When is a Datsun not a Datsun? When it's a Nissan - as in the 1982-model Nissan Stanza, a new global car replacing the old Datsun 510. If that sounds confusing, it really isn't. Simply, Nissan, Japan's No. 2 carmaker, which has been marketing its products to many nations of the world under the Datsun label, is gradually - ''over the next three years,'' according to C. P. King, vice-president of sales for Nissan Motor Corporation in U.S.A. - shifting away from the Datsun logo to the Nissan tag.
The point is, Nissan, as a maker of many products besides cars, is trying to consolidate its image so that the nametag Nissan means not just automobiles, but lawn mowers and anything else in the Nissan line. The Datsun name now is used only on cars.
It'll be a costly name shift, the US subsidiary of the Japanese manufacturer admits.
Indeed, the car dealers, vocal at first, seem to be falling into line even as the cost for changes in the physical plant - company names and product signs, for example - will very likely run into the millions of dollars. While it is not yet clear, the manufacturer will certainly pick up some of the tab for the change.
One US dealership is known as Dat's Datsun, Mr. King said. Will it become Dat's Nissan? Unlikely. Somehow the name loses something in the translation.
All the shots are being called from Japan, reports Mr. King.
Looking at the name change, Arvid Jouppi, a market analyst with Rooney, Pace Inc., New York, sums up: ''It'll be a bonanza for the advertising business.''
Meanwhile, Datsun - no, Nissan - has just launched the new Nissan Stanza as replacement for the aging 510. Next spring the company will bring out a successor to the upgraded 210, but you can be sure it won't be called the 210. Presumably, all the cars will have major revisions or replacement over the next three years and end up with names, not numbers.
Of course, some companies, such as the highly placed West German image cars, Mercedes-Benz and BMW, find that the numbers work just fine, while GM's Pontiac division is taking the numbers route with some of its cars.
Even so, besides the new Nissan Stanza, the Japanese company has put on the road a more powerful front-drive 310, increasing the engine size from 2 liters to 2.2 for 1982. The sporty car needs more performance in line with its intended image so Nissan upsized the engine instead of turning to a more costly turbo. The fuel penalty is about 2 miles a gallon, according to Mr. King.
The top-of-the-line 280-ZX, however, expands its turbo option to include both two-seater and 2 plus 2. With the turbo, the 280-ZX zips from 0 to 60 in just over 7 seconds.
Beginning early in 1982, the turbo ZX will get a manual 5-speed overdrive transmission as standard equipment and the current 3-speed automatic becomes an option.
The first ''talking car,'' the 810 Maxima, which last year gently warned the driver to ''please turn off the lights'' if the ignition key were removed with the light switch still on, adds to its vocabulary for 1982.
Looking at sales, King says the company expects to sell 500,000 Datsuns - er, Nissans - in the US in 1982, up a few tens of thousands from 1981.
But doesn't the Japanese government have a lid on the export of cars to the US? Indeed it does, but Nissan looks for an upturn in total US car sales in the new-model year. If that happens, the Japanese, according to the script, can upscale their sales as a percentage of the total increase.
And more and more of them will be Nissans, no matter what happens on the sales chart.
You can count on it.