SU Carburettor
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SU carburettors were a British manufacturer of constant-depression
carburettors A carburetor (also spelled carburettor) is a device used by an internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the venturi tube in the main meterin ...
. Their designs were in mass production during most of the twentieth century. The S.U. Carburetter Company Limited also manufactured dual-choke updraught carburettors for aero-engines such as the
Rolls-Royce Merlin The Rolls-Royce Merlin is a British liquid-cooled V-12 piston aero engine of 27-litres (1,650  cu in) capacity. Rolls-Royce designed the engine and first ran it in 1933 as a private venture. Initially known as the PV-12, it was later ...
and
Rolls-Royce Griffon The Rolls-Royce Griffon is a British 37-litre (2,240  cu in) capacity, 60-degree V-12, liquid-cooled aero engine designed and built by Rolls-Royce Limited. In keeping with company convention, the Griffon was named after a bird of prey, ...
.


Invention and development

Herbert Skinner (1872–1931), pioneer motorist and an active participant in the development of the petrol engine,Obituary. Mr. G. H. Skinner. ''The Times'', Wednesday, Jan 06, 1932; pg. 12; Issue 46023 invented his Union carburettor in 1904.E A Forward, ''Handbook of the Collections illustrating Land Transport, II. Mechanical Road Vehicles'', Science Museum South Kensington, 1936 His much younger brother, Carl (Thomas Carlisle) Skinner (1882–1958), also a motoring enthusiast, had joined the Farman Automobile Co in London in 1899.Obituary. Mr. Thomas C. Skinner ''The Times'', Saturday, Nov 15, 1958, Issue 54309, p.10. He helped Herbert to develop the carburettor. Herbert's
son A son is a male offspring; a boy or a man in relation to his parents. The female counterpart is a daughter. From a biological perspective, a son constitutes a first degree relative. Social issues In pre-industrial societies and some current c ...
could remember his mother sewing the first leather bellows. It would be given on loan to
The Science Museum The Science Museum is a major museum on Exhibition Road in South Kensington, London. It was founded in 1857 and is one of the city's major tourist attractions, attracting 3.3 million visitors annually in 2019. Like other publicly funded ...
, South Kensington in 1934. In 1905, Herbert applied for a patent,H. Jones: Herbert Wakefield Banks Skinner. 1900-1960. Biographical Memoirs of Fellows of the Royal Society, Vol.6, (Nov. 1960), pp.259-268, publisher: The Royal Society which was granted in early 1906. Later, Carl sold his interest in footwear business Lilley & Skinner and became a partner in G Wailes & Co of
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, London, manufacturers of their carburettor. Herbert continued to develop and patent improvements through to the 1920s, including the replacement of the leather bellows by a brass piston, even though he was a full-time director and divisional manager of Lilley & Skinner.


S. U. Company Limited

S. U. Company Limited — Skinner-Union —Profile Professor Herbert Skinner, ''The New Scientist'' 14 November 1957James Leasor, ''Wheels to Fortune'', Stratus, Cornwall 2001 was incorporated in August 1910S. U. Company Limited, 386—388 Euston Road, N.W. Capital £5,000 in £1 shares. Formed to acquire from G. H. Skinner certain inventions relating to carburettors for motorcars &c
New Companies Registered. Private Companies. The Automotor Journal, 24 September 1910
to acquire Herbert's carburettor inventions, and it began manufacture of the carburettors in a factory at Prince of Wales Road,
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, in North London. Sales were slow. Following the outbreak of war in 1914, carburettor production nearly stopped, with the factory making machine gun parts and some aircraft carburettors. With peace in 1918, production resumed, but sales remained slow and the company was not profitable, so Carl Skinner approached his customer, W. R. Morris, and managed to sell him the business. Carl Skinner (T C Skinner) became a director of Morris's privately held empire, and remained managing director of S.U. until he retired in 1948 aged 65. Production was moved to the W R Morris-owned Wolseley factory at
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,
Birmingham Birmingham ( ) is a city and metropolitan borough in the metropolitan county of West Midlands in England. It is the second-largest city in the United Kingdom with a population of 1.145 million in the city proper, 2.92 million in the West ...
. In 1936, W R Morris sold many of his privately held businesses, including S. U., to his listed company,
Morris Motors Morris Motors Limited was a British privately owned motor vehicle manufacturing company formed in 1919 to take over the assets of William Morris's WRM Motors Limited and continue production of the same vehicles. By 1926 its production represent ...
.James Leasor, ''Wheels to Fortune'', Stratus, Cornwall, 2001


The S. U. Carburetter Company Limited

Manufacture continued, now by The S. U. Carburetter Company Limited, which was incorporated 15 September 1936,
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company number 00318520
as part of the Morris Organization, later known as the
Nuffield Organization Nuffield Organization was the unincorporated umbrella-name or promotional name used for the charitable and commercial interests of owner and donor, William Morris, 1st Viscount Nuffield. The name was assumed following Nuffield's gift made to form ...
. The company became a subsidiary of
British Leyland British Leyland was an automotive engineering and manufacturing conglomerate formed in the United Kingdom in 1968 as British Leyland Motor Corporation Ltd (BLMC), following the merger of Leyland Motors and British Motor Holdings. It was partl ...
, and traded under the name SU Carburetters. The S. U. Carburetter Company Limited of 1936 was voluntarily liquidated in December 1994. In 1996, the name and rights were acquired by Burlen Fuel Systems Limited of
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, which incorporated an entirely new company with the name The S.U. Carburetter Company Limited, which continues to manufacture carburettors, pumps and components, mainly for the
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market.


Applications

S.U. carburettors were widely used not only in Morris's
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and MG products but
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,
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,
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, Riley,
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,
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,
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,
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and Swedish
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, for much of the twentieth century. S.U. also produced carburettors for
aircraft engine An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years many ...
s including the early versions of the
Rolls-Royce Merlin The Rolls-Royce Merlin is a British liquid-cooled V-12 piston aero engine of 27-litres (1,650  cu in) capacity. Rolls-Royce designed the engine and first ran it in 1933 as a private venture. Initially known as the PV-12, it was later ...
, but these were of the conventional fixed-jet updraught type rather than the firm's patented constant-depression design. S.U. carburetors ( American spelling) were also a popular upfit for Harley-Davidson motorcycles, given the superior ability to self compensate for changes in air density/altitude and the "side draft" design. Many owners replaced the stock Linkert, Bendix or Keihin carbs with SU's until the Keihin Constant Velocity carb became stock in 1990 They remained on production cars through to 1994 in the
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and the
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by which time the company had become part of the
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.
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also built carburettors based on the SU design which were used on the
Datsun 240Z The Nissan S30 (sold in Japan as the Nissan Fairlady Z and in other markets as the Datsun 240Z, then later as the 260Z and 280Z) is the first generation of Z GT 3-door two-seat coupés, produced by Nissan Motors, Ltd. of Japan from 1969 until ...
, Datsun 260Z and other
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Cars. While these appear the same, only their needles are interchangeable.


Operating principle

A similar description of the below content can be found here: SU carburettors featured a variable venturi controlled by a
piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tig ...
. This piston has a tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits
fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...
into the airstream passing through the carburettor. Since the needle is tapered, as it rises and falls it opens and closes the opening in the jet, regulating the passage of
fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...
, so the movement of the piston controls the amount of fuel delivered, depending on engine demand. The exact dimensions of the taper are tailored during engine development. The flow of air through the venturi creates a reduced
static pressure In fluid mechanics the term static pressure has several uses: * In the design and operation of aircraft, ''static pressure'' is the air pressure in the aircraft's static pressure system. * In fluid dynamics, many authors use the term ''static pres ...
in the venturi. This pressure drop is communicated to the upper side of the piston via an air passage. The underside of the piston is open to atmospheric pressure. The difference in pressure between the two sides of the piston lifts the piston. Opposing this are the weight of the piston and the force of a spring that is compressed by the piston rising. Because the spring is operating over a very small part of its possible range of extension, its force is approximately constant. Under steady state conditions the upwards and downwards forces on the piston are equal and opposite, and the piston does not move. If the airflow into the engine is increased - by opening the throttle plate (usually referred to as the "butterfly"), or by allowing the engine revs to rise with the throttle plate at a constant setting - the pressure drop in the venturi increases, the pressure above the piston falls, and the piston is pushed upwards, increasing the size of the venturi, until the pressure drop in the venturi returns to its nominal level. Similarly if the airflow into the engine is reduced, the piston will fall. The result is that the pressure drop in the venturi remains the same regardless of the speed of the airflow - hence the name "constant depression" for carburettors operating on this principle - but the piston rises and falls according to the rate of air delivery. Since the position of the piston controls the position of the needle in the jet and thus the open area of the jet, while the depression in the venturi sucking fuel out of the jet remains constant, the rate of fuel delivery is always a definite function of the rate of air delivery. The precise nature of the function is determined by the profile of the needle. With appropriate selection of the needle, the fuel delivery can be matched much more closely to the demands of the engine than is possible with the more common fixed-venturi carburettor, an inherently inaccurate device whose design must incorporate many complex fudges to obtain usable accuracy of fuelling. The well-controlled conditions under which the jet is operating also make it possible to obtain good and consistent atomisation of the fuel under all operating conditions. This self-adjusting nature makes the selection of the maximum venturi diameter (colloquially, but inaccurately, referred to as "choke size") much less critical than with a fixed-venturi carburettor. To prevent erratic and sudden movements of the piston it is damped by light oil (20W Grade) in a
dashpot A dashpot, also known as a damper, is a mechanical device that resists motion via viscous friction. The resulting force is proportional to the velocity, but acts in the opposite direction, slowing the motion and absorbing energy. It is commonly us ...
, which requires periodic replenishment. The damping is asymmetrical: it heavily resists upwards movement of the piston. This serves as the equivalent of an "accelerator pump" on traditional carburettors by temporarily increasing the speed of air through the venturi when the throttle is suddenly opened, thus increasing the richness of the mixture. SU carburettors do not have a conventional choke flap, which in a fixed-jet carburettor enriches the mixture for starting the engine from cold by restricting the air supply upstream of the venturi. Instead a mechanism lowers the jet assembly, which has the same effect as the needle rising in normal operation - namely increasing the supply of fuel so that the carburettor will now deliver an enriched mixture at all engine speeds and throttle positions. The 'choke' mechanism on an SU carburettor usually also incorporates a system for holding the throttle plate slightly open to raise the engine's idling speed and prevent stalling at low speeds due to the rich mixture. The beauty of the SU lies in its simplicity and lack of multiple jets and ease of adjustment. Adjustment is accomplished by altering the starting position of the jet relative to the needle on a fine
screw A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to fa ...
(26TPI for most pre-HIF versions). At first sight, the principle appears to bear a similarity to that of the slide carburettor, which was previously used on many
motorcycles A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle steered by a handlebar. Motorcycle design varies greatly to suit a range of different purposes: long-distance travel, commuting, cruising, ...
. The slide carburettor has the same piston and main needle as an SU carburettor, however the piston/needle position is directly actuated by a physical connection to the throttle cable rather than indirectly by venturi airflow as with an SU carburettor. This piston actuation difference is the significant distinction between a slide and an SU carburettor. The piston in a slide carburettor is controlled by the operator's demands rather than the demands of the engine. This means that the metering of the fuel can be inaccurate unless the vehicle is travelling at a constant speed at a constant throttle setting - conditions rarely encountered except on motorways. This inaccuracy results in fuel waste, particularly as the carburettor must be set slightly rich to avoid a lean condition (which can cause engine damage). For this reason
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ese motorcycle manufacturers ceased to fit slide carburettors and substituted constant-depression carburettors, which are essentially miniature SUs. It is also possible - indeed, easy - to retrofit an SU carburettor to a
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that was originally manufactured with a slide carburettor, and obtain improved fuel economy and more tractable low-speed behaviour. One of the downsides of the constant depression carburettor is in high performance applications. Since it relies on restricting air flow in order to produce enrichment during acceleration, the throttle response lacks punch. By contrast, the fixed choke design adds extra fuel under these conditions using its accelerator pump.


SU carburettor types

SU carburettors were supplied in several throat sizes in both Imperial (inch) and metric (millimetre) measurement. The carburettor identification is made by letter prefix which indicates the float type: : "H": introduced in 1937 in which the float bowl has an arm cast into its base, which mounts to the bottom of the carburetor with a hollow bolt or banjo fitting. Fuel passes through the arm into the carburetor body. The bolt attaches to the carburetor body just behind the main jet assembly. : "HD": introduced in 1954 with the float bowl mounted with its arm fastening directly below, and concentric with, the main jet. The arm has a flange that fastens with 4 screws to the bottom of the carburetor, and sealed with a rubber diaphragm integral with the main jet. : "HS": introduced in 1958 the float bowl can be rigidly or rubber mounted to the main body, fuel is transferred by an external flexible pipe to the jet. The jet moves down to enrich the mixture for cold starting,when the 'choke' linkage is pulled. : "HIF": (1972) the float bowl is horizontal and integral (hence the name) Horizontal Integral Float. Example: 1972-1974 MGB. : "HV" (1929), "OM" and "KIF" types also exist but were less commonly employed.SU technical pages
/ref> The Imperial sizes include 1-1/8", 1-1/4", 1-1/2", 1-3/4", 1-7/8", and 2", although not every type (H, HD, HS, HIF) was offered in every size. There were also H models made in 2-1/4" and 2-1/2", now obsolete. Special purpose-built carburetors (Norman) were made as large as 3". To determine the throat size from the type number: If the final number (after one, two or three letters, beginning with H) has 1 digit, multiply this number by 1/8", then add 1". For example, if the type number is HS6, the final number is 6: 6/8 = 3/4", add 1, total is 1-3/4", etc. If the final number has 2 digits, it is the throat size in mm. For example, if the type number is HIF38, the final number is 38, size is 38 mm etc.


SU fuel pumps

In 1929 SU introduced the Petrolift electric fuel pump which could be fitted as a substitute for the vacuum type pumps common at the time. This was superseded in 1932 by the L type fuel pump which used a
solenoid upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whose ...
to operate a diaphragm pump.


See also

*
Amal (carburettor) AMAL was a British engineering company servicing the motorcycle and other light-engineering motor industries between 1927 and 1993Zenith carburettor The Zenith Carburetter Company Limited was a British company making carburettors in Stanmore Middlesex, founded in 1912 as a subsidiary of the French . In 1965, the company joined with its major pre-war rival Solex Carburettors, and over ...


Notes


References


External links


Burlen Fuel Systems - manufacturer of genuine SU carburetters
* ttp://www.classiccarhub.co.uk/articles/perfomance/guide_to_setting_up_tuning_su_carburettors.html Guide to setting up and tuning SU carburettorsbr>Classic Motorsports magazine's guide to understanding, tuning and rebuilding SUs
{{DEFAULTSORT:SU carburettor Constant-depression carburettors Carburetor manufacturers