Trainer Darren Weir and jockey John Allen have made it back-to-back South Australian Derby victories, with Volatile Mix taking out the Classic at Morphettville.

Weir and Allen combined 12 months ago to win the 2500m three-year-old staying race with Howard Be Thy Name who went in as one of the favoured runners after taking out two key lead-ups.

It was a slightly different story this time around, with Volatile Mix sent out a $16 chance on Saturday when upsetting his more fancied Victorian rivals Ruthven ($4) and Odeon ($3.20) who filled the minor placings.

The win was Weir's eighth at the elite level this season, a tally he shares with Sydney's premier trainer Chris Waller.

The Ciaron Maher-trained Ruthven pressed forward to be outside the lead after being caught wide early in the race and he kicked clear early in the straight.

Favourite Odeon loomed to win and appeared to hit the front at the 200m before fading to finish third, 2-1/4 lengths from Volatile Mix who finished strongest on the outside to beat Ruthven by a half length.

There was pre-race drama with two horses, Waging War and Docker Pav, scratched after playing up in the barriers.

Volatile Mix has now won three of his nine starts.