91 Octane in a 340?

A few things..

If it doesn't knock (unless you have a really noisy exhost you will hear it) you can try different blends.

drive it with 94 till it's almost empty.
1 to 2 gallons in the tank.

Fill it 1/2 way with 91. Do some drive testing. No ping your good to go, if not add 3 gallons of 94 now you have 92 to 93, no ping you know the blend needs to be a 50% 94 and 50% 91 mix.

It can take some effort to keep the mix but it can be done.

BTW that's how the gas pumps make the middle grade thew blend high and low in some percentage.

My 2006 grand caravan 3.1? Needs the middle grade. Under load it pings it's heads off on the lower grade.


Lastly...

Summer vs winter blends and your altitude can make a difference