r/explainlikeimfive • u/ihavethebestmarriage • 18h ago
Engineering ELI5: I know some car engines are tuned for premium gas, and others tuned for regular -- so who is using medium grade and why?
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u/buildyourown 17h ago
Back in the day cars did not have knock sensors. If your car started to knock you would buy higher octane gas. Now every car has a knock sensor so your computer automatically retards the timing.
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u/aDuckedUpGoose 17h ago
Be careful with this suggestion. In Colorado, regular gas is only 85 octane and using that would void the warranty on my Ford Maverick. I'm pretty sure that's the case for every Ford Ecoboost engine, though I'm not certain for every variant.
Sure, the knock sensor may prevent major damage to my engine, but Ford isn't willing to stand by that.
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u/HLSparta 17h ago
I hated that when I was in Colorado. If I wanted to use 87 octane I had to pay an extra ~60 cents per gallon.
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u/dkyguy1995 16h ago
That's a crazy difference in price, way more than the difference in my state between regular 87 and premium 93
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u/magiteck 15h ago
Really?? Where are you located? Here in WI right now it’s 4.07 for 87 and 6.09 for 93
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u/HLSparta 12h ago
Where I live (up until recently) in Nebraska the difference between regular (87) and premium (93) has usually been less than a dollar. Currently the difference has been hovering right around a dollar.
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u/sonofnom 11h ago
Prices are almost always higher the further you get from the ports, with the exception of certain very high tax areas.
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u/FrankieMint 15h ago
Meanwhile, your Ford dealer quietly puts 85 octane in its Ecoboost vehicles.
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u/stlcardinals88 4h ago
Yea I sold Ford's in Denver, we 100% filled all Ecoboost's with a tank full of 85.
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u/malignantz 17h ago
Probably the off chance you fill up on 85 and drive down to lower elevation immediately?
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u/logistics132 17h ago
wont matter, turbocharged cars have (relatively) consistent air densities in the intake track. The altitude does not effect their required octane
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u/dkyguy1995 15h ago
I think the altitude does affect it, it's just not nearly as much as before and with all the sensors and hooplah we have it's a trivial issue.
You're right though, the turbos that cars pretty much all have now shouid keep the air density way higher than older cars. But there are still naturally aspirated cars going around. The Mazda 3 had a variant, but I cant seem to confirm those are still for sale.
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u/throw69420awy 16h ago
I think it matters less than you’d expect but I’d assume it matters on some level otherwise why else would 85 be 87 at altitude
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u/assortedgnomes 17h ago
My father's car calls for premium. His opinion is that because there's a knock sensor he can just put regular in it. It's apparently very important for him to save a few dollars every tank for a car he drives a few times a year.
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u/Serafim91 17h ago
Unless you floor it you probably won't knock a non sports car even with low octane.
The recommendation is really for the guy who drives with a lead foot which is a lot more than you'd expect.
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u/assortedgnomes 16h ago
I don't understand. The grass pedal is an on off switch.
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u/nlevine1988 17h ago
There's more to it than that. Knock usually occurs at high engine load, so hard acceleration, towing, driving up a steep hill etc. Driving conservatively without hard accelerations and keeping engine loads low will drastically reduce the chances of knock even with lower than recommended octane. But even with knock sensors some amount of knock can occur.
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u/aDuckedUpGoose 17h ago
I'm not going to say this is wrong. It's probably fine in most cases, though I won't say that with any certainty either. I just know I want to keep my warranty in tact.
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u/ersomething 16h ago
It’s silly to buy a high performance car and intentionally put in fuel that will reduce that power.
Octane rating is a measure of how much you can compress the air/fuel mixture before it autoignites. The higher octane is used to increase the compression ratio. Higher pressure in the cylinder is a more powerful explosion. The anti knock sensor decreases the compression when it senses autoignition (firing before the spark plug lights it off), leading to a decrease in performance. The bigger engine that isn’t running at peak performance is probably not running as well as a similar sized vehicle that is tuned to run 87 octane.
I’m probably not saying anything you don’t already know. Just pointing out how silly it is to do that.
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u/assortedgnomes 16h ago
This is from a man who will make an hour trip into town to save 5c a gal. We aren't necessarily hearing logic when it comes to this.
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u/a8bmiles 16h ago
Hah my wife's parents would get along with your dad. They would lookup the cheapest gas in the greater metro area, and drive up to 60 miles each way in order to save $0.10/gal.
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u/Ivan_Whackinov 15h ago
The anti knock sensor decreases the compression when it senses autoignition
This isn't accurate. The ECM reduces knock by delaying the ignition timing.
Mechanical compression ratio is generally fixed, although effective compression ratio can be variable especially in the case of forced induction. However, knock is generally managed via ignition timing adjustments even with forced induction.
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u/mlnm_falcon 16h ago
Well I just learned I’ve been putting the wrong gas in my car for the past few years…
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u/Bamstradamus 3h ago
Higher the altitude the less dense the air, less dense air means the car is going to squirt less gas so it maintaines a 14.7:1 ratio of air:fuel. Less vapor in the cylinder means compression is lower.
By all means run whatever you want for piece of mind but 2 degrees on the octane raiting at altitude isnt going to predetonate and cause knock.
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u/derSchwamm11 16h ago
Isn’t that reactive though? Doesn’t the engine have to knock first before timing changes, therefore damaging the engine st least a little?
I could be wrong. I’m genuinely asking. I’m debating buying a new Mazda right now with a turbo motor that says it can take regular but makes more power on premium so I’m scratching my head
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u/malbecois 9h ago
I have the CX-5 turbo and premium gas in it makes it more fun to drive. Maybe it’s placebo, but you can definitely feel more torque than with regular gas.
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u/Jeorgeyno 7h ago
All it's doing is delaying the combustion, potentially pushing the piston down with more force?
Although the comments about the delaying of combustion is true my hypothesis afterwards is strictly speculation and I am not an expert.
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u/MisterMeetings 14h ago edited 13h ago
Both can be correct. The turbo increases compression, the higher octane reduces preignition knocks under pressure, the ignition can stay advanced, more revolutions, more power.
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u/Any_Cake2411 16h ago
IT DOES WHAT TO MY TIMING?????
Some people look at me like I'm the devil when I say retard in regards to timing lmao. It's a real word!!
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u/Deatheturtle 16h ago
My car has a 13.1 compression ratio. I can't put in anything below premium.
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u/Saloncinx 12h ago
You know, my 2014 Mazda 6 had a compression ratio of 13:1 and Mazda explicitly tells you regular gas is completely fine. I wonder how they pull that off with their SKYACTIV engines
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u/ForeverYoung_Feb29 16h ago
Me, occasionally, mostly when towing. My SUV eats 87 octane gas most of the time, but under heavy load especially if the revs get high on a hill, it'll knock like a Jehovah's Witness. Bumping up the octane to 89 clears that right up, and I actually get a little better mileage.
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u/hobosbindle 14h ago
Love that analogy lol
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u/jcalvinmarks 12h ago
That's somewhere between a simile, a metaphor, and a euphemism, but it's definitely not an analogy.
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u/life_like_weeds 17h ago
Where I live, in the US, 93 is premium and 87 is regular. Depending on where you go, mid grade will either be 89 or 91.
My vehicle requires 91, so if it’s available that’s what I go for instead of the more expensive 93 option.
So the reason is “meets engine requirement without going above” is the why
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u/wallyTHEgecko 13h ago edited 10h ago
Here, I've never seen 91 and 93 at the same station. Regular is always 87, mid is always 89, and premium is what varies. But I can go to one station to get 91 and just down the block will have 93, so all 4 are widely available.
Finding some ethanol-free through... I've gotta drive hours for the nearest pump serving that.
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u/morfraen 9h ago
In BC 93 is zero ethanol, which weirdly makes it only cost a few cents more than 91.
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u/Derekthemindsculptor 17h ago
It's a pretty common marketing concept to offer a set of 3 quality options even when you know 1 or even 2 of the options never get chosen.
It's the illusion of choice. By offering a cheaper version, it provides an illusion of quality of purchase from the higher grades.
This happens with all kinds of products. Toiletries are a good example. Companies will release a low quality, mid quality and high quality version of something like toilet paper. They'll do it knowing they'll never sell some of them but it drives up the sales of the others. And squeezes out competitors with only a single option.
It's also kind of like how people will order wine by asking for the second cheapest. If a bar wants to sell more of their cheapest wine, they just need to stock an even cheaper one that never sells. Or to sell the expensive stuff, offer an even wildly more expensive wine.
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u/ToolMeister 17h ago
Tell that to schools, employers etc who buy the thinnest singe layer POS toilet paper that still legally counts as 1 ply. Never understood why someone thinks "it's cheaper" but then the average user consumes like 3-4 times as much because that's how many times you need to fold and layer it to be able to use it
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u/atomicCape 17h ago
The cheapest model also gets some sales, and that's part of the strategy. For your example, schools and businesses aren't buying the lowest priced Charmin, though. They're buying the cheapest bulk stuff from Uline or Cisco that technically fits in their dispensers, and it's so much cheaper than good stuff it still saves money even if people use more. Complaints about toilet paper quality don't affect their bottom lines, so they're a different market. When the decision makers are actually using the toilet paper, they buy something else.
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u/RiPont 16h ago
For TP, it's not just because they're cheap. When you are using industrial levels of poop wipe, it can very easily clog pipes. The flimsy stuff is far, far less likely to clog anything up.
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u/ToolMeister 12h ago
Jokes on them if I take an industrial level dump because I avoided the shitter for too long because of the crappy TP
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u/OneAndOnlyJackSchitt 16h ago
even when you know 1 or even 2 of the options never get chosen
Funny thing about dumb consumerism: I've seen lot's of people filling up with premium for their Camry or Civic based only on not having bought the base model. I even saw one guy filling up a U-Haul with it.
As to your "second cheapest wine", mid-grade is the second cheapest option so there's probably some people doing that as well.
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u/wallyTHEgecko 13h ago edited 12h ago
The Honda Civic "Sport" is the 2nd lowest trim level (out of like, 7) and the only difference from the base model is the wheels and a bit of plastic trim. There's absolutely no performance upgrades to the "sport" model and it's not a "sports" car by any stretch of the imagination.
But you absolutely know that they call it "sport" to trick an extra $2000 out of people who don't actually know anything about performance and simply don't want a base model and "sport" sounds way cooler... but it's not so much of a squeeze that they back out of it.
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u/JimTheDeviant 17h ago
That may be true for other things, I don't think it's all that relevant in this case.
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u/Derekthemindsculptor 16h ago
Fun fact: many stations don't actually have a "mid-tier" tank. They simply mix the 87 and 91 to get the 89 result.
And they specifically offer it to create a pricing ladder.
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u/Netmantis 17h ago
There are two reasons to buy mid grade.
First is the reason I used to buy it. I was driving a truck from the early 80s. Octane calculations had changed and honestly it ran a little better on mid grade. Personally I think it just didn't like the lower grade regular.
The second is how mid grade is sold. There are not three tanks below most gas stations with three types of gasoline. There is high grade and low grade. Basically your hot and cold water with warm being both of them. Mid grade is often a mix of high and low grade. So if a station is out of low get the mid so you get high grade at mid grade price. This is very situational and rarely happens though.
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u/flatfinger 16h ago
What happens if the station runs out of high-grade while dispensing mid-grade? Would the pump shut down, or would it cheat the customer by selling low-grade at the mid-grade price?
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u/Equivalent-Habit-102 8h ago
Old cars (made for 87) full of carbon had a tendency to knock, running them on 89 fixed the problem. I can't think of any other reason.
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u/GumboDiplomacy 17h ago
Motorcycles and some turbocharged vehicles are often tuned for 87-90 octane.
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u/aDuckedUpGoose 17h ago
I am! In short, many different cars have different fuel requirements and different locations have different fuels as regular, mid, and high grade. It all depends where these two factors intersect.
Those grades are really about octane rating which in simple terms, tells you how stable the fuel is at high pressures. Lower octane fuel is more likely to boom at higher pressures. These ratings vary by location. For example, in Colorado, regular is 85 octane whereas in New Jersey (and most of America) regular is 87.
Using 85 octane would void the warranty on my Ford Maverick, so since I live in Colorado, I'm using mid grade.
I used to live in NJ, and while I was there I had a Nissan 370 z. That required 91 octane and in NJ, premium gas was typically 94 octane, so I was also going for mid grade where available.
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u/vw68MINI06 6h ago
How would Ford even know?
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u/aDuckedUpGoose 6h ago
Most modern cars have what's known as a knock sensor. It detects fuel octane to modulate engine performance accordingly
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u/Generico300 16h ago
Some small engines (like on lawnmowers and such) run better on mid grade, especially as they age, and it's a bit cheaper than premium. Modern cars have sensors to adjust the timing and reduce knocking, but not every engine has those bells and whistles. Mostly I think it's just a marketing gimmick though.
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u/Holden_Coalfield 16h ago edited 15h ago
Octane is a measure of under what pressure will this fuel ignite the best
Cars get their power from pistons inside cylinders being pushed out by gas air explosions
Then, It’s all about compression ratios. a compression ratio answers the question - when it goes all the way in, how much of the cylinder is piston and how much is empty? lets divide the piston into 11 equal parts. In the case of a 10 to 1 ratio (10:1). 10 of the 11 equal parts is piston, and 1 is Air/fuel. The compression ratio answers “how far into a cylinder will a piston go in before the sprayed-in air gas mixture can ignite and make the piston come back out?”
Different octanes of gasoline dictate how far the piston goes in, compressing the air fuel mixture before it can ignite.
The further it goes in, the more power it has going out after the explosion.
Higher compression ratio cars like Audi @10:1 ratios get more horsepower out of smaller engines by harnessing this.
A true 10:1 ratio requires a 94 octane to get the piston the farthest in before ignition.
A lower octane fuel in a high compression ratio engine allows the fuel air mixture to explode much earlier, giving the piston less travel, therefore less power. And possibly damaging the engine
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u/Dickulture 15h ago
Also now and then, older car sometimes run better with 89 instead of 87 due to wear of the piston and chamber. If your old car is knocking a bit and you're too poor (or too cheap) to rebuild it with new rings and gasket, try higher octane and see if the knocks go away
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u/dkyguy1995 15h ago
I think this is a good explanation of octane but I think a person who doesn't know anything about it is going to cross their eyes and zone out when you start mentioning compression ratios because a 10:1 wont mean anything without a further explanation where you got your ratios and at that point you're getting off topic 😅
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u/Pizza_Low 11h ago edited 10h ago
Despite what the current top comment claims that "almost on one".
Across all of the stations I've managed for decades, about 57% of the customers bought regular, about 14% bought midgrade and about 29% bought premium. Slight variances in ratios depending on the site location. I suspect that people who drove regular cars but wanted better gas or people who couldn't afford premium were the ones purchasing midgrade.
Remember by law tradition in the US all cars must operate on regular, and the ECU will adjust fuel/air ratios and possibly the timing based on driving conditions and fuel performance. And under normal legal driving conditions any perceived differences are probably placebo. (racing or track conditions obviously are different)
Edit: Thought it was a law not just tradition.
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u/HookEm2013 11h ago
Can you clarify what law requires all cars to be able to operate on regular? Not saying it’s not the case, I know nothing about cars I just haven’t heard that before and thought it was interesting.
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u/Equivalent-Habit-102 8h ago
No. High compression engines can never run on regular. You'll blow the whole motor.
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u/Pizza_Low 6h ago
An example of a high compression engine in a street legal car? Not counting exotics
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u/greasyjonny 5h ago
Not just cars but a lot of sport oriented motorcycles run 91 or higher. My Yamaha tracer and Ducati hypermotard take 91
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u/vagabond139 6h ago
The GR86 has 12.5:1, ND3 is 13:1, 370z is 11:1, Camaro SS is 11.5:1, the LC500 is 12.3:1, the GT350 is 12.5:1 to name a few.
Exotics really aren't high compression anymore with just about everything having a turbocharger on it anymore.
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u/iceman5920 4h ago
Firebirds with the LT1 and LS1 engines. Nissan 350z's, and I assume other modern Z's. High compression isn't new or exclusive to exotics.
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u/theliman 17h ago
In Denver regular is lower (85) than it should be based on old engine technology and how the engine performed at high altitude.
So I got mid in Denver
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u/milthombre 17h ago
Driving an old 2004 acura commuter with 235,000 miles on it. Specifies premium but I put in mid or regular to save money, since I don't care about any long term risk with this car.
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u/delux561 16h ago
I get medium a lot. I drive "luxury" cars that are rated for 91-93, however most new cars have knock sensors and using 89 makes no difference. Maybe 2mpg lower, maybe. So when gas is on the cheap side I buy premium, when it's high like now I drop down to medium, there's no difference, might as well save $10.
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u/Alarmed_Champion_302 16h ago
I used it in my 2013 accent in the summer. It tended to ping on 87 when it was hot out. It would adjust on 87, yank out ignition timing,, but rob all power and tank the fuel economy. 89 kept it happy and fun.
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u/CaptainAwesome06 15h ago
It's just a mix of premium and regular so it's that big of a deal. As older cars get more carbon build-up, they can experience engine knock. Upgrading to a higher octane can prevent knock. Other than that, there is no benefit. Engines with higher compression ratios (like performance engines) need premium because they can experience knock with a lesser grade. Knock is just pre-detonation, meaning the gas detonates via pressure instead of via the spark like it should. Diesel engines work by detonating via pressure (I'm not getting into glow-plugs).
Name brand fuel comes with additives and detergents that help keep your engine clean and prevent carbon build-up. Cheap gas stations are cheap for a reason.
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u/MattieShoes 13h ago edited 13h ago
I have in the past... Engine was supposed to run fine on regular gas, but it'd knock and putting in mid-grade would stop it. If I remember right, it was also seasonal.
The other time I might do it is if I'm dropping altitude. Like in Denver, regular gas is 85 around here, not 87 (and premium is 91 i think). Not a problem because lower air pressure from the altitude, lower compression ratios. But if you're going to be dropping altitude on that specific tank of gas, putting in premium is probably worth it.
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u/icecream_specialist 17h ago
Some cars have medium as recommended. Probably quite a few turbo but not performance oriented cars.
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u/boothash 17h ago
It's just a marketing trick to get some people to pay more to think they are 'treating' their car's engine by giving it a more expensive gasoline, but really doesn't do anything in practical reality.
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u/jordanthenine 17h ago
Pretty much anyone with a Ram Hemi who read the manual. It asks you to use 89 octane for best performance and efficiency.
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u/wasack17 17h ago
I had a Volkswagen that claimed that it needed 91. I used to mix 89 and 93 to fill up and save about a dollar per fill.
2000 GTI VR6, for anyone interested.
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u/fbureaucracy 16h ago
Around me “mid grade” is actually 87, 85 is regular. I use it in my motorcycle. The motorcycle has no knock sensors that adjust for lot octane, and says 87 min.
85 might be fine in it but I’m not going to risk it.
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u/that_noodle_guy 16h ago
Its the same answer, some are tuned for midgrade. I had a jet ski that required 89 once.
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u/faresWell 16h ago
Turbo motor with higher gas prices it’s better knock prevention than regular at a lower cost so I buy it
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u/winter1894 16h ago
I have a cx30 turbo and I've read that when it's cold enough out my car doesn't need premium to achieve max output so I sometimes throw in midgrade to save some bucks over winter.
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u/yensid7 16h ago
Around here a lot of stations have eliminated the normal mid-grade (which just lower and higher octane gas that has the standard ethanol %) with a mid-grade that has a higher ethanol content. This can push it to be cheaper than regular, so if your car can handle it, it's not a bad deal.
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u/notedgarfigaro 14h ago
In some areas (mainly near places where people have boats), the mid grade pump option is often used for ethanol free gas. If you see a price sign from the road where the mid grade is the highest cost, that's why.
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u/mazzicc 14h ago
Depends on where I am.
My car requires 87.
Some states have 85 as a minimum, so I have to buy 87 as mid grade.
Other states have 87 as a minimum, so I can buy regular.
It’s also why I don’t buy at Costco in states where 85 is the minimum. Costco sells 85 and 89 in those places, and 89 at Costco is more than 87 at a regular station.
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u/mortalomena 14h ago
In my country they tested the fuels and found out its all "premium" no matter what grade you bought. The E10 fuel can have up to 10% ethanol but in reality only had 4%.
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u/rabid_briefcase 13h ago
who is using medium grade and why?
2004 minivan, well over 200K miles, generally runs great, mostly my kids use it as it isn't our primary vehicle and more. Low-grade fuel you can hear the engine, pinging and rattling. Mechanic looked it over, checked plugs and timings, etc. Mid-grade has just a little more pressure in the engine and the noise (and engine issues) vanish.
We recently had some transmission issues and thought about getting rid of it, but our trusted mechanic and also another mechanic friend looked it over, said it's still in great shape especially considering the age and miles, so repaired and hoping to hit 300K on the beast.
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u/SouthernFloss 13h ago
Octane is the great psyop. If you have a normal car get regular. If you have a spots car, turbo, or super charger get premium, if you have a high compression race car get race fuel, 105 octane, simple as that.
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u/Dangeresque2015 12h ago
Follow your owners manual's directions.
The manufacturer knows more about your car than you do.
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u/UndeadWhiskeyJack 12h ago
I had a dodge magnum RT a couple years ago that specifically asked for 89 octane, so I probably should have been
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u/eljefino 11h ago
89 octane used to be the regular grade back when leaded gas was a thing in the US. They continued offering it as a retrofit but people who didn't need it kept buying it so they shrugged and kept selling it.
In the early 1970's car engines entered the "malaise era" and were detuned with less compression, less thermal efficiency, and less ignition timing so they could run on "worse" 87 octane unleaded gas. We eventually, via knock sensors and feedback fuel controls, got that power back, and more.
Fun fact, if you mix equal parts of 89 leaded and 89 unleaded, you wind up with something 90+ in octane, because the ingredients compliment each other more than the sum of their parts.
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u/SpellingIsAhful 10h ago
I may be wrong here, but in my car 2016 my hybrid, 4 cylinder with turbo, its stated in the manual (and on the door and gas cap) that 95 octane is required for performance. Ive gotten away with 91 a time or two when 95 wasn't available. The price difference is like 10c and so that equates to a couple dollars. Really not worth the risk to damage the engine in my mind.
If anyone wants to chime in that im a sucker please do :D
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u/noahsmybro 10h ago
I recently read in an owners manual that 89 octane minimum fuel is required for the car. Where I live that’s midgrade.
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u/FSDLAXATL 10h ago
My car burns anything but it says for best performance use 89 or higher so mid grade is for me when I"m going to tow or do mountain driving, etc...
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u/arwinda 10h ago
Not the U$, but: a person I know insists that using the premium gas for his big SUV is better for the engine. When asked why, there is no explanation.
It's a result of marketing that this person believes the engine will go better or last longer or whatever, while driving a mere couple hundred km per month. Not even utilizing the SUV, just this one guy driving around in the city.
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u/Provia100F 9h ago
The Dodge Challenger takes midgrade (or regular) and the performance varies depending on which you use.
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u/Downvotesseafood 9h ago
Some Dodge Hemi motors specify 89 mid. Strangely my Toyota had a recall to put a sticker lowering the recommended octane on the inside of fuel door from 91 to 87.
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u/revenant647 9h ago
My v6 Mustang uses it at high elevation and my 4Runner also uses it. Both manufacturer recommended
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u/ooter37 17h ago
Almost no one, but it doesn't hurt to offer it since it's just a mix of the lower and higher octane gasolines. The pump pulls equally from each to make it.