The young Blues staged a stirring fightback from being 2-0 down at half-time to overcome the League One opposition.
Sixteen-year-old Substitute Jayden Braaf made a huge impact after coming on early in the second half, bagging a brace of goals with skipper Luke Bolton and Taylor Richards also on target.
The victory saw City become only the third Academy side to reach the Checkatrade quarter-finals, emulating Swansea in 2016-17 and Chelsea last season.
And manager Harsley was full of praise for the way his young charges fought back to record another memorable victory following on from our dramatic 5-3 win on penalties at Barnsley last month in the round of 32.
“We’re absolutely delighted with the lads. We’ve said along – players and staff – that everybody loves this competition. It has been really good for us this year,” Harsley declared.
“We changed the front three around positionally at the break and they responded really well in the second half and took the game away from Rochdale pretty impressively.
“At times this season when a game has gone away from us, it has gone away properly but last night we said 'we don’t go under.' At 2-0 down we stressed the next goal was massive and we got it and you could see that from then on, we went from strength to strength.
“Jayden Braaf, a young kid who is just 16, came on and had a dream game and made a massive impact so we were really pleased for the boys.
“Though it’s an Under-21 team we’re probably averaging about 18 years of age and for these lads to be getting these opportunities and exposure is magnificent so, for us, it’s great.”
The City youngsters and coaching staff now await the draw for the last eight, which takes place at 7pm tomorrow night, knowing the Club are just two rounds away from the final at Wembley.
Harsley insisted no-one would become complacent or look too far ahead – but he said the incentive of potentially going on to secure a place in the Wembley showpiece was acting as an extra motivational tool.
“We’re not getting ahead of ourselves, but we know there is a little carrot for us there. In two games time now they could be stepping out at Wembley which would be fantastic,” Harsley added.
“But we are not getting ahead of ourselves. It’s a carrot for them to keep them going and we’ll just see what happens next.
“The next game is a big game and hopefully we can have another good performance.”