Cosworth Vega (1975-1976)

Article by Mark Trotta

In a time when American car manufacturers were grappling with building emissions-controlled performance cars, Chevrolet introduced the Cosworth Vega. Produced just two short years, the CosVeg featured several firsts, including Chevy's first sixteen-valve twin-cam engine, first factory-installed stainless steel exhaust header, and first use of electronic fuel injection.

Cosworth Vega History

The Chevy Vega, designed to compete in the low-priced compact car field, was built from 1971 through 1977. It's innovative inline four-cylinder engine was based on an all-aluminum, sleeveless block. The incentive to use this newly-developed technology was both weight and cost. Using aluminum over cast-iron saved about 35 pounds per unit, and "engineering out" the block liners saved about eight dollars per unit.

Four Chevy Vega models were available; hatchback coupe, notchback sedan, kammback wagon, and panel delivery. The limited production Cosworth Vega was offered as a hatchback coupe only, but it's unique drivetrain and trim packaging set it far apart from the others.

Twin-Cam Motor

Chevy Cosworth Vega twin-cam engine

Introduced as a 1975 model, the Cosworth Vega was conceived four years earlier. Around 1971, Chevrolet contacted Cosworth Engineering of England, and development began on a race-inspired version of the four-cylinder Vega engine. Cosworth has a long, distinguished career in Formula One racing. Starting in the Seventies, the DFX version of the F1 engine won 151 races throughout a 14-year time in the Indy/CART series, racking up ten driver's championships and ten Indianapolis 500 victories.

Cosworth assisted in the 16-valve cylinder head design, which was constructed of aluminum alloy, using sintered-iron valve seats and cast-iron valve guides. Twin camshafts were housed in a removable cam-carrier, which also served as a guide for the valve lifters. Each camshaft was driven by an individual gear found on the front of the head, and along with the water pump and fan, turned by neoprene-rubber belt.

To achieve the desired displacement, the standard Vega engine, which displaced 140 cubic-inches (2.3L), was de-stroked to 122-cid (2.0L) for the twin-cam motor. Chrome-plated piston rings were developed to be used with the forged aluminum pistons. The crankshaft and connecting rods were forged steel.

The first CosVeg engines that were developed delivered an impressive 180 horsepower, and optimistically slated for the 1974 model year. However, the initial compression ratio of 12:1 could not be kept while still complying with emissions requirements.

Production Delays

The Environmental Protection Agency and the State of California required all cars to pass pre-established emission standards, and then pass them again after a 50,000-mile durability test. One of two Cosworth Vega test-cars passed the first part, but an engine inspection after the 50K test revealed several burnt exhaust valves, causing the engine to fail. Engine changes included a drop in compression. Chevrolet then planned to introduce the car with a hopeful 135-140 horsepower, but engine design was again compromised as emissions standards tightened.

The CosVeg motor passed the 50,000-mile durability test the second time, after engineers had added pulse-air tubes and revised the timing specs. After a three-year development program and a year and a half of delays, Chevy began building the Cosworth Twin Cam Vega in March of 1975. With the compression ratio now at 8.5:1, engine output was 110 horsepower with 107 lb-ft of torque at 4600 rpm.

The twin-cam engines were assembled in a special area at Chevy's Tonawanda Engine Plant in New York. Each motor was hand-built and included a cam-cover sticker with the engine builder's signature. A Bendix electronic fuel-injection system featured four injector valves, an electronic control unit, five independent sensors and two fuel pumps. The engine also featured transistorized electronic ignition and a stainless steel exhaust header. After being tested, the motors were shipped to the Lordstown, Ohio facility, were all Vegas were built and assembled.

1975 Chevy Cosworth Vega

1975 Cosworth Vega

All 1975 Cosworth Vegas were painted black, with gold pin-striping applied to the hood bulge, body sides, wheel openings, and rear cove panel. Wheels were 13" cast-aluminum and finished in gold. 'Cosworth Twin Cam' lettering appeared on the front fenders and rear cove.

The interior of the Cosworth Vega has been described as stark but functional, and essentially the same as the Vega GT models. Special trim included instrument bezels, 8000 RPM tachometer, "Cosworth Twin-Cam Vega" steering wheel emblem, and a consecutive vehicle number plate on the dash panel. Interference with the air cleaner and the evaporator housing kept air conditioning off the option list. Lack of space also kept power steering and power brakes from being offered. A floor-mounted four-speed manual transmission was standard.

The CosVeg shared its shocks and springs with the Vega GT. Front disc brakes were standard, as were front and rear stabilizer bars.

Monza Torque-arm Rear Suspension

The eventual successor of the Vega, the Chevy Monza, also debuted in 1975. The new model featured GM's first use of a torque arm rear suspension, which provided solid rear axle power control. In mid-1975, the Vega's four-link axle design was replaced by the Monza torque arm style. The axle carried a 3.73:1 gear ratio, with a limited-slip differential available at extra cost.

Cosworth Vega History

The Cosworth Vega, priced at $5,900, was the second most expensive car in Chevrolet's 1975 model line-up. Double the cost of a standard Vega hatchback, the CosVeg was only $900 less than a Corvette. Approximately 2,061 twin-cam Vegas were built for the 1975 model year.

1976 Cosworth Vega

For 1976, the Cosworth, along with the other Vegas, received a minor restyle with a wider grille and tri-color tail-lamps. A Borg-Warner five-speed manual transmission with 4.10 final-drive axle was added to the option list. The exhaust system, formerly dual outlet, was now a single outlet. Mid-year options included a sunroof, 8-track tape player, and new exterior and interior colors. Anti-rust improvements were made to all Vega models, including galvanized fenders and rocker panels. The chassis received upgrades from the Monza, including the box-section front cross-member and larger rear drum brakes.

1976 Chevy Cosworth Vega

Prompted by an overheating problem on the standard Vega engines, Chevrolet installed an engine overheat protection system on all models, including the Cosworth. If the radiator coolant level became low, or if the coolant temperature reached 260 or higher, a sensor mounted in the radiator activated an "add coolant" light. Another sensor warned the driver if engine oil pressure dropped below 6 psi, activating the temp/press light.

Because of the potential overheating problem (cars properly maintained did not overheat), and with early-car rust problems, the Vega picked up a bad reputation, which caused sales to fall. This ultimately hurt Cosworth sales, with 1976 production falling to 1,447 units. Combined total production for the two years produced was 3,508 units, well short of the projected sales of 5,000 per year. Although meeting the stricter 1977 emission standards, the Cosworth Vega would not be offered in the Vega's final year.


In 1976, Road Test magazine conducted a 'Supercoupe Shootout', with the Cosworth Vega competing against a BMW 2002tii, Alfa-Romeo GTV, Lancia Beta, and a Saab 99. RT wrote about the Vega: "It had the fastest 0-60 time, the fastest quarter-mile time, and tied with the Saab for the shortest braking distance."

Developing maximum power at 5,600 rpm, with a redline at 6,500 rpm, a stock Cosworth Vega engine made about 110-horsepower. Reviewing old road tests, 0-60 mph times varied from 9 seconds to 12 seconds. In unmodified form, the highest top-speed recorded was 112 mph. Gas mileage around town was between 18-22 mpg, with highway mileage near 30 mpg.

Combining the 1975 and 1976 model years, 3,508 Cosworth Vegas were produced.


Cosworth Vega Bonneville Record-Holder

In 2009, a modified Chevy Cosworth Vega set a new class record at the Bonneville Salt Flats. Using modern tuning techniques, owner/driver Frank Sloan and his crew coaxed nearly 260 horsepower (at 9500 rpm) out of the four-cylinder engine, more than double the factory rating of 110. Sloan and his Vega now own the Land Speed G/Pro class record of 156.818 mph.

Chevy Cosworth Vega Bonneville record holder

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