With race gas and enough fuel, you can probably do 23psi...maybe more. But no need to go any crazier. I know of two other dynos that chose to go that high but start lower to be safe, listen for pinging and such. Also make sure you don't have any boost leaks if you can before going in. I'm not 100% sure if the 555s are enough for 20+ psi without having an FPR (I know for TD05s you'll need an FPR to be safe). So tread cautiously and analyze the air/fuel and the steadiness of the dyno at top rpm.
Good luck.
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