04-08-2012, 11:52 PM
Join Date: Sep 2011
Location: Portland, OR
My Ride: 2002 E46
| Wow this is one of the most volatile (get it? wink wink) gas threads I've come across, even as far as gas threads go it's already nasty. . . But I gotta chime in, and to be honest I probably won't even peek at the follow-ups because I don't want to inspire another 8 pages of really poorly thought out "I DONT HEAR MY KNOCK SENSORS, YOU'RE SO STUPID!!!"
Anyway, it says near your gas cap and in your manual what octane gasoline you should use. And yes high elevations brings it down a bit. Besides retarding a little bit for lower octane (hook up a live OBD2 reader and you can probably get enough info off the standard generic OBD2 parameters to see how 87 vs 91 works in your engine). However, I think even more important than the resistance to detonation are the different detergent packages between different gasses. I have the Honda TSB that advises dealers to tell people to use Top Tier gas, I can post if you want. And I don't think Honda makes any cars that require premium, they're talking regular for Hondas.
Someone is pounding out on their keyboard right now "all gas is the same." You're right, the base stock is practically identical, sometimes manufacturers will even fill their base stock off each others refineries. However the detergent packages that get mixed in later are totally different between manufacturers and also between grades of the same manufacturer. Premium gas always gets the best detergent package.
If you're totally cheap, at least do like 5 consecutive fill ups of premium then 5,000 miles on 87, then repeat.
Also you may or may not know that BMW is in a group of manufacturers that has a standard set for their recommended gasoline that is beyond the spec of the current EPA standard. Their engines will run on anyone's gas, but these manufacturers are saying that they run best, and cleanest, etc., etc., with certain gas.
This isn't a deal like where Castrol pays BMW to be the recommended motor oil and get their name on the oil cap, this isn't a pay to play deal.
It sucks because most states have laws that require them to sample different manufacturers gas at different places every single month and do pretty elaborate testing on them, but they don't publish that information. You'd literally have to pay to get gasoline tested yourself if you wanted to test some of the mojo in it. Or, if you have a medium amount of a science background, do a lot of googling, and read the top tier gas manufacturers, and some research on bobistheoilguy.com in the gasoline forums where major science nerds hang out, you'll stick with Shell Premium. Just sharing what I have found through research, you do your own. Or just be a grumpy guy at a keyboard harassing people in all caps.
Also, you're seriously silly if you are making comments like "one tank my car felt like X, and the next tank I filled up with this other gas and it felt like Y and I got 2% better MPG." Do you have any ideas how many times something has to be done in automotive testing to throw out all extraneous influences and verify it as a repeatable circumstance with a delta greater than the margin of error? Hundreds of thousands if not millions of miles when all other factors are identical.
If you don't believe me, let's both change our head gasket at the same time (my e46 has around 50k miles on it) and take photos of the combustion chamber build up. I've used premium all 50k miles, so this would be directed towards someone that hasn't.