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Cosworth Twin Cam Vega Engine Facts		Home

HISTORY
	Any automobile is only as good as the sum of their components. The Cosworth Twin-Cam Vega is no exception to that rule. While looked upon as “just a Vega” by those unfamiliar with the car, it is much, much more than “just a Vega,” as we shall see. History has it that John Z. DeLorean was the one who pushed Chevrolet engineering to get Cosworth Engineering of England involved. Ultimately this would lead to Cosworth attempting to turn out a racing engine based on the cast aluminum Vega block furnished by the Tonawanda NY Engine Assembly Plant. What finally made it through the door at GM was what should have been built all along for the entire Vega family, perhaps not with the dual overhead cams, but using an aluminum head rather than cast iron. It is felt that the cast iron head on the standard Vega aluminum block was behind a high number of engine failures. These failures were only compounded by an inadequate cooling system, lack of a low coolant warning light, and the early cars lack of a coolant overflow recovery system-all items which were later addressed during the Vega’s life span.

Head Design
	The Cosworth-Vega engine as delivered to those who waited long enough for their car was, and still is, an engineering masterpiece. Features include a high-silicon content, die-cast aluminum engine block with iron-plated pistons running on un-lined cylinder walls using stress relived and Magnafluxed Vega connecting rods spinning a forged and tuftrided crankshaft. The cylinder head features what has been referred to as hemispherical combustion chambers and four valves per cylinder. Personally, I prefer to call the head and piston design a Pent Roof Design, very similar to the design Honda uses on its motorcycle engines. The small valves are operated by shim-over-bucket lifters with the shims running directly under the camshafts. Like the standard Vega, the cams are driven off the front of the engine by a unique to the Cosworth crank sprocket. The High-Energy Ignition (HEI) distributor is driven off the intake cam sprocket. The cams and lifters run in a separate cam carrier. For service, this carrier can be removed as a unit, although shim adjustment can be made with the cam carrier in place. All of these exotica are covered by a black wrinkle-finished cam cover signed by each individual engine builder with a raised “Cosworth-Vega” and the trademark Chevrolet Bow-Tie emblem. All engines at time of assembly were stamped on the rear of the cam carrier, head and block in sequential order, Cosworth #0321 was built with engine number 0296 which according to the build date code was completed in Tonawanda NY on May 1 (T0501 - T=Tonawanda, 05=May, 01=Day of the Month) ZCA=Engine code for the 1975 Cosworth Twin Cam Vega. NOTE=ZCB is the engine code for a 1976 Cosworth Twin Cam Vega.

Fuel & Exhaust
	The method used to fuel the production engine was another first for Chevrolet. Working with Bendix and using many parts supplied by Bosch was an analog Electronic Fuel Injection (EFI) system. Unlike today’s systems, there was no oxygen sensor for feedback. Signals were sent from a coolant temperature sensor located near the thermostat and an air temperature sensor installed under the intake manifold which sensed air temperature as air flowed over it. Small fingers that run on a printed circuit board report throttle position. These signals were processed and the computer sent its signals to the special Bosch injectors that would spray fuel down the intake runners every other revolution, except at wide-open throttle when they would fire every rotation. Burned gases exit the engine through a stainless steel header and are routed under the engine to the passenger side. Here they flow through a Cadillac V8 GM bead-type catalyst before crossing over the rear axle and exiting on the driver side just behind the rear wheel well opening.

Shortcomings
	The revealed flaw of this design was the engine block itself. As this was a die cast block there was no way to put a deck on it, and the free-standing cylinder bores would allow the cylinders to move around just enough under a racing load (290bhp @ 9,000rpm) that the head gaskets would abrade and fail. Cosworth went so far as to use gas-filled O-Rings, eliminating the gasket altogether, but leakage was still an issue. The second problem was that the blocks would crack along the base of the cylinders under a racing load and actually separate from the crank portion of the block. GM did finally address the cracking issue with a modified Heavy Duty block, but not until Cosworth had given up on the racing engine project. This HD block was never used in production cars and was only offered over the parts counter as an “Off-Road” part. Today these blocks are almost worth their weight in gold.

#0321Modifications
	Cosworth-Vega #0321 features an engine that is basically as delivered in June of 1975. The original owner rebuilt it, with help from a good friend, in the early 1980’s installing TRW cylinder sleeves, new factory 8.5:1 pistons, Sealed Power piston rings, and new factory cams. Hutton Motor Engineering of Clarksville TN rebuilt the head using Cosworth of England Intake Valves, new factory exhaust valves, and HME bronze valve guides. I have had a replacement header Jet-Hot coated. The original stainless steel header resides in a box in my basement with the like-new Pulse Air Injection Reaction (P.A.I.R.) system. As noted in the photos, the header has a nice silver sheen looking more like a new silver dollar rather than the duller stainless steel. The J-H coating has noticeably reduced under hood operating temperatures, and it will not discolor. All Cosworth engines at time of assembly had a sequence number stamped along the rear of the block, head, and cam carrier. As mentioned previously this car carries engine number 0296 and all components match. The EFI set was installed off line at the Lordstown OH Vega Assembly Plant. These components were also numbered as a set. This car carries EFI set number 0544, and all the major EFI components are so labeled, including the manifold, regulator, MAP sensor, wiring harness and the computer.
This was very high technology for 1975. However, most of this engine package was in the design stages as far back as 1971!