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E21, E30General discussion and technical help for 1975-1991 3 series cars.
BMW mandates 89 octane or higher. 91/92 is usually ur best bet. Ive tried 94 octane and its decent but i dunno if its worthe the extra money. BMW says a higher octane will incrase fuel economy (the distance u can get out of a tank basicly) but i havent really noticed a major difference.
The amount of octane in fuel determines how much the gas can be compressed in the cylinders before it spontaneously ignites. If it does, then the result is engine knock. The higher the octane, the more the gas can be compressed. But that doesn't mean the higher the better. In fact, more octane in your gas can be just a waste of money, since the octane rating is determined by your engine's compression ratio and ECU parameters.
Contrary to popular belief, higher octane ratings does not keep your engine cleaner and does not save the environment any more than having just the right amount. It's the additives put into the gas that make a difference. The Fed Gov't has mandated that all gas must have detergents that clean you injectors and spark plugs, and there is a standard minimum to which this must be adhered to. Even the discount gas stations are held to this, and samples are constantly tested to ensure that this is happening.
In short, it's the quality of gas that matters most, and as long as you're putting in the octane recommended by your owner's manual, you'll be fine. Other E30 owners here should be able to tell you what your car requires, if you don't have the manual. Older cars, like the E30, are actually getting better gas than they need nowadays because the standards for additives were put into place in 1994.
While we're on gas, here's another tip I came across:
"If you're smart, you'll put that debit card away..." Your debit card might be a convenient way to pay for gas, but it's a no-win proposition. When you swipe a debit card at the pump, the bank doesn't know how much money you'll be spending until you've finished pumping. So to make sure you have the funds to cover the purchase, some stations ask banks to automatically set aside some of your money: That amount used to be $20, but with gas prices going up, stations have started asking banks to hold $50, even $100. That means even if you just topped off your tank for $10, you could be out $100 until the station sends over its bulk transactions, which can take up to three days. If your funds are running low, you might end up bouncing a check in the meantime — even though you had the money in your account.
Unfortunately, paying inside with your debit card isn't much of a solution either. Many banks charge their customers between 50 cents and $1 for the privilege of using their debit card in any PIN-based transaction. The American Bankers Association estimates only 13% of consumers pay these fees, but critics say the practice is on the rise and consumers are often unaware of these charges.
i used to put 93/4 in my e30 except when gas prices spike (like now). you wont notice a diff either way (if your running stock). standard compression on an M20B25 is 8.8:1, so it works out fine. i think the B27 has a higher compression ratio tho, i think 9 something to 1. not sure exactly. but i dont think you should have probs with 89 either, i have never experienced knock, even using 87.