Carroll said: "After a few laps the track started getting better and we were able to work on the setup, at the end of the day the car felt great and we managed to put some very fast laps in, it was a very enjoyable day.
“The MSR Houston Road Course is quite technical; it has some very high speed corners where you can really feel the downforce of the Champ Cars and some slower sections where mechanical grip is more important.”
Howard, 25, the reigning Indy Pro Series title-holder, was having his first run in a ChampCar, but was surprised with how easy he found the transition.
He said: “Driving the Champ Car is not as physically demanding as I expected, I was expecting it to be a lot harder in my arms.
“But because of the downforce you can feel a big difference between the slow and the fast corners, the steering just gets a lot heavier on the fast stuff and that's something that I've never really experienced before with the cars I've driven. “
Carroll and Howard were joined by ChampCar Atlantic champion Simon Pagenaud in the testing line-up.
Pagenaud, 22, was also having his first run in a fully-blown ChampCar.
Forsythe were joined at the Texan track by Conquest Racing, who continued their trend of running rookies this winter by giving South African driver Stephen Simpson his first run in a ChampCar machine.
Simpson, 22, who has raced in ChampCar Atlantic and in A1GP this year, was impressed with the speed of his machine.
Simpson said: "It was a very successful and instructive test for me and went better than I'd anticipated. I hope to be back in a Champ Car again next month."
“I was able to acclimatize myself with a Champ Car including getting used to the brakes and downforce which were substantially in excess of anything I'd previously experienced and I ended up being able to run some fast laps that I was pretty satisfied with bearing in mind my experience.”