Some notes analysing expert advice on making a popular vintage sports car perform properly.
The “12 50” Alvis, which was, in continuous prodnction from 1923 to 1929, must still be with us in considerable numbers, judging by the requests we of Motor Sport receive for instruction books and “hotting up” data. Early in 1940 we published an article from John Cooper who served his apprenticeship at Alvis Ltd, and owned a very fine big-port “beetle-back” car, which we suspected would supply all the data on this famous vintage car that even the most avid “12/50” owners could desire. It did, but it has gone out of print. So, to defend ourselves from the renewed floods of Alvis enquiries, we have decided to extract some of the matter from Cooper’s comprehensive article and add some additional data from other sources. Cooper recommended the steel-bronze-steel combination of timing gears, and said that standard valve springs were adequate up to 4,800 r.p.m, – 300 r.p.m. beyond the maker’s peak speed. He liked to open out the masked plug pocket on big-port heads and considered that 6.3 to 1 is about the highest useful compression ratio for touring work. This was realised in the later “SD” and “TH” cars, using the big-port head and h.c. pistons, and these engines also had duralurnin con.-rods. These rods can be used in any “12/50” engine if pads or circlips are devised to replace the pinch-bolts of the normal steel rods. Cooper found that any decent plug sufficed, but preferred K.L.G. L583. Incidentally, the normal compression ratio of all early engines, and also of the “TJ,” was 5.35 to 1, while that of the “TL” “12/60” was 5.8 to 1. Cooper gave much information about closing up those disastrous gaps between the gear-ratios of the common or garden “12/50,” but space precludes repeating this and much of the work would hardly be the thing in this “toil or else” era!
Michael May added a few views of his own to Cooper’s remarks, saying that he preferred all-steel timing gears and did not tamper with the plug masking. He considered you could go to a compression ratio of 6.3 to 1 with the small-port head and 6.9 to 1 with the big-port, above which, even on Discol, the car would only go slower.
May ran a “12/50” with the block bored out to 72 mm., his idea being to attain approx. the bore-stroke ratio of the 68 by 103 mm. sports engine when using the 110-mm. stroke crankshaft, thereby being able to effectively use the big-port head. He used Sylicum pistons, a standard big-port head with inside exhaust system and standard 40-mm. Solex carburetter assembly, bronze-backed main bearings, a 1926 crankshaft with the two-stud fixing for the balance weights, 1926 crankcase and standard camshaft. In this form, with 4-seater body, May’s car would do approx. 85 m.p.h., a s.s. 1/2 mile in 35 sec., and would beat a Ford V8 on acceleration. At one Aston-Clinton speed trial it made 4th f.t.d., beaten only by Dunham’s 20 h.p. track car, Waller’s Special “Silver Eagle” and Powys-Lybbe’s 20 h.p.). T.T. car. It was, of course, very carefully assembled and used close ratios in its gearbox. Powys-Lybbe ran a rather special “12/50” at Brooklands, lapping at 85.13 m.p.h. in 1934. He found dry-sump lubrication essential for prolonged high-speed work. In the main he preferred the old 40 mm. Solex carburetter assembly, although he tried two S.U.s, and he agreed with May that 6.9 to 1 was the optimum cornpression ratio using petrol/benzole and that you didn’t go any faster with it higher, on alcohol, at all events with standard valve timing. Lybbe used standard valves, camshaft, tappet-setting and big-port head, and, so far as he could recall, Champion R3 plugs – May used R10s.
Incidentally, the best lap put up by an old-type “12/50” at Brooklands seems to have been at 90.06 m.p.h., by H. W. Purdy in 1925, while Westbrook’s “duck’s-back” did 88.15 m.p.h. as late as 1933.
So far as carburetter settings are concerned, Cooper gave 27 choke, 55 pilot and 120/51 main for the 40-mm. type “MV” Solex. John Wyer once worked out a combination for using this carburetter on a 69 by 110-mm. engine as: 29 choke, 60 pilot, 160/41 main and No. 6 jet assembly-this for maximum power. Lybbe mentions a 29 choke. For the later type of Solex carburetter on the small-port head Alvis, Ltd., recommend a 26 choke, 52.5 pilot and 125 main jet for power. There seems some difference of opinion re ignition timing, Cooper suggesting 28 degrees before t.d.c., full advance, but Michael May recommending approx. 42 degrees before t.d.c. (14 starter ring teeth before t.d.c.) – the instruction book is discreetly silent on this matter. A tip of May’s is to bore out all oil pipes on the suction side of the pump from 5/16 to 3/8 in. The correct oil pressure is 25-30 lb./sq. in. when really hot, but anything from 10 lb. up to this figure is reasonably safe. We hope those we have quoted in these notes will forgive us; their experiences should, we feel, still be of inestimable value in enabling owners of “12/50s” to get the best from a car that remains deservedly popular. Other applicable data are:
Firing order: 1, 2, 4, 3. Valve timing: Inlet opens at t.d.c., closes 50 degrees after h.d.c.; exhaust opens 55 degrees before b.d.c., closes 10 degrees after t.d.c. One degree on flywheel equals 2.7 mm., i.e., 10 degrees equals 1 1/16 in. Tappet setting: .003 in. warm, inlet and exhaust, or .004 in. for performance. Piston clearances: top, .021 in.; first ring, .013 in.; skirt, .002 in. Main bearing end-clearances: Front and centre bearings, .010 in.; rear, .003 in. Rear axle ratio at 4.77 to 1 equals 9/43. Some early cars had 4.55 to 1 ratio.