There is a double edge sword when the ECU decreases timing due to lower octane fuel. When the ECU starts to pull or retard timing, combustion chamber heat rises which can cause detonation. The ECU can only pull so much timing. Air intake temps, coolant temps, ignition timing, octane, cat temps all contribute to combustion chamber heat. The ECU will pull timing when air intake and engine coolant reach set temps. When the cats reach high temps the ECU adds more fuel to cool them down and run efficiently. Not only lean, but overly rich condition can cause detonation. Adding lower than required octane fuel and you have a perfect storm. A **** poor performing engine on the edge of cracked ringlands or pistons.

The GM engineers require 91 and above octane so the engine can operate at a safe threshold and be managed thru ECU programming. Using lower octane can bring the engine below its safe operating threshold. It's your engine, experiment as you want, but understand what is going on inside an internal combustion engine and its programing.

 

Indeed. I only put Shell V-Power in my C8 as I did my C5 Z06 before it, and every performance vehicle (2 or 4 wheel) I have had. Expensive? sure, but it comes with the territory....I really don't pay attention to pricing since I always use the same fuel. But as Ed mentions, only 93 octane for me, for sure.

 

The ECU can also limit airflow (throttle) into the engine to limit cylinder pressure--which eliminates pre-ignition and detonation.

 

Let's say you drive 10,000 miles a year (which is probably high) and average 15 mpg all around. That means you use 666 gallons of fuel in a year. If premium costs an extra 50 cents a gallon, you're paying an extra $333 a year to use the right fuel. IF you drive that much.

If, like most owners, you drive a lot less, say 5,000 miles a year, you'd spend an extra $167. Even if premium costs a dollar more, that's still only $333. If you can't afford to use the right gas, you can't afford a Corvette.

I think it was Ben Franklin who coined the phrase "penny wise and pound foolish." Sheesh.

Actually, no you can't. Your "tried and proven" method was only marginally useful before the days of knock sensors. Today, if the knock sensors are working correctly, you'll never hear the sound of the engine knocking, and you'll have no way of knowing if the ECU is retarding timing, limiting performance and economy. And even before knock sensors, you couldn't realistically test under all load and temperature conditions. It's fine if 89 octane works in easy suburban driving. That doesn't mean it's going to be fine when the temp is 100F and you're really pushing the car hard.

Is it really worth it to save a pittance? (see actual math, above.)

 

Here is some information from the 2020 Service Manual regarding the operation of the knock sensors. Apparently there are two, and apparently they are piezoelectric vibration sensors.

 

Did they check in high temps, with the engine running near maximum temps and under load? Do you rally think GM recommends premium because they want gas stations to make more money?

 

Exactly. But there might be a way to tell - I am not sure. The 2020 Service Manual talks about DTCs and I think one of them may be for excessive retarding of the timing. And it looks like with a tool there are parameters regarding min and max retardation, for knock alone, and another for overall adjustment (temp etc). I did not include that last one, but below are the other excerpts from the manual about this. Someone more familiar with the DTCs and the diagnostic tools can better determine if this is right.

 

Every car I've owned since 1990 has required 93, or 91 if that's all one can get -- that's 7 Vettes (979,000 miles), a Ferrari (40,000+ miles), and a Porsche (DD).... if one can't afford the gas for one's C8 perhaps making a "Garage Queen" out of it is a good idea.
As for me?

I'll continue to drive them if gas goes to $10 a gallon, and I'm retired on a fixed income.

 

I bet someone with a Bluetooth OBD adapter (like this:https://www.obdlink.com/products/obdlink-mxp/) and the Torque Pro app could determine if timing was being pulled if they wanted to make the effort (I have one but I'm lazy and barely familiar with Torque Pro). It's a lot cheaper than a dyno.

It would be interesting to see if the 2020-2021 MYs pulled timing when running 91 octane on the street since that is all that is available in many areas.

Also, I wonder if the information is available in the PDR data when using Cosworth PI Toolbox.

 

[QUOTE=Chotis Bill;1604842376]There is a TV show "Engine Masters" that did a show on octane. They compared California 87 and 91 and Sunoco 110, 116 and E85. They used a LS3 6.2 with 10.7 CR. For each fuel they did FR sweeps, timing sweeps and adjusted for max HP. They found all octanes made the same HP except for the E85, timing didn't change for any fuel even the E85. The 87, 110, and 116 had a FR of 12.8 while the 91 had a FR of 13.1. The E85 has a different scale. ​​​​​​


Exactly, dyno run in a controlled environment is not real world results.

My stock 2008 C6 LS3 pulled timing with 91 octane on a 75F* day. I ran data logs to verify it. I added Boostane to increase octane to 93-94 and no timing pulled, 0 K.R.

The LS3 will pull timing once IAT's (intake air temps) reach 86F*. How much? 1-3* and increases as IAT's go up. We are not talking ambient air temps, but IAT's.
The LS3 will pull timing once ECT's (engine coolant temps) reach 212F*. How much? 1-2* and increase as ECT's go up.
Add the above with 87 octane and you looking an engine that is not very happy, especially on a warm day.

I have not read the C8 tune yet, but expect more of the same.

 

If you run 87 octane and the engine doesn't knock all this nonsense about knock sensors and ignition timing is irrevelant! You guy still don't get the OP!

 

And you don't get that you can't tell if the knock sensors are retarding timing or not. If you don't care that the engine is running inefficiently, producing less power and using more gas then it should, then fine. But most of us want the car to operate as it was intended. I think it's you who don't get it. Why did you spend $80K or whatever on a car just to handicap it with shitty gas, and take a chance that when you want to have fun with it the knock sensors won't be able to fully compensate for your cheap-*** ways?

Spend the couple of hundred bucks a year and use it as designed.

 

http://bndautomotive.com/aces-formul...oline-formula/
Octane increase
87 to 96
89 to 98
91 to 100
93 to 102

 

Hey Dude, we do get your OP... but speculation about if the engine knocks and if it's OK while running on low octane fuel is just plain not cool. People are worried about gas prices and this is playing into hysteria. Think of the kids and impressionable boomers looking to save a few bucks and destroy their car in the process!

The facts are simple... the engineers specified 91+ octane for a reason. Fancy gadgetry can only protect the user so far. At some point the OEM is allowed to throw up their hands and say "no more!" (no more warranty for those who knowingly run bad fuel)

 

It's a $100,000 car.
if paying 30 cents per gallon more to put the proper fuel in the tank bothers you , you might have the wrong car.

 

You gotta make that special mixture if your state has it. Fill up on 93. Get to a half tank, add half a tank worth of 91. Lose a quarter tank, add 87. It is going to be extremely close to that 91, but you can squeeze out some savings.