Don Knowles can relax now. He's completed the first stage in his effort to win two national amatuer road-racing championships this year. He's qualified to defend his title in the Showroom Stock B class and had clinched an invitation in the A class for the October Sports Car Club of America runoffs.

"Defending champions get automatic invitations if they run three races," the Springfield racer explained. "I did that in my last year's Saab. Then I got a good start with my A car, a turbocharged Saab, and won six straight races early this season. That got me in with that car by winning the Southeastern Division title."

Turbocharged cars are supposed to be too slow out of the corners to make good road-racing machines. Not entirely true, says Knowles, who drives a factory-backed sedan.

"By braking hard and a little longer while keeping the accelerator on hard, you can overcome the usual second or two lag until the turbocharger's boost comes on," Knowles said. "These sedans are a nimble and quick as sports cars now. For top-speed, I get 115 miles per hour. On street tires, that's not bad."

Racing in the very competitive Showroom Stock class in which cars can be only slightly modified, the 32-year-old Knowles has won four straight divisional titles. Off the trackm he's a program analyst for the Agriculture Department. "I'm glad to have earned my invitations early," he said. "My job is going to keep me busy over the summer."

Eighteen Porsche 935s, each worth at least $125,000 or more, are among the 51 entries for Saturday's six-hour world manufactures' championship race at Watkins Glen, N.Y. Sunday's Can-Am race there for modified sports-racing cars has drawn 35 entries.

Le Mans 24-hour winners Bill and Don Whittington and Klaus Ludwig are in the six-hour in their winning car. Paul Newman, Rolf Stommelen and Dick Barbour, who were second to them, also are entered. Indianapolis winner Rick Mears will be teamed with hydroplane racer Preston Henn and the almost unbeatable Peter Gregg. All three teams are in Porsches.

Fourth of July is the date for the 57th annual Barbara Fritchie Classic motorcycle races over the dirt Frederick [Md.] Fairgrounds track. Time trials are at 1 p.m., racing an hour later.