Running 91 because it is 'cheaper' can be counterproductive in two ways:
- it could destroy the engine through detonation
- ign timing has to be retarded to stop pinging [ detonation ], which reduces power AND fuel efficiency. You can use cheaper fuel, but use more it.....
With an engine like this that has a question mark about it's history & internal components, I would use the highest octane pump fuel I could get & tune the engine. Listen for pinging. If none, then try a lower octane fuel. You can do a compression test but it will NOT tell you whether THIS engine will work with THAT fuel. There are many, many variables that will affect whether an engine pings & that is why you are going to here stories like this: my 9:1 engine pings on 9. My 10.5:1 engine runs fine on 91....