As I posted over in MPSIMS, I’m getting an old MGB. Naturally I needed an owner’s manual, so I picked up a re-print of the original one. It says that with the low-compression engine (8.0:1) “premium fuels of 93-octane (Research method) up to 97-octane are required, with preference to 95/97.” The high-compression engines (8.8:1) require 98 octane by the Research method.
But the pumps around here aren’t marked with Research numbers. They’re (R+M)2. (Shouldn’t the "" be a “/”? Or is the number really mulitplied?) So what is the U.S.-common equivalent to 93 octane ® to 97 octane ®?
In the U.S. the research octane number is typically higher than the motor octane number by about 10 points, but there is some variation in that. (The difference between RON and MON is called the “sensitivity”, which I have never noticed labelled on the pump.)
If the (M+R)/2 number is 93, RON should be somewhere around 98 and MON around 88, plus or minus two or so. If the (M+R)/2 number is 91, then RON will be about 96 and MON about 86.
By the way, I get this from “Gasoline FAQ” which I highly recommend. It’s a fascinating read if you’re into that sort of thing. Octane ratings are covered in part 3 http://www.faqs.org/faqs/autos/gasoline-faq/part3/
Okay, so if they recommend 95 RON and MON is 10 less, then MON = 85. So R+M/2 (which is what is displayed on the pumps) would be 90. So I’ll need to use mid-grade or premium.
(When I had a '66 MGB before, and I lived in the desert, regular grade fuel would make the engine knock under accelleration.)
Thanks, bibliophage!