Tuning the 4 ½ litre Lagonda and Invicta

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By L. S. Michael, O.B.E.

A well-found 4½-litre Lagonda or Invicta saloon or drophead of the nineteen-thirties gives a more than adequate performance in standard form, compared even with today’s products. It will out-accelerate and reach a higher maximum than all but the really high-performance modern saloons, and it will out-brake and out-steer most of these. Nevertheless, the ready availability of 90- and 100-octane fuels gives rise to thoughts of improved performance.

The engine in these cars is the 4½-litre six-cylinder Meadows originally designed by R. S. Crump in the period 1925-28. Its bottom end does not encourage really high r.p.m.; the recommended limit by Invictas was 3,500 r.p.m. On the Lagonda, for the 1933 to 1935 models it was 3,800. The reason for the difference in these limits is not clear. Mr. Crump, with whom I spent an afternoon recently, can throw no light on this matter and thinks it largely Lagondas own idea. They had good test-bench facilities at Staines and no doubt carried out extensive trials before issuing their three-year guarantee with such a limit permitted.

On the later LG versions (1936 onwards) the maximum was increased to 4,000 r.p.m., the reasons being modified con.-rod design, giving a stronger rod with directly run-in bearings instead of separate brass shells, stronger and more rigid main bearing caps, a crankshaft with larger crank-pin diameter, and crankcases cast in R.R.50 material which gave slightly greater rigidity than the aluminium alloy originally specified. Lighter valve-operating mechanism was supplied for all Lagondas from the M45R (late 1934) onwards. All these modifications were designed by Lagondas, and were not included in the standard Meadows engine of the same date. They help to account for their greater willingness to rev. compared to the Invicta. Incidentally, no normal LG45 could reach 4,000 r.p.m. in top, and the handbook strongly discouraged maximum revs. in the gears!

The con.-rod is not located in the centre of the piston but is displaced appreciably to one side. Thus a greater bending load is imposed than would otherwise be the case. This design feature according to John Wyer, when applied to the 2.9-litre Aston Martin, a bored-out version of the post-war 2.6-litre Lagonda engine, caused all three cars to break down in the 1952 Monaco race. The bottom-end layout was then altered to permit the rod to be placed in the centre of the piston. Of course the 2.9 was a sports/racing job with nearly three times the specific output of the old 4½, and it must be recorded that the Meadows engine is not in fact prone to con.-rod trouble as installed in the cars under consideration. The point is raised as an indication of the design limitations of the unit.

Mr. R. S. Crump, the then chief designer at Meadows, told me that it was originally built in about 1925 as 63 x 120 mm., the small bore being dictated by the prevailing system of taxation. Later it was enlarged, and by 1928 reached its limit at 88.5 mm., which, in spite of a new block, necessitated the cylinders being bored offset from their original centres, thus giving rise to this rather undesirable feature of the design.

Experience in the 500-Mile Race, the 24-hour race at Spa, Le Mans, and the T.T.s, showed that 3,600 r.p.m. could be held for long periods, and undoubtedly up to 4,000 r.p.m. was used frequently in these races. The several laps in the 1936 500-Mile Race at Brooklands which the Lagonda put in at over 122 m.p.h. show that even on the axle ratio and large tyres in use 4,000 r.p.m. was reached frequently, without disaster.

The foregoing is intended to show that up to its recommended rev.-limit the engine is reliable. My own results when exceeding these revs. for any length of time have been disastrous, crankshafts and con.-rods being broken on several occasions. The question that arises is, what to do to increase the performance of this engine, and how far is it safe to go?

The usual procedure of polishing and matching ports and manifolds, etc., is not too difficult on the Invicta and the Lagonda M45, as the inlet manifolds are detachable and expose the inlet ports. In the case of the LG45 little can be done by the amateur because the carburetters bolt directly on to the head itself, the mixture distribution arrangements being cast inside it, and well hidden from view. LG owners can clean up the junctions between the head and carburetters, and console themselves with the knowledge that their cylinder head was specially designed by Weslake, at the instigation of W. O. Bentley, to give more power than its predecessors, and it certainly achieves this object. This latter information came front a discussion with “W. O.” himself, which took place in May 1959 at his home.

An M45 Rapide was prepared by the works for the 1935 Monte Carlo Rally. This had the then standard Rapide compression-ratio of 7 to 1. The engine had been specially assembled and, judging from the Fox and Nicholl files, this usually included polishing the head and polishing and matching the inlet ports and carburetter flanges. A note from Mr. Bolton—then Works Manager at Lagondas—said that “Test 2” showed better results than the standard Rapide. This test gave a maximum power output of 105 b.h.p. at 3,200 r.p.m. For the next test the ignition was advanced from 40 deg. b.t.d.c. to 46.5 deg. b.t.d.c., and the fuel changed from “Power Premier” (in those days certainly not even 80 octane) to 70 per cent. “High Test” 30 per cent. Benzol. This gave 109¼ b.h.p. at 3.400 r.p.m. All shatteringly low figures when related to the official fiction published about the power developed by the Rapide in 1935. The interesting point lies in Bolton’s remarks relating to “Test 2” which show that specially careful assembly, and polishing and matching ports, produced better results than the standard engine, even though no other modifications were carried out for that test.

The works had a cylinder head copperised, that is copper plated, and the brake test showed some improvement, but as the copperising was accompanied by other unspecified modifications it is impossible to say what was due to that process by itself. It is not an expensive thing to have done; the virtue of the copperised head lies in a more even heat flow, heat being conducted away from areas which might otherwise develop into hot spots, thus reducing any tendency to detonation. This was done to one of my M45 engines and to the LG45 the “Scarlet Woman,” of Donald Overy, when only “Pool” (72-octane) petrol was available.

The only major modification which the works tried on the inlet aide of the LG series was to fit larger carburetters. This involved modifying the head to accept them, as they have a different arrangement of studs. An adaptor must be made up to bolt on to the original studs, carrying ones disposed for the new units. The actual hole in the head is enlarged somewhat and blended into the adaptor, which in turn opens out to present no obstruction to the incoming charge from the new carburetters. In the case of the M45 and Invicta the adaptors are bolted on to the induction manifolds instead of directly on to the head, and it is easier to enlarge and clean up the ports on these earlier engines.

This is a rather expensive modification. The existing H.V.5 carburetters are 1⅝-in. bore; modern instruments of 1¾-in. (H.6s) and 2-in. (H.8s) are available, the cost of the conversion being about £35 in the case of the H.6s and about £45 in the case of the H.8s. The works fitted an engine with 2-in. carburetters but they did not carry out brake teats on the same engine using the standard 1⅝-in. instruments. Nevertheless, the engine which achieved the highest output of all the six-cylinder LG engines tested was fitted with the 2-in. carburetters along with other modifications. At least one LG45 is running fitted with 1¾.-in. H.6s and the owner reports better results than were obtained with the original H.V.5s; while it must be recorded that this is only an opinion, it is supported by test-bench results obtained on another LG45 engine. The most exotic set-up of all was an arrangement of six Amals on a specially fabricated manifold fitted to an M45 head modified to accept neatly the six inlet stubs. This work was done by Hugh Howath, of Manchester, and subsequently sold to M. Willoughby for use on an Invicta. When I saw it, it had not been fitted to a car and no figures existed to prove whether it would be better or worse than the more normal alternatives! Such an arrangement would be impossible with an LG45 head,

Some interesting bench tests were carried out by G. N. Richardson of Hartlebury, on July 29th, 1959, on Lord Dunleath’s LG45 Rapide team-car engine. This engine, like all the team cars and some of the production Rapides, had special con.-rods. These had no split at the small end and were (slightly) deeper at each end than the LG45 rods. This increased their resistance to bending loads. The compression-ratio of the engine tested was approximately 7.4 to 1. Fitted with the original H.V.5 carburetters maximum power was 120 b.h.p. at 3,500 r.p.m., with H.6s (1¾in.) maximum power was 128 b.h.p. at 3,500 r.p.m., and with the 2-in. (H.8) instruments 129.5 b.h.p. was realised at the same engine speed. In each case power fell above 3,500 r.p.m. However, the H.6 gave 124.4 and the H.8s 125 at 4,000 r.p.m., whereas the H.V.5s only produced 111 b.h.p. at those revs. There was little difference between the 1¾-in. and the 2-in. S.U.s but the larger ones gave between 1 and 2 b.h.p. more power at all engine speeds, and both exceeded that produced by the original instruments all the way up the scale. In spite of the slight superiority which the largest carburetters show on the bench, Lord Dunleath has a very strong impression that acceleration is slightly better with the H.6 instruments. Figures produced by an engine removed from the chassis are not always reflected in road performance, especially when they are as close as these, and the opinion of this very experienced competition driver should not be lightly disregarded. Better results were obtained by the works on similar engines in 1936. However, this particular unit was removed from the chassis and tested after nearly two seasons use, the purpose of the tests being to compare the different size carburetters. Mr. Richardson, who has great experience in the bench-testing of engines, considers that if the highest power had been sought, a decoke and valve grind would have resulted in decidedly greater output.

When the exhaust side is considered, some strictly comparative information is available. Bench tests were carried out on an engine (No. M45R/152) on August 19th, 1935, to determine the effect of different exhaust systems. First the standard manifold and silencers were used, then a welded steel tube six-branch manifold and “Brooklands” silencer and tail-pipes. Up to 3,300 r.p.m. there was surprisingly little difference, the standard lay-out giving only one b.h.p. less than the six-branch. Above this speed the engine when fitted with the normal exhaust gave no more power, and at 3,400 the curve began to fall. With the improved arrangement, power continued to rise until it peaked at 3,600 r.p.m., exceeding the highest figure produced previously by 6.5 b.h.p., and even at 3,000 r.p.m. it gave 3.5 b.h.p. more than the production exhaust system permitted at that speed. Therefore there is no doubt that an exhaust system made up to give freer flow will decidedly improve the power of these engines.

There are available a few “100-m.p.h. Invicta” exhaust manifolds which will bolt on to the Lagonda cylinder head without any trouble. These are two separate three-branch manifolds which could each be led into a completely separate silencer and tail-pipe. These manifolds are not terribly well designed as compared with modern practice, nor as compared with the LG45 Rapide swept three-branch manifolds, but they are decidedly better than the original Lagonda effort. Good three-branch manifold could be fabricated by any firm with pipe-bending and welding equipment. The original works racing manifold was a six-branch affair, all the pipes eventually merging into a single outlet three inches in diameter just before the silencer. It is not certain which is better. Some people prefer the two separate three-branch manifolds, because it is impossible to have a tuned length of exhaust for maximum extractor effect with six cylinders all merging into a single pipe. According to the most widely accepted theory a distance of 68 inches from the piston at t.d.c. to the end of the tail-pipe of a three-branch system is required to give maximum extractor effect on these engines at 3,000 r.p.m., or 51 inches if maximum effect is sought at 4,000 r.p.m.

Both modifications, in the case of Lagondas, involve cutting the bonnet side to accommodate the new manifolds, and certainly it is very difficult to make the six-branch set-up look as neat and attractive as the alternative, which was adopted for the production LG45 Rapides. Anyway there is 6.5 b.h.p. being wasted in the standard Lagonda exhaust system, which can be obtained without increasing the internal stresses of the engine. It is hardly worth while modifying the Invicta exhaust unless the ultimate in power is being sought.

A modification much favoured by tuners today is to lighten the flywheel considerably. This technique, which is undoubtedly effective in increasing acceleration, was never used either by the works or Fox and Nicholl. On one engine of mine I not only had holes drilled all over the flywheel to lighten it, but removed the quite heavy vibration damper too, in order to reduce inertia and get increased acceleration. This was the first engine which ever disintegrated under me in a race, and be it noted that I have used higher compression-ratios since, in other engines, without serious blow-ups. My present LG45 Sanction III engine’s flywheel is lightened by having a number of 1¼-in. holes in it, disposed in a symetrical pattern, but the vibration damper has been retained. The engine has been carefully statically and dynamically balanced, and so far it has withstood two seasons’ competition use. It is most desirable to have these engines balanced as far as possible. The whole assembly—crankshaft, con.-rods, pistons, flywheel and clutch complete—should be sent to a reputable firm specialising in this work. I have no precise figures to show exactly what is gained by reducing the flywheel weight (top speed is not affected), but there is no doubt that a lighter flywheel makes the step-off noticeably more rapid, and due to the fact that there is a more instantaneous response to the throttle, gear changes are faster, too. The standing ¼-mile in my car at Silverstone, taken at the B.D.C. meeting, August 1958, was 16.46 sec. The track was not perfectly dry, and the course marked out was not dead flat and had an appreciable curve in it. The best figure from the pre-war road tests was 18.4 sec. for the LG Rapide.

If more power is being sought, regardless of the time and trouble involved, attention to both inlet and exhaust arrangements is desirable, but even if neither of these can receive attention, a worthwhile improvement in performance is obtained at low cost by raising the compression-ratio. This can be done when a decoke is due, by the inexpensive method of machining the cylinder head. If really large increases in compression are desired it may be necessary to machine the block as well. The reason being, that it is not advisable to run too far into the sparking plug holes, which are located rather low down on each side of the cylinder head. The only snag which these modifications will occasion is the need to elongate the holes in the water transfer plates to allow for the now slightly reduced distance between their studs. At the same time an extra water transfer can be made between the back of the head and the block. This is very easy to do. It became standard on the LC45 Sanction Ill and helps to keep cylinders 5 and 6 cool. This modification, though desirable, is not essential. It is a comforting thought that if you do not like the result a compression plate can be inserted between detachable cylinder block and the crankcase to reduce the compression-ratio to any desired figure. This was done during the war to a number of Rapides which found the low-octane fuel then available rather indigestible.

Special high-compression pistons can be made instead of machining, but this is much more costly, unless a complete engine overhaul is to be undertaken. The price of such pistons from the firms who specialise in that work is about the same as the standard ones available from the usual sources.

(To be continued next month)