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For the story from start to finish see - Subaru and the Impreza For other subaru models (Historic) see - 360, 1600, Alcyone, Alcyone SVX, BRAT, FF-1 G, FF-1 Star, Justy, Leone, Loyale, Rex, Vivio For the performance version see, WRX, STI, Subaru Tecnica International For Subaru VIN Codes see VIN Codes For information on AWD see AWD - What is it? For differential information see - Differentials, LSD ~(Limited Slip) For flat-4 configuration information see - Flat-4 What is it? For TurboCharge see - Turbo Charger - What is it?, Turbo operation, Design, Reliability, Lag, Boost, Wastegate For BOV (Blow off Valve) see - BOV For Intercooler information see - Intercoolers For information about Brake Horse Power see- BHP For information about Fuji Heavy Industry See- Fuji For information about Engine Knocking see - Engine Knocking, Detonation, pre-ignition For a list of Acronyms please see - Acronym

Subaru and the Impreza

Subaru, a Japanese car company, is a subsidiary of Fuji Heavy Industries. General Motors is a 20% minority shareholder. "Subaru" is the Japanese word for the star cluster Pleiades that is depicted in the company logo. While an automotive minnow compared to many of its competitors, Subaru has been a highly profitable company for many years. It is noted for the production of conventional-bodied cars with full-time all wheel drive (in fact, in many markets its entire product range has this feature) and its use of the horizontally-opposed boxer engine (flat engine).

Subaru's best-known vehicles are high performance turbocharged versions of its Legacy (known as Liberty in Australia), and Impreza, which offer outstanding performance and handling at relatively affordable prices. Modified versions of the Impreza WRX and WRX STI have been competing successfully in car rallies and the World Rally Championship for many years. Several endurance records were set in the early and mid-ninties by the Legacy.

Subaru cars (especially Imprezas) are sometimes affectionately referred to as "Scooby-Doos," "Scoobies", or "Scoobarus," due to the similarity in pronunciation between "Subaru" and "Scooby Doo".

Recently GM has developed the Saab 9-2X model, based on the Subaru Impreza WRX

The Subaru Impreza is a compact car that was first introduced by Subaru in 1993. In addition to filling the small-car market segment among Subaru's offerings in the Japanese, U.S., European (and various other) markets, the Impreza chassis rose to prominence in rally racing more effectively than Subaru's previous contenders. Formerly, Subaru had fielded its larger, mid-size car, the Legacy, for rally purposes. However, as rally competition shifted toward smaller and lighter chassis, Subaru needed a smaller and faster car-a role in which the Impreza was immediately successful and in which the Impreza continues to compete effectively, albeit in more powerful versions than were initially introduced.

In 1993, the Impreza was offered in either front-wheel drive or all-wheel drive versions and only as a four- or five-door sedan. In later years, the two-door coupé was introduced, although production of the coupé ended with the introduction of the redesigned 2003 model year. The engine that came in the car initially was a naturally-aspirated 1.8 flat-4 (the EJ18), followed in successive years by larger and turbocharged engines. The basic turbocharged motor, the EJ20, produces 211 bhp.

Starting in 1994, Subaru introduced Subaru Tecnica International (STI) versions of the Impreza in the Japanese and European markets, which were upgraded in many categories, including performance-tuned engines and suspensions. The STI versions of the Impreza were immensely successful in rallies and popular among street racers. Its top speed is electronically limited at 250 km/h (155 mph) and it has a 0-100 km/h time of 4.7 seconds. In the United States, release of the basic turbocharged Impreza (the WRX) did not occur until the 2002 model year, and the Impreza WRX STI was delayed until the 2004 model year; furthermore, the U.S. version of the STI includes various departures from the Japanese and European counterparts (such as a standard-turbo 2.5 L motor in the U.S., rather than the twin-scroll turbo 2.0 L motor sold elsewhere).

The Impreza brought Subaru three consecutive World Rally Championship titles (1995-7) and was considered the fastest car in the championship.

The Impreza was Wheels magazine's Car of the Year for 2000.

As of late 2004, the most recent overhaul of the Impreza occurred with the 2002 model year, with controversial "bug-eyed" styling for its headlamps, and the discontinuation of the two-door coupé version. Some customers' and the press's negative reaction to the look forced Subaru to facelift the car by 2003, with less unconventional, squarer headlamps.

Models offered in Australia in 2005 include the GX (2.0 L engine, this model has become the baseline Impreza in Australia), RV (2.0 L engine, styled for active lifestyles), RS (2.5 L engine, sport model) along with the WRX and STI version thereof. The RX model has been abandoned in Australia, and its position in the Impreza lineup has been subsumed by the GX (which had formerly been a sparsely outfitted budget model).

Subaru used the Impreza chassis for the mechanical underpinnings of the Subaru Forester, a small SUV designed to compete in the segment comprising the Honda CR-V and Ford Escape; also, the hatchback-or Impreza Wagon-formed the basis of the Saab 9-2X released in 2004 (given the tongue-in-cheek moniker, "Saabaru").

Subaru 360

The Subaru 360 was the first automobile mass produced by Fuji Heavy Industries in Japan. Production ran from 1958 to 1969.

The Subaru 360 featured an air-cooled, 2-stroke cycle engine of 356 cc capacity mounted transversely at the rear. The engine was designed with a capacity of less than 360 cc so that the Subaru 360 would qualify for Japan's Keicar class. The body was of monocoque construction and featured a fiberglass roof panel, which was considered very advanced in 1958.

When introduced in 1958, the Subaru 360's 356 cc engine turned out 16 hp and Subaru claimed 71 mpg fuel economy-by the end of production power had increased to 25 hp with a 36 hp twin carb engine was an option.

Several variants were produced, including a station wagon (called the Custom), a convertible, and two sports models known as the Young S, and the Young SS. A truck and van called Sambar were also produced using the 360's engine.

The Subaru 360 was replaced by the less popular but more advanced R-2 which was quickly superseded by the long-lived Subaru Rex model.

The 360 is the original mass-production Subaru, and for many years the company searched for ways to capture the car's heritage in a new model. A string of city car concepts in the 1990s were tied to the 360 in various ways. In the Subaru Jusmin from the 1991 Tokyo Motor Show, this was simply a yellowish paint color reminiscent of the 360's. Later concept cars such as the 1997 Elten and 1999 Elten Custom proposed a modern remake of the 360. However, it's not until the subaru R1's release in January 2005, that a production Subaru blatantly cites influence from the 360. The R1's Motor Show teaser, the R1e, wore a 360-like yellow paint.

Subaru 1600

The Subaru 1600 can trace its origin back to the Subaru 1000 of 1966, the first front wheel drive Subaru. The Subaru 1000 evolved into the 1100 (also known as the FF-1), which in some markets was called the Subaru Star. The Star soon added an optional 1300 cc engine-and then came the 1400, and then the 1600 and 1800.

Subaru Alcyone

The Subaru XT, XT6, and Alcyone are sporty coupes sold from 1985 to 1991. The XT was sold in New Zealand and it and the XT6 were sold in North America and Europe. The Alcyone was sold in Japan. All were available in front wheel drive or four/all-wheel drive (depending on the year) but the all-wheel drive XT was the only model sold in NZ.

The XT was replaced by the Subaru SVX in 1992.

The Alcyone is named after the brightest star in the Pleiades star cluster, on which the Subaru logo is based.

Subaru Alcyone SVX

The Subaru Alcyone SVX (also known in the United States simply as the Subaru SVX) was a mid-sized coupe sold from 1992 to 1997. Built as a successor to the first-generation Alcyone (also called the XT), it had a 6-cylinder boxer engine and a unique appearance. In contrast to the boxy XT, the SVX had unusual, curvy lines designed by Giorgetto Giugiaro of ItalDesign. Like all Subarus at the time, it was available in all wheel drive, though many front wheel drive examples were also produced. The SVX is most easily recognized by its unconventional split windows.

Subaru BRAT

Subaru BRAT is a pickup version of the regular Subaru 1600 car from the 1970s. The BRAT was developed directly from the company's four wheel drive station wagon model and was first introduced as a 1978 model. Similar in concept to the Chevrolet El Camino, American versions also had Astroturf carpet and welded-in jumpseats in the cargo area as a tax dodge, as passenger cars imported into the US were charged a 2.5% tariff, while that on trucks was 25%. The BRAT was restyled in 1981 and the jumpseats were removed after 1985. Production continued into the early 1990s but ceased to be imported to North America around 1987.

Subaru FF-1 G

The Subaru FF-1 G (also sold as the 1100 and 1300) was a compact car from the 1970s, replacing the FF-1 Star. It was a front wheel drive vehicle with a typical Subaru flat-4 engine. A fully independent torsion bar suspension and rack and pinion steering were impressive for the time. The inboard front drum brakes was an oddity. Also strange were the dual radiators - the car used only a small radiator (which was also the heater core) on starting, speeding warmup.

The 1.1 L (1088 cc) and 1.3 L (1267 cc) engines found in the 1100 and 1300 had no cooling fan. An electric fan on the small radiator cooled the engine. The engines shared the same short 60 mm stroke, but the larger unit was bored to 82 mm rather than 76 mm. The small engines produced an impressive 80 hp (JIS).

Even in 1972, Subaru boasted about the foul-weather handling of their cars, and they were quite successful. Priced at just over US$2,000 and achieving 29 mpg, the Subaru quickly became one of the top-selling import cars in the United States.

Subaru FF-1 Star

The Subaru FF-1 Star was a development of the original front wheel drive Subaru, the 1000 with an 1100 cc engine. The FF-1 was marketed as the Star in the United States in 1970 and 1971 model years, and was replaced by the FF-1 G in 1971.

Subaru Justy

The Subaru Justy is a compact hatchback automobile produced in the late 1980s and early 1990s.

The Justy was equipped with a 1.2 L three-cylinder engine and either a standard manual transmission or a novel type of automatic transmission known as a Continuously variable transmission, or CVT. The CVT technology was employed because with a conventional automatic transmission performance would have been unacceptable, due to the small 3-cylinder engine.

The manual transmission version of the Justy had excellent mechanical reliability and fuel economy. However, it was small and noisy as one would expect of an economy hatchback. The five-speed manual transmission worked reliably, but was far from smooth to operate because its synchronizers were not very good.

The CVT version was actually able to get reasonable acceleration out of the small engine, but was unfortunately quite unreliable.

Since the novel automatic transmission was so unreliable, and the manual transmission, while reliable, was not enjoyable to operate, the Justy was never a very popular model, and Subaru abandoned it in many export markets after a few years. This is especially true due to the fact that its 1990 model was the last car in the United States that was still carbureted.

In Europe, Suzuki Motors built the Justy for Subaru at its Hungarian plant. It was merely a rebadged Suzuki Swift. In 2004, the Justy name continued in European markets on a rebadged Suzuki Ignis.

Subaru Leon

The Subaru Leone is a compact automobile produced from June 1971 to February 1989 when it was replaced by the Subaru Loyale, which was essentially the same car, only with a different name. The Leone was known as the "GL/DL" in North America. The Leone was offered in sedan format from launch and station wagon format starting in September 1972.

The Leone soon became a success in areas where people wanted four wheel drive (4WD), but didn't want a larger car. It soon became the world's top-selling 4WD. The car is powered by a water-cooled, horizontally-opposed, 4-cylinder OHV engine.

The first Subaru Legacy was originally intended to be a Leone replacement, but the car was moved upmarket. The Leone soldiered on in Japan. It did not have a direct replacement until the Subaru Impreza launched in October 1992.

Subaru Loyale

Subaru Loyale was the name given to the former GL and DL model series in 1990. The name change was triggered by the introduction of the entirely new Legacy line of cars.

Subaru GL/DLs became very popular in the 1980s in snowy areas due to their adaptability to extreme cold (you can count on a Subaru to start at 40 degrees below zero, this cannot be said of all cars), low price, good fuel economy, and most importantly, the four wheel drive option. As Subaru GL/DLs were purchased as utilitarian vehicles, mainly for their practicality in icy environments, it is rare to find front wheel drive examples in areas where GL/DLs were popular, and almost as rare to find sedans or coupes.

However, though these may be the cars that gave Subaru their popularity in northern climates, they were not perfectly well adapted to the cold. CV axles and related parts on these vehicles (even more so than most front wheel drive and four wheel drive cars) had a tendency to break or wear out quickly in cold weather, and were relatively expensive to replace (compared to many other makes). Also, in part due to the width of the boxer engine, which made access inconvenient, repairs to the engine could be notoriously difficult, to the point that even in some areas where these vehicles were very common you could find mechanics who would refuse to work on them at all.

GL/DL-type names had been in use since the mid 1970s. All GL/DLs were powered by 1.6 or 1.8 liter carburated flat-4 engines. 2WD models were all fuel injected (with the exception of the STD and GL Hatchbacks, which remained carbureted until 1989) starting in 1986 (85 for California) and 4WD models were fuel injected in 1987 (1986 in California) although 87 4WD models with carburetors do exist, as the changeover was mid-year.) The Loyale was similar to the GL-10, althuogh their range of options ranged from DL to GL-10 levels. (For example, some Loyales did not have a console surrounding the gear shift, while others had power everything and a sunroof. Turbo Loyales were only available in 1990, which also saw the end of the production of the 3-door coupe, wihch was available as an RS model which was basically an RX without the turbo.

• DL - Base model, with few options. Manual transmission most common, although in later years automatics were available. Most examples were front wheel drive. DL models did not have a tachometer, oil pressure gauge, or voltage gauge (With the exception of the DL XT, which did have a tachometer, but no volt or oil gauges) although most other GL options such as power windows were available. Most 80-84 models were equipped with the 1600cc engine, although some models were available with the 1800cc engine on 4WD models.
• GL - More options than the DL, power windows and locks available, larger engine available on early models, turbo option available on later models. Air conditioning, automatic transmission, and power steering available. 1800cc engine was only available with the automatic transmission in 1980, but was available with the manual in 1981, although it was more common on the 4WD models. Starting in 1985, the new EA82 overhead cam engine was introduced, which replaced the EA81 and EA71 cam-in-block engines, except in the 85-89 hatchbacks and the 85-87 Brats.
• GL-10 - Radios, power door locks and windows, power moonroofs standard. Turbo option available on all years, and very common on later models.
• RX - GL-10 without a moonroof, but with an upgraded suspension, standard turbo, and a 5 speed manual transmission and dual-range 4WD standard. Comparatively rare. There were two major versions: a sedan and a coupe, which differed mechanically as well as in body-style. 85 and 86 RX models were part-time 4WD with "shift on the fly" 4WD, that is the vehicle could shift in and out of 4WD while the vehicle was in motion. It was also possible to shift between "high" and "low" with the vehicle in motion, provided the clutch was depressed. Starting in 1987, the RX switched to a "full time" dual range 4WD in which a center differential was utilized to allow the vehicle to remain in 4WD at all times for added traction. A "Diff Lock" switch allowed the center differential to be locked, to provide a 50/50 power split for added traction. An automatic transmission was available in 1989, although it was not very common. Full time 4WD was also available on the Turbo wagon, sedan, and 3 door coupe but it was only offered with dual range on the RX
• Loyale - Replaced all GL and DL models. Essentially the same as a GL-10, but with a different name.

Turbo models (with the exception of the RX) were not dual range from the factory, rather they had a vacuum-operated pushbutton 4WD system, with a 4WD switch integraded into the gear shift knob. It was possible to special order a Turbo model with the dual range RX transmission, but they didn't come standard that way. Loyales never came with low range, all were pushbutton or full-time 4WD.

Production of the Loyale ended in 1994.

Subaru REX

The Subaru Rex is a subcompact automobile produced from 1972 to 1992.

The Rex was the replacement for the Subaru R-2, which was meant to be the replacement for the long lived, but outdated Subaru 360. Because the R-2 was based largely on the 360, it lasted only a few years. In contrast, the 360 was produced for 11 years and the Rex was produced for 20.

The Subaru Rex represented a fresh start, having little do with the Subaru 360, although they did share a rear-engine layout. The Rex featured a 358 cc rear-mounted two-cylinder, water-cooled, 4-stroke cycle engine, upgraded from the 2-stroke cycle engine used by the 360 and R-2 models). In 1981, the Subaru Rex became front wheel drive, and the engine was enlarged to 665 cc in 1982. The end for the Rex came in 1992, when it was replaced by the Vivio.

Subaru Vivio

The Subaru Vivio is a car. It was introduced in March 1992, as a 660 cc small car (K-car) manufactured by Subaru.

The name Vivio is a reference to the number 6 6 0 written in Greek, and also inspired by the word vivid. The Vivio replaced the Subaru Rex nameplate. It was available in 3 and 5 door versions. The "Vivio Bistro" is a variation with a retro theme, with Mini-esque front and rear-end, upholstery of a matching theme and modifications to the dashboard. The Bistro series was popular and Japan would see a string of modern small cars modified to appear retro.

Like the larger Subarus, the Vivio was available with a variety of supercharged 4 cylinder engines, with an emphasis on performance.

The Vivio T-top is a limited production, coupe-convertible version. It is a two-seater with detachable roof (in two halves).

In October 1998, the keicar regulation changed, prompting manufacturers to release new models. The Vivio was replaced by the Subaru Pleo at this time. The Pleo was subsequently replaced by the Subaru R2 in December 2003.

Subaru WRX

The Subaru Impreza WRX is a high-powered, turbocharged version of the Subaru Impreza, a small all wheel drive sedan. The WRX has become a popular choice for automobile enthusiasts wherever it is sold for its high performance, excellent grip and handling, practicality, and relatively affordable price (approximately $45,000 Australian, or $25,000 US).

Originally introduced in 1992 in Japan, then shortly afterwards in Australia and Britain, the WRX had a turbocharged flat-4 2.0 L engine producing 155 kilowatts of power (208 horsepower). Compared to the base Impreza, the WRX had wider low-profile tyres, larger brakes (further improved in later versions with the fitment of four-piston calipers and larger discs), and firmer, higher quality suspension components. Offering the performance and handling of vehicles of far higher cost, it rapidly became popular with enthusiasts worldwide. Indeed, it gained a reputation as a popular getaway car for robberies because the car's precise, yet forgiving, handling meant that even inexperienced drivers were able to handle it at high speeds.

The current WRX is based on the Peter Stevens designed Subaru Impreza WRC. The now famous instructions given by Prodrive boss David Richards were to make a car that "looked like it was doing 100 mph, even when it was standing STIll in a car park".

A series of low-volume versions with even greater power and grip were built by Subaru's in-house modifier, Subaru Tecnica International (STI). STI versions of the Impreza, which produces 300 horsepower (224 kW) from new, are often raced, largely unmodified, in production car circuit racing and stock classes of car rallies to great success. Introduced in 1994, the STI has proven to be a very popular high(er)-performance machine in the Japanese domeSTIc market.

In the 2002 model year, an all-new version of the car was released, finally appearing in the United States, with an engine rated for 227 hp (169 kW) and 217 ft·lbf (294 N·m) of torque. To celebrate the debut of the STI in the European market, 1,000 special "ProDrive" models were produced with exclusive ProDrive interior items designed by Peter Stevens. The Impreza WRX was on Car and Driver magazine's Ten Best list for 2002 and 2003.

While the new model was more refined and handled better, it was not as popular with enthusiasts since its greater weight (approx 3100 lb or 1400 kg) hampered performance slightly. A higher-powered engine (310 hp or 230 kW) fitted to the Japanese market version remedied that problem somewhat. In addition to the sedan model, a 5-door wagon was offered, and met with brisk sales. Even so, the exterior styling was widely regarded as somewhat unattractive. As a result, in an unusually early revamp, Subaru updated the style again for the 2004 model year.

In the 2004 model year, Subaru began selling the highly performance-tuned WRX STI in the United States, with a 2.5 L engine with AVCS producing 300 hp (224 kW).

The Subaru Impreza WRX is used as the basis for the Saab 9-2X.




Subaru WRX STI

The Subaru Impreza WRX STI (also referred to as simply, STI) is a high-performance version of the WRX">Subaru Impreza WRX, tuned by Subaru Tecnica International. Originally sold only in Japan and Europe, Subaru began selling the STI in North America in 2004. Available in the Japanese domeSTIc market with a 2.0 L engine, the North American STI features a 2.5 L powerplant. The STI weighs in at nearly 3300 lb.

All US, Japanese, and European models claim 300 hp at 6000 rpm and 300 ft·lbf of torque at 4000 rpm, although the US claim 0-60 mph times of 4.6 s whereas the Japanese And European models claim 5.1 s.

The WRX STI has these changes over the standard WRX:

Manual transmission only
Modified body aerodynamics (Including rear spoiler)
Larger Hood Scoop
Intercooler spray
2.5L (US) Engine (Compared to 2.0L)
300HP (vs. 257)
Driver Controlled Center Differential
Larger disc brakes
High Intensity Discharge headlights

The STI is the only new Impreza not being produced with a San Remo Red body paint option.

Subaru Tecnica International

or STI is Subaru's motorsports division, similar to RalliArt for Mitsubishi, TRD for Toyota, AMG for Mercedes-Benz, etc. STI specializes in the preparation of a variety of vehicles for rallies such as the World Rally Championship (WRC). It was founded in 1998 by Subaru to promote the company identity. In recent years, the WRC has been predominantly won by the heavily modified STI 555 Impreza, beating many of its rivals such as the Mitsubishi Lancer Evolution. STI is an acronym for Subaru Tecnica International, most people misspell Tecnica with Technica.

The Subaru Impreza WRX also comes in a high performance STI edition, designed by STI.



Subaru Vin Codes

1 Subaru VIN code 1.1 Line types 1.2 Body styles 1.3 Engine types 1.4 Models 1.5 Restraint types 1.6 Plants and transmission types 1.7 Subaru WMIs

 

The line type is specified in character four of the Subaru VIN for passenger cars.

 

The body style is specified in character five of the Subaru VIN for passenger cars.

The engine type is specified in character six of the Subaru VIN for passenger cars.

 

The model is identified in character seven of the Subaru VIN for passenger cars.

The restraint type is specified as character eight of the Subaru VIN for passenger cars.

 

The plant and transmission type is specified in character eleven of the Subaru VIN for passenger cars.

SIA=Subaru-Isuzu of America FHI=Fuji Heavy Industries

AWD - What is it?

Four wheel drive, 4WD, 4x4, all wheel drive, and AWD are terms used to describe a four-wheeled vehicle with a drivetrain that allows all four wheels to receive power from the engine simultaneously. While many people think exclusively of off-road vehicles, powering all four wheels provides better control on slick ice and is an important part of rally racing on mostly-paved roads.

Four wheel drive (4WD or 4x4 for short) was the original term, often used to describe truck-like vehicles that required the driver to manually switch between a two wheel drive mode for streets and a four wheel drive mode for low traction conditions such as ice, mud, or loose gravel. The "all wheel drive" term (AWD for short) was invented to diSTInguish vehicles that are capable of driving all four wheels on normal roads without causing poor control and excessive tire and drivetrain wear.



Flat-4 - What is it?

A flat-4 is a four cylinder internal combuSTIon engine where the cylinders are arranged in a flat configuration. This is not a common configuration, but some brands of automobile have favoured such engines and it is a common configuration for smaller aircraft engines such as made by Lycoming. Although they are considered to be superior to "inline-fours" in terms of refinement and vibration, they have largely fallen out of favor because their larger overall dimensions are not suited to modern small cars. The shape of the engine suits it better for rear engine designs, where the low center of gravity is an advantage; in front engine designs the width interferes with the ability of the front wheels to steer.

The open and exposed design of the engine allows air cooling over water cooling, and in air cooled applications fins are often seen machined into the external cylinder block walls.

Volkswagen used air-cooled flat-4s extensively in their early days, in the VW Beetle and most early VW designs. Porsche also used the VW engine in the early Porsche 356. This engine was replaced by a Porsche designed Flat-4 in the late 356s and the 912. The 914 that replaced the 912 was built in partnership with VW using a VW engine.

Citroën used an air-cooled flat-4 on the GS and GSA.

Alfa Romeo introduced a water cooled flat-4 on the Alfasud. That engine was later used on the Alfa Romeo Arna, the Alfa Romeo 33, the Alfa Romeo Sprint, the Alfa Romeo 145/146.

Lancia also used a water cooled flat-4 on the high-end Lancia Gamma.

Subaru produces water-cooled front mounted flat-4 engines that are confusingly marketed as H-4, by which they mean Horizontal rather than the H cross-section normally meant by H engine.

Differentials

When powering two wheels simultaneously, something must be done to allow the wheels to rotate at different speeds as the vehicle goes around curves. When driving all four wheels, the problem is much worse. A design that fails to account for this will cause the vehicle to handle poorly on turns, fighting the driver as the tires slip and skid from the mismatched speeds.

One could avoid the following mechanical complexity by simply using one electric motor per wheel, with speeds under computer control. This is not normally done though, so...

A differential allows one shaft to drive two output shafts with different speeds. The differential distributes torque (angular force) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the input shaft. Each powered axle requies a differential to distribute power between the left and right sides. If all four wheels are to be driven, a third differential can be used to distribute power between the front and rear axles.

Such a design would handle very well. It distributes power evenly and smoothly, making it unlikely to start slipping. Once it does slip though, recovery will be difficult. Suppose that the left front wheel (of a design that drives all four wheels) slips. Because of the way a differential works, the slipping wheel will spin twice as fast as desired while the wheel on the other side stops moving. (the average speed remains unchanged, and neither wheel gets any torque) Since this example is a vehicle that drives all four wheels, a similar problem occurs between the front and rear axles via the center differential. The average speed between front and rear will not change, torque will be matched, torque goes to zero, speed at the rear goes to zero, and the speed at the front goes to double what it should be... making the left front wheel actually turn four times as fast as it should be turning. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a patch of slick ice or raised off the ground. The simpliSTIc design works acceptably well for a 2WD vehicle. Since a 4WD is twice as likely to have a driven wheel on an icy patch, the simpliSTIc design is usually considered unacceptable.



Limited Slip Diferential (LSD)

A Limited Slip Differential (LSD) is a modified or derived type of differential gear arrangement that allows for some difference in rotational velocity of the output shafts, but does not allow the difference in speed to increase beyond a preset amount. In a car or automobile, such limited slip differentials are sometimes used in place of a standard differential, where they convey certain dynamic advantages, at the expense of greater complexity.

The main advantage of an LSD is found by considering the case of a standard differential where one wheel has no contact with the ground at all. In such a case, the contacting wheel will remain stationary, and the non-contacting wheel will rotate at twice its intended velocity - the torque transmitted will be zero and the vehicle will remain stationary. In normal everyday use on typical roads, such a situation can usually be assumed to have a vanishingly small probability, and so a normal differential suffices. For more demanding use however, such as driving off-road, or for high performance vehicles, such a state of affairs is undesirable, and the LSD can be employed to deal with it. By limiting the velocity difference between a pair of driven wheels, useful torque can be transmitted as long as there is some friction available on at least one of the wheels.



Turbocharger - What is it?

A turbocharger is a compressor used in internal-combuSTIon engines to increase the power output of the engine by increasing the mass of oxygen and fuel entering the engine. A key advantage of turbochargers is that they offer a considerable increase in engine power with only a slight increase in weight.

A disadvantage in gasoline engines is that the compression ratio should be lowered (so as not to exceed maximum compression pressure and to prevent engine knocking) which lowers engine efficiency when operating at low power. This disadvantage does not apply to specifically designed turbocharged diesel engines. However, for operation at altitude, the power recovery of a turbocharger makes a big difference to total power output of both engine types. This last factor makes turbocharging aircraft engines considerably advantageous-and was the original reason for development of the device.

Principle of operation

A turbocharger is an exhaust gas driven supercharger. All superchargers have a gas compressor in the intake tract of the engine which compresses the intake air above atmospheric pressure, greatly increasing the volumetric efficiency beyond that of naturally-aspirated engines. A turbocharger also has a turbine that powers the compressor using waste energy from the exhaust gases. Compressor and turbine have the same shaft, similar to a turbojet aircraft engine.

The term supercharger is very often used when referring to a mechanically driven supercharger, which is most often driven from the engine's crankshaft by means of a belt (otherwise, and in many aircraft engines, by a geartrain), whereas a turbocharger is exhaust-driven, the name turbocharger being a contraction of the earlier "turbo-supercharger"

The compressor increases the pressure of the air entering the engine, so a greater "charge" (fuel/air mixture) enters the engine in the same time interval (the increase in fuel is required to keep the mixture the same ratio). This greatly improves the volumetric efficiency of the engine.

The increase in pressure is called "boost" and is measured in pascals, bars or PSI. The energy from the extra fuel leads to more overall engine power. For example, at 100% efficiency a turbocharger providing 100 kPa (1 bar or 14.7 PSI) of boost would effectively double the power of the engine. However, there are some parasitic losses due to heat and exhaust backpressure from the turbine, so turbochargers are generally only about 80% efficient because it takes some work for the engine to push those gases through the turbocharger turbine (which is acting as a restriction in the exhaust).

For automobile use, normal maximum boost pressure is 80 kPa (0.8 bar), but it can be much more. Because it is a centrifugal pump, a typical turbocharger, depending on design, will only start to deliver boost from about 2500 engine rpm (1800 in automotive turbo-diesel engines), while a supercharger will supply some boost at most engine speeds.

A main disadvantage of high boost pressures for internal combuSTIon engines is that compressing the inlet air increases its temperature. This increase in charge temperature is a limiting factor for petrol engines that can only tolerate a limited increase in charge temperature before pre-ignition occurs. The higher temperature is a volumetric efficiency downgrade for both types of engine. The pumping-effect heating can be alleviated by intercooling or aftercooling, or both.

Design details

When a gas is compressed, its temperature rises. It is not uncommon for a turbocharger to be pushing out air that is 90 °C (200°F). Compressed air from a turbo may be (and most commonly is) cooled before it is fed into the cylinders, using an intercooler or a charge air cooler (a heat-exchange device).

A turbo spins very fast-10,000 to 150,000 rpm depending on size (using low inertia turbos, 190,000 rpm), weight of the rotating parts, boost pressure developed and compressor design. Such high rotation speeds would cause problems for standard ball bearings leading to failure in a turbo. Most turbo-chargers use a fluid bearing. This is a flowing layer of oil that suspends and cools the moving parts. The oil is usually taken from the engine-oil circuit and usually needs to be cooled by an oil cooler before it circulates through the engine. Some turbochargers use incredibly precise ball bearings that offer less friction than a fluid bearing but these are also suspended in fluid-dampened cavities. Lower friction means the turbo shaft can be made of lighter materials, reducing so-called turbo lag or boost lag. Some car makers use water cooled turbochargers for added bearing life.

Turbochargers with foil bearings are in development (see picture above). This design eliminates the need for bearing cooling or oil delivery systems.

To reduce the possibility of damage to the engine and to also reduce the amount of time required for the turbo to spool-up and increase the boost after the quick increase in throttle opening, turbocharged engines are usually equipped with a blowoff valve or a bypass valve. This allows the upper-deck air pressure to be maintained within limits that ensure engine efficiency without danger.

To manage the upper-deck air pressure the turbocharger's exhaust gas flow is regulated with a wastegate that bypasses excess exhaust gas entering the turbocharger's turbine. This regulates the rotational speed of the turbine and the output of the compressor. The wastegate is opened and closed by an electromagnetic coil or oil pressure that is regulated by pressure of the compressed air from turbo (the upper-deck pressure) through some form of Automatic Performance Control or the engine's electronic control unit.

Reliability

As long as the oil supply is clean and the exhaust gas does not get too hot (ultra-lean mixtures) a turbocharger can be very reliable but care of the unit is important. Replacing a turbo that lets go and sheds its blades will be expensive. The use of synthetic oils is recomended in turbo engines.

After high speed operation of the engine it is important to let the engine run at idle speed for two to three minutes before switching off. Saab, in its owner manuals, recommends only 30 seconds. This lets the turbo rotating assembly run down in speed and cool from the lower gas temperatures in both the exhaust and the intake tracts. Not doing this will also result in the critical oil supply to the turbocharger being severed when the engine stops while the turbine is STIll turning at high speed, leading to coking (burning) of the lubricating oil trapped in the unit and, later, failure of the supply of oil when the engine is next started causing rapid bearing wear and failure. Even small particles of burnt oil will accumulate and lead to choking the oil supply and failure. A turbo timer is a device designed to keep an automotive engine running for a pre-specified period of time, in order to execute this cool-down period automatically.

Diesel engines are much kinder to turbos because their exhaust gas temperature is much lower than that of gasoline engines and because most operators allow the engine to idle and do not switch it off immediately after heavy loading.

Lag

A lag is sometimes felt by the driver of a turbocharged vehicle as a delay between pushing on the accelerator pedal and feeling the turbo kick-in. This is symptomatic of the time taken for the exhaust system driving the turbine to come to high pressure and for the turbine rotor to overcome its rotational inertia and reach the speed necessary to supply boost pressure. The directly-driven compressor in a supercharger does not suffer this problem. Conversely on light loads or at low RPM a turbocharger supplies less boost and the engine is more efficient than a supercharged engine.

Lag can be reduced by reducing the rotational inertia of the turbine, for example by using lighter parts to allow the spin-up to happen more quickly. Ceramic turbines are a big help in this direction. Another way to reduce lag is to change the aspect ratio of the turbine by reducing the diameter and increasing the gas-flow path-length. Increasing the upper-deck air pressure and improving the wastegate response help but there are cost increases and reliability disadvantages that car manufacturers are not happy about. Lag is also reduced by using a precision bearing rather than a fluid bearing, this reduces friction rather than rotational inertia but contributes to faster acceleration of the turbo's rotating assembly.

Another common method of equalizing turbo lag, is to have the turbine wheel "clipped", or to reduce the surface area of the turbine wheel's rotating blades. By clipping a minute portion off of the tip of each blade of the turbine wheel, less restriction is imposed upon the escaping exhaust gases. This imparts less impedance onto the flow of exhaust gasses at low RPM, allowing the vehicle to retain more of its low-end torque, but also pushes the effective boost RPM to a slightly higher level. The amount a turbine wheel is and can be clipped is highly application-specific. Turbine clipping is measured and specified in degrees.

Other setups, most notably in V-type engines, utilize two identically-sized but smaller turbos, each fed by a separate set of exhaust streams from the engine. The two smaller turbos produce the same (or more) aggregate amount of boost as a larger single turbo, but since they are smaller they reach their optimal RPM, and thus optimal boost delivery, faster. Such an arrangement of turbos is typically referred to as a "twin turbo" setup.

Some car makers combat lag by using two small turbos (like Toyota, Maserati, Mazda, and Audi). A typical arrangement for this is to have one turbo active across the entire rev range of the engine and one coming on-line at higher RPM. Early designs would have the smaller turbocharger active up to a certain RPM limit, after which the exhaust gases were shunted away from the small turbo to the larger one. Being individually smaller they do not suffer from excessive lag and having the second turbo operating at a higher RPM range allows it to get to full rotational speed before it is required. Such small/large combinations are referred to as "sequential turbos". Sequential turbochargers are usually much more complicated than single or twin-turbocharger systems because they require what amount to three sets of pipes-intake and wastegate pipes for the two turbochargers as well as valves to control the direction of the exhaust gases.

Lag is not to be confused with the boost threshold, however many publications STIll make this basic mistake. The boost threshold of a turbo system describes the minimum turbo RPM at which the turbo is physically able to supply the requested boost level. Newer turbocharger and engine developments have caused boost thresholds to steadily decline to where day-to-day use feels perfectly natural. Putting your foot down at 1200 engine rpm and having no boost until 2000 engine rpm is an example of boost threshold and not lag.

For racing the motor avoids <2000rpm and therefore spool-up. A variable geometry avoids variable rpm for the rotor and therefor lag.

Boost

Boost refers to the increased manifold pressure that is generated by the intake side turbine. This is limited to prevent detonation by controlling the wastegate which shunts the exhaust gasses away from the exhaust side turbine


Wastegate

A wastegate is a valve that diverts exhaust gases away from the turbine wheel in a turbocharger. Diversion of exhaust gases causes the turbine to lose speed, which in turn reduces the rotating speed of the compressor. The primary function of the wastegate is to stabilize boost pressure in turbocharger systems.



Blow Off Valve

A blowoff valve is a vacuum operated valve that is located after a turbocharger on an internal combuSTIon engine, but before the throttle body butterfly valve and intake manifold. Its use is to vent extra pressure being developed by the turbocharger when the throttle is closed, such as during a shift. During a shift in a car with a manual transmission, the throttle plate is closed. The pressure from the turbocharger has nowhere to escape to. The pressure pressing against the throttle plate will produce a pressure wave in the opposite direction, causing possible damage to the turbocharger turbines and may also slow or even stop the turbine, thus causing turbo lag when the throttle is pressed again.

A blowoff valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, a strong vacuum develops in the intake manifold after the throttle plate and "sucks" the blowoff valve open. The excess pressure from the turbocharger is vented into the atmosphere or into the intake. Blowoff valves have a very diSTInguised "psshh" sound that is desired by many who own turbocharged cars. Some blowoff valves are marketed with trumpet shaped exits that amplify the "psshh" sound. It is also sometimes called a "dump valve", "bypass valve" or "hooter valve".

Blowoff valves are generally not required on automatic transmission vehicles. Automatic transmission vehicles shift without closing the throttle but are STIll fitted by a lot of manufacturers, this is so the is able to provide boost sooner if if the throttle is only releasd for a second.

Intercooler

An intercooler is a device used on turbocharged and supercharged internal combuSTIon engines to improve the volumetric efficiency and increase the amount of charge in the engine, thereby increasing power. The inter in the name refers to its location compared to the compressors; the coolers were typically installed between multiple stages of supercharging in aircraft engines. Modern automobile designs are technically aftercoolers because they appear at the very end of the chain, but this name is no longer used.

Turbochargers and superchargers compress incoming air, causing it to become heated (see the universal gas equation). Since hot air is less dense than cooler air, the total charge delivered to the cylinders is less than it could be. By cooling the charge after compression, even more charge can be delivered, increasing power. Additionally, intercoolers help to increase the total amount of boost allowable prior to the beggining of detonation in the cylinder by decreasing the temperature of the air charge.

An intercooler is essentially a radiator tuned for high flow rates and the increasing density of the charge as it cools. Most designs use ambient air for cooling, flowing through the radiator core, and often colocated with other radiators for oil or cooling fluid. An alternate design, often referred to as a charge cooler, uses water to cool the charge, then cools the water in a separate radiator. While heavier and more complex, charge coolers can often make arranging the rest of the engine much simpler.

Intercoolers need to be mounted so as to maximize air flow and promote efficient cooling. Cars such as Saab or Mitsubishi Lancer Evolution use front-mounted intercoolers mounted vertically near the front bumper, in line with the car's radiator. In contrast, cars such as the Subaru Impreza WRX mount the intercooler horizontally on top of the engine and use a hood scoop to force air over the intercooler. Some World Rally