Octane's only purpose is to prevent pre-ignition knock. That's is. Higher octane levels just allows your engine to take advantage of any power adders to increase engine horsepower. These adders could include tweaking timing, supercharging, methanol injection, etc. Think of octane as setting a threshold level for your engine. It sets the operating limits for the amount of power you can generate from various adders. The octane itself does not generate HP.

 

Yes, but.

Yes, using higher octane than required won't increase HP.

But, using lower octane than required will, under certain conditions (high heat, high load, high rpm) cause the ECU to retard timing to eliminate knock. Retarded timing LOWERS HP produced in those conditions. GM recommends 93 for the C8. So using 91 will, under those conditions, cause the timing to be retarded and reduce max HP.

 

Yes, I understand this. We're singing from the same sheet of music. The octane will limit how much HP you can use for the current conditions you mentioned. Octane limits what HP you can safely use before causing engine damage. The ECU is monitoring the combustion process and power output. Think of hearing knock as causing internal damage to your beautiful engine. Your engine ECU will try to prevent you from doing this.

 

Got a link that says higher octane means slower burn?

 

Octane is there to prevent post-ignition knock. If you've got pre-ignition knock, you have other issues not related to octance.

 

Let me take you back to what I was responding to: "Octane has ZERO to do with horsepower. ZERO. ZERO. ZERO." Those are your words. Octane clearly affects the horsepower your engine produces. You set up your engine and tune it to 93 octane like the LT2, and give that engine a control that will reduce spark timing to some degree if it detects knock. Now you reduce the Octane to 87 Octane, Under high load, the engine will produce knock causing timing to be pulled, it will produce less what? Less Horsepower, so yes Octane has a more than zero impact on Horsepower. If what you sad were true, any change in Octane would have no impact on horsepower. To take it to an extreme, if somehow you could find 65 Octane gasoline and run it through that engine under load, after the ECU pulled all the timing it could and you continued to run it underload, the knocking would eventually damage the engine so bad it would become undrivable, and produce no horsepower. Your premise is false.

The other reason we know Octane has a non-zero impact on power is because GM (the manufacturer of aid engine) tell us so directly in the manual. Who should we believe, GM and physics, or you? I know who I am going with.

 

Second paragraph:

https://www.eia.gov/energyexplained/...e-in-depth.php

 

I'll confess I'm not seeing a reference to burn rate. Can you copy and paste the relevant part?

 

So you're not making a connection between fuel stability and burn rate?...you should.

 

No.

Show me in either of the two links mentioned in this thread where octane and burn rate are inversely related.

 

https://www.thoughtco.com/definition...-number-604586

 

This is somewhat accurate. Using 91 alone does not promise to reduce power or HP. It depends on several variables, like what is the outside temps. How hard is the car being pushed and so forth. The ECM uses 2 main timing tables, a low octane and high octane timing table along with several modifier tables. The ECM starts off in the high octane table and as long as there is no detonation/KR recorded it stays there, if it sees knock/detonation/KR then it drops into the low octane timing table until the knock event decays out, then it transitions back into the high octane timing table.

During the winter and cooler months you will be able to use 91 with zero issues, during the summer months and warmer times the ECM might see some light KR events. While GM does test and certify the LT2 on 93 the car has to be built to be able to run on 91 since the entire USA does not have access to 93 octane these days. I live in OKC and I can maybe name off 5 gas stations in a 50 mile radius from downtown OKC that stock 93 octane at the pump.

 

Here (paragraph #4), you need more? I can get as many as you need.

https://nasaspeed.news/tech/engine/o...es-more-power/


Here (paragraphs #7 & #10):

https://www.hemmings.com/stories/201...r-priced-fuels

 

This is wrong, octane is used to stop pre-ignition knock. It does this by slowing the flame front down so they dont collide inside of the combustion chamber and cause knock. The higher the octane of fuel, the slower that fuel burns, that is exactly how octane works.

Think of it this way. There are sharp edges inside of the combustion chamber (edges of valves, edges of pistons, edges of valve reliefs in pistons, tip of the injector and so on) and those edges start to heat up faster than the flat surfaces do inside of the chamber, more so when you add more timing to the combustion cycle. As you add timing it can cause the combustion cycle to begin before the piston reach TDC. Add in some of those really hot edges I just mentioned and then inject in some fast burning fuel and what end up happening is the edges of those items ignite the fuel/air mix before the actual spark plug does. This starts the flame front across the piston, then the spark plug ignites the fuel/air mix as well. Now you have to 2 distinct and separate flame fronts moving across the combustion chamber, when they collide it causes "knock/detonation" which is the pinging noise you hear. The damage that can happen under severe knock is like taking a ball peen hammer and hitting the tops of the pistons. It can leave mark and remove material as well.

There are several ways to combat this, either reduce compression (loss of HP), reduce the ignition timing (loss of HP) or use a higher octane fuel that burns slower. If the fuel alone cant control knock then either the ignition timing has to be reduced/retarded or the compression has to be reduced.

That is how octane and ignition knock relate in a nutshell.

 

I read as far as the first bolded, and then stopped. Preignition is not an issue in even-moderately healthy engines. If you do have pre-igniton, you have engine problems that need to be addressed, not octane issues.

I read a little farther. I bolded the second thing that made me shake my head. In any correctly timed engine, combustion ALWAYS begins before TDC. Where are you getting the idea that there are engines that don't, or aren't supposed to, start their combustion cycle before TDC?

The third bolded: Just FYI, I spent several years during my career flying to automotive proving grounds around the world doing pre-production engine knock detection/calibration/evaluation on a wide variety of engines. This topic (knock/detonation/timing/octane) is not just a hobby for me. It was part of my career.

 

That link says nothing about burn rate.

 

That link was for your edification as you cleary not familiar with octane. My other 2 links did address burn rate.

 

Seriously? Hemmings? I suspect the guy just has a journalism degree, not an engineering degree, as he's spewing crap too often in his article.

Quit wasting my time posting nonsense from non-experts. Find something from a fuel manufacturer, or a genuine automotive engineering facility that can back up their claims with data, and doesn't just spew out old wives' tales.

 

Read the last paragraph of post #35.

 

OK, time for you to produce any credible reference or experimental evidence that you have stating fuel burn rate is not diminished with higher octane. I figured you would come back with your lame response. I suspect you have no scientific background.