Estimates of the turnout in yesterday's election indicate that Cambridge Convention '75's strategy of increasing registration among young voters may pay off with a fifth liberal seat on the City Council.

Approximately 28,000 people voted in the election, Edward J. Samp Jr., a Cambridge election commissioner, said last night--about 2000 more than turned out for the 1973 municipal election.

He said the voting pattern appears to be favorable to Cambridge Convention '75.

Samp added, however, that a breakdown by district reveals that although many Harvard Students registered to vote few showed up at the polls yesterday.

By tomorrow evening the commission will have the results of an informal count of the ballots in the City Council race. "We should have a pretty good idea who's going to be elected then," Samp said.

Pretty High

The turnout was "pretty high," Cambridge Mayor Walter J. Sullivan, a member of the conservative Independent group, said last night, and he conceded that "would give some favor to Cambridge Convention '75."

Sullivan added that although it is too early to predict the outcome of the race, there is no doubt that the "big voter registration" will "make a difference" in the vote.

Cambridge Convention '75, a coalition of nine liberal candidates for the city council, enrolled nearly 5000 new voters before the election, mostly in the Central and Harvard Square areas.

By presenting a unified slate and increasing their constituency, the candidates hope to tip the political balance in their favor and capture five of the nine seats on the council.

Throughout the race, convention leaders have maintained that a high turnout in this year's basically issueless and lackluster campaign would indicate success for the Convention.

David Sullivan, one of the convention's organizers, said last night that things were "looking really good; I think we are going to win."

The only major voting irregularity alleged yesterday was a challenge by Cambridge Convention '75 of 150 absentee ballots filed in East Cambridge, a traditional Independent stronghold.

Francis H. Duehay '55 a convention-endorsed incumbent, said last night that the challenged votes were from residents not entitled to file absentee ballots because they were neither ill nor out of the city on election day.

"Some candidate apparently went around signing up people with absentee ballots," Duehay said.

Final tabulation of yesterday's votes will take as long as a week because of the elaborate process required by the Proportional Representation system of voting that the city uses.

Voters yesterday ranked in order their nine choices for council seats. Under the system, if any of the 25 candidates receives 10 per cent plus one of the first-choice votes, he is elected.

The excess votes of winners and the first-choice votes recorded for the lowest candidates are then redistributed to each voter's next choice. The process continues until nine candidates make it over the quota and are elected.

Beginning this morning the election commission will tabulate the votes for city council, holding off on Cambridge School Committee ballots until Saturday.

If the convention does take a fifth council seat, it will be result of the group's strategy rather than any real shift in the thinking of Cambridge voters.

Throughout the campaign, the conservative Independent councilors have maintained that there were no real issues this year and confidently predicted victory, relying on the solid working class constituencies they have built up over that last few years.

"We got our vote out," Sullivan said, "and that is the important thing."

With an extensive publicity campaign, however, the convention told Cambridge residents that an Independent victory would mean the end of rent control and the firing of the city's liberal city manager.

Publicly, the Independents have all but igonored Cambridge Convention '75, but about a half hour before the polls closed yesterday, Sullivan acknowledged that the group "had certainly done its homework.