Home Forums Hunting & Shooting Hunter's Campfire Cars that require premium gas

toying around with the idea of buying a well used Lexus IS250 AWD.

It has a "high compression" engine that requires, not recommends 91 octane.

So I start reading up on it a bit, because , well, if you haven't figured out, I'm so tight my shoes squeak when I walk, and I'm not interested in putting 93 octane for $50 more a month in a $10K car with 125K miles.

What I found in reading is that the crux of it is, lower octane fuel ignites more easily so in high compression engines, which results in uneven compression. To maximize power and fuel efficiency in a high compression engine, auto makers "require" premium fuel.

But any car made after 1996 has a knock sensor that retards the timing when knocking occurs and will do so continuously as long as knocking is an issue,even for awhile if you switch back to premium.

You'll find no shortage of people who swear up and down that you *will* damage the engine eventually if you burn 87 in a car requiring 91, but logically this Car Talk thread explains the fallacy in that.

https://www.cartalk.com/content/premium-vs-regular-1#4

Q. So, I should use high-octane gas to prevent pinging?


A. High-octane gas will prevent pinging.

But these days most cars have something called a knock-sensor, that will - under many circumstances - obviate the need for premium gas. Just about any car that's more recent than about 1996 will have a knock sensor.

Q. How does a knock-sensor work?


A. The knock-sensor uses auditory detection to actually "hear" the knocking or pinging. When necessary, it uses this information to delay the spark and to minimize knocking or pinging.

That's good news. That means you can use gas with a lower octane rating. And, if your engine detects a knock or ping, it can usually adjust the timing of the spark until the knocking is gone.

Q. You're telling me that modern cars automatically detect this problem, and therefore won't ping?


A. Right - as long as the load is not too great.

Load is the amount of work you're asking the engine to do, and it's affected by a number of factors, including climbing steep hills, pulling heavy weight, hard acceleration or driving in hot weather. Any combination of these factors can affect load. So, if you live out in the desert Southwest, it's 115 degrees out and you're hauling your mother-in law up, you're going to need premium.

Why? Because there's only so much that your car's knock sensor and computer can do to adjust the engine's timing.

Q. Is it a big deal if my car pings for a few seconds here and there, under these circumstances?


A. No, it's not.

Q. So, are there any circumstances under which premium gas would be a good idea?


A. If you plan to haul that aforementioned mother-in-law in hot weather, or are going to be driving up monstrous mountain passes with a heavily laden car, then you might consider filling up with a tank of premium gas.

Q. But, under most normal operating conditions, do I lose anything if I use regular unleaded instead of premium?


A. In many cases, no, but you'll have to experiment with your own car. If you take our advice and drive judiciously - without lead-footing or overloading your car - and you don't hear pinging, you can use regular fuel. Of course, you'll lose that warm feeling of giving oil companies an average of 20-30 cents more per gallon of gas. And, you will lose performance that under most driving situations won't be noticeable.

here is consumer guide, they have the same information but still conclude you should buy premium

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As mentioned, it’s rare that new cars require premium-grade fuel, but two test vehicles that passed through here recently did—or at least, that’s what it said on their fuel doors. As both were Lexus models, we called the company to see what they’d recommend if someone accidentally filled one with regular.


According to Doug Herbert at Lexus, “Required” is a bit overstated:
Herbert said that both vehicles (an RC 350 sports coupe and an NX 200t compact SUV) will run on regular-grade fuel without damage. But since both the advertised power ratings and the EPA fuel-economy figures were attained using premium, that’s what they have to put as the required fuel. However, he also warned that continued use of regular-grade gas would cause the engine’s computer to back off on the ignition timing—thus adversely affecting both power and fuel economy—and that it would continue to do so for a time even after the car is filled with premium.

It’s important to note that Herbert’s advice only applies to Lexus vehicles, as he obviously can’t speak for other manufacturers—and we can’t, either. And while our stance is that you should use premium-grade fuel if it’s stated to be “Required” or “Only,” here are some thoughts if you accidentally fill it with regular.

Detonation is your concern, and as it mostly occurs under hard acceleration, tread lightly on the throttle. Listen carefully for the telltale knock or ping that indicates detonation, which means you should try and keep other noises (such as the radio or conversation) to a minimum so you can hear it. As soon as you get down to a quarter or half a tank, fill it with premium, which will raise the average octane level fed to the engine.

One more thing. Since premium-grade gas often costs about 20 percent more than regular-grade gas, some folks have thought of trying to save money by using a $4 bottle of an octane-booster product to raise the octane level of regular-grade gas to that of premium. Don’t even think about it. While we don’t want to make any blanket statements here, we feel pretty confident in saying that a bottle of an octane booster—even one costing far more than $4—isn’t going to raise the octane level of a tank of fuel by four whole numbers (87 to 91). Some brands may say on the label that they’ll raise the octane level by “3 points,” but that usually means tenths of a point—so from 87 octane to 87.3 octane—which is certainly not worth it. If you’re a manufacturer of one of these products who wants to refute that claim, please write to us; we’d be happy to eat our words to save our readers money.

But for now, the best way to save money is to use the lowest-octane fuel your owner’s manual says is OK, even if the fuel door “recommends” that you use premium. But if premium-grade fuel is “Required,” bite the bullet and put in the good stuff.

Do NOT rely on a factory knock sensor to keep an engine safe from detonation. If the car requires high octane because it has high compression, use the proper octane.

Using lower octane gas than what is recommended will be noticed! If the car requires 91 octane then anything less will ping, knock and maybe occasionally stall. You’ll probably quickly notice the loss of power as well.

If you really don’t want to buy premium gas then buy a vehicle that doesn’t require premium gas.

Is it possible to re-tune the engine to run with a lower octane fuel? With direct injection it may be possible.

I'm running a 2022 with a Super Charger, and I run high Octane or nothing....that's what the manual says, so that's what I do.

been using regular on my 2009 lexus rx 350 for 3 years , no problem so far

yea I don't think anyone is arguing a turbo or super charger shouldn't - the assumption is you are getting on it when driving and need that octane

naturally aspirated grandma cruisers - maybe not so much

Octane boosters are not worth it. Sure, if you light throttle around and dont pull any hills or steep inclines, you can run it on 87. Why chance it? If 91-93 octane is a deal breaker then you shouldnt buy the car to begin with. My 6.2 at4 requires 93. Its a little painful on fill ups, but would never run anything else. The factory GM computer is supposed to recognize lesser octane and pull timing, but it isnt a fail safe. Just run the proper octane or pass on the car. Best advice I can give

I don’t know much about octane boosters. Can you dump a can of that in everytime you fill up?
I’ve also read recently that gas will soon be 15% ethanol, which is sure to play hell with internal combustion.
Looks like Gunchamp answered my question as I was posting it!
7mm

If the computer will let a tuner in, yes. At a cost of power and most of the time, fuel mileage

I think the RX's are ok with 87.

nope, just checked, they say its 91

I also heard they are going up to %15 ethanol, there goes most small engines in America.

I had a Mustang with 5.0L Coyote V-8 and it was actually rated at two different HP figures for regular vs. premium fuel. The computer would adapt to either, but I generally ran 90+ octane in it. I did find that some stations had a much larger upcharge for premium than others, even though their prices on regular unleaded were comparable. I've also noticed that two Toyota trucks I owned with 24v DOHC V-6 engines really needed premium in dead summertime if I was pulling a trailer or putting any kind of load on the truck. They'd clatter otherwise.

Air pressure plays a big part in it. If you're over about 2000' in elevation, you can drop a couple points in octane. We're at about 3500'. All the regular gas in the area is 85. Go over to Boise which is about 2500, they all have 87. Boise is ok at 85 but many of the people there are regularly in towns farther west and into OR which is pretty marginal for 85.

i have a 2010 Ford flex 3.5(no turbo) that detonates like a mthr fkr at part throttle when the engine is cold or half warmed up(87 octane)... head wrench at the ford dealer scanned it several times, no codes, all systems fine... he says it cant ping as long as the knock sensor is working... Bullschit!...

If the engine calls for 91 octane, I'm betting you'll see better fuel mileage with 91 than 87. Maybe not enough to completely offset the price difference. But enough to ease the pain some at the pump. In other words, I bet using 87 in that engine won't save you as much as you think.

None of the owner's manuals in any of my vehicles specifically states, " use 87 octane". They state, "use at least 87 octane." I know that some vehicles specifically state to use 91 octane when towing but 87 is fine otherwise. I experimented with 91 octane for about a year and even though it isn't required in my vehicles there were modest gains in performance. The trucks would hold in higher gears climbing the same grades than when using 87 octane. If fuel mileage was any better it was nominal, and I couldn't tell it. But the gains in performance were worth the additional costs to me.

At least until gas prices went to $5-gallon last summer. Prior to that I could shop around and find 91 octane for only 30-40 cents more than 87. But after that it was as much as $1 more, and $5/gallon hurt enough.

In parts of the country above about 5000' elevation they sell 85 octane as regular, 87 is mid-grade and 89 is premium. I made the mistake of using 85 octane once. We filled up in NM heading into southern Colorado. My truck ran horribly until I burned through most of that tank. I went to 89 octane for the rest of or time in Colorado and the difference was significant.

My bmw mini has a supercharger, needs premium, and top speed of 134 mph.


I have had it up to half that speed.

My 2019 chevy with a 6.0 gas hs a tune in it to tow my fifth wheel, I can put reg gas when not towing or under load. Though I have found the gas millage and performance really suffers with reg gas. So I just run it on premium all the time and the performance and extra milage make up for they extra cost of fuel.

I have owned two cars in the past twenty-five years that had high compression engines and required 91 or higher octane. One was a '99 Olds Aurora, the other a 2013 Lexus LX570.

Both would run less smoothly with 87, and would trigger the dreaded "check engine light" within a few miles after filling up with 87.