Alain Menu led home a Chevrolet 1-2-3 to take his third World Touring Car Championship win of the season
The Swiss driver held off a concerted challenge from teammate Rob Huff in the first two corners of the race to take a lead he would never relinquish.
Huff held station in second for the rest of the race, with the heavily-ballasted Nicola Larini completing the podium after holding off a challenge from Gabriele Tarquini at a restart.
"It's a great day for RML and Chevrolet," said Menu. "They have done a very good job with the car. Look at where we started in 2005 and where we are now."
Despite his success, Menu believes that he will not be able to translate his race-winning pace into a title challenge.
"There are some tracks where the Chevrolet is good," he said. "But the BMW is good everywhere. I know I'm not fighting for the championship - I'm just taking each race as it comes."
Tarquini was unable to challenge the Lacettis on his way to fourth, while SEAT teammate Yvan Muller proved that it is possible to overtake on the streets of Porto by jumping the GR-Asia SEAT of Tom Coronel on the run into Park Corner on the last lap to earn fifth.
Behind the two SEATS, the three championship protagonists fought it out for seventh, eighth and ninth places.
Schnitzer Motorsport's Augusto Farfus missed out on the reverse grid by finishing ninth, shadowing teammate Jorg Muller to the flag but unable to launch a serious challenge.
Defending champion Andy Priaulx will start alongside Muller on the front row after taking seventh.
Farfus had earlier clashed with the ROAL-run works BMW 320si of Alex Zanardi, spinning the Italian down to the back of the field on the first lap.
Italian Stefano d'Aste continued his strong run for the Wiechers-Sport BMW team, taking victory in the independents' trophy in 11th overall.
Points leader Pierre-Yves Corthals took second, while the independent ranks were depleted on the second lap when GR-Asia's Emmet O'Brien hit the barrier and was rear-ended by the unfortunate Sergio Hernandez.
Pos Driver Team Time 1. Alain Menu Chevrolet 34:07.853 2. Robert Huff Chevrolet + 0.837 3. Nicola Larini Chevrolet + 1.618 4. G.Tarquini SEAT + 1.929 5. Yvan Muller SEAT + 5.418 6. Tom Coronel SEAT + 6.090 7. Andy Priaulx BMW + 6.638 8. Jorg Muller BMW + 6.970 9. Augusto Farfus BMW + 7.270 10. James Thompson Alfa Romeo + 12.082 11. Stefano D'Aste BMW + 12.564 12. M.Jourdain Jr SEAT + 12.896 13. P-Y.Corthals SEAT + 13.160 14. Jordi Gene SEAT + 13.975 15. Tiago Monteiro SEAT + 14.675 16. A.Zanardi BMW + 15.043 17. Felix Porteiro BMW + 24.319 18. M.Ceresoli SEAT + 26.127 19. Miguel Freitas Alfa Romeo + 26.557 20. Luca Rangoni BMW + 45.258 21. L.P.Magalhaes SEAT + 51.277 Not classified O.Tielemans Alfa Romeo 7 laps Roberto Colciago SEAT 2 laps Emmet O'Brien SEAT 1 lap Sergio Hernandez BMW 1 lap
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