What is the highest octane recommended for a supercharged engine

[markmurfie]

I always wondered how this would work. Some race fuels have a stoic of 13.0 or so. How far off can the PCM account for?

This is one of those hard questions to answer, because its asked so open endedly.

-stock tune.
Long term fuel trims will learn up to 24% rich, and down to 29% lean before a check engine light. You don't want to rely on the ECU to "learn" the fuel stoich through the trims. Trims break the air model into "Cells" so teaching the model the stoich in all conditions would require hitting each cell for long enough for short terms to learn and correct the long terms. This is slow and in certain conditions like low RPM or high load could be dangerous with the air fuel ratio starting so far off.

-The PCM has flexfuel logic in it.
Flex fuel logic works in a similar matter to long term fuel trims, just with out the need to hit the dangerous areas as it know its learning the fuels stoich, not error in the air model. Stock flex fuel logic,according to your capless fuel neck, is 0%-15%. 14.7:1 to 13.8:1 fuels. Can you get away running a fuel with 15: 1 or 13.5:1 and be fine, probably. Don't start going out side of that though with out a tune for them, it quickly becomes more important than benefits from a boost in octane.

-Probably the answer you should hear in the boosted section of this forum.
Most tunes, aftermarket and OEM for boosted cars avoid O2 sensor feedback learning as much as possible. Windows for check engine lights, limp modes, ect. should get smaller. This is for good reason. Whats in the tank and whats in the PCM really should be in agreement when in cylinder air density is increased so significantly above NA, there is just no room for error and remaining safe.

-No one cares
Air fuel ratio control is the most important thing for making any amount of power, small or large.
A density increase in air and fuel maintaining this ratio is the second most important thing for determening how much power is going to be made and if it is safe or unsafe.
Density of the air doesn't determine the ratio
Density of the fuel doesn't determine the ratio
They are both only one part of the combustion reaction.
The ratio is determined by the molecular makeup of the reactants and products, being balanced before the reaction and after. Stoich is a perfect balance where no fuel or air is left out of the reaction, and only the products of their reaction remain.

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[BrianH87]

Thank you for taking the time to explain that!

 

[EFI]

Some race fuels have a stoic of 13.0 or so. How far off can the PCM account for?

Not enough for that, especially safely. You don't want to rely on a stock tune that's calling for 14.1 AFR to try and run 13.0 AFR. You'll be running extremely lean which is dangerous.

Any fuel that has a different stoich than pump gas will need a tune.

 

[Cory S]

Not enough for that, especially safely. You don't want to rely on a stock tune that's calling for 14.1 AFR to try and run 13.0 AFR. You'll be running extremely lean which is dangerous.

Any fuel that has a different stoich than pump gas will need a tune.

ECU reads from 02's in Lambda and coverts so the gauge reads AFR. This is why the specific gravity of fuels doesn't matter with closed loop fueling.

 

[beetle6986]

With a stage 2 Roush at the stock boost level, I think your best route would probably be a good octane booster like Boostane. It would only take a 1/4 can of professional per table of 91 to hit 97 octane, and you really shouldn’t meet more than that to clean up any knock you might have.

This is pretty much what I follow. I add 3-4 oz of boostane to a tank of 93. I think it puts me around 96 or so. Just a little safety against bad fuel. My car is on an aftermarket 93 octane tune. Without the boostane it usually -1 to +1 for knock and around 16 degrees timing. With just a few ounces of boostane knock is around -3 to -4 and timing is 19-20 degrees of timing.

 

[HKusp]

ECU reads from 02's in Lambda and coverts so the gauge reads AFR. This is why the specific gravity of fuels doesn't matter with closed loop fueling.

So just to clarify for the students in the back of the room who have decided to try and play catch-up, are you saying the ECU can account for bad fuels when using closed loop fueling?

 

[Cory S]

So just to clarify for the students in the back of the room who have decided to try and play catch-up, are you saying the ECU can account for bad fuels when using closed loop fueling?

Define “bad” fuels……

 

[HKusp]

My bad, poor choice of wording. Less than ideal octane rating. The ECU can accommodate for less than desired octane reading and/or lean conditions?

 

[engineermike]

Octane doesn’t change the mixture ratio.

 

[Cory S]

My bad, poor choice of wording. Less than ideal octane rating. The ECU can accommodate for less than desired octane reading and/or lean conditions?

Yes. Active knock adder/subtracting and will always try to target commanded lambda.

 

[TRP45ACP]

My 2017 Roush RS3 on a Roush Phase 2, 727hp and I watch the Knock activity with my N Gauge and my car does good with only 4oz of Boostane with 15 gallons of 91 Octane.
N Gauge shows - knock, so adding timing, so it's happy.
If I don't put any Boostane in and have just the 91 octane, it will show + knock sometimes.
We do have one Fuel Station here that has 93 octane, but my car did not like it.
Just because it says 93 does not mean it's Good Gas.
.

In my experience, when I was running gas, Shell 93 octane V-power is very good fuel.

 

[Markuaw1]

@stanger1 just because it shows negative knock retard doesn’t mean it’s avoiding knock. You have to plot knock retard over time at wot and watch the behavior. If there are sudden increases in retard, even if it stays negative, then it recorded and responded to knock. This can happen whether the correction is positive OR negative. What’s your total timing?

I'm a little confused about what you're saying because when my car is reporting negative knock it is definitely adding timing on the other hand when it is reporting positive knock it is pulling timing so I'm not sure what you're talking about

 

[beefcake]

thats more of an opposite question, question should be what is too low, you really can't have too high of an octane

most important is to keep it unleaded

 

[engineermike]

I'm a little confused about what you're saying because when my car is reporting negative knock it is definitely adding timing on the other hand when it is reporting positive knock it is pulling timing so I'm not sure what you're talking about

The timing control is fairly complex on the engines, but I’ll try to simplify for my explanation. At wot, timing typically starts at the borderline knock value. Then knock advance starts adding timing. It will automatically stop advancing when it hits another limit (preignition, mbt, cylinder pressure) or it hits the knock advance maximum. If it adds, say, 4 deg timing then most scanners will show -4 deg retard. If it senses knock then it will take away typically 1-3 deg from one or all cylinders depending on the intensity and the calibration. So if you’re sitting at -4 deg retard and it step changes to -2 deg, then you just had a knock event, even if the retard number is still negative.

 

[Markuaw1]

The timing control is fairly complex on the engines, but I’ll try to simplify for my explanation. At wot, timing typically starts at the borderline knock value. Then knock advance starts adding timing. It will automatically stop advancing when it hits another limit (preignition, mbt, cylinder pressure) or it hits the knock advance maximum. If it adds, say, 4 deg timing then most scanners will show -4 deg retard. If it senses knock then it will take away typically 1-3 deg from one or all cylinders depending on the intensity and the calibration. So if you’re sitting at -4 deg retard and it step changes to -2 deg, then you just had a knock event, even if the retard number is still negative.

Thanks for trying to clear that up but that still doesn't sound right to me

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