The end is in sight for the venerable V-8 engine, 75-plus years old and a high volume centerpiece of the auto industry for more than half a century.

General Motors Corp. has told the National Highway Traffic Safety Administration (NHITSA) that the V-8 will most likely be dropped in about five years as the primary power plant for family cars.

Today just under 70 per cent of all American-made cars are now being built with V-8 engines. About 20 per cent have six-cylinder power plants.

GM's reason for the V-8 demise is to be able to meet the federal laow now on the books requiring vast improvements in the miles per gallon for all new cars sold through 1985 and after.

Oddly, the GM document was written before President Carter announced his tax-and-rebate program for new cars based on their fuel economy. His program could only serve to accelerate the changes envisioned in GM's statement to NHTSA.

The auto maker's quiet document detailing the drastic alterations likely in coming U.S. cars was drawn up in response to an NHTSA query about how GM planned to meet the required fleet average boost in fuel economy.

In it, GM made these points:

No 1984 model GM car would have a V-8, and only the 1983 Corvette would have one.

The 3.8 litre, roughly 230 cubic inch, V-6 would be the largest engine sold in 1984 models.

GM's just-downsized hugely successful 1977 model big cars would again be cut in size for the 1983 model year. After that they would weigh roughly 3.200 pounds - or approximately what the present compact Chevrolet Nova weighs.

Meeting the 1985 mileage requirement will enormously degrade acceleration and performance. None of the 1985 cars will to go from a standing start to 60 miles per hour in less than 15 seconds, and most will average about 18 or 19 seconds.

While the "drastic scenario" so carefully delineated would meet the mileage required by law, "there is no guarantee that the consumer will buy the products," GM said.

Better radial tires should increase mileage for four-fifths of the 1985 cars by a surprising 4 per cent.

The General Motors submission is heavily spotted with blacks - things GM insists NHTSA treat with confidentiality. The document is not one GM is offering freely or widely in this city but is available to the public at the government's Freedom of Information library in Washington, D.C.

In adddition to have surprising things that GM sees in its crystal ball, the auto maker for first time has specified the precise cost of downsizing its 1977 large cars.

"The downsizing applicable on 1977 (GM) regular and luxury cars was about as modest a program as possible, yet it cost about $1.1 billion.

"As we go forward, downsizing may require the use of front wheel drive components, all new engines, all new transmissions, etc.

"The cost of such programs will be substantially higher than our 1977 program, which was largely able to utilize many carryover mechanical components," GM's staffers wrote.

That last statement may be the most important in the entire GM submission. It is pregnant with the implication that another huge round of large inflationary price increases for new cars is in the offing.