The experienced co-driver spent the majority of the hour-long session working on race set-up in the #25 Walkinshaw Holden, hovering around the lead group as times dropped into the low sixes.
He then jumped long-time pacesetter Garth Tander with a late green tyre run that yielded an impressive 2m05.459s.
"We had a good programme so far this morning, we just kept chipping away from where Chaz [Mostert] ended up yesterday with the set-up of the car," said Luff.
"We just kept improving it. The car is really comfortable. We're still looking at more of a race car than a qualifying [car], he'll focus on the Shootout [in the] next [session], but I'm very happy."
Brad Jones Racing's Thomas Randle was another late improver, sneaking into second with a 2m05.741s, while Tander slipped back to third with his 2m05.817s set with 25 minutes to go.
James Moffat was the best-placed Tickford driver in fourth, followed by David Russell (MSR Holden), Dale Wood (Kelly Ford), Will Davison (Tickford Ford) and Will Brown (Erebus Holden).
Zane Goddard made it two MSR cars in the Top 10 with the ninth quickest time, edging Bathurst rookie Jordan Boys (BJR Holden).
There was some speedy spanners in the DJR Team Penske garage, meanwhile, after a transaxle issue left Tony D'Alberto stopped at the last corner after 20 minutes.
Remarkably the crew managed to swap the faulty unit for a new one before the end of the session, D'Alberto returning to the track with a little over three minutes to go.
"I went to grab second corner at the final corner and it didn't allow me to," D'Alberto explained. "Then it felt like I had some neutrals, so I just decided to shut it off. It all sounded a bit funky to me.
"Before that, though, the car was really good. I was on full tanks, trying to a do a race run, and it felt really nice."
He ended up 16th fastest, directly behind teammate Tim Slade and Triple Eight's Craig Lowndes.
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