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The Armstrong Siddeley immediate post-war 2.6 litre aluminium car engine

by Richard Hodgson

This amended 1994 article first appeared in the Rolls-Royce Heritage Trust's (Coventry Branch) magazine "The Sphinx".  Please use your Web Browser's "Back" button if you want to return to this article after visiting any hyperlink which are shown in bold dark blue.  Please click here to go to the list of articles on this site's home page.


In the last edition of "The Sphinx", an account was given of the Sid Allen’s report on the post-war 16hp car engine.  He had been asked to investigate methods of increasing its power output, as the 16hp was widely considered within the company to be underpowered. Bastow has stated that such feeling was generally shared by the industry.  Allen’s urgent report was so pessimistic that the design of a brand new engine was immediately put in hand.  For reasons that remain less than clear, Sid Thornett appears to have been in charge of the project - the rather more able Fred Allard leaving ASM in a bit of a huff.   Allard had been responsible in many respects for the large pre-war light alloy Siddeley Special engine, though S. M. Viale of the aero-engine side had given "technical assistance".  An understanding of the problems faced by ASM on the 2.6 project and its ultimate abandonment will, I suggest, place the subsequent Bastow/Bentley involvement in a clearer context and, above all, highlight the success of the Sapphire engine (and chassis).

Thornett suggested, or rapidly adopted the suggestion, that the new post-war engine be a wet-linered light alloy 6 cylinder unit.  It would be perceived as being state of the art.  In any case, lighter weight and higher output were highly desirable for the 16hp cars.  To reduce weight yet further and lower the mean piston speed, Thornett chose almost square dimensions.  It was considered that anyone who might purchase an Armstrong Siddeley was not likely to worry about the small (in comparison to purchase price) additional annual taxation that this would cause.  The RAC horsepower taxation regime that penalised short strokes still existed in 1946.  In any case, such taxation was mostly unknown abroad and the government of the day had decreed that car manufacturers must export much of their output.

Thornett started design work in early 1946.  The first prototype was ready for test by the end of October, a short time in the circumstances of the period.  The engine was often referred to as the "2" though it capacity was 2.6 litres.  Despite Allen’s criticism of the 16hp’s porting, [the late] John Densham, the car division’s experimental engineer, recalls that a very similar design was adopted.  The 2.6’s in-line overhead valves were operated by push rod.  In his book "W.O. Bentley ~ Engineer" (Haynes 1978), Donald Bastow makes a point of stating that the aluminium 2.6 had fairly typical push rod operation.   This is may be due to the existence, at least on paper, of a very advanced 90 + bhp 2.3 litre 4 cylinder 4 main bearing all-alloy sleeve valve engine designed by Stewart Tresilian during 1938/9.  He was chief engineer and "co-manager" of that furthest outpost of Hawker Siddeley, Templewood Engineering, founded for him by Devereux and which was then developing a highly advanced a light alloy car with rubber [sic] suspension.  That "aluminium" engine design (or rather the crankcase/crankshaft part of the design) was, one way and the other, offered after the war.  The author does not know whether ASM considered it in 1946.  The Templewood unit was later to form the basis, with ohc valves, of the BRM P25 power unit, and in late 1959, of a 4.5 litre ohc aluminium V8 design.


The known leading particulars of the 2.6 ohv aluminium engines as built were:

        6 cylinders 80mm bore x 86mm stroke;  7 main bearings

        2593.7 cc capacity;  Tax rating: 23.8 hp RAC

        Max power output: 90/92 bhp @ 4400rpm [but see below]

        Compression Ratio: 7.0 : 1 [standard combustion chamber]
                                       6.8 : 1 [average of the "squish" combustion chambers]

        Mean piston speed: 2257 ft/min @ 4000rpm

        Inlet diameter: 1.475" in 1.375" port     } Final "optimal" dimensions
       
Exhaust diameter: 1.5"                          } of early 1948

        Valve lift: 0.350"

        Inlet valve opens: 10 BTC, closes 54 ABC         } Initial design, as per 16hp;
       
Exhaust valve opens: 48 BBC, closes 16 ATC    } timing frequently changed

        Spark plugs: Champion NA-8 long reach

        Carburettor: 1 Stromberg 42mm downdraught

Unfortunately, weight remains unknown.  No drawing has been located at the time of writing [or since].


From the outset, the 2.6 proved to be exceedingly troublesome.  Only a partial list of the defects will be given here.  The unit leaked liberal quantities of water into the sump as well as having numerous oil leaks.  The water leaks occurred even when the engine was not running!  Fuel consumption was considered to be too high.   Breathing, as might be expected given the general design of the porting, was poor.   However, given the lower mean piston speed, mechanical efficiency at 82-83% for most of the running speed was higher than the 16hp’s very low top end figure of 72.5%.  But as a seven main bearing design had been chosen, the result of the stresses caused by this layout could not only be felt but also, so it is said [by John Densham], seen!  This obviously did not help with the oil leakage problems and mechanical efficiency.  To this day [1994], John Densham (who protested at the time) does not know why Thornett chose this arrangement rather than a 4 main bearings one which if designed properly causes less trouble, especially in a light alloy car engine with a less than substantial crankcase.  As a partial remedy to some of the problems, a "squish" combustion chamber was also developed in parallel with the standard head from about March 1947 with certain benefits.  The breathing difficulties included very poor fuel distribution around 1250-2500rpm, leading to a very high specific fuel consumption at this important engine speed range.  Experimental wanted completely external porting - as suggested by Allen for the 16hp - but this does not seem to have been adopted.  Volumetric efficiency was not helped by the 2.6 having a tendency to overheat.  There were difficulties with valve timing, clearances and springs.  Hydraulic, special hydraulic and ordinary tappets were tried.  There was also quite severe problems of cylinder wall wear after times as short as 28 hours running.   Roughened bores were one of the measures adopted in an attempt to reduce such wear.

Another major problem was that the engine was very sensitive to small variation in ignition timings at lower speeds.  Maximum power, about 90bhp with the standard head, could only be obtained by tuning and timing that caused heavy detonation up to 3000rpm.   The later squish head gave about 92bhp with less detonation.  It was specially adapted to give up to 100bhp in March 1948 on better fuel.

Experimental and developmental work carried on in earnest until about the end of February 1948 and some work continued until at least late August 1948.  Valve timing and clearance problems - there was a loss of lift - were still not sorted out by September 1947.  This was one of the causes of the continuing poor volumetric efficiency.  Overheating remained.  A saloon, FDU 113, was fitted with a 2.6 engine with both two and four blade fans, and with a standard bonnet and a bonnet with special louvres on the bonnet side.  It was found that the radiator temperature did not vary by any appreciable amount.  Finally, the valance was cut away, and a slight improvement was obtained.   However, whilst there was an increase in power output up to 3000rpm, there was a very small decrease thereafter, no doubt due to the fan absorbing rather more power at higher speeds.

During the 2.6 project, John Densham carried out a large number of tests and modifications, both on the test bed and on the road.  His view does seem to have been that if there were actually any way of making a success of the 2.6, he intended to find it.  However, it is fair to say that he, and others, had grave misgivings about the engine’s design from its inception.  He has not [1994] changed his mind after 48 years.

Though ASM had tried fuel injection on the 16hp and started work on the 18hp stop-gap variant, it considered that a new engine was urgently needed.  By the end of 1947 it was clear that the 2.6 would take a long time and much more money to develop into production job.   In about February 1948, ASM was either offered the rights to the new Bastow/Bentley designed Lagonda 6 cylinder 2580cc, or invited to test it with a view to making a suitable offer [i.e. not the Bastow/Bentley engine that was to be commissioned by ASM].   Comparative tests with the 2.6 were carried out on 15th March 1948.  For much of the running speed, the Lagonda unit showed a 6 to 8 bhp advantage, save at the highest speed when the 2.6 produced 2 bhp more.  The Lagonda’s torque fell below the 2.6’s above about 3500rpm and was about 10% lower at the top of the running range.  However, it was felt that the Lagonda unit was a better engine as, inter alia, it did not have assorted kinks in its performances curves, had better fuel consumption, did not get so hot and was far less prone to leakage.  All in all, it was considered [by both John Densham and Mervyn Cutler] a rather more refined unit, if maybe just a little old fashioned.

It is not known how much the Lagonda engine would have cost ASM, but by now ASM wanted an engine that could produce at least 110-120 bhp.  ASM viewed the design of Lagonda unit as a good basis from which to make a quick start.  Further, Bastow and Bentley were or were about to become legally "free" to offer their services to anyone prepared to pay their fees.  Either Chapman or Sopwith, or most likely both, considered that given the problems with the 2.6 and the comparative refinement of the Lagonda, it would be sensible to hire Bastow, Bentley and their small team.  A very tentative suggestion that Bentley act as a consultant had already been made in 1939 by his former V12 chief designer at Lagonda.  There would of course be the very important commercial advantage that Bentley’s name could, with some legal care, be attached to the car!   This last fact was probably the most important reason for hiring them to design what was immediately and proudly referred to in internal memos as the "A.S. Bentley".

I would like to thank [the late] John Densham and [the late] Mervyn Cutler for assistance in preparing this article, and, in the case of John Densham, making original test reports and memos available for inspection.


Richard Hodgson 1994, 2000  -  may not be reproduced in whole or in part without permission