RAC Horsepower

Physical unit
Unit name	RAC Horsepower
Unit symbol	${\ displaystyle {\ text {(RAC) hp, (RAC-) PS}}}$
Physical quantity (s)	Performance of reciprocating - combustion engines
Formula symbol	${\ displaystyle \ P}$
dimension	${\ displaystyle {\ mathsf {M \; L ^ {2} \; T ^ {- 3}}}}$
system	Anglo-American system of measurement
Named after	British Royal Automobile Club (RAC) , average work horse performance
Derived from	Diameter of the cylinder bore, number of cylinders, indirect: several other factors
See also: horsepower , tax horsepower , DIN horsepower , British horsepower

RAC horsepower (at German : RAC - horsepower , abbreviated often called) RAC hp is an outdated measure for the performance of reciprocating - combustion engines . It was developed especially for motor vehicles by the British Royal Automobile Club (RAC) at the beginning of the 20th century and was in use throughout the British territory including the British colonies and later throughout the Commonwealth for decades . A related equivalent in the German-speaking area was most likely the unit of measurement tax horsepower .

overview

The traditional club badge of the British Royal Automobile Club

The unit of measurement, newly created in the first decade of the 20th century, was primarily used in the United Kingdom of Great Britain and Ireland and in the successor states to standardize and help to better compare the performance of internal combustion engines. The calculation formula explicitly only took into account the cylinder bore and the number of cylinders; On the other hand, the piston stroke and the speed were not taken into account, at least at first glance . Initially, the abstractly determined values largely agreed with the actual engine output. With improvements especially in the combustion processes , the fuel quality and higher engine speeds, the actual engine output deviated more and more from the abstract value of the RAC Horsepower .

Nevertheless, from around 1909 onwards, the new unit of measurement increasingly prevailed against competing calculation methods from other clubs and associations . Until the First World War , some organizers of British motorsport events adopted the unit of measurement and determined their racing classes accordingly . From 1910 to January 1947, British motor vehicle tax was levied using the RAC Horsepower .

Many British manufacturers derived the model designation of their motor vehicles from the RAC Horsepower value, in the further course also increasingly through a combination of RAC Horsepower and actual performance in the exemplary form "16/60" . Some of these model names were traditionally retained until the early 1970s or were even revived after being abandoned in the mid-1950s.

Because the RAC calculation formula - supposedly - does not take the piston stroke into account, it is often held responsible for British engine construction that was technically backward until the 1960s. Both the changes to the motor vehicle tax based on the RAC Horsepower and its replacement led to the rapid rise or fall of individual automobile brands in Great Britain several times.

Historical background

The prestigious, almost 70-meter-long front of building 89–91 Pall Mall , London, the headquarters of the British Royal Automobile Club since 1911

On August 10, 1897, Frederick Richard Simms founded the Automobile Club of Great Britain in London , which in 1907 was renamed The Royal Automobile Club (RAC) by Royal Decree of Edward VII . Reliable test beds for combustion engines at that time were not yet widespread or not invented yet easy-to-use devices. The contemporary motor vehicle manufacturers therefore used different measurement procedures and calculation methods to determine engine performance.

The variety of methods and procedures led in practice to contradicting results. Some manufacturers tended to outbid each other in terms of performance for marketing reasons ; Others deliberately set the performance low so that their passenger cars did not fall into the category of regular automobiles , but into that of the smaller, in several ways privileged cycle cars . They were subject to certain weight and , at times, additional displacement and power limits. As a result, it is often difficult to compare the manufacturers' performance data.

The RAC - initially known as the Automobile Club of Great Britain and Ireland - reacted to this in 1906 and introduced a new abstract formula for determining the engine performance of reciprocating internal combustion engines, as well as a new unit of measurement that became known when the club was renamed RAC Horsepower in 1907 .

The calculation formula

An early Heenan & Froude hydraulic dynamometer from 1937

The RAC formula explicitly takes into account the cylinder bore and the number of cylinders of a reciprocating internal combustion engine. The piston stroke and the engine speed do not appear explicitly in it. The formula is:

{\ displaystyle {\ text {RAC hp}} = {\ frac {d ^ {2} \ times n} {2 {,} 5}}}

Here d is the diameter of the cylinder bore in inches and n is the number of cylinders ; the divisor 2.5 is a constant that was considered appropriate at the time , which indirectly records several factors such as the working pressure in the cylinder during combustion that was usual at the time, the piston speed that was usual at that time and the efficiency that was usual at the time .

The formula was easy to use because the number of cylinders was easy to determine and the diameter of the cylinder bore was comparatively easy to check. On the other hand, the RAC omitted the piston stroke, which is more difficult to determine, and a laborious calculation with the - irrational - circle number ( Pi ) in the formula . The biggest drawback of the RAC formula, however, was that the specific engine speed was not taken into account. As the engine designers advanced into ever higher speed ranges in the years and decades that followed, the discrepancy between actual performance and the abstract RAC performance values increased increasingly . ${\ displaystyle \ pi}$

Regarding the - at least from today's point of view - seemingly arbitrary number 2.5 , the RAC pointed out that “it will turn out to be fairly and sufficiently accurate for comparison purposes”, “so that the general public can arrive at an approximate strength of engines compared to others could ". However, the club emphasized from the start that the RAC rating "should not be viewed as an accurate or scientific calculation of actual horsepower". A fixed conversion from RAC hp to kilowatts or DIN hp is therefore not easily possible.

In addition to the above formula, there are occasionally other representations in publications that can be obtained by simple mathematical transformation .

The formula with the divisor 2.5 expressly refers to the engine power on the flywheel with a cylinder bore specified in inches. When specifying the cylinder bore in millimeters, it should be divided by 1613 (mathematically correct would actually be 2.5 × 25.4² = 1612.9). In order to determine the power that is actually transferred from the drive wheels to the road, the divisor 3 was given (when given in inches) ; Accordingly, in the first two decades of the 20th century , the RAC assumed the usual loss of power, in particular due to frictional resistance in the area of the clutch , gearbox , cardan shaft or drive chain , differential and the like in a considerable amount of around 16.67 percent.

The same formula used starting from about 1908, the American Association of Association of Licensed Automobile Manufacturers (ALAM) and its successor organization, National Automobile Chamber of Commerce (NACC). According to another source, the ALAM had already developed this formula in 1905 after evaluating the engine data of its member companies; Although only around half of the US automobile manufacturers were affiliated with it, most of them were considered to be modern and progressive. According to this, however , the British RAC was the organization that had examined the formula more closely for plausibility, endorsed it and was the first to apply it.

Abstract performance assessment and comparability as original goals

A modern engine test bench from the Japanese manufacturer Horiba with a clamped-in reciprocating internal combustion engine for concrete measurements instead of abstract calculations (around 2007)

The Royal Automobile Club and all other institutions that had set themselves the goal of determining the performance of an internal combustion engine faced several problems at the beginning of the 20th century. On the one hand, sufficiently precise, easy-to-use and inexpensive engine test stands for determining the physical unit of horse power were not yet widespread. On the other hand, it was not even clear whether the power (as braking horsepower ) should be measured on the flywheel of the engine or, better, on the drive wheels; alternatively, the performance produced in the cylinders ( indicated performance ) came into consideration. It was also unclear how horse power should be specifically defined as a unit of performance.

The idea, which seems bizarre today, of wanting to mathematically determine the performance based on certain less characteristic data of an internal combustion engine , if necessary only for comparison purposes , was therefore quite obvious from the point of view at the time. A wide variety of formulas were created, some of which required more, some less computational effort. In the first years of motorization up to the 1920s, they provided some fairly precise and some less precise approximate values for the actual output. It must be taken into account that the engine output even of luxury class and racing vehicles was often only in a range of up to around 60 HP / 44 kW. Slight deviations in the formulas were therefore of little consequence at first; In addition, there were major differences in performance within the scope of the series spread due to comparatively large manufacturing tolerances , fluctuating material quality and simple processing methods.

As recently as 1914, British motor vehicle enthusiasts were presented with various formulas in publications, with which they could approximately check the engine performance: The so-called Red Book of 1914, an early form of an automobile catalog, presented the following formulas:

Braking horsepower = displacement in liters × maximum speed × 0.0064;
PS = cylinder bore² × piston stroke × number of cylinders × speed / 12,000 (dimensions in inches);
PS = (cylinder bore + piston stroke) ² × number of cylinders / 6500 (dimensions in millimeters).

The formula of the British Society of Motor Manufacturers was quite common and popular, but a little more complex to calculate :

Brake PS = bore × (bore - 1) × (2 + stroke / bore) × number of cylinders / 5 (dimensions in inches)

In comparison, the Royal Automobile Club formula was much simpler and quickly found acceptance and dissemination. The simplicity was based primarily on simple empirical values from that time. Traditionally, these engines were side-controlled with side-mounted valves ( SV valve control ). As a result, the piston stroke was usually significantly larger than the cylinder bore, often in a ratio of around 4 to 3, a so - called long - stroke design . In addition, the engines of this time typically achieved an average piston speed of up to 1000 feet per minute (304.8 meters per minute or 5.08 meters per second) and their peak performance at around 1500 crankshaft revolutions per minute. These empirical values are reflected in the simple constant 2.5 of the RAC Horsepower formula (for an exact derivation see below).

At the same time, this shows the weaknesses of the RAC formula: The RAC power value remains the same, although doubling and tripling the engine speed in the following years and decades could double to triple the engine power. The RAC performance value also remained the same when the designers, in response to this formula, designed engines with ever longer-stroke engines with larger displacement and thus higher actual output. Improvements through optimized combustion processes with higher compression , better ignition , increased fuel quality , lower manufacturing tolerances and the like also lead to an actual increase in performance without changing the RAC values.

Against this background, the RAC Horsepower formula in this form was not undisputed, even within the association. In individual publications from around 1910 to 1924, especially those that deal with motorcycle and airplane engines, another performance formula ascribed to the RAC can be found:

RAC hp = (bore + stroke) ² × n / 9.92 (dimensions in inches), corresponding approximately

= (Bore + stroke) ² × n / 6400 (dimensions in millimeters).

Officially, however, the RAC Horsepower formula was never adapted to the changed technical conditions, especially not for the automotive sector. The main reason was to maintain continuity in the most important use case, vehicle taxation.

Abstract formulas similar to that of the RAC Horsepower were already in use in the United Kingdom before 1906 and in parallel until the 1920s as nominal horse power, especially for steam engines and turbines as well as diesel engines , especially for stationary engines and as marine engines . Another related equivalent to the RAC Horsepower was the tax horsepower in German-speaking countries. For four-stroke engines between 1922 and 1928 the following formula applied:

Tax HP = 0.30 × number of cylinders × bore² × stroke (bore in centimeters, stroke in meters)

For two-stroke engines , the different factor was 0.45 instead of 0.30.

Then as now, the RAC Horsepower formula enables vehicles from the same era to be compared with one another and to form vehicle categories. Furthermore, the exact RAC Horsepower value of a vehicle can easily be used to determine its cubic capacity, according to which vehicles today are often compared with one another and assigned to certain categories. The formula RAC hp × 5/8 × × piston stroke in inches gives the displacement in cubic inches or the formula RAC hp × 5/8 × × 2.54² × piston stroke in centimeters gives the displacement in cubic centimeters. ${\ displaystyle \ pi}$ ${\ displaystyle \ pi}$

The derivation of the constant divisor 2.5

On closer inspection, the Royal Automobile Club did take into account the piston stroke and the engine speed in its Horsepower formula, in the form of the constant divisor 2.5. The starting point for the development of the RAC Horsepower formula was the general physical formula for power and James Watt's definition of horsepower.

In order to determine the drive power of a multi-cylinder reciprocating internal combustion engine, several factors are required,

the mean working pressure in the cylinder during the working cycle (p _m ),
the area of the piston crown ( × r² = × (d / 2) ² = × d² / 4 with d as the diameter of the cylinder bore), ${\ displaystyle \ pi}$ ${\ displaystyle \ pi}$ ${\ displaystyle \ pi}$
the distance between the top and bottom dead center of the piston (= piston stroke h),
the number of downward movements of the piston per unit of time (= crankshaft revolutions per minute [rpm]),
the ratio of the work cycles per crankshaft revolution (= 1 for two-stroke engines; = ½ for four-stroke engines) and
the number of cylinders (n).

In addition, there is the definition of British horsepower according to James Watt with 1 hp = 550 pound-foot per second = 33,000 pound-foot per minute. Finally, there is a correction factor for engine efficiency (efficiency), because a small part of the power generated in the cylinders during combustion is lost through frictional resistance, as waste heat, through vibrations and the like, and so only a part is lost as usable power (= braking horsepower ) is available at the end of the crankshaft for the drive.

So the following results for the (indicated) power generated in the cylinders of a four-stroke engine:

hp power = p _m × ( × d² / 4) × h × rpm × ½ × n / 33,000 ${\ displaystyle \ pi}$

= p _m × × d² × h × rpm × n / (33,000 × 4 × 2)

{\ displaystyle \ pi}

= p _m x x d² x h x rpm x n / 264,000

{\ displaystyle \ pi}

≈ p _m × d² × h × rpm × n / 84.023 (rounded and simplified as 3.142)

{\ displaystyle \ pi}

For further simplification, the Royal Automobile Club made three assumptions based on empirical values at the time:

1. The RAC uniformly assumed 90 pounds per square inch (psi), corresponding to about 6.3 bar, as the mean working pressure at maximum output . By 1930, however, the usual mean working pressure was already around 125 psi, about 8.75 bar.
2. As a standardization , the RAC assumed 1000 feet per minute as the mean piston speed (= 2 × h × rpm) at maximum power , corresponding to 304.8 meters per minute = 5.08 meters per second. So the club took into account that an engine with a short stroke has experience shown that it achieves its maximum output at higher speeds than an engine with a long stroke. For engines with a stroke of three, three and a half, four and four and a half inches, corresponding to 76.2 mm, 88.9 mm, 101.6 mm and 114.3 mm, the RAC simply assumed an engine speed at maximum power of (partly rounded) 2000, 1714, 1500 or 1333 crankshaft revolutions per minute. By 1930, however, the usual mean piston speed was already around 2,000 feet per minute, and by the mid / late 1930s it was around 2,750.
3. For the degree of efficiency, i.e. the question of what percentage of the power generated in the cylinders by combustion arrives at the end of the crankshaft, the RAC took a unified 75 percent.

By inserting the values assumed by the RAC into the above formula, the result is:

Braking PS = 90 × d² × 1000 × n × 0.75 / (84.023 × 2)

= 67,500 × d² × n / 168,046

= d² × n / 2.489…

Under certain circumstances, 1 RAC hp corresponds exactly to one British horsepower (bhp) = 0.7457 kW, namely at an average working pressure at maximum power of around 6.3 bar (6300 hP), an efficiency of 75 percent, and a speed of , from which results in an average piston speed of 1000 feet per minute. In the case of deviating values, however, the relation changes by the factors x ₁ = p _m / 6.3, x ₂ = v _m / 5.08 and x ₃ = / 75. The general conversion factor between bhp and RAC hp thus depends on the mean working pressure , the speed , the cylinder stroke and the efficiency as follows: ${\ displaystyle \ eta}$ ${\ displaystyle \ textstyle {\ frac {152.4m} {h}} rpm}$ ${\ displaystyle \ eta}$ ${\ displaystyle \ textstyle p_ {m}}$ ${\ displaystyle \ textstyle Upm}$ ${\ displaystyle \ textstyle h}$ ${\ displaystyle \ textstyle \ eta}$ ${\ displaystyle RACh.p. = {\ frac {\ eta \ cdot p_ {m} \ cdot Upm \ cdot h} {720.09 {\ frac {barm} {min}}}} bhp}$

The importance for early motorsport

Participants in the London - Edinburgh 1920 race of the Motor Cycling Club (MCC) , one of the last major long-distance journeys with separate classification for cycle cars

A 1913 British AC 10 hp , one of the most solid models in the large cycle car category; sometimes with a special sports body, it was also used in races

The classification based on the RAC Horsepower gained importance in motorsport in the first two decades of the 20th century. In the early days of motorization and early motorsport, sheer engine power played a subordinate role. Common were long-distance races and reliability trials , known as Trials , also in terms of skill rides . Reliability, endurance and vehicle control were in the foreground, while the road conditions did not allow particularly high speeds due to a lack of asphalt or similar . The RAC appeared early on - initially as the Automobile Club of Great Britain and Ireland - as a motorsport organizer, for example in 1900 at the 1000 Mile Trial or in 1905 at the first Tourist Trophy .

Especially at smaller British motorsport events, motorcycles and conventional automobiles often competed in open classes in terms of power and weight; in the case of large starting fields, however, classes were formed from around 1906 on the basis of abstract performance formulas such as that of the RAC . For example, the RAC Horsepower formula was used in June 1908 at the Royal Automobile Club's 2000 Mile International Touring Car Trial to classify the 50 cars registered.

In addition to regular automobiles and motorcycles, two categories of cycle cars emerged around 1908. The smaller class, mostly three-wheeled vehicles, was often limited solely by their weight, the larger cycle car class by their empty weight and the power or displacement of the engine. In Great Britain and Ireland at that time, the cycle car races were particularly popular because the starting fields were usually much larger than for regular automobiles. The spectators were also more likely to identify with these drivers because of the significantly lower purchase and maintenance costs.

Organizers such as the RAC , the Auto-Cycle Union (ACU) , the Motor Cycling Club (MCC) and the local Cyclecar Clubs also used weight up to the end of 1912, in some cases up to around 1920, for classifying regular automobiles and cycle cars abstract Horsepower formulas like those of the RAC . From 1913, however, such horsepower formulas gradually lost their importance in British motorsport and were replaced by displacement limits: First, the institutions belonging to the Fédération Internationale des Clubs Motocyclistes (FICM) decided in December 1912 to set a limit of 1,100 cubic centimeters displacement for international standardization. which other British motorsport organizers gradually joined. Immediately before the First World War and until around 1921, a displacement limit of 1,500 cubic centimeters finally prevailed internationally; in the United Kingdom the popular light cars replaced the cycle cars .

The importance of UK road tax

An early British “Volkswagen”, an inexpensive Austin 7 from 1929, named after its (rounded) RAC horsepower value

From 1910 to January 1947, automobile or vehicle tax in the United Kingdom was levied solely on the basis of the RAC Horsepower . Before that, taxation was mainly based on weight.

The British automobile tax from 1910

In 1909, the British Chancellor of the Exchequer David Lloyd George presented a budget which for the first time included an annual automobile tax based on the RAC Horsepower formula. Tax revenue from this should be entirely the cost of the construction and the maintenance cover of British streets which makes as a "Road Fund License" became known. The law passed for 1910 also contained a first-time tax on fuel , so that the term "Vehicle Excise Duty" was also in use. According to the graduated scale valid from January 1, 1910, the tax for an automobile with an output of 12 RAC hp increased comparatively moderately to 4 guineas (£ 4-4s-0d) per year, for a vehicle with 60 RAC hp , for road vehicles the highest value reached at that time, to an enormous 21 British pounds sterling . This corresponded to about one to two percent of the respective new price.

British Motor Vehicle Tax from 1921 (RAC Horsepower Tax)

Thanks to the favorable RAC Horsepower classification, an affordable passenger car, a Morris Eight Saloon from 1935

The British Budget Act, which came into force in January 1921, brought about a fundamental change . With it, a vehicle tax was introduced, which became known in Great Britain as the "(RAC) Horsepower Tax" : Although the separate tax on fuels was no longer applicable, a tax of £ 1 per commenced RAC Horsepower was due from now on. Average British passenger cars like the Morris Cowley with 11.9 RAC hp went up to 12 pounds per year, the popular, inexpensive Ford Model T with 22.5 RAC hp to an enormous 23 pounds per year. The British government wanted on the one hand to absorb the high debt burdens that had arisen from the First World War; on the other hand, it wanted to take advantage of the economic upturn that became apparent after the end of the war, but which then lasted only a few months and finally led into a year-long global recession .

However, there were also individual, sometimes strange exceptions to this tax rule. For motor vehicles that were already registered before the First World War, only half the fee was due; Vehicles that only used public roads to bring domestic staff to church or voters to polling stations were completely tax exempt .

The British RAC Horsepower Tax was modified in 1934, as the release of 1 pound (= 21 shillings ) to 15 shillings per RAC hp year was reduced: After the global economic crisis should thus the onset of development to a British mass motorization also by the state get supported. The next change to the RAC Horsepower Tax took place in 1940: In addition to a one-time purchase tax of 33.3 percent on new cars , the annual tax rate was increased to 25 Schillings per RAC Horsepower commenced . The additional income was needed to offset the sharp rise in armaments spending associated with rearmament and Britain's entry into World War II . Taxation according to RAC Horsepower lasted until January 1947.

It followed:

from January 1947 an annual vehicle tax of 1 pound for every 100 cubic centimeters or part thereof; this was intended to promote the development of modern, short-stroke engines.
From the beginning of 1948 for every new passenger car registered after January 1947 an annual flat tax of only 10 pounds, plus a purchase tax doubled from 33.3 to 66.6 percent for new cars with a price over 1,000 pounds. This was intended to support the development of more powerful vehicles that were required for vehicle export and the foreign exchange income to be generated from it . At the same time, a kind of “ luxury tax ” was introduced for the domestic market in the form of the staggered purchase tax to compensate for the tax shortfalls that were expected from the uniform vehicle tax.
from 1951 an annual flat tax of £ 10 for all - including older - passenger cars while maintaining the graduated new car purchase tax. This was intended to make it easier to continue using pre-war vehicles with a larger displacement, as long as the high export quotas meant that there were not enough new vehicles available for the domestic market; and
from 1955 a return to the displacement-dependent vehicle tax, after a sufficient supply of new cars for the domestic market again existed.

In addition to taxation, the RAC Horsepower was also the benchmark for fuel rationing in Great Britain during World War II and for older motor vehicles during the Suez crisis of 1956/57.

The influence on the model names of British automobiles

One of the last vehicles whose model name was still derived from the RAC Horsepower value, an Alvis TF 21 Drophead Coupé from 1966

One of the last vehicles whose model name combined the RAC Horsepower value with the actual performance, a Wolseley 18/85 from around 1971

In the early years of motorization, British manufacturers mostly only used the horsepower information to describe a vehicle model in more detail or to differentiate between their own vehicle models. At first it remained unclear which measurement method or calculation formula was used for the horsepower. With the introduction of the automobile tax according to RAC Horsepower from 1910, the motor vehicle manufacturers increasingly decided on the RAC Horsepower formula for their model names . Other model names, for example with the cubic capacity in liters as in the case of Bentley or separate names such as " Silver Ghost " in the case of Rolls-Royce , remained brand-specific exceptions - even as a mere supplement.

Many passenger car models were named after the RAC Horsepower values, which made a significant contribution to mass motorization in the United Kingdom and, with their economic success, to the flourishing of individual automobile brands. Examples are the Austin 7 from 1922 to 1939, the Morris Eight from 1935 to 1952 and the Singer 9 from 1932 to 1952. From the 1920s, there were increasing numbers of model names in which the RAC horsepower values were combined with those of the actual performance were. Examples are the Alvis 12/50 from 1923 to 1932 and the Wolseley 21/60 from 1928 to 1935.

Even after the end of the RAC Horsepower Tax in January 1947, many British automobile manufacturers continued to use model names that were based on the RAC Horsepower values. At Alvis there were the three-liter models TA 21 to TF 21 until 1967 and at Armstrong Siddeley the model 18 (as Whitley and Hurricane ) until 1953. At Austin , the sedan series Eight , Ten , Twelve and Sixteen ran out by 1948 However, such model names were revived with the A30 Seven from 1951 to 1956 and the Seven 850 from 1959 to 1967 (the original Mini ). At Lea-Francis the models 12 , 14 and 18 still existed until 1954, at Singer the 9 Roadster until 1952. At Standard the models Eight , Twelve and Fourteen ran out in 1947, but there were new models called Eight and Ten again from 1953 to 1959 and as a Kombi Ten Companion until 1961. Wolseley revived the model names that were abandoned in 1948 with combined hp values for the 15/50 , 15/60 , 16/60 and 18/85 models from 1956 to 1972.

The influence on the technical development of British engine construction

One of the last British models to have a long-stroke, side-actuated valve engine, a 1959 Ford Popular

Despite the long-stroke engine design, the Jaguar D-Type , which was used from 1954, was very successful in racing

The RAC Horsepower formula and the vehicle taxation based on it are seen as the reason why engine construction in Great Britain was technically backward until the 1960s. In fact, the formula supported a long-stroke engine design; Although this enables high-torque engines, it affects the revving and the use of higher speeds, so that the specific engine power remained below average.

The incentive to make the cylinder bore small and the piston stroke large, and thus to keep the vehicle tax low, actually led some manufacturers to stick to conservative engines for a comparatively long time; The hallmarks of this were valves that stood on the side instead of hanging and L- or T-shaped instead of modern hemispherical combustion chambers . The background to this was that it was difficult to reconcile sufficiently large intake and exhaust valves with small cylinder bores when hanging. For example, the inexpensive British Ford models Prefect and Popular used engines with side-mounted valves until 1959 and 1962, respectively, while Rover used vertical exhaust valves in its six-cylinder sedan models until 1967.

On the other hand, British automobile manufacturers like Alvis switched to modern engines with overhead valves ( OHV valve control in series from 1923) and even to those with an overhead camshaft ( OHC valve control in series from 1928). For the British automobile manufacturer AC Cars , the designer John Weller designed and built the first four-cylinder and from 1919 six-cylinder engines with OHC valve control as early as 1916 ; they were so advanced that the six-cylinder units were still available in new AC vehicles until 1962 . The privilege of engines with narrow, high cylinders by several engine manufacturers also led to progressive designs; In their case, the valves were not hung parallel to one another, but rather inclined in a larger V-shape to one another, which also resulted in more favorable roof-shaped combustion chambers.

The frequent assertion that the RAC Horsepower formula is responsible for particularly long-stroke engines and the temporarily backward engine construction in Great Britain is countered by specialist book authors with reference to parallel developments in other European countries. The specialist book author John Mclellan refers to the AC inline six-cylinder from 1921; With a bore of only 65 millimeters and a stroke of 100 millimeters, it has exactly the same cylinder dimensions as the engine of the Fiat racing car Tipo 804 Grand Prix of 1922, which was repeatedly classified as brilliant and designed by the Italian engineer Giulio Cappa . Jaguar was also able to compete in the Won the prestigious Le Mans 24-hour race in 1953 and 1955 to 1957 , although the engines with a bore of 83 millimeters and a stroke of 106 millimeters were unusually long and therefore less easy to turn than the favored Ferrari racing cars.

The influence on the rise and loss of importance of individual automobile brands

Profited greatly from the introduction of the RAC Horsepower Tax , the long-stroke Morris Cowley 11.9 hp "Bullnose" , here a vehicle from 1924

In the United Kingdom suffered heavily from the RAC Horsepower Tax from 1921 , the less long-stroke Ford Model T , also from 1924 here

When an automobile tax was introduced in the United Kingdom in 1910, which was first based on the RAC Horsepower , only members of the wealthy upper class could afford cars. The realigned tax had hardly any noticeable impact on this group of people and the sales figures were still so low that they had no significant impact on the British automotive market.

The RAC Horsepower Tax of 1921, however, led to fundamental market shifts: Until then, the originally American Ford Model T dominated the British market. It was assembled in Ford's first UK factory in Manchester and was available from £ 220 in 1919. When the British road tax for this relatively large, less long-stroke model with 22.5 RAC hp was raised to 23 pounds a year in 1921, almost 10.5 percent of the new price, its sales plummeted. The Morris Cowley 11.9 hp , however, experienced an enormous rise. In the few months of the boom after the First World War, the model with the nickname “Bullnose” initially cost at least 465 pounds, with an annual vehicle tax of only 12 pounds and significantly lower fuel consumption however, its maintenance costs are significantly lower. In addition, the base price was lowered in two stages to 299 pounds by the end of 1922. Influenced by this, sales of the Morris model rose to 54,151 units in 1925, which corresponded to a remarkable 41 percent of British car production. Also, Austin experienced a similar upswing after the law was the 1,921th

At the same time, this was accompanied by significant processes of concentration in the British motor vehicle market, when the up-and-coming manufacturer Morris took over independent suppliers for radiators , bodies , engines, axles and carburetors , which were previously essential for other, smaller car manufacturers. These included the body construction company Hollick & Pratt , the British subsidiary of Hotchkiss et Cie as an engine manufacturer and the carburetor manufacturer SU .

On the other hand, the British Ford subsidiary experienced a major boom when, after the end of the RAC Horsepower Tax for the 1950 model year, it presented a new generation of four- and six-cylinder mid-range models with modern short-stroke engines. As a result, the British motor vehicle market changed again dramatically: In order to secure its own body demand, Ford took over the country's largest, until then independent body construction company Briggs Motor Bodies , whereby the financially troubled automobile manufacturer Jowett Cars lost its supplier and ceased operations. The British Motor Corporation (BMC) responded that same year by taking over the large independent coachbuilder Fisher & Ludlow , which brought its other major customer, Standard - Triumph , into trouble. In 1954 this group contractually secured the complete body production of Mulliners of Birmingham before finally taking over the company in 1958. Small series manufacturers such as Alvis , Aston Martin and Daimler lost an important bodywork supplier, which led to further mergers.

literature

Peter JS Dennet: The Decline of the British Motor Industry - The Effects of Government Policy 1945-1979 (Routledge Revivals) . Routledge, Abingdon, Oxon, United Kingdom 2011, ISBN 978-0-415-67938-1 (English).
Robert D. Dluhy: American Automobiles of the Brass Era: Essential Specifications of 4000+ Gasoline Powered Passenger Cars, 1906-1915, with a Statistical and Historical Overview . McFarland & Company, Inc., Publishers, Jefferson, North Carolina, United States and London, United Kingdom 2013, ISBN 978-0-7864-7136-2 , pp. 35 ff . (English).
James J. Flink: The Automobile Age . MIT Press, Cambridge, Massachusetts, United States / London, United Kingdom 1990, ISBN 978-0-262-56055-9 (English).
Kenneth Richardson: The British Motor Industry 1896-1939. A Social and Economic History . The Macmillan Press Limited, London and Basingstoke, United Kingdom 1977, ISBN 978-1-349-03390-4 (English).
Jonathan Wood: Wheels of Misfortune: The Rise and Fall of the British Motor Industry . Sidgwick & Jackson, London, United Kingdom 1988, ISBN 978-0-283-99527-9 (English).
Don Goodsell: Dictionary of Automotive Engineering . Butterworth-Heinemann, Oxford, United Kingdom 2013, ISBN 978-1-4831-0206-1 , pp. 119 (English).

Web links

Richard Hodgson: The RAC HP (horsepower) Rating - Was there any technical basis? designchambers.com/wolfhound.org.uk, 2001, accessed June 13, 2016 .
The RAC Horsepower unit on the diracdelta.co.uk web portal , accessed on June 12, 2016.

Doug Castle: The RAC HP Rating. austin7.org, 2009, accessed June 13, 2016 .

Johnny Oversteer: Stroke me - The origin of the species and the evolution of the British sports cars. mossmotoring.com, April 1, 2015, accessed June 13, 2016 .

Individual evidence

↑ ^a ^b Overview of the history of the British Royal Automobile Club on its website royalautomobileclub.co.uk , accessed on June 12, 2016 (English).
↑ ^a ^b ^c ^d ^e Jonathan Wood: The Enthusiast's Guide to British Postwar Classic Cars . Osprey Publishing Limited, London, 1980, ISBN 0-85045-377-1 , page 19 (English).
↑ ^a ^b ^c ^d page 2 of the Red Book for 1914, reproduced on the web portal gracesguide.co.uk , accessed on June 12, 2016 (English).
↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Richard Hodgson: The RAC HP (horsepower) Rating - Was there any technical basis? on the subpage wolfhound.org.uk of the web portal designchambers.com , 2001 , accessed on June 12, 2016 (English).
↑ ^a ^b ^c ^d ^e ^f The RAC Horsepower unit on the diracdelta.co.uk web portal ( memento of the original from October 17, 2015 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , accessed on June 12, 2016. @1@ 2

↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Background information on RAC Horsepower on the web portal austin7.org , accessed on June 13, 2016 (English).

↑ ^a ^b The ALAM Horsepower unit on the sizes.com website , accessed on June 12, 2016 (English).

^ Robert D. Dlouhy: American Automobiles of the Brass Era: Essential Specifications of 4000+ Gasoline Powered Passenger Cars, 1906-1915, with Statistical and Historical Overview . McFarland & Company, Inc., Publishers. Jefferson, North Carolina, United States and London, United Kingdom, 2013. ISBN 978-0-7864-7136-2 , pp. 35 f. (English).

^ Victor W. Pagé: Early Motorcycles: Construction, Operation and Repair . Dover Publications, Inc., Mineola, New York, United States, 2004, ISBN 0-486-43671-3 , pages 97 to 100 (99), reprint of 2nd edition of a work published in 1st in 1914, 2nd in 1920 2nd edition and 1924 as a reprint (English).

^ NN in: Automotive Industries (magazine), 1910, volume 23, page 593 (English).

^ John B. Rathbun: Airplane Engines in Theory and Practice . Stanton and Van Vliet Company, 1921, p. 298.

↑ Kevin Atkinson: The Singer Story . Veloce Publishing Limited. Poundbury, Dorchester, Dorset, United Kingdom, 2016, ISBN 978-1-874105-52-7 (English).

↑ John Mclellan: Classic ACs - Car Carrier to Cobra . Sutton Publishing Limited. Thrupp, Stroud, Gloucestershire, United Kingdom, 2000, ISBN 978-0-7509-2042-1 , pages 3 to 9, 13 and 19 to 29 (English).

↑ ^a ^b ^c ^d ^e ^f ^g Jonathan Wood: The Enthusiast's Guide to British Postwar Classic Cars . Osprey Publishing Limited, London, 1980, ISBN 0-85045-377-1 , page 20 (English).

^ Nick Baldwin: The World Guide to Automobile Manufacturers . Facts on File Publications. New York City, New York, United States, 1987, ISBN 978-0-8160-1844-4 , page 533.

↑ ^a ^b ^c ^d ^e ^f ^g ^h Jonathan Wood: The Enthusiast's Guide to British Postwar Classic Cars . Osprey Publishing Limited, London, 1980, ISBN 0-85045-377-1 , page 50 (English).

↑ NN: Notes and Comments - The wrong way to ration , in: New Scientist (magazine). Reed Business, November 29, 1956, p. 6.

^ ^A ^b Roger Gloor: All cars of the 50s, 1945-1960 . Motorbuch Verlag, Stuttgart. 1st edition 2007. ISBN 978-3-613-02808-1 as well as all cars of the 60s . Motorbuch Verlag, Stuttgart. 1st edition 2006. ISBN 978-3-613-02649-0 (each sorted alphabetically).

↑ John Mclellan: Classic ACs - Car Carrier to Cobra . Sutton Publishing Limited. Thrupp, Stroud, Gloucestershire, United Kingdom, 2000, ISBN 0-7509-2042-4 , pages 9 and 17 f. (English).

↑ John Mclellan: Classic ACs - Car Carrier to Cobra . Sutton Publishing Limited. Thrupp, Stroud, Gloucestershire, United Kingdom, 2000, ISBN 0-7509-2042-4 , page 70 (English).

↑ John Mclellan: Classic ACs - Car Carrier to Cobra . Sutton Publishing Limited. Thrupp, Stroud, Gloucestershire, United Kingdom, 2000, ISBN 0-7509-2042-4 , page 22.

↑ Jonathan Wood: The Enthusiast's Guide to British Post War Classic Cars . Osprey Publishing Limited, London, 1980, ISBN 0-85045-377-1 , pages 54 to 57 (English).

Remarks

↑ In the mostly English-language sources, the term “Horsepower” is abbreviated differently then as it is today; Alternatively, upper and lower case letters and abbreviations with and without periods are used. The abbreviated form RAC is sometimes not used.
↑ According to the web portal austin7.org , the tariffs amounted to 2 guineas for vehicles up to 6.5 RAC hp , 3 guineas for vehicles below 12 hp and 4 guineas for vehicles below 16 hp. For more powerful vehicles below 26 hp, 6 guineas fell for those under 33 hp 8 guineas and under 40 hp 10 guineas. This source names 21 guineas for vehicles under 60 hp.

[rac-2] Overview of the history of the British Royal Automobile Club on its website royalautomobileclub.co.uk , accessed on June 12, 2016 (English).

[Wood19-3] Jonathan Wood: The Enthusiast's Guide to British Postwar Classic Cars . Osprey Publishing Limited, London, 1980, ISBN 0-85045-377-1 , page 19 (English).

[gracesguide-4] ↑ ^a ^b ^c ^d page 2 of the Red Book for 1914, reproduced on the web portal gracesguide.co.uk , accessed on June 12, 2016 (English).

[wolfhound-5] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Richard Hodgson: The RAC HP (horsepower) Rating - Was there any technical basis? on the subpage wolfhound.org.uk of the web portal designchambers.com , 2001 , accessed on June 12, 2016 (English).

[diracdelta-6] ↑ ^a ^b ^c ^d ^e ^f The RAC Horsepower unit on the diracdelta.co.uk web portal ( memento of the original from October 17, 2015 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , accessed on June 12, 2016. @1@ 2

[austin-7] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Background information on RAC Horsepower on the web portal austin7.org , accessed on June 13, 2016 (English).

[sizes-8] The ALAM Horsepower unit on the sizes.com website , accessed on June 12, 2016 (English).

[9] Robert D. Dlouhy: American Automobiles of the Brass Era: Essential Specifications of 4000+ Gasoline Powered Passenger Cars, 1906-1915, with Statistical and Historical Overview . McFarland & Company, Inc., Publishers. Jefferson, North Carolina, United States and London, United Kingdom, 2013. ISBN 978-0-7864-7136-2 , pp. 35 f. (English).

[10] Victor W. Pagé: Early Motorcycles: Construction, Operation and Repair . Dover Publications, Inc., Mineola, New York, United States, 2004, ISBN 0-486-43671-3 , pages 97 to 100 (99), reprint of 2nd edition of a work published in 1st in 1914, 2nd in 1920 2nd edition and 1924 as a reprint (English).

[11] NN in: Automotive Industries (magazine), 1910, volume 23, page 593 (English).

[12] John B. Rathbun: Airplane Engines in Theory and Practice . Stanton and Van Vliet Company, 1921, p. 298.

[13] Kevin Atkinson: The Singer Story . Veloce Publishing Limited. Poundbury, Dorchester, Dorset, United Kingdom, 2016, ISBN 978-1-874105-52-7 (English).

[14] John Mclellan: Classic ACs - Car Carrier to Cobra . Sutton Publishing Limited. Thrupp, Stroud, Gloucestershire, United Kingdom, 2000, ISBN 978-0-7509-2042-1 , pages 3 to 9, 13 and 19 to 29 (English).

[Wood20-15] ↑ ^a ^b ^c ^d ^e ^f ^g Jonathan Wood: The Enthusiast's Guide to British Postwar Classic Cars . Osprey Publishing Limited, London, 1980, ISBN 0-85045-377-1 , page 20 (English).

[17] Nick Baldwin: The World Guide to Automobile Manufacturers . Facts on File Publications. New York City, New York, United States, 1987, ISBN 978-0-8160-1844-4 , page 533.

[Wood50-18] ↑ ^a ^b ^c ^d ^e ^f ^g ^h Jonathan Wood: The Enthusiast's Guide to British Postwar Classic Cars . Osprey Publishing Limited, London, 1980, ISBN 0-85045-377-1 , page 50 (English).

[19] NN: Notes and Comments - The wrong way to ration , in: New Scientist (magazine). Reed Business, November 29, 1956, p. 6.

[Gloor-20] A ^b Roger Gloor: All cars of the 50s, 1945-1960 . Motorbuch Verlag, Stuttgart. 1st edition 2007. ISBN 978-3-613-02808-1 as well as all cars of the 60s . Motorbuch Verlag, Stuttgart. 1st edition 2006. ISBN 978-3-613-02649-0 (each sorted alphabetically).

[21] John Mclellan: Classic ACs - Car Carrier to Cobra . Sutton Publishing Limited. Thrupp, Stroud, Gloucestershire, United Kingdom, 2000, ISBN 0-7509-2042-4 , pages 9 and 17 f. (English).

[22] John Mclellan: Classic ACs - Car Carrier to Cobra . Sutton Publishing Limited. Thrupp, Stroud, Gloucestershire, United Kingdom, 2000, ISBN 0-7509-2042-4 , page 70 (English).

[23] John Mclellan: Classic ACs - Car Carrier to Cobra . Sutton Publishing Limited. Thrupp, Stroud, Gloucestershire, United Kingdom, 2000, ISBN 0-7509-2042-4 , page 22.

[24] Jonathan Wood: The Enthusiast's Guide to British Post War Classic Cars . Osprey Publishing Limited, London, 1980, ISBN 0-85045-377-1 , pages 54 to 57 (English).

[1] In the mostly English-language sources, the term “Horsepower” is abbreviated differently then as it is today; Alternatively, upper and lower case letters and abbreviations with and without periods are used. The abbreviated form RAC is sometimes not used.

[16] According to the web portal austin7.org , the tariffs amounted to 2 guineas for vehicles up to 6.5 RAC hp , 3 guineas for vehicles below 12 hp and 4 guineas for vehicles below 16 hp. For more powerful vehicles below 26 hp, 6 guineas fell for those under 33 hp 8 guineas and under 40 hp 10 guineas. This source names 21 guineas for vehicles under 60 hp.