Originally Posted by shammi
Today was my first time refueling my 997.2 cab. I noticed that the minimum recommended octane rating (RON + MON)/2 was 93. However the highest available octane rating here in the Seattle area is 92 and that's what I used.
What are the implications of doing that? Also does brand make a difference? I used Chevron today. Finally, does anyone know of anyplace in the Seattle area that caters 93 octane fuel or higher?
Using a lower than spec'd octane grade of gasoline under some condition -- part throttle (most cars spend the majority of their time at 30% throttle or less so part throttle is nothing to be laughed at) higher loads -- the lower octane gas will result in improper combustion which is knock. Knock sensors detect this as they should and the engine controller dials back ignition timing to save the engine from the damages of knock.
I might add these engines with their very high compression ratios run just short of incipient knock alot.
This ignition timing retardation reduces pressure in the cylinder/combustion chamber but it reduces the amount of chemical energy that is converted to mechanical energy. It also raises the exhaust gas temperature.
Thus a high compression engine run on say 92 octane vs. 93 suffers some loss of torque, fuel economy suffers, driveability may suffer, and all components in and downstream of the combustion chamber are exposed to higher temperatures.
Now, the difference between 93 and 92 is almost certainly slight enough that even after years of a steady diet of 92 the engne will be no worse for the experience. Gas mileage over this time will be affected but by no federal deficient scary amount.
91 vs. 93 more of the above, but again the difference is slight. I have used 91 octane for the last 150K miles (since moving to CA) in my Boxster and I see no effects from this. Gas mileage has problably suffered a few percent drop (if that).
89/90 same thing but the drop in fuel mileage and the possibility of driveabilty problems increases. Exhaust gas temperatures have to go up even more.
And as for 87, IIRC both my Boxster owners manual and my Turbo owners manual do not recognize 87 octane gas as suitable for use in either car's engine.
If you can buy 92 octane consider yourself and your car lucky. Just shop at a name brand station to get gas with a good detergent/chamber deposit fighting additive package and buy station (to get the freshest gas) and enjoy your car for a long time.
I have found no difference between Chevron or Shell or Union 76, Unocal, or Philips 66 and in some areas of the country Quik Trip and a few other chains.
After filling up for several tanks worth wit USA and Rotten Robbie sometimes upon filling up with Shell or Chevron I notice the engine's a bit snappier, perkier. But hard to quantify.