Date registered: Apr 2010
Vehicle: '97 SL500, 06 Subaru Outback, LL Bean 3.0
Location: Knoxville, TN
Mentioned: 0 Post(s)
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I don't think regular gas affects the cat converter. Leaded gas is what damages it. The only difference between the 3 grades of gasoline is the octane (anti-knock) rating, i.e.the resistance to the pre-ignition or detonation that becomes more pronounced the higher the load on the engine. Severe knocking or pinging can cause engine wear over time. In general the higher the compression ratio, the higher the octane requirement.
My '97 SL500 which I just got and have hardly driven, has a compression ratio of 10.0 which is lower than my 3.0 Subaru which also requires 91 octane (the highest) gas, but the dealer said the 89 was ok and I've yet to hear a ping even though I pull a light boat and am not hesitant about passing.
Bottom line: I'm gonna try to get by on the 89 (middle grade) simply because a gas station very nearby is the only one I can find that sells 100% gas (no alcohol) and it only has the 87 & 89.
Hope this helps.