What really happens if one uses regular gas instead of premium in a car on a regular basis?
This is a topic I been coming back to over the years. I have 2010 Lexus RX 350 and 2017 ES 350. Both mention to use "premium". However, in the 2010, I use regular more often without any visible issues. Occasionally, I would also use ethanol free gas.
What makes "premium" gas premium, and why some car manuals state to use it?
https://www.reddit.com/r/askscience/...g_premium_gas/
Not a direct answer but an explanation of the “rating”. FWIW, but not Japanese, I never used premium in three different Audis and never saw any ill-effect, and I was finicky about those things.
You’ll see a lot of chatter and even some mechanics disagree.
Last edited by PNWTO; 11-18-2022 at 12:11 AM.
Short Answer is: Maybe.
Longer Answer: You'll have to evaluate if your particular vehicles have significant differences in fuel economy (the best indicator for efficiency changes), and/or are notably down on power. If not then you probably don't need premium, particularly at sea level.
Longest Answer: Understand what octane does - Increased octane allows more timing advance to happen prior to detonation (knocking) occurring. Detonation is when then air/fuel mixture is ignited not by the spark plug, but by burning vapor still present in the cylinder.
Remember what an internal combustion engine is - it's a big air pump. Octane helps resist detonation, reducing (eliminating) knocking - this let's the engine run more timing (more efficient mating between spark plug and air/fuel ratio). In a perfect world, timing is set at the point at which the cylinder is perfectly filled with the correct amount of compressed air/fuel such that the ignition causes 100% combustion. That is not only physically impossible, but also impossible given varying load conditions placed on IC engines. To compensate, variable timing has existed for more than a century. Today, it's controlled by many sophisticated sensors and much better than back in the day. But even still, extra octane can allow the engine to run more timing, producing more power and more efficiency.
It maybe counter-intuitive, but an engine makes all the power it is capable of making at maximum efficiency. That's all "tuning" really is - finding places to improve engine efficiency. So, if your engine can resist knocking and have more timing, it will be more efficient (to a degree). Because physics and real world, it's never perfect.
In modern engines you can get away with running lower octane fuels because the computer will retard timing based on input from knock sensors before damage occurs. However, retarding timing costs you power and efficiency. Climing a steep incline in scorching summer heat is not the same as idling along on city streets in mid autumn. In the first scenario, you'll certainly see a difference as the heat soaked engine that's working hard will start to have knock earlier with lower octane fuel. In the second scenario, you'll very unlikely to note a difference.
I run premium in my Camaro because that's what the LS3 calls for and I don't want my car going to the low octane timing charts. I bought the HP I paid for. Same reason I ran mid-grade in my 5.7L Hemi equipped Ram. My 6.4L Hemi equipped Ram calls for 87 because the lower compression means the motor won't take advantage of the higher octane, as the 6.4 is built with a bias for longevity over outright performance.
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Toyota uses the exact same block assembly (11401-09600) and head assemblies (11102-09142 & 11101-09262) in the 2010 V6 Camry as Lexus does in the 2010 ES350, but Toyota specifies 87 octane while Lexus says to use premium. Lexus does use a different ECU/PCM ( part number 89661-33E72 vs 89661-33E61) suggesting it gets a bit sportier tune allowing it to make a bit more power & torque:Originally Posted by Andy T
The 2010 Lexus ES350 was rated at 272 hp @ 6200 rpm and 254 lb-ft @ 4700 rpm
The 2010 Camry 3.5L was rated at 268 hp @ 6,200 rpm and 248 lb-ft @ 4,700 rpm
Every Lexus manual I've seen says you can use 87 octane if nothing else is available, but to use premium normally. I imagine it won't make a difference in whether the engine makes it 100,000 miles, but it may affect whether you make it to 300,000 on original head gaskets.
Modern ECUs can detect the slightest knock and detune the engine to protect it, but only after knock has occurred. Hundreds of thousands of miles of these minute knocks can cause head bolts/studs to stretch and the head gasket to fail (more severe knocks can harm the piston & rings, but on Toyota it is usually the head gasket that fails)
Last edited by 0ddl0t; 11-18-2022 at 04:11 AM.
I put premium in my 87 octane car a few months ago (skipping the why)
Got worse gas mileage and the stink from the CAT took 2 more tanks to get rid of.
Don't do that!
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If your car has forced induction I'd strongly recommend against using anything lower than recommended. I've seen two of my former bosses crater engines because they put regular fuel in them. Both supercharged.
One was a Ford F150 Harley Edition. That one actually got replaced under warranty but I know it was simply the dealer doing it because of who he was. We told him it would happen but he didn't believe us, he said it "ran fine". 8k later he had holes in 3 pistons.
The other one was a factory ordered big block Chevy Suburban 4X4 with a Whipple bolted on after delivery. He loaned it to his sister and forgot to tell her about the gas. That was a painful mistake.
I had a buddy back in the 1980's who had a Saab 900 Turbo. He monkeyed with the wastegate and the boost ran much higher than factory spec. The car pulled HARD under boost! He sold it to a friend who knew what he was getting and liked the fact that the car had great performance. Within a couple months the new owner had a trashed engine. There was so much pre-detonation due to the increased boost that holes were burned through the pistons.
I have a 2006 Miata that prefers premium. I had to put a tank of regular in it on a long drive and noticed an immediate performance hit, it came back a little as the car relearned things but it was much happier and back to normal after a few tanks of premium again. I started keeping a bottle of octane boost in the trunk after that.
Ken
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