Does the C7 Corvette’s engine control system allow it it take advantage of say 93 vs 91 octane gas? Thanks, Don.

 

When I took delivery the dealer put in a tank of 87 unleaded and it didn't seem to do anything different when I started filling up with premium. I could not hear any audible knocking with regular in the tank

 

Normally, you do not hear knock because the knock sensors reduce timing. Significant knock will cause the ECM to reduce timing values to lower timing table. In others words, reduced performance.

 

I understand that, still interested to know if the ECM can take advantage of higher octane to provide better performance. Don

 

The dealer put 87 oct in your C7?

Even the fuel door states "premium fuel recommended". It will run on 87 oct, but that is pretty cheap for a dealer to fill up with less than what GM recommends. ECU will just switch to the low oct timing map...

 

The advantage with 93 vs 91 octane, as long as you do not experience any K.R., the ECM will remain in the higher timing map performing at its peak potential. However, higher octane gas, 95+ ignites a bit slower requiring more ignition timing. So to really take advantage of higher octane gas you may need to add timing to your tune.

 

The octane rating of a fuel has no correlation to its flame speed...higher octane fuel does not burn slower

 

It's more a matter of withstanding compression/boost before detonating, rather than rate of combustion, but rate of combustion is affected by octane a certain degree.

 

I Disagree:

Quote from "Engine Management Advanced Tuning" by Greg Banish. page 58, Paragraph 2. " Fuel octane is a number representing the ratio of octane to heptane in gasoline. Higher octane numbers indicate a stronger concentration of the slower, more stable burning octane molecules than the more volatile heptane. Since high-octane fuels have a slower burn rate, larger amount of ignition lead can and should be used..."


Maybe TurboLX will chime in and add some more of his knowledge?

 

I think Greg Banish may be confused about "octane". It's not added to fuel as a way of increasing the "octane rating" or resistance to detonation. Actually, the octane rating of "pure" octane (referred to as n-octane) is -17...kind of a paradox isn't it.

The Parrafin family (also called alkanes) have open-chain chemical structures with the general formula C(n)H(2n+2).
(Ignore the dots, it was the only way I could make everything line up. The vertical lines link the H atoms to the C atoms.)
...H
....l
H-C-H would be Methane CH4
....l
...H


...H..H.H..H.H..H.H.H
....l...l..l...l..l...l...l..l
H-C-C-C-C-C-C-C-C-H would be Octane C8H18
....l...l..l...l..l...l...l..l
...H..H.H..H.H..H.H.H

These normal members of the parrafin family are identified by the prefix "n": n-octane. Isomers of each normal member of the family have the same chemical formula as the normal member but with different molecular structure and properties. Isomers have the suffix "yl" added. For example, the name 2,2,4 trimethyl pentane is one specific isomer of octane. Tri means three and methyl means the radical CH3. So three methyl groups are attached to the pentane base (5 carbon atoms) at carbon atoms 2, 2, and 4 (numbered left to right) giving:


........CH3....CH3
....H....l...H....l...H
.....l....l....l....l....l
H--C--C--C--C--C--H
.....l....l....l....l....l
....H....l...H...H...H
........CH3
This is the structural formula for 2,2,4 trimethyl pentane or iso-octane. These terms are used synonymously since this particular isomer of octane is of importance in combustion engine work as you'll see below.

The critical compression ratio of the parrifin family for audible knock in a spark ignition (SI) engine decreases rapidly as the length of the chain of the normal members is increased. The normal parrafins in the volatility range of gasoline are poor SI fuels due to very low octane ratings...remember n-octane has an octane rating of -17. Catalytically cracked fuels have branched parrafins (like above) making the carbon chain shorter resulting in higher critical compression ratios.

Finally, the octane rating is a reference scale to measure SI knock and has been estabilished by arbitrarily selecting two primary reference fuels. They are isooctane (2,2,4 trimethyl pentane) which has been arbitrarily assigned an "octane rating" of 100 and n-heptane which has been arbitrarily assigned an "octane rating" of 0. The "octane rating" of a fuel is found by comparing its' knock intensity with various mixtures of n-heptane and isooctane. So an octane rating of 93 means a fuel has the same knock intensity in a standard CFR engine (google "astm CFR engine", you can vary compression ratio while it's running ) and at standard conditions is equivalent to a mixture of 93 parts isooctane and 7 parts n-heptane (by volume). Gasoline is made of up to 500 different compositions, it isn't a simple ratio of iso-octane to n-heptane...Mr Banish has a distinct lack of knowledge when it comes to gasoline but if you want to take what he says as gospel, you're welcome to ignore the facts too.

The octane rating of any fuel has absolutely nothing to do with the flame speed of the fuel. Octane is a measure of a fuel's resistance to detonation and that is ALL.

Check out this link:
http://www.faqs.org/faqs/autos/gasol.../preamble.html
The last line of paragraph 6.3: "Flame speed does not correlate with octane."

More good reading:
http://www.faqs.org/faqs/autos/gasol...section-1.html

Also, as flame speed decreases, the mixture is more susceptible to detonation since the end gases have more time for pre-flame reactions to occur which is counter to what you want with higher octane fuels. Remember, combustion is just a chemical reaction that releases heat.

Lead in the old days delayed the preflame reactions to increase detonation resistance and get octane ratings higher than 100. Adding the fuel n-octane would have the opposite effect.

And by all means, have TurboLX chime in and refute these facts.

 

Pretty smart guy...you should tell Banish how to tune while your at it. I know I am impressed. I always listen to folks that have to knock some else to make themselves appear wise.

 

That was a completely inappropriate thing for the dealer to do. I think you should have taken it back and told him to siphon the garbage out of the car and give you $ for an additional free tank afterward.

Maybe the ECM can deal with 87 octane but if it has issues and doesn’t pull the timing back, the engine could suffer. Don

 

Sorry, but "glass slipper" is spot on.

 

I love this detailed presentation on gas and octane and compression. I agree and confirm. As the director of one of the largest Ethanol development programs in the US (in Illinois in the 1980s) we worked a lot with the blending of gas and ethanol to change the fuel's ability to withstand compression without self-ignition. We now have blended fuels with ethanol to help "octane numbers". unfortunately we also got real crappy gas in that same process for a host of reason all having to do with the ethanol "fuel" characteristics. I say "fuel" because anyone who studies this in detail knows ethanol will never be a great fuel in a spark ignition engine that also needs to keep NOXs low (low compression). Needless to say, as an old "expert" I stay as far away from these ethane blended fuels as I can get in my C7. Again, thanks for this very super great post. Nick

 

TurboLX may want to refute, but I am not nor qualified. Thanks, glass slipper for taking the time with a thorough explanation.

 

Whew, I bet you're sorry you asked !!! I'm no chemist or engineer, but I would like to know if the C7 or any ECM controlled engine will adjust the timing back up after you've run on 89 or lower octane fuel?

 

I believe that the system will only retard timing if it senses knock. It is always “listening” for knock/detonation and will leave timing alone if nothing is sensed.

With 11.5 to 1 compression, I won’t ever use less than 91 octane unless I’m out of gas and have no other choice, an unlikely situation for me. Don

 

Tuners typically optimize the tune for a specific octane. If you optimize the tune for 93, you may get knock detected at 91. The retard that happens when knock is detected will normally over compensate. The best tune will have minimal or no knock detected at the desired octane.

If you (or GM) optimizes the tune for 93 octane, you'll not have the best tune possible for 91 octane (and visa versa). The OP's question is a good one, did GM optimize the tune for 91 or 93 octane. I would guess it is 91.

 

I read the C6's were tuned for 93. This can and did become a problem when I experienced knock retard on my LS3. 91 octane is the highest in CA. Yes, the knock detected will normally over compensate. Significant knock retard will put the ECM in the lower timing maps. Not sure for how long or how many start up cycles? A custom tune will clean up these issues.

I do not know about the octane tune on the C7? However, direct injection will allow for more compression ratio due to direct fuel spray cooling in the combustion chamber. This is one reason why the C7 is running 11.5:1 compression.

Interesting enough: If the GM tune on the C7 is 91 octane, then the aftermarket tuners have taken advantage with the 93 octane in their gains. I am sure those gains are less in CA with 91 octane.

 

While I certainly appreciate someone using my book as a reference, please keep in mind that it was written not as a college text, but rather as a medium to educate those with little or no engineering background about the underlying principles of engine calibration. I intentionally avoided going too deep on some subjects to keep from losing interest from the majority of readers. As it stands, I already get enough complaints along the lines of "it's too technical, I just want to know X..." When I have questions, I reach to my copy of . It's 930 pages of college textbook glory and hasn't failed me since my days in engine design class nor through my current role as an OEM calibration engineer.

Now back to the topic at hand. The quote from my book above is taken a little out of context here. I wasn't referring to the difference between 87 and 93 octane pump fuels, but rather pump premium versus dedicated race fuels such as VP C16, where there is a radical shift in the blend of chemicals. Many race fuels have higher percentages not only of 2-2-4 trimethyl pentane, but also things like aromatics (think Benzene rings) that are even harder to break than simple H-C chains. Changing fuels certain DOES have an effect on laminar flame speed, as seen in Heywood's figure 9-25 on page 403:



Even there, one can see a difference in flame speed between isooctane and "gasoline" with a mean difference of roughly 4cm/s. (table 9.2 shows isooctane at 26.3 cm/s and gasoline at 30.5 cm/s) So changing which chemical cocktail one burns in the engine can have significant impact on required timing if you wish to maintain a CA50 number of roughly 7-9dg ATDC under the same conditions. Am I being specific enough here? I think it's no secret that pump gas is a cocktail of chemicals, which includes some octane, but also plenty of other things like detergents, stabilizers, light ends that evaporate over time, oxygenates, etc. Each can have its own effect upon final performance, so you'll see that factory calibrators spend a lot of time making sure the ECU is flexible enough to handle these, especially as seasons and fuel specs change.

Back on point again. To the original question... To the best of my knowledge, while I was at GM "premium fuel" for us as 91 pump octane. This means that cars with either "premium required" or "premium recommended" had their high octane spark tables calibrated for 91 octane pump gas and the low octane table for something less (usually about 86 or 87 pump number). The ECU has a knock learn factor that is uses to determine how far in between these two tables it should be at any given time. In the presence of knock, one would see this knock learn factor swing toward the low octane setting. If after some time and higher load events without knock, it would creep back toward the "good fuel" table. The GM logic did not have the ability to add base spark beyond the high octane table values like some other OEMs do.

That's all great info, but what about my car? Can I add timing and make more power?

Probably not. It has been my experience that N/A engines like this don't gain much reliable, consistent power from adding the last one or two degrees of spark advance available at full load due to the difference between 91 PON and 93 PON. You'll see bigger differences run to run than the actual change due to improved cylinder pressure profile if you're honestly running the tests under identical conditions for IAT, ECT, EOT, cam position, lambda, and sweep rate.