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Octane to Use

My friend called me and told me this morning on Car Talk a question was asked about using lower octane gas if living at a higher elevation. The advice was if the car manual called for 87 octane, 85 could be used. I too live in Denver and my car calls for 91 or 93 octane. What is the lowest octane I can use? Thank you.

Use the highest octane available in your area.

Higher octanes are used to prevent preignition caused by the heat generated by higher compressions. Compressing matter creates heat. At high elevations, there’s less air per volume, so there’s less matter being compressed by the pistons. Therefore, less heat is generated and lower octane can be used.

Your car, however, calls for the highest octane available. I’m guessing it has a turbocharger. Since it’s calling for the highest octane available, that’s what you should use…even at higher altitudes.

If you want a fast car, ya gotta pay the freight!

  1. What does the owner’s manual say?

  2. What car is it? Tell us make, model, year, engine, and anything else you care to.

The owners manual says to use 91 octane. The engine is a 3.0L V6 DOHC 24V VVT-I Engine. I live in Denver (greater than 5,000 feet). Thank you.

People really find value in using a fuel that costs less. It must be a “embarsssment” thing as in being embarassed that you bought a car that requires fuel that costs more. Notice I said costs more not more expensive becuause the price difference between the cheapest gas and the most expensive gas does not make the gas that costs more “expensive”. I have noticed things like gaining or losing a few mpg or paying .30 cents a gallon more for fuel really sets people off.

It depends since it can vary by make and even the characteristics of the same make and model may be different.
What you do NOT want is to hear any faint rattling while the engine is under a load; passing, going uphill, etc.

If rattling is heard then either up the octane ante or have the EGR system on the vehicle carefully inspected and repaired as necessary.

Ninety-one octane is recommended for my Lincoln (never sees it either) and during half a dozen trips to Colorado I’ve used 85 octane quite a bit. Even with pulling 10,000 foot mountain passes in the summer I’ve never had a single peep out of it.
Fuel economy remains the same, spark plugs burn normal, etc. The only difference is a slight power loss due to the thin air but gasoline octane has nothing to do with this.
(And it has the same slight power loss even with 93 octane to head that one off. Any car suffers a bit at altitude.)

Again, it depends.

If you’ve never used the recommended fuel how can you come to the conclusion that using the recommended fuel is not worth the added cost?

If an engine does not detonate using 91 octane fuel, chances are it will not detonate when using 87 octane fuel…If it does rattle a little on 87, it will be barely detectable and not destructive. To answer the OP, try a tank of 87 or even 85 and see if you can detect any slight rattling/clicking sound when accelerating. If not, you are not damaging or harming your engine. There MIGHT be a SLIGHT difference in fuel mileage, it’s up to you to decide if the mileage and performance boost, if any, is worth the extra cost of premium fuel…

My high frequency hearing is so bad (noise induce hearing loss,shop tools and hunting weapon gunshots) I cannot hear even extreme pinging/detonation. Many older people suffer noise induced hearing loss.

I’m not so sure about that. On my Mustang you absolutely have to use 93 octane. I lent the car to my brother who wanted to take his date out in it. I told him specifically that he could use the car, just fill it up with 93 octane upon returning it. The car had about 1/3 a tank when I gave him he the keys. The next day I went to go out to run some errands, and sure enough when I started the car, there was a symphony of pings and rattling come from the engine. I called my brother and asked him what he did to my car. He said that he put regular in it, since the high test was some 40 cents more expensive per gallon. I was livid and after berating him. I told him to get back over so he could drop the tank and remove the offending gas. Which he did. We put the 15 gallons or so of 89-ish octane fuel in gas cans for use with the lawn mower. And after jaunt up to the gas station with an empty gas can we returned with 6 gallons of 93 octane. I started the car, and after a few minutes of continued pinging, the noises subsided and it ran normally again.

My Bronco has GT-40P heads and a circa-1995 intake off a Lightning pickup. It has no knock sensors. It exhibits minor to moderate knocking on 87 octane, and some very, very light knocking on 89, and no knocking at all on 91 or better. I generally use 89 octane since it only knocks under heavy load and I don’t drive it that often anyway.

The family Triumph TR6 does not run well on 87 octane at all. It makes all kinds of noises when running on it. Running 93 octane helps a quite a bit, the manual specs 95 octane, but you can’t get that at many places anymore.

Most newer cars are equipped with a “knock” sensor. If you use a lower octane fuel in a car so equipped the spark will be retarded if the sensor detects knocking. This protects the motor from internal damage, but also results in a slight loss of power.

Your car is designed to perform best on “Premium” fuel. Whatever octane premium gas is in your area that’s what you should use for best performance. If the car has a knock sensor you can use the lower octane gas without harm, but your mpg might be a bit worse than with premium.

Daddy, give me a break…NO engine will detonate at idle when under no load… “when I started the car, there was a symphony of pings and rattling come from the engine.”

What you describe is simply not going to happen at high manifold vacuum. There is very little air to compress, nothing to trigger detonation…Detonation occurs ONLY when accelerating under moderate to heavy load with manifold vacuum below 6 inches…

If you REALLY need high-octane gasoline for a pre-1975 high compression engine, your local airport will sell you (sometimes) 100 octane low-lead av-gas if you sign some “off the road use only paperwork”…The only problem with av-gas is starting in cool/cold weather. You may need a shot of starting fluid to get things going…

I should add that my car has an aftermarket supercharger and has a custom ECU tune, in which certain perameters of the stock program are overidden. The custom tune disables the knock sensors and O2 sensors among other things. My car is very sensitive fuel’s octane level. Using your logic, I could fill a funny car on 87 octane pump gas, start it up and nothing bad will happen.

Because I have used the recommended octane, and even higher octane, repeatedly in test situations. Zero, zilch, and nada difference as to performance, fuel economy, etc.
While my hearing has suffered a bit over the years, my wife has perfect hearing and I’ve asked her (and my sons) to verify there is no rattling going on. In all cases, none.

Even pulling a 7% mountain grade in summertime heat with 85 octane does not produce one single pre-ignition rattle.

And I totally agree with Caddyman; an engine should not rattle upon startup and at an idle with no load on it.
If your vehicle is rattling under those circumstances then there are issues other than gasoline octane.
There’s often a computer code for that situation when the ECM allows too much spark advance upon startup.

Also, I inspect spark plugs about every 10-15k miles (with a magnifying glass) and that’s the best indicator of what is going on inside the engine. In all cases, the plugs show no evidence of anything wrong. Completely normal.

I am constantly baffled by folks not wanting to put the fuel the engine was designed for in their vehicle. Cars aren’t inexpensive. You laid down probably at least $20K for the car and you are worried about $4 extra to fill the tank? Even with the anecdotal information you get from some folks about having no problems with their cars, the owner’s manual is the authority here. If it calls for 91-93 octane, then feed it 91-93 octane.

BTW, trying to listen to the detonation pinging is getting more difficult as vehicles become better sound insulated (or if you have the stereo on). Don’t count on that, and instead fuel it appropriately.

Caddyman should clarify his comment by saying “No stock street legal engine designed to run on X octane fuel will detonate at idle when under no load”. With certain modifications, higher octane will be required to prevent detonation, i.e. your Bronco. A funny car engine is so extreme that you couldn’t hope to run it with 87 octane fuel and not hear it ping.