# Does 89+93 = 91 octane?



## Severe550i (Dec 2, 2009)

If you fill up your tank with half 89 and half 93 fuel, does that equate to 91 octane? 


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## TopDownInFL (Apr 25, 2008)

If only it were that simple...


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## TerraPhantm (Nov 22, 2004)

I think so. AFAIK gas stations sell 89 by mixing 87 and 93, so the same logic should apply to mixing 89 and 93 i think.


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## AdmKlondikebar (Jul 30, 2010)

Is there a place that lets you do that without having to swipe your card again? I've tried at a BP before without luck.


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## Fast Bob (Jun 4, 2004)

Who would go through that to get 91 octane as a result??? Why bother....just run straight 93 and get it over with....BMW`s engine-management system is "elastic" enough to utilize octane ratings up to around 94 or 95 ROM, so why not feed it the best fuel available ?


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## cwsqbm (Aug 4, 2004)

On a related note, if I mixed bottled water and a good beer, is it now light beer?


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## dms540i (Mar 27, 2010)

cwsqbm said:


> On a related note, if I mixed bottled water and a good beer, is it now light beer?


No.


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## compusatman (Apr 14, 2010)

What do you get when you mix a bitch and a ho?


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## Fast Bob (Jun 4, 2004)

compusatman said:


> What do you get when you mix a bitch and a ho?


Lindsey Lohan, silly....


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## Andrew*Debbie (Jul 2, 2004)

BMneWbie-530I said:


> If you fill up your tank with half 89 and half 93 fuel, does that equate to 91 octane?
> 
> Sent from my iPhone using BimmerApp


Not really. You get something close to half a tank of 89 and half a tank of 93. You haven't mixed them together.


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## JimD1 (Jun 5, 2009)

I googled this question and found a variety of pieces. They all seem to agree that they mix linearly. In other words, 91+87 in equal volume = 89. But as the last responder indicates, there is a question about how effectively they will mix in your tank. An article by Bruce Hamilton (I do not know who that is) said they wouldn't mix well and recommended filling with the tank half full. So maybe you put in premium every other fillup. 

I just put 89 in my 128i. I put 93, the premium around here, in for the first couple thousand miles and then tried 89. I can't tell a difference. Maybe the next autocross I run I will fill up with 93.

If you feed your engine less octane than it needs to avoid pinging - detonation (too rapid burning) - it will compensate by reducing ignition timing. That reduces power, however. If you feed it more than it needs to avoid pinging, it doesn't do anything bad for the engine, it just costs more. So it's better to be a little high.

Jim


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## tturedraider (Nov 11, 2005)

BMneWbie-530I said:


> If you fill up your tank with half 89 and half 93 fuel, does that equate to 91 octane?
> 
> Sent from my iPhone using BimmerApp


Yes, that's exactly what you get. It's simple averages.



TopDownInFL said:


> If only it were that simple...


It is that simple.



Andrew*Debbie said:


> Not really. You get something close to half a tank of 89 and half a tank of 93. You haven't mixed them together.


Yes you have.


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## SARAFIL (Feb 19, 2003)

Is it really worth the trouble to do this? 

I would guess your car has a gas tank somewhere in the neighborhood of 14-16 gallons, without actually looking it up.

What are you going to save? $0.10 a gallon on 7 or 8 gallons?


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## Severe550i (Dec 2, 2009)

SARAFIL said:


> Is it really worth the trouble to do this?
> 
> I would guess your car has a gas tank somewhere in the neighborhood of 14-16 gallons, without actually looking it up.
> 
> What are you going to save? $0.10 a gallon on 7 or 8 gallons?


my car has an 18 gallon tank. 
93 is 20 cents more per gallon than 89.
I do a lot of highway driving and spend a lot on fuel. If I can save a couple dollars on each tank, I try it. But usually filling up halfway and then having to fill-up the other half tank is more trouble than it's worth. I've tried it and haven't seen any difference in gas mileage and never have pinging. I was just wondering if it made any difference and if anyone else tried it. I should have known better than to ask here because most people driving BMW's aren't concerned with saving a dollar or two per fill-up.


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## SARAFIL (Feb 19, 2003)

BMneWbie-530I said:


> my car has an 18 gallon tank.
> 93 is 20 cents more per gallon than 89.
> I do a lot of highway driving and spend a lot on fuel. If I can save a couple dollars on each tank, I try it. But usually filling up halfway and then having to fill-up the other half tank is more trouble than it's worth. I've tried it and haven't seen any difference in gas mileage and never have pinging. I was just wondering if it made any difference and if anyone else tried it. I should have known better than to ask here because most people driving BMW's aren't concerned with saving a dollar or two per fill-up.


I'm all for saving a dollar as long as it doesn't require me to do extra work... 

My only point was one you also made... the extra effort required to do it is just not worth it. :thumbup:


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## Tom K. (May 10, 2008)

I don't understand the rationale for saying they won't mix in the tank as there is nothing to prevent it. 

Many years ago, when lead was being phased out and US octane levels were dropping, BMW motorcyclists routinely mixed 89 octane leaded with 91 octane unleaded. Because of some chemical quirk, the resulting mix was about 92.5 to 93 octane, just enough to prevent those air cooled opposed twins from knocking severely. (Knock sensors were still years away - the other fixes were thicker base gaskets to lower compression and dual plugging the heads.)

Tom


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## Klamalama (Oct 6, 2007)

I do that mix when the price of 93 goes over $3. You can even mix 87 with 93 to get 90. It's only 1 point low and runs just fine.

Take into account the weather and terrain you'll be driving. Hot weather and steep climbs need more octane. Flat lands aren't as fussy.

My experience is that the non-turbos (supercharged) will even run with 87 (but a bit less performance and mileage). If you do not hear engine knock then you are doing no harm.

With any type of supercharged engine, knocking is more problematic. There I would strictly stick with the manufacturer's recommendation.


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## dalekressin (Sep 3, 2008)

BMneWbie-530I said:


> my car has an 18 gallon tank.
> 93 is 20 cents more per gallon than 89.
> I do a lot of highway driving and spend a lot on fuel. If I can save a couple dollars on each tank, I try it. But usually filling up halfway and then having to fill-up the other half tank is more trouble than it's worth. I've tried it and haven't seen any difference in gas mileage and never have pinging. I was just wondering if it made any difference and if anyone else tried it. I should have known better than to ask here because most people driving BMW's aren't concerned with saving a dollar or two per fill-up.


Truthfully the cost of driving your 530i is only partially gas expense. If you feel the need to save look for a diesel.


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## Vince_nj1 (Apr 21, 2010)

I'm pretty sure that Sunoco pumps mix high octane fuel 94 with low octane 87 to come up with the different grades they sell. I don't know the ratios, however,


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## AdmKlondikebar (Jul 30, 2010)

Just went to Sunoco today and filled up with 91 octane. It was $2.86, while 93 was $2.88. I filled up 12 gallons, only saving $.24. If I drive 12,000 miles a year, at 25MPG, that's 480 gallons. I would only save $9.60 in a year. It's not worth it.


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