Safe boost level on 91?

[Mr Revhead]

these cars are designed to run on 100 octane in factory trim.

you work it out

[Malcolm]

I run 91 with about . 8 bar ( area ) = 11 or 12 psi on my stock
Gt-4 how ever its not a daily driver as i have a company car and If i blew up the 20 year old motor its just motivation to seek a Higher gen motor

but ive done Fineeeee on it for the past 5 months no visble issues ( other than Engine Mounts )

that's because you can't see the top of the pistons

seriously, i know it's tempting but you shouldn't run any japanese turbo engine on 91, unless you have an aftermarket ecu and it's tuned to run on 91.
The improvement in fuel economy will offset the cost per litre of higher octane fuel, with the added bonus of not destroying your pistons.

[Dell'Orto]

but ive done Fineeeee on it for the past 5 months no visble issues ( other than Engine Mounts )

Have you pulled the plugs yet?