91 octane -- mixing?

[WorthaTry]

Greeting EVers,
I just picked up my new to me 2001 Eurovan weekender yesterday and drove it 600 miles home. I'm sure I will be reading more of your writings in the days ahead. I have so much to do to get it ready for summer adventures.

Meanwhile my first question: I note the sticker saying minimum 91 octane. The two stations I refueled at had 85, 89, and 93 options, so yeah I went with 93. But it occurs to me to wonder if in the future I could save a few bucks every fillup if I get half 89 and half 93. Is the math as simple as that and is there any reason not to do this, other than the extra time running 2 transactions??

See you on the road!

[67rustavenger]

AI Overview says,

[bikes!]

According to this article, mixing equal volumes of 89 and 93 octane fuel, the result is 91 octane. The linked article has a formula to calculate non-equal volumes.
How different is the price per gallon of 89 and 93?
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VWs past: 66 Kombi, 76 Westfalia Camper, 17 GTI; present: 02 Eurovan MVW, 17 Alltrack SEL & 19 Golf R

[HeyCrutch]

In Western North Carolina premium is 93 and mid-grade is 89 -- no 91 octane fuel (at least at the stations I frequent).

My practice is to alternate fill-ups between 89 and 93. There's always 5-8 gallons in the tank when I fill up, so I assume I'm at a mixed octane rating somewhere in the 90-92 range most of the time.

Out west I saw pumps with 91 octane fuel all the time.
_________________
-2003 EV MV Wk - Forest - 51k
-2002 EV Camper - Ghost - 82k
-2003 EV GLS - Fluffhead - 176k

-1997 EVC - 12vVR6 (SOLD 5/2025)

[postjosh]

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[WorthaTry]

[WorthaTry]

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[SambaBurnsRed]

Higher octane is more energy, so you should see improved mileage from it. (therefore close to a no-cost upgrade)

The additive package is increased for more cleaning power.
And in some cases, 0% ethanol (ehtanol isn't great)

Pinging is like hitting the top of the piston with a ballpeen hammer. it's nasty.

Something as slug heavy as the EV with a small motor, I run the highest octane I can find

[Endopotential]

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High octane is actually slightly lower energy per gallon than lower octane, but is rarely marketed that way.
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[bcolins]

Yes, as mentioned above by a few,.....higher octane actually could be considered to have LESS energy than regular,.......the higher octane number controls the timing of fuel ignition, to a later more appropriate time,.......when the spark plugs fire,......where a lower octane fuel could possibly allow premature spontaneous fuel combustion to occur while the piston is still moving UP. This is what causes that metallic PINGING sound. More likely to occur when a vehicle is under load,.....pulling a heavy load, or climbing steep hills/ mountains etc,.....also more likely in hot weather. Many modern engines have something known as a knock sensor,....this can retard the ignition timing when lower octane fuels are used,.....and eliminate pinging. When Ignition timing is retarded,...then fuel economy and performance can suffer to a slight degree. Both my 2018 BMW R1200GS and 2007 BMW Z4 state that any fuel can be used but that when using 87 octane,....fuel economy and performance may suffer. I run Regular 87 in both vehicles all the time and still get very good fuel economy (45 mpg on the bike and 25-30 on the Z4) and performance. Why our Eurovans don't have this functionality mystifies me, since they certainly have knock sensors (I know my 95 EVC has them,...and I run regular in it all the time also)........Maybe the cylinder compression ratio on the VR6 is just too high to run reasonably well with retarded ignition on the VR6. I think anything higher than 10-1 or 10.5-1 will generally require premium grades. That said. I have run regular a few times as a test when driving through Texas, and Arizona,......and had zero issues with pinging.

[SambaBurnsRed]

I just asked the engineers in the lab where I worked (major fuel distribution and processing terminal, $5B a year throughput). They say you'll get close to getting your $ back - but I didn't question that advice.

If not from then pure energy in the fuel, then from the ability to extract, as we know the engine can run a higher state of tune, adv timing, then can more efficient.

[WorthaTry]