# Octane question



## Balsamic Vinegar (Oct 26, 2021)

I drive a Mercedes with a high compression expensive gasoline engine. In California premium gas is almost always 91 octane and I'm on a road trip across the country and in a lot of the rest of the country premium gas is 93 octane...

I think my car runs better on the 93. Am I imagining this?


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## TCar (Aug 4, 2019)

Good question. My toyota corolla seems to run better on 93 octane as well.
When it was cheap, last year, i ran 93. 
Now i always run 89 octane.
Just seems like better food for the car. 
All mechanics are in disagreeance on wheather this is good or not.
Going to ask my new mechanic tomorrow when i get my car for latest oil change.
Going with what they say. 
Be Peace!


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## Older Chauffeur (Oct 16, 2014)

My understanding from having driven full time for a fellow who ran a dealership and talking to other clients who owned Mercedes cars, the electronic engine management system will make adjustments to run at its best for the fuel you put in it. A Mercedes will run decently on 87 octane, but there is a noticeable improvement if you put 91 octane in the tank. i think that may be the reason that you’re noticing a difference. I used to have a client who put 87 in his to pinch pennies. His car ran okay, but not like those of my other clients, including his son’s.🤷🏼
There’s a reason drag racers like high octane aircraft fuel.😊


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## FLKeys (Dec 27, 2018)

My understanding is the higher the octane the more compression it takes to combust. You should use the octane rating the car manual says to use. Using lower octane in cars that require higher octane can cause early combustion which can cause engine damage.


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## Disgusted Driver (Jan 9, 2015)

FLKeys said:


> My understanding is the higher the octane the more compression it takes to combust. You should use the octane rating the car manual says to use. Using lower octane in cars that require higher octane can cause early combustion which can cause engine damage.


Exactly this, putting in higher octane than you need does nothing but cost you more. Putting in lower octane that required will damage your cylinders.


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## Balsamic Vinegar (Oct 26, 2021)

I can explain the engineering behind it. It's the psychology I was wondering about.

All gasoline/air mixture in your cylinder will explode when compressed enough by the piston. In a gasoline engine this is not desirable. Unlike a diesel engine a gas powered engine uses a spark to ignite the fuel/air mixture. This occurs when the fuel/air mixture is at its maximum compression. If it ignites before then its called pre ignition and throws the timing of the engine off. Also causes balancing issues which can result in the piston banging against the cylinder wall which if bad enough can seriously mangle your engine.

They used to add lead to gasoline to prevent pre ignition but of course they can't do that any more but higher octane fuel can be compressed more than lower octane fuel without spontaneously igniting. This is why you need it in high compression engines.

My question was about the difference between 91 and 93 octane. Does that really matter or am I just j***ing off?


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## FLKeys (Dec 27, 2018)

What octane rating does the manual call for? If it calls for 93 then my bet is it runs better on 93 then 91. If it calls for 91 then I would think it running better on 93 is your imagination.

Now as mentioned some newer cars self adjust for the octane rating detected. At that point who knows.


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## ANT 7 (Oct 14, 2018)

The ecu will adjust for octane.

You are imagining it.

Use the 91.


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## FLKeys (Dec 27, 2018)

ANT 7 said:


> The ecu will adjust for octane.
> 
> You are imagining it.
> 
> Use the 91.


If the ECU does adjust for the octane rating than wouldn't the performance be better with the higher octane?


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## Balsamic Vinegar (Oct 26, 2021)

I think the manual says premium. To be clear I don't have a choice in which premium grade fuel I buy. It's 91 in most places in California and 93 in most other places I go. I don't doubt the ecu will adjust to whatever octane I put in there but it is a high compression engine. I'm using the premium fuel regardless. And I think it runs better on 93 than 91. I wouldn't have thought that before but I think that now


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## ANT 7 (Oct 14, 2018)

FLKeys said:


> If the ECU does adjust for the octane rating than wouldn't the performance be better with the higher octane?


No.

Engine compression ratio and other factors in design determine what octane rating is required.

Putting 94 octane into an engine designed to running 87 will result in a slimmer wallet and nothing else.

Dumping 104 octane race gas into your tank will give you nothing as well.


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## CarpeNoctem (Sep 12, 2018)

If you look at the back of the napkin calcs you will see 91 to 93 is not much difference. If the ECU adjusts based on octane, there will be no performance boost. You might see a little better gas mileage but at best will be about 2% (91 to 93). It would take a lot of miles to actually see that difference. Say 27mpg to 27.5mpg. Wind gusts could account for that difference.

If the ECU doesn't adjust, say the engine does 300hp. Again, at best with a 2% increase it would be 306hp.

These are highly theoretical numbers as engine performance is unlikely to be that linear.

Is a 2% performance gain of 6hp or .5mpg worth a 60%(?) increase in cost of fuel?

So, yep, it is all in your mind... A car wash and the correct tire pressures will likely be a more perceptive difference.


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## sumidaj (Nov 2, 2020)

Just put in what your car requires. use a good quality gas with detergents / additives etc... Higher compression engines require the higher octane to prevent detonation etc... 
Putting a higher octane than your car requires is just burning money. 

Anything you "perceive" as better is really just your imagination unless you back it up with data, numbers / mileage etc.. 

the only exception is a very worn . carbon filled motor...you might see some gains with that as the higher octane might work more efficiently in the dirty / carbon filled chamber...but even then its probably not going to be a huge difference and if you drive one of those clunkers....i do  .............you won;t recognize any benefits over the added higher octane costs.


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## JeanOcelot0 (Dec 30, 2020)

Balsamic Vinegar said:


> I drive a Mercedes with a high compression expensive gasoline engine. In California premium gas is almost always 91 octane and I'm on a road trip across the country and in a lot of the rest of the country premium gas is 93 octane...
> 
> I think my car runs better on the 93. Am I imagining this?


That's an interesting question. The only time I've had to get 91 because 93 wasn't sold is out in the Rocky Mountains; for whatever reason, Dalhart, TX seems to max out at 90.  Of course, the air in thinner, but I think I have detected a little less punch in my VW 1.8T engine there. AIUI, turbo engines (for any manufacturer) are designed to be optimal no matter what the octane rating of the fuel, so running 93 will be better than running 91; even base 87 can be run, but I've never abused my car with that.

BTW, it's LONG overdue for there to be an interstate highway from Dallas to Denver. US-287 from Ft. Worth to Amarillo (and 40 miles north) could easily be upgraded, so there would only be about 150 miles of upgrading the 2-lane highway there.


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## BrainDead Driver (Dec 15, 2021)

Now i do not know yor car . But i can tell you i ride sport bikes that require high octang.
One company will say joes gas has 92 highest they sell . . My bike will run alright but when i push it into the red line 13000 rpm you for sure can feel the difference from the 94 mobile sonoco shell gasoline . Driving it like a old lady i would not notcie any difference . Now for that car that requires 92 and up if a person ran will say 87 this is not a issue . If that person did drive it like grandmom going to church. But if gradmomther said ow shit !!! I for got to turn the tea pot off and it will burn ! Hits the gas petal to freeway accelation speeds this will hurt the engine . It will have severe preignition . That chatter you might of heard before getting onto the freeway . Not good .
That high octang fuel might not sell at one station losing octang . So you buy a 92 but it comes out as a 90 . Not many cars use higher then 87. Here in detroit a lot of stations only sell 87


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## Balsamic Vinegar (Oct 26, 2021)

BrainDead Driver said:


> My bike will run alright but when i push it into the red line 13000 rpm


How does anything on a engine work at 13000 rpm? If I was friendly with Mr Internal Combustion Engine when he first invented it and he explained the concept to me I'd say how in the hell are you ever going to get the timing right at 13,000 rpm. How is that crank shaft not going to fly right out the engine like a bullet?


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## Balsamic Vinegar (Oct 26, 2021)

JeanOcelot0 said:


> BTW, it's LONG overdue for there to be an interstate highway from Dallas to Denver. US-287 from Ft. Worth to Amarillo (and 40 miles north) could easily be upgraded, so there would only be about 150 miles of upgrading the 2-lane highway there.


I've been driving all over the country and loving it and recently I drove from Asheville NC up to part of the Appalachian trail and was surprised to find out that there were two state highways that were dirt roads. And several miles long and btw one of them extended into Tennessee where I'm not sure if it turns into a Tennessee state highway or a street with the name of "shitty dirt road you probs don't want to be on but in NC they're state highways."

You're right tho a lot of infrastructure could be upgraded


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## JeanOcelot0 (Dec 30, 2020)

Balsamic Vinegar said:


> I've been driving all over the country and loving it and recently I drove from Asheville NC up to part of the Appalachian trail and was surprised to find out that there were two state highways that were dirt roads. And several miles long and btw one of them extended into Tennessee where I'm not sure if it turns into a Tennessee state highway or a street with the name of "shitty dirt road you probs don't want to be on but in NC they're state highways."
> 
> You're right tho a lot of infrastructure could be upgraded


I was on a work road trip in the southern Appalachians, and went skiing in NC for the weekend. One of the places was Sugar Mountain and the other was Beech Mountain. For the latter, I thought I was on paved road getting to it, but like the frog in the proverbial pot, I soon realized I was heading down a wet, muddy dirt road with nowhere to turn around.    Eventually I got stuck and had to get a tow truck to get me down (I have no idea how he did it!). So I know EXACTLY what you are talking about.


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## JeanOcelot0 (Dec 30, 2020)

Balsamic Vinegar said:


> How does anything on a engine work at 13000 rpm? If I was friendly with Mr Internal Combustion Engine when he first invented it and he explained the concept to me I'd say how in the hell are you ever going to get the timing right at 13,000 rpm. How is that crank shaft not going to fly right out the engine like a bullet?


No need for timing when the combustion container is its own cam:


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## SpinalCabbage (Feb 5, 2020)

JeanOcelot0 said:


> No need for timing when the combustion container is its own cam:


They're great little engines.


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## Balsamic Vinegar (Oct 26, 2021)

JeanOcelot0 said:


> No need for timing when the combustion container is its own cam:


Engineers are smarter than me


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## JeanOcelot0 (Dec 30, 2020)

SpinalCabbage said:


> They're great little engines.


Hmm ... they can spin at very high angular velocity, but compared to a canonically pistoned, turbo engine, they are less powerful & fuel-efficient. Oh, and they necessarily burn a lot of oil. They only seem to be a good choice for small aircraft, where weight is very important.


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