An NCAA committee studying possible modifications of Proposition 48 yesterday voted to recommend a reduction in one of the test score requirements, a move certain to stir debate if it is proposed at the annual convention in January.

Proposition 48, which becomes effective for the 1986-87 academic year, requires a minimum test score and a 2.0 grade-point average in a core curriculum of 11 academic courses for an athlete to be eligible for first-year competition. The committee, meeting at the Marriott Twin Bridges, decided that a score of 13 (out of a maximum 36) on the American College Test is more equivalent to 700 (out of a maximum 1,600) on the Scholastic Aptitude Test than 15, as was passed in 1983.

According to committee data, one of every two black athletes who took the ACT in 1977 and 1982 would have qualified with an ACT test score minimum of 13. Only one of five would have qualified using 15.

In its reports to be made to the NCAA Council, the committee also is expected to endorse at least two options that would allow more black athletes to qualify. One is an Eligibility Index Formula that would allow higher grade-point averages in high school to offset lower test scores. The other would allow schools at which more than half the student body does not qualify under Proposition 48 to use their own formula to determine first-year eligibility.

Wilford Bailey of Auburn University, NCAA secretary-treasurer and chairman of the committee, declined to discuss what recommendations would be made to the NCAA Council to consider at its Aug. 14-16 meeting. According to Bailey and NCAA President Jack Davis of Oregon State, the recommendations then will be released publicly, but no proposal is expected to be written before the Presidents Commission meets in October.

John Ryan, president of Indiana University and chairman of the Presidents Commission, said after attending yesterday's meeting that he did not consider any committee actions to be a watering down of Proposition 48.

Davis, reached at his home in Corvallis, Ore., said there "is a sensing that 13 represents a significant decrease in quality. Some presidents will have trouble with 13."

One leading advocate of Proposition 48 predicted that using 13 instead of 15 would make it easier to qualify under the ACT test than the SAT, and that coaches at schools normally using the SAT would steer athletes into taking the ACT.