How to get 91 octane gas in an 87/89/93 area?

October 22, 2009 5:00 PM

How can I get 91 octane gas in an area that seems to sell only 87/89/93? What sort of damage might I have caused to a new vehicle by filling it with 87 for a little while?

I recently purchased a 2009 Volkswagen CC VR6 (and am loving it). A few fill ups ago I realized that the manual & gas cover both say to be using 91 octane gas. The problem is there are no gas stations in the area that sell 91! Every station I've been to only sells 87, 89 and 93. I've been using 93 since, but I feel like I'm paying for more than I need to. Can I just half & half 89 and 93 and get a tank full of 91 - or is there a more complicated mixing method, or does it just not work this way and I'm stuck paying for 93 to get the 91? Does anyone know of a station or chain of stations in Northern Virginia (preferably somewhere along the Fairfax County Parkway) that sells 91?

Relatedly - how much damage could I have conceivably have caused by driving the first 3000 miles only filling with 87? It seems to me like the answer would be "not much, if any." I don't hear the engine knocking, I didn't feel any measurable loss of power while driving with 87, etc. But realistically what might I be looking at? Does it make sense to bring the car in for a once over just in case?

Also (love these multi-part questions); VW's Carefree Maintenance has free oil changes at 10k, 20k and 30k. Their documentation says that due to the synthetic oil being used, and the "advanced European engineering" that I don't need an oil change every 5k miles - that every 10k is fine. I'm planning to pay for a 5k anyway (until Carefree Maintenance came along, they were recommending 5k changes, so I think this 10k thing is a bit of bull) - but any thoughts on the validity to that claim?

posted by ish__ to travel & transportation (21 comments total)

Lots of new cars don't need oil changes nearly as often as the older ones. If you bring it to a reputable dealer and your car doesn't need an oil change, they will probably just tell you to come back later. I have been told it's not great for the engine to change the oil too frequently in new cars (by a mechanic).
posted by shownomercy at 5:12 PM on October 22, 2009

Damage: none. Modern cars have anti-knock sensors that detect knock and cause the engine's computer to correct for it, I believe by running a richer air/fuel mixture, but Jon-O will probably come in with the correct answer :)

Power decrease: some under load (uphill, accelerating). Also mileage decrease.
posted by zippy at 5:17 PM on October 22, 2009

Roughly: engines knock (that is, the fuel detonates irregularly) when the fuel which you use is of lower octane than the engine ought to have. The danger of having an octane rating that's too low is that your vehicle might end up having engine knocking.

However, this is very unlikely with a modern vehicle and 87-octane gasoline. There is probably some knocking going on, so you may see the effects on a VW if you run it on 87-octane fuel for, say, ten years. However, running it on 87 for 3000 miles should not have hurt it much. I really don't think that a mechanic can give a car an easy once-over and see any damage from that kind of thing, by the way. We're talking about wear and tear on the cylinders themselves, and it's not likely that there's enough wear and tear yet for it to be obvious without taking them apart.

Using 93-octane gasoline is great for your car. In fact, it should be better than using 91. I would guess that the manual and gas cover recommend 91-octane not because that's some ideal, perfect octane rating but because in most of the US it's the highest-octane gas you can buy.

But, yes, technically if you mix 50/50 87-octane and 93-octane, you would get 91. That just seems like unnecessary trouble to me, but it's your call.
posted by koeselitz at 5:24 PM on October 22, 2009

Can I just half & half 89 and 93 and get a tank full of 91

Yes, you absolutely can. Lots of people do so.

As I understand it, they essentially refine 105 octane and 85 octane, and then get all the numbers in between by simply mixing those two. I don't recall if those are the specific numbers, but I do know that they only refine two, and mix the rest.

how much damage could I have conceivably have caused by driving the first 3000 miles only filling with 87?

New, computer-controlled cars will retard the spark timing to prevent knock. So it's not likely to be doing any damage if the engine isn't turbocharged. If it's turbocharged, then the whole thing is more complicated, and you should use the recommended gasoline--apparently, a hot turbo can actually cause low-test gasoline to detonate in the intake manifold.

On the other hand, you should keep in mind that your car has reduced performance and fuel economy when operating in this mode. You might not actually be saving money on the cheaper stuff, given the decreased efficiency. And it probably has subtle effects, like increasing spark plug fouling or taxing the catalytic converter. It's an emergency feature ("Oh shit, 3am in Bumblefuck, AZ and they don't sell 91 octane!"). It's not good for the car for longterm use.

On preview:
Using 93-octane gasoline is great for your car. In fact, it should be better than using 91.

Ehh... if the car's designed and tuned for 91 octane, then putting 93 octane in isn't going to improve things. Yes, it's just that much less likely to knock. But, if it's not going to knock at 91 octane, what does it matter if you put in 93? I've never met a car that will tune itself more aggressively for higher octane (though people tell me they exist). Now, if you rechip your car, and so have control over your own mix and timing, then you can definitely take advantage of the higher octane.
posted by Netzapper at 5:30 PM on October 22, 2009

I could swear my VW GLI said "91 and above octane" and when I lived in Texas ran 93 just fine (and my other VW/Audi turbo cars) on just fine. I was actually surprised when I moved out west that there was only 91 because I thought 93 was everywhere. Now I'm running 91 and the car still purrs like a kitten.
posted by birdherder at 5:54 PM on October 22, 2009

I heard a call in to The Car Guys and they said you can absolutely mix 50/50 and get the midpoint octane.
posted by plinth at 6:09 PM on October 22, 2009

I could swear my VW GLI said "91 and above octane"

It's only a recommendation.


No damage will be done, but 87's horrible. It's the automotive equivalent of wearing a condom.

And those VR6's are high-reving. Mix it.
posted by Zambrano at 6:20 PM on October 22, 2009

You aren't going to find 91 octane out east. I think there's a California law that states that anything higher than 91 octane is considered racing gas, and is taxed differently. As such, when a manufacturer means "premium", they just say 91. Almost the entire midwest and east coast refers to "premium" as 93 octane.

Not that you'd save anything, anyway. We pay the same for 91 octane gas (about 20 cents over 87) that you pay for 93.
posted by hwyengr at 6:44 PM on October 22, 2009

Sunoco has 91. But I would go for most convenience and mix.
posted by JohnnyGunn at 7:08 PM on October 22, 2009

First of all, as long as you are using the right type of fuel (in this case, unleaded gasoline) there is no situation in which using fuel with a higher octane rating than that recommended will damage your engine.

The VR6 in your CC has a compression ratio of 10.5:1, which is a somewhat high compression ratio. I think that using anything under 91 will definitely result in retarded timing in order to reduce knock, which will reduce power (and that seems to be the consensus around here). Using 89 or 87 for the cost savings when your car's engine is designed for 91 is not worth it - if you don't want to pay the extra cost, you might as well sell the CC and buy a cheaper car with less aggressive compression and timing.

Contrary to what a few have said, I do think that some modern high compression normally aspirated (and certainly many forced induction engines) will allow for slightly or somewhat advanced timing with 93 octane gas, which will slightly increase power. Also, many formulations of 93 include detergents and some fuel additives which will generally be good for your fuel system and injectors. In my area, 93 is usually $.07 - $.10 / G more expensive than 89, which ends up being about 3-5% of fuel costs. The variation related to time of day, locations and stations probably is twice as large.

My view is, you should get the oil changed on a schedule recommended by the manufacturer, and use the fuel they recommend as well. If you have the option of 93 instead of 91, I would get it every time. My math says, for 15k miles, using 93 instead of 89 (assuming $.08/G difference and 20MPG) costs you $60 extra. That's money far better spent than on extra oil changes (an extra one every 10k miles puts you at 1.5 extra changes for that 15k miles, and 1.5 extra changes will probably cost you more than $60), plus you'll have better performance, and appropriate timing, for all 15k miles.
posted by iknowizbirfmark at 8:31 PM on October 22, 2009

In my '03 GTI VR6 I have always used 89 which is the local 10% ethanol blend. I have had it about 5 years now (30k miles maybe?) and have always had sticker-or-higher mileage and smooth running in all weather. Like yours it says "91" on the gas flap. We also don't have 91 around here.

I have read that we aren't comparing apples and oranges with these numbers, and some stuff on the wikipedia Octane page backs this up. Quoting one relevant sentence buried in there:

... the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-92 in Europe.

If you can find in your manual or on your flap exactly which 91 they mean, you can use the table on that page to translate it into your local rating if it's different.
posted by fritley at 8:47 PM on October 22, 2009

Netzapper: Ehh... if the car's designed and tuned for 91 octane, then putting 93 octane in isn't going to improve things.

That's true. But 93 won't hurt anything. In fact, the only production-model cars that I know of which actually use those two more octane percentage points are turbocharged cars.
posted by koeselitz at 10:15 PM on October 22, 2009

Fritley, they mean the US 91. Another vote to use 93. Mix if you really want to, but that seems like more trouble than its worth. All my German cars have said to use 91 or higher, so I think the 93 is what you are supposed to use.
posted by Joh at 10:53 PM on October 22, 2009

You can just MIX the two octanes IN YOUR TANK!....

I first read about it here and now almost exclusively mix my own 91 octane special, sometimes you need to use two different credit cards or get the second charge approved by the cashier in-store, but it's worth it.
posted by stratastar at 11:26 PM on October 22, 2009

koeselitz writes "Using 93-octane gasoline is great for your car. In fact, it should be better than using 91."

If your car runs well without knocking or retarding on 91 running 93 will confer no benefits except ever so slightly fast acceleration caused by a lighter wallet. Personally for the saving I wouldn't bother mixing down the 93 for what amounts to a few cents a gallon or less than 60 cents per full 18.5 gallon tank of gas. Especially since the specifications I've found say the engine has a 12:1 compression ratio.
posted by Mitheral at 2:52 AM on October 23, 2009

Mitheral: If your car runs well without knocking or retarding on 91 running 93 will confer no benefits except ever so slightly fast acceleration caused by a lighter wallet. Personally for the saving I wouldn't bother mixing down the 93 for what amounts to a few cents a gallon or less than 60 cents per full 18.5 gallon tank of gas. Especially since the specifications I've found say the engine has a 12:1 compression ratio.

Again, that's absolutely true; there's one and only one case I have ever heard of where a production car ran noticeably different using 93 rather than 91 octane gasoline, and that's in the case of turbocharged cars. There are some turbochargers which hardly work at all unless you've got 93-octane gasoline. However, that isn't even most turbochargers, let alone most cars, so it's hard to see how there could possibly be any noticeable difference, even if you ran the car on 93-octane for years on end. 91 works just as well in nearly every case.
posted by koeselitz at 3:20 AM on October 23, 2009

I had the same experience 3 months after buying my '07 GTI... the salesman at the dealership had actually told me it only needed 87, but I happened to glance down at the inside of the gas cap when I was filling the tank one day. If you look closer, you'll notice it says "minimum octane rating 91," so if you're nervous, fill it with 93 (or do what I do, and alternate filling it with 89 and 93). Like other people have said, it probably doesn't matter in practice... I use higher grade stuff when it's hot outside, because I feel like there's the slightest of performance differences, but I'm pretty sure that's just in my head.
posted by Mayor West at 4:46 AM on October 23, 2009

It's probably not in your head, Mayor. The same properties in higher octane fuel that allow it to resist knock and tolerate higher compression would allow it to keep its density until combustion on a warm day.

To the OP: just use 93. You'll get more anti-knock protection and you may even get a little more power out of the engine, depending on whether or not your engine's CPU can sense the difference and adjust timing on the fly.
posted by notyou at 7:48 AM on October 23, 2009

I would mix 89 and 93. That's fine.

As for the oil, check around on a Volkswagen blog for people who have sent in their oil for analysis. That's the sure-fire way to tell if oil is spent. For my car, even though my on-board interval reads 15,000 miles, oil analysis has shown it in need of changing at 7000 to 8000 miles.
posted by luckypozzo at 1:27 PM on October 23, 2009

Can I just chime in to say your petrol sucks? Here in AU we get 91, 95 and 98, though we pay a lot more tax on it.
posted by polyglot at 5:36 AM on October 24, 2009

Can I just chime in to say your petrol sucks? Here in AU we get 91, 95 and 98, though we pay a lot more tax on it.

Doesn't suck, we just measure octane differently. But you're right about the taxes.
posted by Netzapper at 5:53 AM on October 24, 2009 [1 favorite]