Choosing Your Octane

Updated: Nov 6, 2019


When coming for a tune, it's important to have your tank filled with the fuel you will primarily use. We will be using data gathered from burning that fuel to calibrate your ECU. Any other fuel will make the car run differently, and less efficiently.



What is octane, and why do I care?


"Octane", simply put, is a measure of how resistant a fuel is to detonation (Knock). Fuel in a gasoline motor normally burns in a smooth expanding wave, pushing against the piston. Different octanes of fuel will break down explosively at different cylinder pressures, destructively impacting the hard parts inside of your motor. The higher the octane number, the more resistant the fuel is to this breakdown.


Each motor's octane requirement is different, as there are many factors affecting the sensitivity of a fuel to knock...

  • Cylinder Head Design - An efficient cylinder head design provides some turbulence, which helps mix air and fuel for a more consistent stratified burn.

  • Compression ratio - Gasoline engines compress an air/fuel mixture to accelerate the combustion process. The more the mixture of air and fuel are compressed, the more sensitive the mixture is to detonation.

  • Temperature - Heat increases a fuel's knock sensitivity. Hotter air and fuel mixtures will be more prone to detonation. This heat may come from the air / fuel temperature entering the motor, or heat transferred to the mixture from the motor itself.

While cylinder head design and compression ratio aren't subject to change, temperature will vary greatly; as will fuel quality. The OEMs have developed sophisticated knock detection systems to identify fuel instability, and correct for it, before catastrophic damage occurs.


Knock sensors, specially tuned microphones, are attached to the sides or valley of modern motors, to listen for the resonant frequencies produced by knock. The ECU samples these sensors every combustion event. Should the microphones be noisier than they are supposed to be at a given engine speed, the ECU considers the motor knocking, and issues a command for "knock retard".


Cylinder pressure in a motor can be varied by adjusting ignition (spark) timing. To an extent, more ignition advance, before the motor reaches TDC (Top Dead Center), yields more pressure, which generates more torque. If pressure is too high, and knock is measured, the ignition advance is retarded, delaying the spark closer to TDC for the firing cylinder. Power consequentially decreases.


Having the right octane fuel is important for us to be able to get as much power out of your motor as we can, without having to back off timing to avoid detonation.


Is there such a thing as too much octane?



Yes and no... Octane comes at a price. Considering traditional gasoline, switching from 91 octane to 93 octane costs $.20/gal or more. Race gas is even more expensive! It's important to rely on research or your tuner's experience to know how much octane your car really needs, and to understand what fuel meets your goals.


When tuning the ignition timing of a vehicle, you begin with a conservative amount of spark advance and incrementally increase that value until you reach your mean best torque spark (MBTS) value, and stop gaining power, or until you reach a "borderline" amount of advance, where you start seeing knock.


If you can reach your MBTS value, and no additional timing increases power output, you do not need any additional octane.


For example,

On an octane limited motor:

Run 1 - 19deg - 320hp

Run 2 - 20deg - 326hp (up 6)

Run 3 - 21deg - 330hp (up 4)

Run 4 - 22deg - 332hp (up 2) *Knock measured during the pull*

In this scenario, timing is set back to 21*, as we cannot safely go higher. 21* is our borderline. More octane will allow us to make more power. Given the slope of the power increase per degree of timing, we're sacrificing ~3hp peak with this octane fuel.


On the *Same* motor with sufficient octane:

Run 1 - 19deg - 320hp

Run 2 - 20deg - 326hp (up 6)

Run 3 - 21deg - 330hp (up 4)

Run 4 - 22deg - 332hp (up 2)

Run 5 - 23deg - 333hp (up 1) MBT Reached

In this scenario, MBTS has been found, and additional timing will not make any more power.


"But I don't hear my stock '07 RCSB 6.0 truck knock on 87!"


Factory ECUs are clever in their knock detection and remediation, and will compensate for your choices at the cost of performance. I'm aware it says that premium is just "recommended" in your owners manual, and that it's not "required".


A motor will still live a long happy life if it has occasional bouts of knock at low engine speeds and low load... It, however, won't like pulling a heavy trailer, or racing away from a stop light, knocking itself to death. Uncontrolled knock under load leads to heat, which leads to engine failure.


Knowing this, the OEMs have calibrated most modern ECUs with multiple tables to handle varying octane. The factory high-octane ignition tables are designed with enough ignition advance in low load / low RPM areas that they will knock if they are on an inferior fuel. When the ECU hears this knock though the knock sensors, it learns towards a lower octane timing table.


This doesn't mean your car can adapt to whatever octane you're throwing in it -- It just means it can protect itself to some extent.


Comparatively, you would be better off tuning for 87 octane than letting the truck pull timing out. To illustrate that, below are the High-Octane and Low-Octane tables from a 2007 truck's calibration, and a hypothetical explanation of what happens when running lower grade fuels:


Highlighted, are a number of cells that you may find yourself using during a wide-open throttle (WOT) pull. The high-octane calibration calls for roughly 23* of timing. Had this truck been run on 87, it would learn down to the Low Octane table, which is commanding roughly 13* of timing. A 10* decrease in timing is significant on any motor, and will greatly decrease power output.


If the truck above had been tuned specifically for 87 octane, commanding a lower amount of initial timing to prevent knock, it would likely be able to run somewhere in the ballpark of 19-20* at WOT instead, delivering better performance on the same fuel.


At the end of the day, MBTS on this motor is near 24*, and you will not get there without at least 91 octane. There are other factors such as fueling to consider, but on a stock vehicle, 91 octane may more significant than a tune:

  • 24* - Tuned at MBTS Target

  • 23* - Stock calibration with quality fuel

  • 20* - Tuned for 87 octane

  • 13* - Stock calibration on 87 octane

Does ethanol content matter?


Absolutely -- with every 10% change in ethanol content, there will be a 4% change in how much fuel is needed to achieve the right air fuel ratio.


If you were tuned on E10, and you switch to E0, you will be 4% rich.

If you were tuned on E0, and you switch to E10, you will be 4% lean.


Because of the difference in chemistry and energy density affecting your air fuel ratio, it's important to pick one and stick with it (or at least as much as possible).


Many newer cars assume a stoichiometric (complete burn) air fuel ratio of 14.1:1, which corresponds to E10, rather than the 14.7:1 air fuel ratio for gasoline. On these vehicles, E10 is preferred.


The only real reason to chose E0 or E10 definitively would be availability. Consistency is important, and we need to be able to tune for the fuel you plan to use. Pick either, and stick with it. I run E10 in my own vehicles, as I have it more readily available, tune them for it, and it saves me money.


So what should I run??


While the majority of the cars we tune are LS based, we hope to develop some documented guidelines for what fueling to run on different applications. These guidelines aren't absolute, but should give you some guidance on what octane to strive for on different factory setups.


Chevy

  • Gen 3 LS - 91 octane E0 or E10 fuel

  • Gen 4 LS - 91 octane E0 or E10 fuel

  • Gen 5 LT - 91 octane E10

Ford

  • 2v 4.6 - 91 octane E0 or E10 fuel

  • 3v 4.6 - 91 octane E0 or E10 fuel

  • 4v 4.6 - 93 octane E0 or E10 fuel

  • Coyote - 93 octane E10

Dodge

  • New HEMI - 91 Octane E0

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