After taking pole in the opening two races of the year at Okayama and Fuji Speedway, Subaru was looking good to make it a hat-trick after Iguchi set the fastest time in the opening segment of qualifying on Saturday.
That was until Yamauchi pulled into the pits during Q2 on his hot lap with a technical issue, which was later diagnosed as a loose turbo hose - leaving the reigning champions a lowly 15th on the grid.
From there, the works R&D Sport-run BRZ struggled to regain ground, only managing to regain three positions over the course of the race to finish 12th despite numerous other cars hitting trouble.
It marked Subaru's worst race result since last year's Okayama season opener.
Yamauchi admitted that this year's Balance of Performance for the GT300 class, in which the BRZ had its turbo boost pressure reduced by four percent, has made overtaking almost impossible.
"This year’s car has even better aerodynamics, and I felt with that strength we could fight, but when we get stuck in the group, that merit disappears, and it was a painful race," said Yamauchi.
"Also, the power reduction from the BoP is having an effect. If you can put your nose ahead at somewhere like the S-curve, you can get ahead with a cutback, but getting to that stage was difficult."
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Iguchi added: "The car was really good in qualifying, but we experienced overheating in the race and the grip dropped quickly.
"Also, when I was in the group, there was no downforce and it was hard to challenge anyone on the brakes, which made it tough."
Subaru general manager Masahiro Ozawa admits that qualifying up front is the only way for the BRZ to be sure of a good result.
"The BRZ GT300 has good cornering speed, but when we are stuck in a group, the downforce disappears." said Ozawa. "We don’t have enough speed to compensate for that loss, so we were able to catch up but not pass.
"Therefore, the only way we can compete is by qualifying at the front and not get caught up in the group and demonstrating our speed.
"We have about two months until the next race at Fuji, so we’ll make changes so we can improve our level."
Yamauchi and Iguchi head to the next race at Fuji in August eighth in the GT300 drivers' standings on 9.5 points, 23.5 down on points-leading Kondo Racing Nissan pair Joao Paulo de Oliveira and Kiyoto Fujinami.
