Why British engines have traditionally been small-bore designs

An enduring mystery to me has been why Jaguar stuck with typical pre-war engine design, typified by a long stroke and a small bore, until well into the '60s. At Le Mans in 1957, the 3.8 litre Jaguar D-Type arguably beat the much more powerful Ferrari 315 S, also with a 3.8 litre motor, mainly due to having disc brakes -- once Ferrari adopted discs, it was all over for the classic Jag engine. (Some references also credit the Jag with better aerodynamics.) Let's look at some numbers. 

The Jag 6 and the Ferrari V12 are within a couple of cc's of each other. But the similarities end there. First is bore to stroke ratio. The Jag, at 87 x 106 mm, is undersquare at a bore:stroke ratio of 0.82, while the Ferrari, at 76 x 69.5, is oversquare at 1.09. Power is also quite different: the Ferrari manages 360 horsepower at 7800 RPM, versus 306 at 5500 RPM from the Jag engine.

But the real interesting concept is taxable horsepower, how it was calculated in different jurisdictions, and how it affected engine design.

In France, the approach that generated the famous 2 CV appellation is based on displacement. (Note actual 2CV power output, in the 602 cc version, was 29 hp). There is also a fiddle factor and a rather bizarre RPM calculation. The formula in place from 1956 was CV = 0.00015*n*D^2*L*R, where n is number of cylinders, D is bore in cm, L is stroke in cm, and R=20 is another fiddle factor. So far so good; this is essentially displacement but replacing π/4 with 20 x 0.00015. (For details of the truly bizarre calculation for R, see here, in French). Both the Ferrari and the Jag come out at about 14.5 taxable hp. 

In most other European jurisdictions the calculation was displacement-based. (Today it is based on CO2 emissions). But in Britain, the RAC taxable horsepower calculation only took bore into account; big bore motors were thus taxed more heavily than small bore motors at the same displacement. The formula is P = n*D^2/2.5, where n is number of cylinders and D is bore in inches. The Jag is thus rated, for tax purposes, at 28 hp, but the Ferrari comes in at a whopping 43 hp. 

British manufacturers often provided both taxable and actual horsepower in the model name, hence the Wolseley 14/60 with 60 real hp but taxed at 14 hp; the Alvis 12/70 managed the trick of more actual power (70 hp) at a lower tax bracket (12 hp), presumably via a smaller bore.

This also acted as a non-tariff barrier to imports: the Ford Model T was rated at 22 hp and was thus quite a bit more expensive than an equivalent British car of similar displacement or performance.

So obviously British manufacturers built tall, narrow, small-bore motors to minimise the tax hit. And this persisted well into the '60s, when the classic Jag 4.2 litre had bore and stroke of 92 x 106; the 4.0 litre AJ6 motor that replaced it wasn't much better at 91 x 102. The 5.3 litre V12 was far more modern, at 90 x 70; even the 7.0 litre version was still oversquare at 94 x 84.

So here you have it: unintended (or maybe not) consequences of a poorly thought-out regulation.