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Why Octane Ratings Matter

August 21, 2021


Knock knock, who's there? Judging by the title of this blog, I'm sure you can guess where I was going with that. 

And what better way to fuel the discussion on octane ratings and engine knock?! Okay, I'll stop. 

In all seriousness, octane ratings can get really complicated, especially when it comes to tuning and potential power gains.

Still, it is important to understand what octane ratings actually mean and why they're important, as well as their part in the tuning process. 

Quick Links:

- What Is Octane Rating?

- Why Are Octane Ratings Important?

- What Is Ethanol?

- What Is E85?

- Tuning Potential With Higher Octane Ratings

- Final Thoughts


A purple 2015 Dodge Dart filling up at the gas station

2015 Dodge Dart SXT




What Does Octane Rating Mean?

Simply put, the octane rating of your fuel is a measure of your fuel's stability—in other words, it's a measure of your fuel's resistance to pre-detonation, which causes engine knock. It kind of sounds cool, but I can promise you it's not ideal.


White 2010 BMW 328i (M-Sport) filling up at the gas station

2010 BMW 328i (M-Sport)

At your typical gas station, at least here in Wisconsin, you'll see octane ratings of 87, 89, and 91.

You'll also see 93 octane and E85, but those are generally more difficult to find, depending on the area you live in. 

The higher the octane, the more "stable" it is and less prone to pre-detonation it is because it can withstand higher temperatures and pressures within your combustion chamber.




a silver 2005 BMW 325ixT filling up at the gas station

2005 BMW 325ixT

If you aren't familiar with what "knock" actually means, this is when your air-fuel mixture ignites prematurely in the combustion chamber.

Remember, in the engine cycle of your 4-stroke engine, you have intake, compression, combustion, and exhaust.

When your engine experiences "knock,"  the pressure within your cylinders becomes super high during the compression cycle, causing the temperature to increase and your air-fuel mixture to ignite without a spark. 


A silver 2005 BMW 325ixT filling up at the gas station

2005 BMW 325ixT




Why Are Octane Ratings Important?

So, you're running tight on cash, and you're considering just pumping 87 or 89 octane instead of the manufacturer-required 91+ octane because that extra $10-20 difference would be nice to keep. 




Red 2004 Mazda Miata with AVID Av1 wheels, Continental ExtremeContact DWS06 Plus, and Tien air suspension

2004 Mazda Miata 

If you take anything from this blog, let it be this: don't put a lower octane gas into your car if the manufacturer requires 91+ octane. 

A simple cost-benefit analysis will show you just how much you'll screw yourself by doing this. Here's why. 


A Mustang and BMW at the gas station filling up!

Mustang & BMW 135i 

Every corner of your engine was specifically designed that way for a reason.

In your combustion chamber, everything is perfectly timed for a controlled combustion cycle under certain circumstances—intake, compression, combustion, and exhaust.

When these circumstances are disrupted by a low octane fuel, for example, expect to plan a funeral for your engine. 

That might be a little dramatic, but this situation is certainly not ideal. 




Two Miata's at the gas station filling up

NA Miata Party At The Gas Station

The octane rating of your fuel matters because low octane gas pumped in a car that takes premium is more prone to pre-detonation and will disrupt this controlled combustion cycle—this is called Knock. 

It cannot withstand the higher temperatures and pressures due to your cylinder and head geometry, along with the timing of your engine.


Black 2018 Lexus GS F with Variant Krypton wheels, Michelin Pilot Sport 4 S tires, and Coilovers

2018 Lexus GS F

Spontaneous combustion, or knock, can cause significant damage to your engine in the form of melted or ruptured holes in your combustion chamber causing metal fragments to flow into your oil system. 

Depending on the type of pistons your engine uses, they can fail and crack too. 




BMW 135i filling up at the gas station at night

Jake's 2008 BMW 135i Tuned on E85

Now, if you own a car that takes regular gas (87 octane), and you want to put a premium in there, go for it. 

There's no harm in pumping higher octane gasoline, to a degree. In fact, you might even see slightly better gas mileage. 

And in all honestly, if you accidentally put 87 or 89 instead of 91 or 93 in your car as it requires, it may not have immediate repercussions, but if you continue to do so or drive aggressively, the consequences are not pretty. 


Another night shot of a BMW 135i filling up at the gas station

Jake's 2008 BMW 135i Tuned on E85



What Is Ethanol?

Corn. Well, kinda.

The ethanol in your pump gas, "Contains up to 10% Ethanol," for example, is liquid alcohol made from fermented starch and sugar that primarily comes from corn and sometimes wheat—about 94% of the ethanol in the US is made from the starches extracted from corn. 




Yellow 2021 Toyota GR Supra Premium with Rays 57 cr wheels, Hankook Ventus V12 tires, and HR lowering springs

2021 Toyota GR Supra Premium

So, why is ethanol added to gas in the first place?

Ethanol has a high octane level, so it's blended with lower octane gas to increase its octane rating to 87, for example. Without ethanol, 87 octane would only be 84 ish octane. 

Essentially, it's an octane booster that's better for the environment.

Pure ethanol actually has an octane rating of between 100 and 113. 


Mini cooper at the gas station filling up


Producing ethanol also requires much less energy compared to making fuel, which is better for the environment.

It not only burns cleaner compared to petroleum-based octane boosters but since ethanol is made from either "waste" (left-over crop residues) or created using corn, which meets renewable energy laws, it's not necessarily harmful to the environment.

It's a win-win for everyone! 




A Black 2018 Lexus GS F with Variant Krypton wheels, Michelin tires, and coilovers

2018 Lexus GS F




What Is E85?

I'm sure you've either seen a sticker on an enthusiast car or on license plates that says "Corn Fed," and that refers to E85 fuel. 

E85 is a type of "flex-fuel" that enthusiasts go nuts for because it does indeed have a lot of tuning potential, especially for boosted cars and high compression N/A engines, but as I said before, it's not for every car. 


BMW 335i filling up with E85 at the gas station

BMW 335i tuned on E85 | CornF3D

E85 contains a super high level of ethanol, between 51% and 83%. 

Like any octane fuel, you won't always get "87" or even "91" octane fuel.

It depends on the source and the season.

Some areas will have a winter-blend of gas which differs from the gas you pump in the summertime. 




BMW 335i filling up with E85 fuel at the gas station

Jake's 2008 BMW 135i Tuned on E85

Also, like 87 or 91 octane fuel, E85 is made from fermented sugar and starch from corn. See, basically corn. It just contains more pure ethanol. 

 Finally, E85 is becoming a little more popular, but it can still be hard to find depending on where you live.


A black BMW 135i getting E85 fuel at their local gas station

Jake's 2008 BMW 135i Tuned on E85



Tuning Potential With Higher Octane Ratings

I'm sure you've been at a car meet and overheard someone talking about how they just got their Corvette or 350z tuned on 93 octane or E85.

What does this mean, and why do people want to spend the extra $1/gallon more on gas (at least right now)?? 

The potential HP gains and whether a 93 or E85 tune is worth it are debatable, but let's tackle the facts. 

It's also worth mentioning that since every platform is different, I'd highly suggest reaching out to people on your platforms (FB groups or forums) to see what they have or have not gained from a 93 and/or E85 octane. 

Als, you can't just go to a tuner and ask them to tune your car on E8%, for example—it's not that simple




Mazda Miata filling up at the gas station

1996 Mazda Miata 

As I previously mentioned, a higher octane fuel is less prone to pre-detonation.

That extra resistance to detonation allows the fuel to ignite later in the combustion cycle.

Using a tune, you can manipulate ignition timing and air-fuel mixture to use the higher octane to your advantage for more power. 


Gas station picture with the police and an Evo X


So, for example, with a 93 octane, you'd add ignition timing (in degrees) until you'll see engine knock, and then you'd remove a few degrees to be safe because you don't want knock as we've discussed. 

The higher octane fuel alone won't get you the extra ponies you've been dreaming about, but it's one piece of the puzzle to increase efficiency and pump out some more power. 




2005 BMW 325ixT

As far as tuning with E85, the process is similar.

You manipulate ignition timing, air-fuel mixture, and with forced-induction engines, boost as well.

Now, there's a lot of variables that affect tuning on E85 vs. 93 octane. While naturally-aspirated engines generally see more power gains with a 93 octane, boosted engines respond best to E85 fuel. 



The higher octane rating allows you to add more timing, and with forced-induction engines, you can also adjust boost (usually running less boost).

Since E85 is highly oxygenated, you can essentially fill the cylinders more with less boost. 

E85 is also much better at cooling, which is especially important with forced-induction engines because its higher ethanol content vaporizes, absorbing heat and reducing inlet air temps. 




Blue 2006 Subaru WRX with Aodhan DS07 wheels, Federal SS595 tires, an Raceland air suspensiond

2006 Subaru WRX 

If you have a super high compression N/A engine, E85 is generally a better option compared to 93 octane as well.

Even from the factory, higher compression engines usually require higher octane fuel because lower octane fuel is more likely to experience pre-detonation and knock, which we already discussed.

In this case, E85 has an extremely high octane rating and can withstand those higher temps and pressures that a higher compression engine is naturally going to have. 


2004 Toyota Celica GTS with Rota RS with Achilles ATR Sport 2

2004 Toyota Celica GTS w/ 11:1 CR




Final Thoughts

On the surface, octane ratings are just differences in ethanol and gas content, but the good stuff reveals itself when you start talking about tuning and how a higher octane can mean more power. 

If you own an STI or any boosted car and are looking to make some serious power, you're going to need more than just an E85 tune, but as I said before, it's one piece of the puzzle.




2004 Mazda Miata parked next to another Miata

Miata Party Pt. 2

Is your car on an E85 or 93 octane tune?

What kind of power and torque did you gain through your tune? If you're considering a tune, comment down below! 

Don't forget to reach out to other people on your platform to see what common tunes people are running and why.

Also, make sure you go to a reputable tuner if you choose that route. Adjusting things like air-fuel mixtures and ignition timing is not something easily achieved by a noob. 

Thanks for reading! 




Comments (1)

AAudi S7 Owner


It should also be noted that you can't just start using E85 in any car. It needs to be a "flex fuel" vehicle or otherwise designated, and designed for high ethanol fuel blends. What may happen if you use E85 in vehicles not designed or retrofitted for it? You can expect to see seal failures, fuel line failures, fuel pump failures, as well as other issues. Can you expect to see these problems on your first tank? Probably not, but repeated use will guarantee expensive fixes in the near future. Thanks to the ingenuity of the tuner scene, most popular tuner cars have aftermarket E85 conversion kits available, and are a necessary investment if you're planning on tuning for E85.

*Subject to approval within a 24-48 hours time frame.

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