Lots of us are concerned with gas prices and our C8's needing 91 or higher octane per Corvette. That's the expensive gas for sure. So do you really need 91 octane?

Octane helps to reduce spark knock or premature detonation within the cylinder. Every time you hear a ping (knock) what is happening in the cylinder is that something other than the spark plug ignited the gas prematurely.

This is bad! Imagine a flat piston top being pounded by a hammer. That's what happens because the piston hasn't reached TDC and the blast wants to push the piston back against the crankshaft instead of with it. Not at all good for rods, bearings or crankshafts either!

Here's what you can do to test if your engine needs 91 octane. Put some lower octane in the car and listen for pings. The best way to test this is to floor the accelerator in a gear where you are just lugging the engine. If it pings under these conditions, you need more octane. If not, try some even lower octane. Eventually you will find the level of octane your engine needs to operate without spark knock.

Let the critique begin...

Dennis

 

Any modern engine will run off of lower octane gas by retarding the timing. You likely won't get any knock or ping, but will lose power.

 

https://www.corvetteforum.com/forums...mium-fuel.html

 

I have had every model except the C1, C2 C3 needed the good stuff I have never put a dime into anything but regular for my C 4 5 6 7 8 and never had an issue absolutely nothing !!

 

My owners' manual says required 93 octane. I buy SUNOCO Ultra 94. Yes, gas is expensive now. However, so is your Vette.

 

Thats about the dumbest thing I've read so far on this forum. To purposely use low octane gas to save a few dollars.
Just sell the car and get a cobalt.

 

Changed in the 2022 Owners Manual:

 

I'll just continue to use the proper/recommended octane rated gas for my car - no desire to do a test. At approximately $0.50/gallon difference in price for premium, I'll just spend the extra $9/ tank.

 

My guess, and that's exactly what it is.. A GUESS .. is that some part of the reason for the 93 octane "requirement" called out in the manual (2020s and 2021s) but will accept 91 is that the knock sensor being used or the programming itself can't/won't retard the advance to accommodate lower octane than that.

 

Another guess is that the way the "requirement" is written in the Owner's Manual is affected by EPA emissions and fuel economy protocols. My guess is that the manufacture must recommend the same (R+M)/2 as was used in certification tests, Part of these tests are used, in a rather complicated way, to derive the sticker mpg and make CAFE calculation. Modern engines with ECMs will produce slightly more power and have higher fuel efficiency (less CO2) with higher (R+M)/2 gasolines. The EPA is strict and vigilant on these matters. They would see certifying on one gasoline and than advising owners to use a different gasoline as cheating.
Besides, as others have pointed out, spending the $$$ to buy a C8, and then saving a few dollars using a lower (R+M)/2 gasoline, seems like a strange choice.

 

I think there is a difference between pre-ignition and detonation. At least many web sites say so, and it might explain why going below 91 octane is a problem. In a normal situation, timing is advanced well before top dead center of the piston, but it takes some time for the air fuel mixture to fully ignite, and expand the gases to push the piston back down. Timing is adjusted so that the pressure is fully developed at or after TDC. If the pressure is well developed before TDC, you get the ping or the knock. That is where either pre-ignition or detonation comes in.

Pre-ignition happens when the air fuel mixture ignites spontaneously and before the spark. It can be due to high compression (the way a diesel ignites) or some hot deposits in the combustion chamber. If the octane rating is too low in a high compression engine, that can happen. In that case, reducing ignition advance timing doesn't help because the spark is not what ignites the fuel. Maybe that is what will happen if the octane gets below 91, but I do not know that for sure.

Detonation is when the air-fuel mixture burns too quickly (after ignited by the spark) so that the maximum pressure is happening well before TDC. And again lower octane fuel will burn too quickly causing that. But in this case, it can be accommodated by reducing the amount of ignition advance.

 

Use what octane that the owners manual instructs you to, why would anyone buy a 70+ k dollar Corvette & buy 87 octane fuel? Even if you here no spark knock, you will be losing performance & performance is why you bought this car, right?

 

Both engine knock/engine detonation and pre-ignition/dieseling happen without a spark from the plug. Read about it here: https://www.oreillyauto.com/how-to-hub/engine-knock

 

Thanks. Other sites say (define these terms) differently. I knew there were differences in how it is explained. Regardless, if you have pinging or knock (the sound), caused by low octane fuel, and it can be compensated for by adjusting the spark timing, that is different from the type that occurs without the spark. It you have a pinging or knocking that results from air-fuel ignitions without a spark, you cannot adjust ignition timing to solve it.

Here are two sites as examples:
https://resources.savvyaviation.com/...-pre-ignition/

https://belleshell.com/professional-...power%20stroke.

 

In normal ignition the combustion flame front propagates from the spark at subsonic speeds in a controlled manner providing gradual loading on the piston.
Detonation is a supersonic explosion that abruptly loads the piston and can cause damage. That is the knock you hear.
Preignition happens when the air/fuel mixture combustion starts before the spark occurs, usually from a hot edge or surface. It can lead to detonation as the combusting mixture is still being compressed as the piston continues to move upward.

 

The ECU has a number of knock algorithms to prevent or react to knock. There is a knock sensor. There also is a Low Octane ignition table when it does sense knock, will automatically pull timing until it goes away.

 

Yes - and because a change in the ignition timing eliminates it, it means it was after the spark occurs - what RKCRLR called a supersonic explosion. Lower octane fuel tends to do that more, and higher octane fuel tends to do it less.

 

Gasoline does not explode. It can only burn. It all happens at just below the speed of sound, and so quickly, that lots of folks think it's an explosion and describe it as such. In a normal ignition cycle, the spark from the plug starts a controlled burn that propagates throught the combustion chamber. The combustion chamber's design, spark plug position, piston top configuration and compression ratio all determine how the burn propagates. There's a lot on engineering in just the combustion chamber as the designers try to equalize all the forces evenly across the piston top. And, yes, some combustion chamber designs are prone to knock due to poor design.

Regardless of what other sites say, this is the true story of what is happening in your engine:

"Engine knock or engine detonation occurs before the spark plug ignites the air-fuel mixture in the engine's cylinder. When the pressure or temperature in the cylinder's combustion chamber is too high, pockets of fuel and air may combust before the spark plug fires. The result is multiple points of combustion of the air-fuel mixture before the proper point in the combustion cycle, and when these forces collide it produces a high-frequency pressure wave in the cylinder. This is what causes the metallic pinging or knocking sound in the engine.".

Like I said in my OP, it's like pounding on the piston top with a hammer.

Everyone is still missing the point of my post. You only need as much octane as your particular engine demands. You can easily test this to see how much octane your engine requires. And folks, this test will not void warrranties or destroy your engine! If you hear knocks, go fill it with premium and take it easy for the next tank. I was a gear head in the late 60's when the majors were coming out with super high hp engines. There was a great debate about octane requirements at that time. The test I mentioned is what most of us did to determine what our engines needed. "Don't buy any more octane than you need" was the mantra at the time.

Do what you want, but my OP is a tried and proven way to determine octane requirements.

 

That "high-frequency pressure wave" is the detonation/explosion. Definition of detonation:
https://en.wikipedia.org/wiki/Detonation
If that isn't an explosion I don't know what is.

And I agree that higher octane than required doesn't do you any good. The engineers that designed the engine have determined that a minimum octane of 91 is required for the engine to run properly. Lower than that and the car will start pulling timing to the extent it can and perhaps cause damage to the engine under high load conditions.

 

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. ​​​​​​One thing ​they didn't check was fuel economy using BSFC.