Like a top jockey, Silver Spring drag racer Mike Mortimer has a clock in his head, and it was on time last week when, the 22-year-old steered a borrowed car to the Steet Eliminator class title in the Winston World Championships at Ontario, Calif.

Mortimer beat an invited field, hitting about 100 mph at the end, of the quarter-mile dash in Plymouth Duster legal for street use. Bob Maxey, Clinton, Md., raced in another class, but was eliminated in an early round.

They complete in "Bracket" racing, a handicap system equalizing all cars regardless of their speed potential. Before each match race, drivers "dial in" their predicted elapse time for the race. The faster car gives the slower a head start. If the winning car beats its predicted time, it loses. If the loser also is faster than its forecast, then the car closest to its dialed-in time wins.

Mortimer's virtue is consistency. He almost always runs at or slightly slower than 14.3 seconds. In his own car, which he drives to the races, he has won his class the past two years at 75-80 Dragway, Monrovia, Md. This season he won the Northeast Divisionals, earning $2,000 and the expense-paid trip to California. His victory there was worth only $300.

Mortimer, a college student, does his own mechanical work with some help from his father. A racer for six years, he plans to step up to a Super Pro dragster next season. He can still use the clock in his head for that class.