In common with other early car engines, Thornycroft engines were extremely low revving, with a normal speed (to use the company's term) of 900 to 1,000rpm, depending on model - ie. little more than the idling speed of a modern car. As a result of the leisurely rpm, power outputs were very low relative to engine size.
In its 1908 brochure, Thornycroft said that "the standard gears of 30 and 45hp Models give speeds as follows with the engine running at normal speed: 1st speed - 10mph, 2nd speed - 21.5mph, 3rd speed 35.5mph (16.1kph, 34.6kph, 57.1kph). Reverse speed - 10mph (16.1kph)".
But Thornycroft then went on to comment, "These speeds can be considerably exceeded with the engine accelerated." But by how much? We can find a hint of an answer from the 30hp L4 engine. This 5,213cc, four-cylinder engine was guaranteed to give 30bhp at 1,050rpm, with a maximum of 35bhp at unspecified rpm - an estimate of 1,200 to 1,300rpm is probably reasonable.
RAC hp explained
For many years, cars in the UK were taxed according to the power their engines would give assuming a piston speed of 1,000ft/min (304.8 m/min) and a brake mean effective pressure of 67.3lb/in2 (4.64bar). This led to the RAC, or Treasury formula: bhp = 0.4 d2N
Where d = cylinder diameter in inches, N = number of cylinders.
A bmep of 67.3 lb/in2 (4.64bar) is less that half of what can be achieved today on naturally-aspirated production engines, so a century ago cylinder volumetric efficiencies were poor by modern standards.
In the early days of motoring, the RAC formula gave a fair indication of the bhp produced. But, as engines were developed over the years to give increasing power for their size, it became normal for an engine to give several times its RAC rated bhp. Thus, for instance, a small car in 1946 rated at 8hp for taxation purposes would have an engine giving over 20bhp. Eventually, the RAC bhp taxation system was discarded.
The RAC formula shows that the most tax efficient engine would, for a given swept volume, have a narrow bore and a long stroke - i.e. undersquare. And, over the years, the RAC taxation system resulted in a tendency for manufacturers to produce cars with long stroke engines, until a revised taxation system freed manufacturers from tax efficiency constraints and, as a result, short stroke engines eventually became normal.
A point of confusion are the designations given to each Thornycroft car type, i.e. 10hp, 14hp, 20hp, 24hp, etc. which do not necessarily correlate with either the RAC rating or the bhp developed.