Eng, whose BMW is run by Rahal Letterman Lanigan Racing, beat Sebastien Bourdais (Cadillac V-Series.R) and the second BMW of Connor De Phillippi in the 90-minute practice session.
This weekend’s 2h40m event is being run for GTP and GTD machinery only.
After the Porsche Penske Motorsport 963s of Nick Tandy and Felipe Nasr swapped the top spot between them early on, Connor De Phillippi took over in the #25 BMW M Hybrid V8, lapping in 1m14.916s inside the opening 20 minutes.
Eng then made it a BMW 1-2 in his #24 sister car, lapping in a session-topping 1m13.349s as De Phillippi improved to 1m13.722s just before the halfway point.
Bourdais broke up their party in the #01 Chip Ganassi Racing-run Cadillac, taking the second spot with 1m13.573s, 0.224s off the fastest time.
Ricky Taylor suffered an early off at Turn 3 in the #10 Wayne Taylor Racing with Andretti Acura ARX-06, the car that set the pace on Friday, and also suffered a flat tire later. His off-track moment was mirrored by Renger van der Zande in the #01 Cadillac in the second half of the session.
The session was red flagged with 15 minutes to go when Jarrett Andretti went off at Turn 5 in the Andretti Motorsports Porsche 911 GTD entry.
At the resumption, Eng remained on top, ahead of Bourdais and De Phillippi. Dane Cameron made a late improvement to take fourth in the #7 factory-run Porsche 963, ahead of the sister car of Mathieu Jaminet.
Pipo Derani was sixth in the Action Express-run Cadillac that Jack Aitken crashed yesterday. Ricky Taylor was seventh, ahead of the sister Acura of Louis Deletraz, who improved late on.
Richard Westbrook (JDC-Miller) and Gianmaria Bruni (Proton) rounded out the GTP class in their customer Porsche 963s.
#14 VasserSullivan Lexus RC F GT3: Jack Hawksworth, Ben Barnicoat
Photo by: Jake Galstad / Motorsport Images
Lexus beats Corvette in GTD
In GTD, Jack Hawksworth set the pace in Vasser Sullivan’s #14 Pro class Lexus RC F with 1m20.376s, ahead of Nicky Catsburg in his #4 Chevrolet Corvette Z06 GT3.R.
That’s the way it stayed until the end. Marvin Kirchhofer (Pfaff Motorsports McLaren 720S) was third, just pipping the top pro-am entry, the Wayne Taylor Racing with Andretti Lamborghini Huracan of Danny Formal.
Qualifying starts later at 7:35pm ET.