Re: Gas
Generally I agree with the comments here concerning octane, but I want to bring something up worth discussing. Although the original question concerned an '88 Merc 5.7, the answers have been more general in nature sometimes coming close to inferring that octane is just this side of meaningless. Octane is a rating of knock resistance and I agree that with older, mechanical, carbureted engines that if it doesn't knock with 87 then great!! Save some money. My confusion (or desire for discussion) comes from the fact that modern, completely electronic (MPI and EFI) engines usually employ a knock sensor. Doesn't this allow the ECM to push up against knock limit and then back off? If so, wouldn't your engine operate at peak efficiencies and power if it was allowed to advance the timing as far as is safe?<br /><br />If you throw in a couple of other knock inducing factors; load and ambient air temp, I have decided to run higher octane gas in my boat AND my tow vehicle when towing. We deal with up to 117 F towing from LA to the Colorado River and climb up to 8% grades. The boat runs under the same ambient conditions and when pulling skiers, or blasting a WOT run, the boat is often running at full load conditions too. It just seems to make sense to me to give those engines as much help as possible giving me the highest performance they can.<br /><br />Wouldn't the mid-grade also enhance performance as compared to 87 in the examples I noted? Yes, I know they will both run with the 87, and I have previously believed it is a waste, but again, these engines can adjust for the octane, right?
<br /><br />There's no question that computer controlled cars are a completely different animal, and I certainly understand your thought process, but I still feel that any more octane than recommended for any stock vehicle just means you're unnecessarily spitting dimes outta the exhaust pipe.
