Octane question
Safety Car
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The 2022 manual states the minimum recommended octane is 91. The 2022 manual also states that the rated horsepower is 490 (495 w/NPP). There is no mention in the 2022 manual that fuel with an octane rating greater than 91 is required to get the rated horsepower.
The original 490/495 HP SAE rating was performed using 93 octane fuel.
It is possible that the 2022 manual is incorrect. Or it is possible that it was determined that the 490/495 HP rating can be maintained using 91 octane fuel.
This would be a good question for Ask Tadge.
It will be interesting when they release the octane requirement for the Z06 engine with an even higher compression ratio.
The original 490/495 HP SAE rating was performed using 93 octane fuel.
It is possible that the 2022 manual is incorrect. Or it is possible that it was determined that the 490/495 HP rating can be maintained using 91 octane fuel.
This would be a good question for Ask Tadge.
It will be interesting when they release the octane requirement for the Z06 engine with an even higher compression ratio.
Just like GM till recently only published fuel economy based on the non-Z-51, they publish power by the best available gas.
There’s no benefit to GM for quoting the lower hp from the lower octane, just like they don’t publish what the hp is for higher altitude.
Just watched an episode of Engine Masters (California) and they tested a LS3 with 87, 91, 110 and 116. The engine wanted 29 degrees of timing for all 4 fuels and the horsepower and torque varied by less than 2. The C8 is designed to run on 90+ octane and there will be virtually no difference in power between 90 and 93. Don't lose sleep over it. Facts are facts.
If that is the case then then GM is intentionally misleading people.
Fact: The 2022 owners manual states 91 octane is the minimum fuel requirement.
Fact: The 2022 owners manual states the engine has 490 horsepower (495 with performance exhaust).
Fact: There is no mention in the 2022 owners manual that fuel with an octane rating higher than 91 is required to obtain the rated horsepower.
Fact: The 2022 owners manual states 91 octane is the minimum fuel requirement.
Fact: The 2022 owners manual states the engine has 490 horsepower (495 with performance exhaust).
Fact: There is no mention in the 2022 owners manual that fuel with an octane rating higher than 91 is required to obtain the rated horsepower.
They didn’t lie, it’s just common marketing.
Last edited by Majestic94; Nov 6, 2021 at 12:43 PM.
Race Director
Joined: Aug 2016
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From: Garden Valley CA
It's quite possible you would only see a reduction in HP during track events since you can't maintain constant WOT on the street for very long, and time at WOT can be a big driver for when timing gets pulled. And, technically, that would only effect the Z51 since track use is only approved with that option. But if that is the case they should mention it it the track preparation section.
Safety Car
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It's quite possible you would only see a reduction in HP during track events since you can't maintain constant WOT on the street for very long, and time at WOT can be a big driver for when timing gets pulled. And, technically, that would only effect the Z51 since track use is only approved with that option. But if that is the case they should mention it it the track preparation section.
Race Director
Joined: Aug 2016
Posts: 12,844
Likes: 10,254
From: Garden Valley CA
Likewise there’s no mention that it gets 495hp power at 91 octane.
Just like GM till recently only published fuel economy based on the non-Z-51, they publish power by the best available gas.
There’s no benefit to GM for quoting the lower hp from the lower octane, just like they don’t publish what the hp is for higher altitude.
Lol, there’s a naive statement.
They didn’t lie, it’s just common marketing.
Just like GM till recently only published fuel economy based on the non-Z-51, they publish power by the best available gas.
There’s no benefit to GM for quoting the lower hp from the lower octane, just like they don’t publish what the hp is for higher altitude.
Lol, there’s a naive statement.
They didn’t lie, it’s just common marketing.
Burning Brakes
Joined: Mar 2006
Posts: 753
Likes: 241
From: New Hampshire
A engine with 11.5 to one compression won’t run on regular gas without taking a considerable amount of power out of the engine. Engines equipped with carbs didn’t have electronic engine management to retard the timing therefore you would hear the detonation that was harming your engine. The only reason you aren’t hearing the ping in your c4 c5 c6 c7 and c8 is because the engine has knock sensors in it to retard the timing enough so that the worst of the detonation is not audible. This doesn’t mean that you’re not still doing damage to your engine. When detonation occurs what is happening is the fuel is igniting prior to the spark plug igniting trying to drive the piston back down in the opposite direction it’s going, causing a loss of horsepower. In the process it is having a negative impact on the bearings crank rods pistons and rings from excessive pressure. Extreme forms of detonation can break any of these parts. For a guy that has had several high performance cars I would think u would better understand a high performance engine. Give your engine a break and spend another 30 cents a gallon on fuel and you will get some more horsepower in the process. Both of u will be happier.
Safety Car
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If car was designed for 87, it will say 87 is required. If it was designed for 91, but will not destroy the engine on 87, they’ll recommend 91 but require 87. If it was tuned for 93 but won’t get destroyed by 91, they’re say 91 is required but 93 is preferred. They’re not going to publish their hp number using the bare minimum gas.
Likewise a popular mod in 93 octane states for cars that prefer 91 octane, is to change the engine tuning to use the higher octane. In the C8’s case they just designed it for 93 octane so we can skip the tune that we can’t do anyway.
Team Owner
Joined: Oct 2004
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It's quite possible you would only see a reduction in HP during track events since you can't maintain constant WOT on the street for very long, and time at WOT can be a big driver for when timing gets pulled. And, technically, that would only effect the Z51 since track use is only approved with that option. But if that is the case they should mention it it the track preparation section.
They can still legitimately claim a max output of 495 because that's what it tested at on 93 octane using the accepted SAE protocol, and the manual still states "91 or greater." I'm sure there are also temperature and atmospheric conditions where it doesn't make that on 93.
Last edited by Foosh; Nov 6, 2021 at 05:21 PM. Reason: typo
Race Director
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The one thing that changed was the fuel injection system which gives better city mileage. Maybe that has something to do with it. Also, at higher altitude states only 91 octane gas is available. The main reason at altitude a lower octane is needed is because there is less air to compress.
What you say about higher altitude is true, but the manual change doesn't say anything about altitude. 91 Octane is also the highest pump gas available on the coast of California, at sea level.
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Race Director
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The 2022 manual states the minimum recommended required octane is 91. The 2022 manual also states that the rated horsepower is 490 (495 w/NPP). There is no mention in the 2022 manual that fuel with an octane rating greater than 91 is required to get the rated horsepower.
The original 490/495 HP SAE rating was performed using 93 octane fuel.
It is possible that the 2022 manual is incorrect. Or it is possible that it was determined that the 490/495 HP rating can be maintained using 91 octane fuel.
This would be a good question for Ask Tadge.
It will be interesting when they release the octane requirement for the Z06 engine with an even higher compression ratio.
The original 490/495 HP SAE rating was performed using 93 octane fuel.
It is possible that the 2022 manual is incorrect. Or it is possible that it was determined that the 490/495 HP rating can be maintained using 91 octane fuel.
This would be a good question for Ask Tadge.
It will be interesting when they release the octane requirement for the Z06 engine with an even higher compression ratio.
Can't really compare the octane requirements of the LT2 and LT6. The 4 valve heads, larger bore and relocated fuel injectors changes everything about combustion.
Race Director
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If that is the case then then GM is intentionally misleading people.
Fact: The 2022 owners manual states 91 octane is the minimum fuel requirement.
Fact: The 2022 owners manual states the engine has 490 horsepower (495 with performance exhaust).
Fact: There is no mention in the 2022 owners manual that fuel with an octane rating higher than 91 is required to obtain the rated horsepower.
Fact: The 2022 owners manual states 91 octane is the minimum fuel requirement.
Fact: The 2022 owners manual states the engine has 490 horsepower (495 with performance exhaust).
Fact: There is no mention in the 2022 owners manual that fuel with an octane rating higher than 91 is required to obtain the rated horsepower.
Race Director
Joined: Aug 2016
Posts: 12,844
Likes: 10,254
From: Garden Valley CA
That’s exactly what manufacturers do.
If car was designed for 87, it will say 87 is required. If it was designed for 91, but will not destroy the engine on 87, they’ll recommend 91 but require 87. If it was tuned for 93 but won’t get destroyed by 91, they’re say 91 is required but 93 is preferred. They’re not going to publish their hp number using the bare minimum gas.
If car was designed for 87, it will say 87 is required. If it was designed for 91, but will not destroy the engine on 87, they’ll recommend 91 but require 87. If it was tuned for 93 but won’t get destroyed by 91, they’re say 91 is required but 93 is preferred. They’re not going to publish their hp number using the bare minimum gas.
At the minimum, the way the 2022 owners manual is worded, the 2022 C8 EPA mileage rating would need to be performed with 91 octane fuel. I'm sure the Feds would not look kindly on having an undocumented fuel requirement to achieve the rated mileage. And my guess is if the 2022 C8 got better fuel mileage using 93 octane, GM would have left the wording in the 2022 manual the same as the 2020/2021 and taken credit for the higher mileage rating.
Perhaps the 2022 fuel injection change eliminates the mileage advantage 93 octane would have. And perhaps that got buried in the switch to the Z51 mileage rating for 2022.
I just believe there is more to the story than a wording change in the 2022 manual. The bigger question is how this relates to the 2020/2021s.
Last edited by RKCRLR; Nov 6, 2021 at 02:10 PM.
Race Director
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From: Garden Valley CA
The ECU won't pull timing under all conditions. Under testing conditions, without high temps and high loads, the engine no doubt still makes rated HP with 91 octane fuel. That doesn't mean it will still make that when the air temp is 100F, you're on the track, and coolant temps are 230F or higher.
And I agree with the wording change in the earlier post, I fubar'ed.
Last edited by RKCRLR; Nov 6, 2021 at 02:18 PM.
Safety Car
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But having 87 minimum octane required without mentioning anything else would also mean that the EPA fuel efficiency certification would need to be performed with 87 octane fuel. With as much as manufacturers do to increase their CAFE ratings these days I doubt they would leave that on the table.
At the minimum, the way the 2022 owners manual is worded, the 2022 C8 EPA mileage rating would need to be performed with 91 octane fuel. I'm sure the Feds would not look kindly on having an undocumented fuel requirement to achieve the rated mileage. And my guess is if the 2022 C8 got better fuel mileage using 93 octane, GM would have left the wording in the 2022 manual the same as the 2020/2021 and taken credit for the higher mileage rating.
Perhaps the 2022 fuel injection change eliminates the mileage advantage 93 octane would have. And perhaps that got buried in the switch to the Z51 mileage rating for 2022.
I just believe there is more to the story than a wording change in the 2022 manual. The bigger question is how this relates to the 2020/2021s.
At the minimum, the way the 2022 owners manual is worded, the 2022 C8 EPA mileage rating would need to be performed with 91 octane fuel. I'm sure the Feds would not look kindly on having an undocumented fuel requirement to achieve the rated mileage. And my guess is if the 2022 C8 got better fuel mileage using 93 octane, GM would have left the wording in the 2022 manual the same as the 2020/2021 and taken credit for the higher mileage rating.
Perhaps the 2022 fuel injection change eliminates the mileage advantage 93 octane would have. And perhaps that got buried in the switch to the Z51 mileage rating for 2022.
I just believe there is more to the story than a wording change in the 2022 manual. The bigger question is how this relates to the 2020/2021s.
Race Director
Joined: Aug 2016
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From: Garden Valley CA
Are you saying the owners manual doesn't have to include the fuel requirement to get the EPA rated mileage? I find that a little strange.
Safety Car
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Up until recently they didn’t tell you the published gas mileage was for non-Z-51’s, 1LT Coupe’s either. EPA guidelines are pretty slack.
Team Owner
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See Zymurgy's post #11 for the exact wording of all 3 MY owners manuals (20-22). 20-21 both clearly state 91 can be used but with reduced power. 22 deleted the part about reduced power but says "91 or higher" octane fuel, which is just a different way of saying 91 and 93 are OK, minus the negative connotation for use of 91 octane for those folks who can only get that. It doesn't imply power output will be the same as with 93 either, just nothing. That is thrust of this lengthy conversation.
So the question is why did GM make that wording change? My theory is they just wanted to stop a lot of questioning and complaining. The only change to the '22 cars is higher pressure injectors and fuel pump, which allows AFM to deactivate cylinders at slower speeds. I can't think of a reason why that change would produce more power on 22s with lower octane fuel. If it did, then theoretically it would have increased output at the top. It is simply presented as a fuel economy enhancement.
So the question is why did GM make that wording change? My theory is they just wanted to stop a lot of questioning and complaining. The only change to the '22 cars is higher pressure injectors and fuel pump, which allows AFM to deactivate cylinders at slower speeds. I can't think of a reason why that change would produce more power on 22s with lower octane fuel. If it did, then theoretically it would have increased output at the top. It is simply presented as a fuel economy enhancement.
Last edited by Foosh; Nov 6, 2021 at 04:46 PM.
Melting Slicks
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From: Covington LA
I never said the C8 had a LS3. The point was that octane didn't make any real difference in the horsepower and torque of a basically stock engine. They did say the engine would not tolerate 87 octane under the hood and out in the real world driving around with the all the heat associated with it. What was really telling for me is that 116 octane made less horsepower and torque than the 87 and the 91 (which was appropriate for the engine) made as much as the 110 race gas. The point being that the C8 is tuned for max performance for 93 octane but 91 won't hurt it and there won't be any significant difference in output. The E85 was a whole other story being significantly better but then the ECU has to be tuned for that.
Safety Car
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I never said the C8 had a LS3. The point was that octane didn't make any real difference in the horsepower and torque of a basically stock engine. They did say the engine would not tolerate 87 octane under the hood and out in the real world driving around with the all the heat associated with it. What was really telling for me is that 116 octane made less horsepower and torque than the 87 and the 91 (which was appropriate for the engine) made as much as the 110 race gas. The point being that the C8 is tuned for max performance for 93 octane but 91 won't hurt it and there won't be any significant difference in output. The E85 was a whole other story being significantly better but then the ECU has to be tuned for that.
LT2 was designed for 93 octane with 91 being the “minimum but with degraded performance” octane. The two aren’t the same.
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I would use the 93 octane if it was available to me, the 10% ethanol will have no effect on your car's performance
Melting Slicks
Joined: Jan 2009
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From: Covington LA
LS3 was designed for 91 octane, so going to 93 or higher isn’t going to benefit you without a tune to take advantage of it. 87 was the “minimum but with degraded performance” octane.
LT2 was designed for 93 octane with 91 being the “minimum but with degraded performance” octane. The two aren’t the same.
LT2 was designed for 93 octane with 91 being the “minimum but with degraded performance” octane. The two aren’t the same.