I live in the high elevation West. I drive an an F-150 with EcoBoost engine, and the owner’s manual says to put 87 octane “regular” gas in it. The manual specifically says that if you live in an area where a lower octane “regular”is sold—like the 85 octane gas here, that you should use 87. I use Mobil 1 oil and put 87 in it because I want to keep the truck.

I also have a car that requires premium gas. The manual actually says 93 octane, all I can buy is 91. I wouldn’t think of putting anything lower octane than that in it. Don’t buy a car with a high performance engine in it, if gas cost is a consideration.

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Originally posted by TMats:
Don’t buy a car with a high performance engine in it, if gas cost is a consideration.

Not sure why someone would buy the performance engine and then run crap gasoline. You don't get the performance you bought.

My wifes VW requires premium. Its what we put in it. A while back she started complaining about that requirement. I suggested we trade the thing in for a slug. She liked everything about her car except the fuel requirement. She knew about it going in. I solved the problem the same way guys often do. I just gas her car now. As long as I pay the difference, there isn't a complaint. Best of both worlds for her. She doesn't even have to pay regular gas price.

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Originally posted by Sig209:

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Originally posted by doublesharp:
Around here there is a 50-60 cent price difference between reg and premium. That's the same as legalized theft. Anyone remember the old days of 29.9 regular and 31.9 for premium?

I was thinking about that the other day.

I recall the 'norm' being about 10 cents difference between the fuel grades. The difference now is crazy.

Yup.

When I bought the new-to-me Jeep with a Hemi in it, I didn't think about fuel grades. Yup: Wants 89 octane. So cruised through three local gas stations on the way from here-to-there. Average of 30¢/gal. bump from regular to mid-grade, then from mid-grade to premium .

But... average mileage on one of the trip displays when I acquired the truck was 13.8 MPG. Driving sub-rural (not quite suburban, not quite rural) I was getting 15-1/2 to 16 MPG before it got cold and the snow flew. Getting ±15 MPG now.

I noticed that, as I added more-and-more mid-grade or premium to the tank, mileage was increasing.

N.B.: Costco premium is less expensive than regular station mid-grades.

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Originally posted by dangho731:
I used to run a mix of aviation 105 and lower rating and got good result.

How did your catalytic converter do with the leaded aviation gas?

93 in my EcoBoost F150 (manual calls for it with frequent loads & towing)
87 in our N/A Flex

My '00 TL never ran less than 91/93

My '96 Infiniti J30 wouldn't tolerate less than premium. I ran it about 1,000 miles on 89 & it started knocking. Bumped back up to 91/93 & it went back to purring nicely. Hurricane Ike got me to sell that car. $4.50/gal for premium on a broke college student's budget didn't work out.

I remember, back in the day, my dad driving our '57 Chevy Bel Air into the gas station, the guy walking up to the driver's window, and my dad saying to him "fill it with ethyl".

I had a Mini Cooper that was supercharged and the engine required premium gas. I tired regular once and it ran like shit.

And yet again, this whole issue could be resolved permanently by killing ethanol. The pure gas I run in my lawn equipment, I occasionally run through my old truck, and the performance and mileage improvements are noticeable from the higher octane.

As to whether running regular instead of premium will damage an engine, I highly doubt it given I ran regular in my wife's POS Pathfinder for years without issues. Unfortunately that was about the only thing on that vehicle that didn't give me issues.

My 2006 Honda S2000 requires premium gas. Most of our premium in NW Montana is also real gas, free of corn. The car runs beautifully on the good stuff. I don't mind spending an extra 10-15 cents per gallon.
Mike

Today's engines use technology that enable much higher compression ratios - like in the 11.5 to 12:1 range.

As a result, a new phenomenon has manifested itself - it's called Low-Speed Pre-Ignition (LSPI). LSPI is a very severe form of pre-ignition that has been known to damage pistons very quickly. GM had a problem with this just a couple of years ago.

There is no way I would even consider using lower-octane gas in an engine with Direct Injection and either Turbo or super charging. And knock sensors are minimally effective at preventing damage from this type of pre-ignition - once it happens, the engine can be damaged immediately, so traditional knock sensors do not help as much.

I would follow manufacture's recommendations today. In the past, you could get away with using regular where premium is specified, but no longer. Even motor oil formulations are changing as they can have an effect on LSPI.

My 2014 Mazda 6 with the Skyactiv 2.5L I4 states that it will run on regular 87... but with a 13.0:1 compression ratio, it makes me incredibly wary.

It exceeds its window sticker mpg with 93 octane ; in euro markets they have either a 13.5 or 14.0:1 with the same family and it requires premium. The old car guy in me still defaults back to 93 for such high compression, even with computer whiz-bangery adjusting the spark and timing.

The fascinating thing is how Direct Injection facilitates the higher compression ratios.

Older high-compression engines without Direct Injection begin ingest of the gasoline/air mixture starting at the beginning of the Intake Stroke. As a consequence, gas had to be high octane to resist premature detonation - since ingestion into the cylinder begins at the start of the intake stroke, the mixture had plenty of time to heat up from the hot environment within the combustion chamber - from the remainder of the intake stroke, through the duration of the compression stroke - at which point it is ignited by the spark plug.

But with Direct Injection, the gasoline is injected directly into the cylinder at the near end of the compression stroke - so the gasoline/air mixture has much less time to heat up and prematurely detonate. In fact, the act of injecting gas can actually cool the combustion chamber temperature, reducing pre-ignition. I believe the Eco-boost technology, and also Honda's Direct-Injection technology actually increase the richness (amount of gas) during injection to suppress pre-ignition when it is detected. As a consequence, some Ecoboost and Honda engines with Direct Injection tend to have excessive (but seldom problematic) amounts of unburned gas in their oil.

Ethanol-Free Gas

If only these weren't always concentrated near lakes and with commensurate pricing....

I have a Buick Turbo and factory says regular s fine and I get 29 mpg average

My BMW S 1000 XR has only one problem: Premium gas required. At least corn free is fairly easy to come by in the Yoop.

Octane rating is a measure of gasoline's volatility. The method posted on pumps is traditionally the average of a particular fuel's research and motor octanes. For this reason, it's possible two gasolines with the same octane rating will perform differently in the same engine.

The higher the octane rating the lower the volatility of that fuel, meaning the less likely it is to combust under heat and pressure in the absence of spark. When a high octane fuel is specified, that's generally because the engine is designed to run at higher compression and the manufacturer wants to prevent the gas from igniting too prematurely, known as detonation, knocking or pinging. When detonation occurs it stresses engine components as inertia fights the force of explosive gasses and is a fairly common cause for engine failure.

Modern engines frequently are engineered with knock sensors to prevent detonation and protect the engine. You can expect lower performance as others have stated, though I have not seen any definitive data on the longevity of high compression engines with knock sensors that run low octane fuel routinely. Suffice to say, it wasn't engineered to run that way.

FWIW, it's also possible for engines to exhibit detonation when the miles creep up. Cylinder head deposits that form in older engines retain enough heat to ignite fuel before the piston reaches the top of it's stroke, so it may become necessary to step up in octane to reduce the occurrence of knocking.

Theoretically, high compression engines in higher altitudes could get away with a more volatile lower octane fuel, but modern electronics sense and compensate for all that. But if you own a '69 Yenko Super Camaro, then have at it.

quote:

Originally posted by jjkroll32:
My 2014 Mazda 6 with the Skyactiv 2.5L I4 states that it will run on regular 87... but with a 13.0:1 compression ratio, it makes me incredibly wary.

It exceeds its window sticker mpg with 93 octane ; in euro markets they have either a 13.5 or 14.0:1 with the same family and it requires premium. The old car guy in me still defaults back to 93 for such high compression, even with computer whiz-bangery adjusting the spark and timing.

I have a 2015 Mazda 3 with the same engine. I have never ran premium in it but I have averaged 32.73 MPG since it was new. About a third of its miles were driving in Mexico where the gas is??

I’ll try a tank or two of corn free premium.

My `16 Challenger (5.7L) recommends 91 octane but says it will run on 89. I've had it for 5 months now and spent some effort running a comparison between 89 and 91, and found that if there is any advantage to 91, it's barely noticeable and definitely not worth the price difference. I get no knocking or other issues when I run 89 in it. I haven't tried 87 and probably won't.

That said, if I go to Costco I can buy 91 octane for the same price or lower than other places charge for 89 (they only offer 87 and 91, don't have 89).

Let's say one is driving 20,000 miles per year with a vehicle that gets 22mpg.

20,000 mi/year / 22 mi/gal = 910 gal/yr.

Assuming that the price difference between Premium and Regular is $0.2 then we're talking about $181/yr difference. Seems like it's not a big deal when a car that requires Premium probably costs north of $30,000 (over 10 years, the cost of the car is $3000/yr). So, less than 10% of the cost of car in gas price delta.

I drive less than 10k miles per year, so the cost delta is even less. To me, this has not been a sway factor in deciding what car to buy. Gas type is immaterial to me; gas mileage is more significant.

I have 2 Nissans with the VQ 3.5L engine. The Maxima requires 91 octane but the Muarno is fine to run 87 octane. I guess it's in the tune of each car.