Tax Horsepower - Britain

Britain

The so-called RAC horse-power formula was concocted in 1910 by the RAC at the invitation of the British government. The British RAC horsepower rating was calculated from total piston surface area (i.e. "bore" only). To minimise tax ratings British designers developed engines of a given swept volume (capacity) with very long stroke and low piston surface area. Another effect was the multiplicity of models: Sevens, Eights, Nines, Tens, Elevens, Twelves, Fourteens, Sixteens etc each to fit with a taxation class. Larger more lightly stressed engines may have been equally economical to run yet, in less variety, produced much more economically.

British cars and cars in other countries applying the same approach to automobile taxation continued to feature these long thin cylinders in their engine blocks even in the 1950s and 1960s, after auto-taxation had ceased to be based on piston diameters, partly because limited funds meant that investment in new models often involved new bodies while under the hood/bonnet engines lurked from earlier decades with only minor upgrades such as (typically) higher compression ratios as higher octane fuels slowly returned to European service stations.

The RAC (British) formula for calculating tax horsepower:

where

D is the diameter (or bore) of the cylinder in inches

n is the number of cylinders

The distortive effects on engine design were seen to reduce the saleability of British vehicles in export markets. While the system had protected the home market from the import of large engined low priced (because produced in such high volumes) American vehicles the need for roomy generously proportioned cars for export was now paramount and the British government abandoned the tax horsepower system with effect from 1 January 1947 replacing it at first with a tax on cubic capacity itself in turn replaced by a flat tax applying from 1 January 1948.