A Different Octane Question!

[MikeSho]

So the 3.5L in my 2015 has a compression ratio of 10:1.

If I remember my compression basics correct, at 10:1 the engine should be screaming for 100-Octane Gas. If 10:1 runs on 87, the timing must be retarted quite a bit.

I take it this is one of the main purpose of the tunes? To advance the timing to take advantage of the compression potential?

 

[mknittel]

I assume there is a logical timing curve based upon detonation detection. I know I see at least a 2 MPG difference when I run 93 vs 87 in the summer, and since octane by itself does not equate to milage, I have to believe the spark curve is retarding due to detonation on the 87 in the summer. Interestingly enough in the coldest of months in the Missouri winters I can get away with 87 with little to no difference in mileage between the 93 and 87, but that could be some other factor than the colder denser air in the low 20s.

 

[MikeSho]

I assume there is a logical timing curve based upon detonation detection. I know I see at least a 2 MPG difference when I run 93 vs 87 in the summer, and since octane by itself does not equate to milage, I have to believe the spark curve is retarding due to detonation on the 87 in the summer. Interestingly enough in the coldest of months in the Missouri winters I can get away with 87 with little to no difference in mileage between the 93 and 87, but that could be some other factor than the colder denser air in the low 20s.

I know they have basic knock sensors, so that definately sounds plausible. I wonder how agressive it can self-adjust the timing.

 

[StealBlueSho]

So the 3.5L in my 2015 has a compression ratio of 10:1.

If I remember my compression basics correct, at 10:1 the engine should be screaming for 100-Octane Gas. If 10:1 runs on 87, the timing must be retarted quite a bit.

I take it this is one of the main purpose of the tunes? To advance the timing to take advantage of the compression potential?

Yes. The 2010+ SHO (and most cars at this point) use knock sensors to adjust spark based on detonation. This is called Octane Adjust Ratio or Learned Octane Ratio...

Basically the ECU will keep adding spark until it the knock sensors report excessive knock. Which is why there is an * next the to power rating on most cars stating “93 octane”...

So if you a running 93 octane and your wife puts 87 in it the knock sensors will start lighting up and the ecu will drop to a less aggressive spark table. Or vice versa...

Of course in the programming it will only allow it to adjust timing to certain point so there is a limit to how much advance you get even on 93.


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[krewat]

To add to what's already been said. Variable cam timing has a lot to do with the "dynamic compression" and what you can get away with, octane-wise. 10:1 is the "static compression". Fiddle with cam timing, and you can get an 11.5:1 to run on 93 octane - The 390 I built for my '74 highboy was like that

 

[brucelinc]

There is a diminishing return with octane. As Steal says, there is a limit to the timing advance. For example, the stock 3.5 ecoboost cannot take advantage of 100 octane fuel. I have experimented with higher octane than what my car was tuned for and it actually hurt performance.

 

[StealBlueSho]

There is a diminishing return with octane. As Steal says, there is a limit to the timing advance. For example, the stock 3.5 ecoboost cannot take advantage of 100 octane fuel. I have experimented with higher octane than what my car was tuned for and it actually hurt performance.

True... there are a couple schools of thought with the Transverse 3.5L Ecoboost...

A lot of tuners suggest that 23degrees advance under WOT is the most aggressive you should go due to some design limitations with the motor. I know there have been a couple tuners who have dyno tested that... the results were basically you the cylinder pressure went up significantly more vs the extra HP you get... IE it’s not worth going further... tune to 23 degrees and then add boost if you need more...

Other school of though is MBT tables for the 3.5 suggest that 23 degrees isn’t a stopping point... and that the age old rules of MBT are still valid. Which suggest under certain loads spark advance can still yield significant gains past 23 degrees...

Either way, most people will be fuel starved before they are knock limited on 93 octane. So as Bruce said.. anything more than 93 on a stock tune is a waste of money.


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[rubydist]

The other piece of the puzzle that I did not see addressed above is that aluminum heads typically allow 1+ additional point of compression ratio compared to iron heads. The "old school" compression ratio v. octane rule that you mentioned in the first post is for iron heads.

So, the combination of aluminum heads, variable valve timing, knock sensors with dynamic timing adjustment capabilities, improved combustion chamber design etc. allows for a lot higher static compression ratios than that "old school" rule would suggest.

 

[MikeSho]

Thank you for all the responses!

Lots of great information! The car I learned to work and drive on was a '65 Mustang I restored (1990-2000) so I have a lot of learning to do regarding the newer technologies.
Ahh memories, back when I could fill up with 100LL at my local airport for $2/gal.

 

[mrhighcaliber]

The main contributor to this engine being able to run 87 with a 10:1comp with boost on pump gas is the direct injection. DI cools the combustion chamber which greatly increases knock resistance. Making regular 87 behave more like 91+ premium.

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