Octane effect on MPG

[tmorris1]

Ethanol actually increases octane, so maybe the E15 starts with an even lower octane base fuel.

No. The extra 5% ethanol provides the increase

 

[KYMISTWALKER]

Chemically, lower octane has more energy.

Then why on a turbo car or engine do they recommend 93 octane

 

[WeyounDS9]

Chemically, lower octane has more energy.

This has been casually bandied about on this website but its all just kinda/sorta true.
Yes, the inferior fuel may in some cases have more btu (sligthl) but the higher octane fuel can get more energy extracted per gallon.

 

[y_p_w]

This has been casually bandied about on this website but its all just kinda/sorta true.
Yes, the inferior fuel may in some cases have more btu (sligthl) but the higher octane fuel can get more energy extracted per gallon.

It’s not a direct correlation. Octane rating can be boosted by using 10% ethanol at a cost of maybe 3% lower BTU/unit volume. There are many ways to increase octane rating using components with different energy content.

 

[WeyounDS9]

It’s not a direct correlation. Octane rating can be boosted by using 10% ethanol at a cost of maybe 3% lower BTU/unit volume. There are many ways to increase octane rating using components with different energy content.

Right. so as I said. even though the btus may be lower, the higher Octane rated fuel can get more energy etxracted.
I will add this for clarity: the higher Octane rated fuel can get more mechanical energy extracted.

 

[CleanSump]

Then why on a turbo car or engine do they recommend 93 octane

To prevent pre-ignition or pinging.

 

[melk]

My 2010 Northstar consistently turns in 2-3 mpg better with 93 Octane on our long road trips.

 

[GSCJR]

Manual says 91 premium fuel is required, but there is ample evidence of people going 300,000 plus miles on this model with regular 87 octane and I'm hard pressed to find real world examples of damage

Our is a 2010 RX350 that has been on a steady diet of regular unleaded since new. When my wife got it from her parents, it had 70 or 75k kms, which is when we switched it over to Shell almost exclusively. Beginning in the summer of 2024, I began treating the fuel system with V-power 91, but only during the summer months. It currently has 245k kms and has never skipped a beat, it still has the original spark plugs and engine coolant, the 2GR-FE has a good reputation. I've changed the oil every 5k kms except for once due to the fact that it is a short tripper. An air filter and serpentine belt are the only things I've changed when it comes to engine maintenance. Approximately every 2-3 years I do a complete drive line service; that's it! It now sits in the driveway awaiting a for sale sign, because my inlaws gave my wife their 2020 RDX 2.0L turbo (Premium fuel recommended) with 90k kms.

 

[Fabulous50s]

I would highly recommend replacing the original spark plugs at 245,000 km. Those gaps would be huge by now! Thats not helping your ignition coils. That coolant is shot at 16 years old too. Should have been changed 2x by now.

 

[y_p_w]

Right. so as I said. even though the btus may be lower, the higher Octane rated fuel can get more energy etxracted.
I will add this for clarity: the higher Octane rated fuel can get more mechanical energy extracted.

Relative to energy content though. There’s still energy in the fuel, and many engines have higher efficiency by taking advantage of advancing timing. But higher efficiency can’t necessarily overcome less energy. Like E85 is never capable of the same fuel economy as E10, although it’s pretty high relative to the theoretical amount of energy of the fuel.

 

[y_p_w]

To prevent pre-ignition or pinging.

Knock sensors prevent that. But if it has to retard timing to do that, performance and fuel economy goes down.

 

[CleanSump]

Knock sensors prevent that. But if it has to retard timing to do that, performance and fuel economy goes down.

Knock sensor react to preignition, they don't prevent it.

 

[y_p_w]

Knock sensor react to preignition, they don't prevent it.

Pre-ignition is generally pretty bad when it happens. But mild knocking is usually from detonation. And most knock sensors can detect knock well before it’s noticeable to the human ear.

 

[WeyounDS9]

Relative to energy content though. There’s still energy in the fuel, and many engines have higher efficiency by taking advantage of advancing timing. But higher efficiency can’t necessarily overcome less energy. Like E85 is never capable of the same fuel economy as E10, although it’s pretty high relative to the theoretical amount of energy of the fuel.

E85 is an extreme examples as it smostly ethanol.
Looking for apples to apples comprison.

 

[fins]

At my local station E15 88 is 30 cents lower than E10 87

 

[y_p_w]

E85 is an extreme examples as it smostly ethanol.
Looking for apples to apples comprison.

There are way too many variables, where the base fuel is likely the most important. But for a direct comparison, it would require identical energy content, then applying whether or not higher antiknock properties helps timing advance. And even then, there are more variables since antiknock properties can vary depending on the load. But this has been simplified into research and motor octane.

 

[Mackelroy]

I have over 100,000 miles on my 2011 RX350 and have the following observations which are completely counterintuitive. Manual says 91 premium fuel is required, but there is ample evidence of people going 300,000 plus miles on this model with regular 87 octane and I'm hard pressed to find real world examples of damage. Here is what I find for driving 72-3 mph on interstate highways with wind not a factor:

On premium 93 octane: 23 mpg. The engine definitely has more torque and idles smoothly.
On regular E10 87 octane: 23.5 mpg. Notice minor loss of power and idle isn't as smooth
On Regular E15 88 octane: 24.2 mpg!

To me this makes no sense, but I swear it is what I've consistently seen. I thought it could be that premium gas has lower energy content (Btu/gallon) than regular but a refinery guy on BITOG provided documentation that this is not the case. And E15 has to have the lower btu content than the others but it gives the best gas mileage and is lowest cost. Just seems to be a sweet spot.

You would think my turbocharged EcoBoost Fiesta would benefit from higher octane even though it isn't required, but I can't say I can see a difference. I had a 2002 Nissan Maxima that recommended premium, but I had similar experience as with the RX.

Any technical explanation how this could be?

My son 30 years ago got a new Mazda, I told him not to run 87, even though says its does.

I drove the vehicle one time, and it was clattering all the time under load , yep he was running the cheap stuff.