Premium Gasoline

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I recently purchased my first BMW -- a 1994 352i. I think it's a great car. However, I notice that the owner's manual indicates that I should fill my tank with high-octane ("premium" here in the United States)
gasoline. I've never before used premium gas, figuring that it was not necessary and never previously having had a problem with regular (87 octane) unleaded with my other cars (mostly Volvos). Now, with the price of gasoline reaching all-time highs here in the U.S., I want to make quite sure that my "new" BMW really needs 89- or 92-octane gasoline. Any perspectives on this issue would be much appreciated. Thanks in advance.
Jeff Morris Arlington, VA USA
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M y '94 352s (sic) get mid-grade gas.

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There have been many discussions on this subject in this group, and also IIRC in rec.autos.driving - you should google them. Basic story is: you probably can use regular without engine damage. You *will* get less power, as BMW engines are optimized for premium.
Given that, I'm not sure that the M50 engine in your car (I presume you actually have a 2.5L not 5.2L engine ;->) has as *good* a set of knock sensors and engine controller as the current crop of BMW engines do: if your car pings *at all* on regular, move up to mid-grade and then premium.
FloydR
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On 24 Jun 2006 16:43:14 -0700, " snipped-for-privacy@gmail.com"

Premium car = premium gas. If you don't want to, that's your decision. The cost over time isn't really all that much, especially when compared to out-of-warranty engine repairs.
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"Toby" wrote

Being a '94 model year, I doubt he still has warranty. :)
Besides, a knock sensor would most likely pull timing and not allow the engine to get damaged. However, given the worse performance and possibly higher fuel consumption, I agree that using a low-octane gas is hardly a bargain.
Cheers,
Pete
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"Pete" wrote

Nevermind that comment. Too many beers tonight...
Pete
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Then I'll say it. Being a '94 model year, I doubt he still has a warranty.
I run the mid-grade in my 3 Series car and have done it for more than 120,000 miles and approaching 6 years. He won't exercise his warranty because of the gas he uses.
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On 6/24/2006 7:42 PM, Jeff Strickland went clickity-clack on the keyboard and produced this interesting bit of text:

This brings up a question I've recently begun to ponder. From what I've read all the octane level means is that it changes the pressure under which the fuel will detonate. Higher octane is harder to detonate. The purpose of this is so that the fuel won't detonate early in high compression engines.
The question: What constitutes a high compression engine, low compression engine and mid-compression engine? I suspect the engines in the M cars are going to have higher compression engines, but what about the other ones? My M60 has a compression ratio of 10:1. Is this high?
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Sorta.
Your hunch is correct, higher octane will not spontaniously combust the way that low octane fuel will, this is how high octane gas prevents ping. When the car has a knock sensor (knock and ping are two words that are used interchangeably, however I've been told that they are not really interchangeable words -- but that is another post). Anyhow, if the knock sensor detects knock (as when low octane fuel is introduced) the computer can retard the timing so that the engine fires closer to TDC. This gives slightly less power because the spark comes when the piston is already on the way down, instead of just peaking over the top. A result of retarding the timing a few degrees is that the motor runs a bit cooler to avoid pre-ignition that comes from heat, but the cost of retarding the timing is a loss of power. Frankly, the average driver will not miss the lost power on his daily commute to and from the salt mine.
10:1 is on the edge of high compression. I have said several times that I use the mid-grade fuel with never a hitch.
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"Jeff Strickland"wrote

Yeah, I think Toby realized that and that's why he wrote "out-of-warranty".
Pete
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Okay, now I get it.
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On Sat, 24 Jun 2006 19:42:25 -0700, "Jeff Strickland"

Where exactly did I say that the OP had a warranty? Quit inferring. Read what is written.
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You might do that yourself. Where above does he attribute that statement to you?
-- Larry
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On Sun, 25 Jun 2006 11:57:58 -0400, pltrgyst

You're too stupid to give any more comment to.
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Toby wrote:

Bzzzzzzzzt!! Wrong answer. But thanks for playing.
Premium gas is defined by various m,arkets around the world and varies through tme.
The 2.5L M50 engine in your 1994 325i requires 89 AKI (RON+MON/2) gasoline. That is what is meant by "premium" in the owners manual.
You can use 87 in a pinch and the knock sensors will retard the timing if/when there is pinging, but when this occurs the conventional wisdom is that the mileage will suffer.
Considering that the price of fuel in the US is about $3.00 a gallon and the difference between grades is only 10 cents, you only need to realize a 3% improvement in mileage to break even buying the 89.
--
-Fred W

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Thank you all for the very helpful advice. I certainly agree that paying a few cents extra per tankful to get full performance from my 325i is well worth it. But it is also good to know that if necessary I can, on occasion, run on mid-grade gasoline without damaging the engine.
Fred W wrote:

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That's why we should talk only in octane and not use uncertain adjectives.
DAS
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Exactly. Back in the late l970's, the eastern U.S. mostly had two types of gas - leaded "regular" at 89 (that is [R+M]/2) octane and leaded or unleaded 95 octane premium. Amoco was one of the first companies to drop the octane of their premium from 95 to 93, and the Maryland state comptroller decreed that they could no longer market it as "premium" in the state!
I don't understand how 89 has now become premium rather than "mid-grade", unless were talking about high altitude areas of the west.
Tom K.
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It's always struck me as strange that the lowest octane in the UK (95) is described as Premium - especially as under the old star system it would have been Regular...
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Quite so. Am glad to note that I am not the only one scratching the head over that one...Probably the price today (in non-infllated-adjusted terms) is at a premium over the one in force when a putative 95 (since in the old days there was no 95 IIRC) cost less...
Similar/greater confusion exists in other countries, especially where 91-octane (European/international) is still available.
Devalues the language. What is premium when there is a super? Super petrol at a premium price? Or peremium petrol at a super price?
What about the hotels whose basic room in the broom cupboard by the clanking lifts is called "superior" and the next one up is "executive".
Superior to what? A bed in a toilet? The youth hostel next door?.
You can just see it, can't you? No more "good, better, best". Just "superior, executive, junior suite".
DAS
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