Hirakawa was one of the most consistent performers in the Japanese single-seater series during the SF19 era, having finished in the top four of the championship standings in each of the past three seasons.
But this year's revised SF23 package, which boasts less downforce than its predecessor in a bid to create more exciting racing and all-new Yokohama tyres, threatens to change the competitive landscape.
Numerous drivers said they had noted a shift towards oversteer during this month's sole pre-season test at Suzuka, and Hirakawa admitted his surprise at how different the SF23 was to drive compared to the prediction he made based on last year's Motegi development test.
While he set the fifth-fastest time of both the opening day of the test and overall, the Toyota World Endurance Championship star gave a downbeat assessment of how Impul is faring so far.
"It’s hard to manage everything compared to last year," Hirakawa told Motorsport.com. "Still it’s very hard to understand it. Having less rear downforce with this car makes it more difficult.
"For me, last year I was always rear-limited, fighting the rear. We never had understeer. That’s why I am struggling with this year’s car. I didn’t spin [at Suzuka] but I had three or four moments. It’s very hard to drive.
"Also, there is more tyre degradation. We didn’t do more than about 10 or 15 laps on a long run, and it was difficult to manage the rear. We were surprised that the set-up from last year didn’t work. I’m nervous, actually."
Hirakawa added that with passing expected to be easier with the new car, particularly at Fuji where the slipstream effect is likely to be powerful, finding an improvement in race pace will be crucial.
"Fuji should have less tyre degradation [than Suzuka], so we may struggle less there," he said. "But if it’s hotter, it could be more difficult.
"This car is also supposed to make passing easier, so that makes race performance more important [than qualifying].
"To find more rear grip is the urgent thing we have to improve. This is the key."
Despite his struggles in pre-season testing, Hirakawa believes that he can't afford to sacrifice too many points during the opening double-header weekend of what is now only a nine-race series on April 8-9.
"My target for Fuji is to be on the podium for both races," said the 29-year-old. "It’s over 20 percent of the championship, so even if the car is not working well, I need to do as much as I can. If I finish on the podium, I will be happy."
