Indy Lights championship leader Jack Harvey declared himself happy with his first IndyCar test after narrowly beating Nelson Piquet Jr and Matt Brabham to lead the non-regulars at Sonoma.
Seven drivers turned out on Thursday for the special test, which was allowed under an IndyCar rule that grants teams two additional on-track days if an Indy Lights driver completes at least 50 per cent of the running in each car.
Harvey completed 55 laps in a Schmidt car set up by Ryan Briscoe and finished with a fastest lap of 1m18.93s, which was good enough for sixth overall in the morning session.
Formula E champion Piquet was just behind him on the unofficial times at 1m19.02s for Team Penske, while Brabham managed a 1m19.03s with Andretti Autosport in the afternoon.
"I loved every minute of it," Harvey said.
"In Indy Lights last year I won both races [at Sonoma], so I was really excited when I knew this track was going to be my first test.
"I'm pretty happy with what we've achieved.
"This car is so much bigger, so much more power.
"I mean at the end of the day it's still a race car, but the power and the weight of the car and the size of it were the things I noticed straight away."
His title rival Spencer Pigot, who ran alongside Piquet for Penske, was also impressed by his first run in an IndyCar.
"After a few laps I was able to start pushing more and more," he said.
"I was able to get comfortable fairly quickly.
"We went through a lot of video and data beforehand, and I talked to Will [Power] a lot.
"Team Penske has helped me a lot to get up to speed quickly.
"This car carries so much speed through the corners and carries so much downforce. It's a new experience and a lot of fun."
Also testing were Sean Rayhall (with Scott Dixon for Ganassi), Ryan Phinny (alongside Sebastien Bourdais at KV), and Ed Jones (with RLL's Graham Rahal).
Additionally, Mikhail Aleshin completed a refresher test with Schmidt ahead of his IndyCar comeback at Sonoma, a year after he was injured in a crash at Fontana.
"I feel like I'm home," he said. "It's a great team and a very nice car.
"The difference is very big compared to last year.
"There's more downforce now so it's not very easy, but I have some experience handling it, so I am enjoying it."
Unofficial times put Power fastest overall with a 1m17.37s in the afternoon.
The teams will return to Sonoma Raceway for the double-points season-finale at the end of this month.