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A 10.1:1 compression engine should have no problem running the 91 octane. It should even be safe for close to 89 to 87 for short and light load traveling. I wouldn't risk anything lower than 89 for sure though. If you do use the 91 and feel it isn't right, have it tested. There are several places that can test the octane content and give you an analysis (much like oil analysis'). If the fuel is consistantly off in rating and measurement, use some booster in a full tank and have it re-tested until you find the proper booster to fuel ratio. Sounds like a lot of crap to go through, but it shouldn't take but about 3 weeks of light driving and however long it takes to get a sample of each to a lab. May be on the expensive side, though. Well worth it versus another engine.
4cyl. Chevy S-10's have about 10:1 compression (if not mistaken) and I used regular with never an issue for 7 years. Newer cars may have different results though.
To clear some things up, a ratio of 13:1 compression will have a hard time running smooth off of 93 octane - pump gas. 13:1 Mind you. Si's have 11.1 or 11.3:1 compression. That's perfectly capable of using 91 octane with no problems.
Running higher octane other than 91 may give you some mileage gains, but unless you're running 103 octane you won't notice any difference between the Cal. normal 91 and the other 93 ratings.
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