, mainly in the low lift area, took place for the 6 litre introduced in 1993 and tappets also changed to the lighter squeeze cast type. Finally in early 1995 the cam base circle diameter was increased by 0.002” to allow a reduction of specified tappet clearance by the same amount to 0.010” – 0.012” which fits with earlier findings about the connection with possible noise issues.

It is interesting that the same 15,57;57,15 (252 degrees duration) cam profile also carried over to the early AJ6 3.6 engine which, with much more valve area, was quite peaky and delivered poor torque below 3000 revs. Refinement of these early engines was also criticised. In subsequent years the AJ6 engine moved towards milder cams with 16 degrees less duration (7,49;49,7) and 0.36” lift for the 4.0/3.2 engines and the resulting torque figures were improved beyond proportion.

For further noise refinement in 1993 the cam duration was extended by 6 degrees (11,51;51,11) and lift reduced to 0.35” which carried over at first to the AJ16 engine. Complaints of poor idle quality led to an 8 degrees reduction of overlap by changing to 7,55;55,7 in mid-1995.

Different cams.

There are two particularly significant features about the timing of valve events provided by any camshaft arrangement. The first is the inlet valve closing point because that strongly influences the engine speed where volumetric efficiency will be most effective. The compression process cannot commence until the inlet valve closes so with a late closing cam at low speeds not only will reverse flow take place but combustion efficiency will also be poor. Only when the speed is high enough for the dynamics of the induction system to overcome these obstacles will the engine begin to perform effectively. On the other hand an early closing cam will produce good efficiency at low speeds but will become restrictive at higher speeds.

It was in the 1950s that cam timing reached extremes with engines like the Manx Norton. With the inlet valve only closing 98 degrees after BDC this left just 82 degrees for compression of the charge to take place. Similarly the power stroke only lasted 95 degrees so the entire power generating part of the engine cycle lasted less than half a revolution of the crankshaft.

Of course, one reason such long valve opening periods were necessary was the need to keep inertia loads in the valve mechanism of a high speed racing engine at a tolerable level with the materials and simple cam profiles of the time.

That such an engine could achieve a b.m.e.p. figure of 200 p.s.i. demonstrates the great importance of induction and exhaust dynamics. It is also clear why the engine became totally gutless outside its narrow power band.

The second feature is the overlap period where both inlet and exhaust valves are open. A high speed racing engine can exploit plenty of overlap to improve the entire breathing process but at lower speeds unwanted flow reversals will take place that will degrade both breathing and combustion efficiencies. Good idling qualities, light load drivability and low emissions are all best served by having minimal overlap although in moderate load more overlap can be useful to reduce NOx emissions by providing a measure of internal exhaust gas recirculation (EGR).

It is because of these contradictions that so many modern engines have variable lift and timing systems. Because inlet and exhaust cams are all on the same shaft the V12 does not easily lend itself to such devices.

In the writer’s opinion the Jaguar V12 would have been a better power unit for the majority of its customers if it had been equipped with camshafts having about 20 degrees less duration whilst maintaining similar lift of around 0.375”. Adequate valve lift is important because it is desirable that the inlet valve should open sufficiently far that the port can direct flow into the cylinder to produce the required charge motion for promoting good combustion.

Fuel efficiency, emissions and drivability would all have been better. On the other hand, there was a risk that the more fierce accelerations of such a cam might have aggravated the noise problems. The extra mid-range torque would have been far more use than a few b.h.p. at the top of the speed range when driving through the three speed automatic transmission that virtually all 5.3 engines were attached to. It is ironic that the most torquey V12 of all, the 6 litre, was the only one to be mated to a four speed automatic transmission.

It has been mentioned above that the production cam profile used for the V12 can be traced back beyond the XK. It is quite well known that the XK originally had 5/16” cam lift instead of 3/8”, still with the same duration, because of fears that clumsy service work and unfamiliarity with an OHC engine might result in bent valves. What is far less well known is that the 240 version of the Mk 2 saloon had cams with 5/16” lift and almost 30 degrees less duration. Despite also having the straight port head with oversize porting suited to the 4.2 as a consequence of rationalisation, this engine significantly out-performed the earlier 2.4 engine, thus adding support to the hypothesis that small over-square cylinders work better with milder cams for a given speed range than might be used with larger cylinders. Had the 240 retained the curved port head it probably would have been better still!

One might reasonably conclude that the ideal cams for the smaller over-square cylinders of the V12 would have been of shorter duration than had been used for the XK. Many years ago we produced short period cams for the V12 which significantly improved acceleration but they were hard to sell because they did not give more top end power – which few people ever use, but which everyone seems to think they want.

Of course the usual direction for a change of camshaft is to longer duration and more lift but this inevitable leads to loss of torque at lower speeds. For a track car this may not be a problem at all but for a hefty road car it can be a backward step especially if still driving through a three speed transmission.

A car equipped with a five speed manual transmission would be more tolerant of a narrowing of the power band but a lot of people like to use a manual box to obtain relaxed cruising with an overdrive top gear and to exploit the ability to accelerate smoothly from as low as 1000 r.p.m. in everyday driving. Obviously changing the cams will adversely affect such a capability.

A typical ‘fast road’ cam upgrade offered would be 35,65:65,35 (280 degrees duration) with about 0.406” lift but torque would be reduced below 4000 revs and gains at higher speeds would be very dependent on how much the induction and exhaust systems had been worked on. Such a cam might not be a good choice for a car with less than four gear ratios. It is also likely that emissions and idle quality would worsen because of the extended overlap period and lambda feedback corrections might become unstable and aggravate the poor idle condition. The ECU mapping would need to be revised to match the changes of volumetric efficiency across the speed range – needing less fuel at low speeds and more higher up.

A ‘full race’ cam might be 45,85;85,45 (310 degrees duration) with about 0.435” lift but without serious induction and exhaust upgrading this would be pointless and low speed torque would be almost non-existent. The lack of a reliable manifold vacuum signal would require engine load to be monitored by an EFI system according to throttle angle.

There have also been some really brutal race cams of about 60,90;90,60 (330 degrees duration) and about 0.5” lift which can only be used with larger tappets and carriers made for the XJR6 race cars. Cams like this are very specialised and would not be noted for longevity or refinement. The trend in recent years has been for cams to have shorter duration and more lift than in the past and even the very high revving F1 engines of the mid-2000 era had typical durations of only around 290 degrees – but with lift of about 0.6” (16 mm). Long overlap was no longer possible with enormous valves over huge cylinders with ludicrously short strokes and the need for compression ratios of around 13:1, so engineers had to find ways of operating valve mechanisms beyond previously accepted boundaries. This was achieved by adopting very light and stiff valve assemblies, specialised surface treatments, pneumatic springs, and advanced cam forms with carefully controlled acceleration properties. 

Perhaps modern cam technology would have made possible a short duration, high lift cam, as hypothesised above that might have been more ideal for the Jaguar V12.