In the small world of the horse show circuit, Norman Dello Joio is known as Stormin' Norman.

This afternoon Dello Joio lived up to his nickname when he guided his mount, Johnny's Pocket, owned by the Coker Farms, to victory in the $15,000 Mercedes-Benz Grand Prix. It was his fourth win on the American Grand Prix circuit this year, and he earned an $8,000 gold watch as well as the $4,500 purse.

Dello Joio, 24, of South Salem, N.Y., toured the seven-fence jumpoff course in an extremely fast time of 32.854 seconds. He was the seventh of 10 riders qualified to go against the clock.

Rodney Jenkins was the eighth rider to enter the ring at the Salem farm. Jenkins, who won in 1974 and 1979, had to settle for second place on Second Balcony, owned by Edie Spruance. Jenkins, 37, of Orange, Va., finished in 32.976 seconds.

"When I left out a stride between fence three and four I had it planned to jump the next fence on the left-hand side, so that I could keep galloping," Dello Joio said.

Jenkins agreed it was the extra one stride between fences that accounted for the narrow time difference. "My horse is better turning, but he (Dello Joio) took six strides there and I did seven," said Jenkins, who also won third place on Coastline, owned by Elizabeth French.

Dennis Murphy, who rode Lyrical Lou, owned by Mr. and Mrs. Dennis Murphy and John C. Adams, summed it up while waiting to receive his fifth-place ribbon: "If you don't have a racehorse, you don't have a chance." f