Obviously you checked the power limiter? Also, some standards still need a break in. I bought my Le Mans Quattro with 150ish miles and had to put 180 more on it.
I can't replicate 971 by swapping parts around so it must be a run in issue. Can't remember what car it was (might be the speed 12) but something I had needed a few more miles. Put 55 miles on it with a run on b spec extreme > dream car > indy (20 min ish) or SSRX like Trybal said and oil change then also restore the engine and let us know if that sorts it 👍
i've had this happened to me before, check your parts and if it still don't do it the get rid of the car, get another one or use restore coupon and break it in again. It happened with my rx7, no matter what i do it won't get 605 hp, it will stay at 599 hp, even thou i had like 300miles in it, one thing i notice, it said 599hp but it was beating everybody just like a 605hp, it could be a neutral glitch, eh.. good luck
It does depend on where you get the miles from though. You can't just do an Aspec race. Tire wear is a must. Either way, just do 2 quick laps and and oil change to see if it increases.
I usually do a few laps first to make certain everything is as it's suppose to be. Check the hp and, so long as there was a gain, continue. Beats driving for an hour for no gain lol.
I really need to tune. I have a ton of cars that need to be brought up to par. I may get on and race later if you guys are still on. But it wont be until 11 or 12.