Two Ollie Watkins goals, one each from Florian Jozefzoon and Neal Maupay and a Marc Roberts own goal saw the Bees romp to an emphatic win in a scoreline that flattered Birmingham.
It was a nightmare first return to Griffin Park for Blues trio Harlee Dean, Maxime Colin and Jota, who all moved on August transfer deadline day.
It could have been a night to rival the hosts’ 9-0 record win over Wrexham in 1963 but for a string of David Stockdale saves and some lacklustre finishing.
Brentford were dominant in a sometimes frantic first half which also saw Blues have their chances.
Maupay was first to show with 10 minutes gone, but his volley at the end of a slick three-man move flashed just wide. The Frenchman went close minutes later with an almost carbon-copy effort but snatched at the shot which fizzed past the upright.
On-loan City striker Sam Gallagher saw a strong penalty shout waved away when he went down after cutting inside Andreas Bjelland after 14 minutes, but should have done better minutes later when his close-range snapshot was blocked by Daniel Bentley.
The hosts took the lead just after the half-hour when Blues keeper David Stockdale let Watkins’ 25-yard piledriver squirm through his hands to nestle just inside the post.
And 10 minutes later Maupay robbed Jason Lowe on halfway and ran clear to feed Jozefzoon, who picked his spot with a low angled drive from the edge of the box.
After the interval Brentford were on a different level to Blues and carved the hesitant rearguard open almost at will, though Gallagher was first to go close when he fired over from close range from a rare Birmingham attack.
Jozefzoon turned provider with a low cross for Maupay to tuck home from close range and Stockdale was culpable for a second time on 54 minutes when he fumbled the ball for Watkins to square for Maupay, whose wayward first-time drive was deflected in by the hapless Roberts.
The goal rush saw Dean Smith’s Bees oozing confidence but Stockdale made amends with a superb double save to deny Watkins on the hour.
Jozefzoon curled a teasing shot just wide before Watkins missed a sitter from point-blank range as Birmingham looked increasingly dispirited.
Stockdale tipped a Watkins header wide and repelled Jozefzoon’s low drive as he staged a one-man rearguard to keep the scoreline in single figures.
But Jozefzoon was at the heart of things again 10 minutes from time when his whipped cross was nodded home by Watkins to make it five.
The travelling City fans had to wait until injury time for their side’s best effort on goal. Gallagher’s powerful header was blocked by a brilliant reflex Bentley save before Che Adams blazed the rebound over.
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