The AC Light Six:
John Weller's Engine, 1919–1963

Weller's design, 1919

John Weller had been AC's chief engineer from the company's earliest days, and the six-cylinder engine he laid down for the marque in 1919 was unusually forward-looking for a small British firm of the period. Where most contemporaries stuck to cast-iron side-valve fours, Weller specified an aluminium cylinder block with wet cast-iron liners, a seven-bearing crankshaft, and a single overhead camshaft driven by a triplex chain from the front of the crank. The bore and stroke were 65 mm by 100 mm, giving a swept volume of 1,991 cc — a long-stroke, torque-rich configuration typical of the era, but wrapped in a light, modern architecture that would outlast almost every one of its rivals.

Why the overhead cam mattered

A single overhead camshaft, in 1919, was the province of racing cars and expensive continental machinery. Weller adopted it in part for its breathing advantage — the direct valve actuation allowed higher lift and more accurate timing than a long pushrod-and-rocker train — and in part because it suited the aluminium block: keeping the valvegear at the top of the head simplified the casting and left the block itself uncomplicated. The layout also gave the engine its distinctive tall, narrow appearance and its characteristic mechanical note.

Four decades of development

The engine's longevity is the story in itself. From its 1919 debut it was refined continuously — better bearings, a stiffer crankshaft, improved oiling, revised combustion chambers, and eventually triple SU carburettors — but the essential architecture remained Weller's. It saw AC through the vintage era, the 1930s, the war, and the whole of the company's post-war revival. By the time the last examples were built alongside the final Ford-engined Aceca 2.6 in 1963, the design was in its forty-fourth year of production: an innings almost unmatched in British engine-building and, arguably, exceeded only by a handful of designs anywhere.

Specification comparison

A handful of dimensions were laid down by Weller in 1919 and never altered in forty-four years of production. What did change, repeatedly, was the state of tune: compression ratio, carburation and the resulting output. Use the tabs below to compare the 1919 launch specification, the fixed-core architecture common to every Light Six, and the final 1954–1963 Aceca state of tune.

Camshaft and valve timing

The single overhead camshaft ran the same two-valve-per-cylinder layout throughout, with valve timing progressively opened up as the engine's state of tune rose. The late Aceca cam is noticeably wilder than the pre-war profile, though still mild by 1950s sports-car standards.

The engine in the Aceca (AE and AEX)

Acecas built with the AC engine carry the AE prefix, or AEX for overdrive cars; these were the coupé's first specification when production began in 1954 and remained available until the Bristol engine took over as the enthusiast's choice from 1956. In late Aceca tune the two-litre six was fitted with three SU carburettors and produced, in road specification, in the order of 85 to 100 brake horsepower depending on state of tune and year — enough to give the light coupé a top speed of around 100 mph and the kind of flexible, torquey delivery that suited long-distance road use.

The AC unit was outclassed, on paper, by the Bristol six that replaced it in the model range — more power, more revs, more competition pedigree — but it was smoother than the hard-edged Bristol, cheaper to buy, and, importantly, entirely of AC's own making. To an original-minded enthusiast the AC-engined car is the pure Aceca: the coupé exactly as it was conceived at the 1954 Earls Court Motor Show.

What survives, and why they matter

Of the 328 Acecas built in total between 1954 and 1963, the AC-engined AE and AEX cars are outnumbered by the Aceca-Bristols. They are the rarer specification of an already-rare car, and the direct line to Weller's 1919 design gives them a technical interest that goes far beyond the model's own production run. Under the bonnet of a matching-numbers AE Aceca is an engine whose architecture predates almost every other unit still in series production at the time it was built — a genuine piece of motoring history, quietly outlasting the more famous power units around it.

Reading the chassis prefix

The AC engine's presence in a given car is decoded from the chassis prefix: AE for the standard car, AEX for overdrive. The BE and BEX prefixes denote the Bristol-engined Aceca-Bristol, and RS the final Ford Zephyr-powered Aceca 2.6. For the full scheme, see the chassis number guide.