Fuel Octane Ratings: How to Choose the Right Fuel for Your Tuned Honda or Acura
Off the bat, this is an easy question to answer! Just put the highest available octane rated fuel in your Civic and turn that boost up. But wait! Things are not so simple.
In this blog, we’ll dive deeper into choosing what fuel to use for your specific application whether that’s 91 OCT, 93 OCT or something else altogether like Ethanol blends.
Let’s start with the basics. What is Octane and what Octane is generally available at the pump at your nearest Chevron or Mobil station.
“Octane” is simply another term for how stable a fuel is and at what pressure it will “combust”. You know, that thing that happens when your engine spins and you smash the gas. The higher the number, the more stable the fuel and the more compression that the fuel can withstand before detonating
So now you know what Octane is. Let’s look at what’s available typically at your nearest station.
Regular: 87 (Sometimes we see as low as 85 in parts of the country)
Mid Grade: 89
Premium: 91-93 (Sometimes even 94. Mainly, this is sold by Sunoco in limited locations)
These aren’t the end all and be all and you can find very specific blends sold by distributors elsewhere, but let’s focus on what’s available mainstream. This also applies mainly to the United States and Canada. Our overseas counterparts have similar Octane, but ratings on different scales (RON).
Ok, so you’re asking yourself “why does this all matter to me”? Well, if you're driving your 10th or 11th gen L15 or K20C powered Civic without any intent to maximize performance then the Honda recommended minimum Octane ratings of 87 or 91 are just fine, but you wouldn’t be reading this if that was you. We as enthusiasts want to squeeze out all the power and reliability we can. Leave nothing on the table, right?! We tune the vehicles for maximum performance.
So, how is tuning affected by Octane you might wonder next? Well, that is the crux of it all.
Knock Resistance & Ignition Advance:
Once your engine senses a knock (check out our blog on knock), in most modern cars, the PCM will either add fuel, reduce timing, lower boost, or a combination of all three in order to get the knock to stop. Higher octane fuel is more resistant to knock meaning the potential for more power and consistent power is there (i.e we move parameters close to the limit).
Then there is Ignition Advance. Top dead center (TDC) happens when each piston is at the top of the stroke. Add fuel, spark, and air and you have combustion. If this happens too early, you have the potential for pre-ignition followed by intense heat and pressure which can cause engine damage and so forth. If the combustion even happens too late, then you simply leave power on the table and the combustion event is chasing the piston on the down stroke. So, we want to maximize the timing of this event and higher octane fuel allows us to start this combustion event just ahead of TDC giving us the best chance at getting all the power out of the engine. The reason this is done is because it takes time for the flame front to expand and combust the air fuel mixture. Lower Octane fuel will require us to retard timing(closer to TDC) to maintain safe operating parameters and in turn lower the limit of possible power (mainly through lowering pressure in the combustion chamber).
There are also many other variables that go into tuning like boost targets, air/fuel ratios, etc that impact these equations which we won’t get into here.
So, let’s recap. Octane is a measure of fuel being resistant to combustion. Higher = better. For daily driving, 87/89 or 91 is fine (follow Honda’s recommendation). For performance or track duty, you want the highest octane rating possible to ensure you’re getting all the power out of your sedan or hatch.
The caveat here is that you need to be able to take advantage of that higher octane with an appropriate tune. If you put 91 or 93 fuel in a vehicle meant to run on 87, that simply won’t do much for you besides burn money. Alternatively, an updated tune from Hondata, Ktuner, or your favorite off-the-shelf map provider is the key here as they can tailor these maps to run on specific Octane(read:higher) blends pushing the needle on performance.
A final question you may be asking yourself is, does 93 Octane make more power then 87 Octane? Well, the fuel itself does not make any more power, but when you are tuning your car, a tune able to utilize a higher octane fuel like 93 will be able to approach the limits of your vehicle more so compared to a lower octane fuel. Think of 93 as the benchmark and everything else as a possible downgrade in power. In practice, if you purchase a sick part from us like an upgraded 10th or 11th Gen turbo, you can expect solid gains even running 91 octane, but even bigger gains from running 93 or even Ethanol (read below).
Bonus read: How does Ethanol fuel play into all of this? We love us some “E”! Well, “E85” has an Octane rating of ~105!
That means we can dial up the boost and timing to really take advantage of the fuel's ability to be very stable. It’s not uncommon to see 20, 30 or even 40+ horsepower gains compared to 91 Octane just by making use of this fuel type. Ethanol fuel is also able to absorb heat better once in your cylinders and cooler/denser charge air means more power. Now, you will need a few things to run ethanol fuel like a flex-fuel kit which is basically a sensor to measure ethanol content as well as a Ktuner or Hondata as this fuel requires a specific tune not found from the factory, but it’s so worth it!!!
If you’ve enjoyed reading this piece and want more, just leave a comment below.
Till next time. Mod on!
-Vlad R.
President