Its a really great point and I did think of that. I have to speculate because the service manual does not give any detail on the algorithm. But my thought is it detects and corrects it very quickly at a level that might not be perceived as hard knocking. Perhaps imperceptible and harmless. We don't know how sensitive it is. But it also says (and this is not speculation) that "If the control module has determined that knock is present, it will retard the ignition timing to attempt to eliminate the knock. The control module will always try to work back to a zero compensation level, or no spark retard." So the question is how does it do that. It would have to advance the timing a little to see if it knocks any more. Does it do that continuously? Or Does it do that periodically? Or once per drive cycle? Does it over-correct and then slowly move back toward zero compensation? It does not say. But if its really continuously trying to work back to "no spark retard", then it would continuously have tiny bit of knock. So again, speculating, I'm thinking it doesn't do it that way.

I use 93 all the time by the way. I've never tried 91.

 

I think it's more using the fuel available in your area versus picking one of the other. Our fuel comes in a pipeline from Gulf Coast refineries. It's all the same with different additives put in the tank delivery trucks based on Brand. Most tank trailers have multiple separate sections so can carry several Brands. No choice the base gas is all 93 or 87 octane with intermediates mixtures done at the pump. There is no 91 octane available around here. In some areas, no choice, 91 octane is the maximum octane available.

I recall knocking in my first car, stick shift, was easily detected going slow up hill in top gear! With the little or no overlap cams of the day, using any significant throttle opening at low speed the cylinders were full of the fuel air mixture on the intake stroke. That caused a maximum compressed mixture pressure as TDC was approached. We also used light distributor advance springs for more power quicker. That combo cause knocking usually felt going up hills. At high rpm with WOT the cylinder was not completely filled. Also less advance. So shifting to a lower gear stopped knocking! Fuel air mixtures from carburetors being used were also slightly richer at larger throttle openings and high rpm. That helped cool the mixture and reduced knocking. Carburetors, unlike say DI did not have perfect mixing so a richer setting assured no lean areas that were more prone to knocking.
Vacuum Advance
Distributors also used vacuum advance increasing timing for more responsive during light acceleration. They use manifold vacuum so increasing throttle opening decreased the added timing. See SIDEBAR for what I had to do in my 8.2 Liter carbureted crate engine, vacuum advance distributor to reduce engine temps at idle!
Bottom Line:
So wonder if the C8 might not use a richer mixture after pulling timing and knocking is still detected? The excess fuel does cool the mixture.

SIDEBAR (a Shaggy Dog Story)
The instructions that came with the 8.2 Liter engine I built and installed in my street rod said to use "Ported Vacuum" to connect to the distributor vacuum advance diaphragm. That port is above the throttle butterfly so there is essentially no vacuum in that port to activate the distributor advance mechanism at idle.

An old Peterson publication reviewed the history of "Manifold" versus "Ported" vacuum. "Ported Vacuum" came about during the start of pollution controlled vehicles to reduce nitrous oxide emissions with lean air/fuel idle mixtures. They state that by using "Manifold Vacuum" the spark at idle will occur sooner and "less cylinder wall is exposed to the heat (of combustion), thereby lowering coolant temperature." Interpreting their statement, with more advance at idle, the combustion starts on the upper portion of the cylinder on the compression stroke. Flame front travels very slowly at the low cylinder pressure that occur when idling. Therefore the cylinder head and upper cylinder walls will absorb most of the combustion heat and the lower portion of the cylinder on the power stroke will be cooler. This transfers less total heat to the cooling system.

My engine temps with the car moving were fine but when stopped at idle, they increased. Had a large 6 blade puller fan attached to the engine and added two electric fans in front of the Rad. The electric fans were only needed when stopped at idle and the engine temps were still increasing.

I found an additional ~ 10 degrees advance occurs at idle when the "Manifold Vacuum" port was connected to the distributor vacuum advance rather than the "Ported Vacuum" location. This provides about 25 degrees advance in total when idling! The engine idled considerably faster compared to when the recommended "Ported Vacuum" was employed. The throttle butterfly's had to be closed about 1/2 turn on the idle adjustment screw to maintain the ~900 RPM idle speed a good indicator of more efficient use of the fuel. All else being equal, less butterfly opening means less fuel consumed which equates to less total heat to transfer to the coolant.

I also found info that some emissions engines using "Ported Vacuum" employed a thermal vacuum control switch to deliver ported vacuum to the distributor until the engine reached operating temperature, then switched manifold vacuum to keep the engine cooler at idle." So I used that vacuum source.

Measuring the engine temperature on a very hot day (95 degrees in the shade, with the air-conditioning off and the electric fans on) with the car idling at 1100 RPM in PARK, it slowly rose to 220 deg F when "Ported Vacuum" was used . Under the same conditions using "Manifold Vacuum" for the distributor vacuum advance the temperature maxed out at 210 degrees, a 10 degree lower coolant temperature!

 

Why would you consider putting that in your C8?

 

I've done tons of DLs when I was tuning. When setting up the primary timing table, the knock sensors are key. the ECM will instantly adjust timing with a knock sensor event and commensurate with the intensity of the knock. Although the adjustment to retard timing is instant, the ECM will crawl back to the correct timing from the primary table once the first event is over (and if it doesn't return). Return is not instant.

Another comment about the 93 octane or higher statement in the event of tracking. The primary timing table has a ceiling. The engine cannot advance past that point if higher octane is used. This is why octane higher than what the maximum timing is set for is considered not useful in terms of more energy produced, But, because of the difference in burn rate with higher octane than the max required can help with a little cooler cylinder as when a car is tracked the engine is spending a lot more time at mid to max RPM where heat can become an issue.

 

Thanks! So the answer is, it "slowly creeps back" and will back off again if necessary.

Regarding the limit in the timing table, I was aware that there is ceiling, but I did not know where it is set, I believe you are saying that it does reach the ceiling when 93 octane is used. Does it reach that ceiling with 93 octane, or is the some headroom left if a higher octane fuel is used? I'm asking that because a while back someone reported monitoring that advance, while using a higher octane than 93 and I thought they reported that it did advance further. They may have corrected that in a later report though.

Also, the earlier manuals "required 93 octane" but then goes on to say "If unavailable, unleaded gasoline with a posted octane rating of 91 may be used, but with reduced performance and fuel economy." But in the Track Events and Competitive Driving section of the earlier manuals it said 93 octane is required, with no caveat about using 91 for tracking. The wording was changed in the later manuals to say 91 octane is required (rather than ok to use if can't find 93) but in the Track Events and Competitive Driving section it says 93 octane is required for that.
I don't think the engine changed, it just the way they chose to present the information. You could always use 91, and 93 was always required for tracking. But....does the timing advance reach the ceiling in the timing tables when using 91 octane?

I really brought this up because of statements that you should not use an octane higher than necessary to eliminate detonation. I don't think that is quite accurate any more (or at least requires a little clarification) because if one uses 91 octane, the car is not going knock (at least under most non-track use conditions). But you do get reduced fuel economy and performance, relative to if you used 93 octane according to the earlier manuals. But i do understand that if you are at the timing advance ceiling with 93 octane fuel, then higher octane will not be useful.

 

The primary timing table has a ceiling in that the numbers from which the ECM chooses are fixed. So for instance, if I have a cell in the table for a specific RPM and spark air mass, lets say 12* advanced, 12* is the maximum advance I can have at that particular cell. The system has no way of making that 13*, but is is very capable of reducing it to 10* or 8* or whatever it needs to react in kind to a knock sensor event.

The issue with a ceiling, the car is tuned and the table created where there is a certain % of SAFE efficiency given average driving conditions. A certain % is important BC the timing table is the first place tuners go if they want to get more out of the stock factory tune. Tuners essentially are eliminating that factory built in % which, in part is a safety and longevity component.

Along with a factory recommendation for fuel (93 octane in this case) comes a long list of driving and environmental conditions some of which are altitude humidity, Bar pressure, etc. The factory established timing table has to consider any or any combination of these conditions can exist. To complicate it, the primary TT is adjusted by a number of adjunct tables to accommodate certain conditions. One simple example of this is IAT. Just Physics, but anything past about 88* ambient air temps, sections of the primary timing table are impacted by a timing adjustment from the IAT table that also varies by ambient air temp. So, a good tuner dialing in a primary timing table after a performance add understands that a timing advance # on a data log will not match the corresponding value in the primary timing table cell for that RPM and spark air mass.

Then comes fuel and its relationship to ignition. Lets forget closed loop for a minute and go where the action is in power enrichment, If 93 octane works and doesn't produce a knock event and I begin to fiddle with the AFR and go leaner than a typical PE average around 12.5:1, a leaner mix can start to produce knock events.

I think the challenge of getting wording correct in an owners manual is quite a job. With all the information out there, even an average knowledge reader is going to try to interpret what 93 octane is required, but 91 octane can be used at reduced...means.

 

Bye Bye warranty.

 

Run the octane it's tuned for. Higher octane lets you run more timing and maybe more power. But without some pretty expensive custom tuning, I don't think you'd see any gains. My truck has an aftermarket 87 octane tune. I can put cheaper gas in it and it still gets a little extra performance over stock. Last I knew, lower octane means it's more combustable .