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EDIT: Read post #7 for a big change in GM's position on later C7s. This negates what I wrote before, although I'm leaving it here just for discussion purposes.
There's a difference between required and recommended octane ratings. Recommended is 91/93; and I don't know if there's a required rating (IOW you can drive your car with 87 in a pinch).
So, why do you need a given octane rating? At lower power requirements (like cruising on an Interstate, or city driving), even 87 octane is all that's required. Get onto a fun 2-lane road, or pass a car on that 2-lane road, and you'll quickly get into incipient detonation. The PCM will pull timing, but only to a certain extent. Higher octane fuel gives more resistance to that detonation.
Jump deep into the throttle and you'll need everything that 91/93 octane can give you.
HTH, and have a good one,
Mike
Last edited by VetteDrmr; 02-20-2018 at 07:54 AM.
Reason: Got new info from Foosh in post #7
The owner's manual for my 2018 recommends 93 octane, however I just looked up the 2014 owner's manual and it recommended 91, I'm not sure what model year started the switch to 93. It makes me wonder if the engine tuning got slightly more aggressive? Or did they find that with 91 octane there was too much spark retard occurring in the real world so they needed to change the recommendation to 93 to reduce potential power loss?
The owner's manual for my 2018 recommends 93 octane, however I just looked up the 2014 owner's manual and it recommended 91, I'm not sure what model year started the switch to 93. It makes me wonder if the engine tuning got slightly more aggressive? Or did they find that with 91 octane there was too much spark retard occurring in the real world so they needed to change the recommendation to 93 to reduce potential power loss?
Right you are, and I hadn't noticed the change. It was always 91 or greater previously. The '18 manual does go on to say on page 206:
If unavailable,
unleaded gasoline with a posted
octane rating of 91 may be used,
but with reduced performance and
fuel economy. If the octane is less
than 91, the engine could be
damaged and the repairs would not
be covered by the vehicle warranty.
That's quite a significant change in my view. The problem for many people in the western US is that 93 octane is not easy to find, particularly in CA. 93 is usually the only premium available anywhere east of the Mississippi.
I just did. It sounds like Tadge needs to read the '18 manual, and take appropriate action, or at least take another crack at explaining the change in manual guidance. That was not one of his better efforts.
Right you are, and I hadn't noticed the change. It was always 91 or greater previously. The '18 manual does go on to say on page 206:
If unavailable,
unleaded gasoline with a posted
octane rating of 91 may be used,
but with reduced performance and
fuel economy. If the octane is less
than 91, the engine could be
damaged and the repairs would not
be covered by the vehicle warranty.
That's quite a significant change in my view. The problem for many people in the western US is that 93 octane is not easy to find, particularly in CA. 93 is usually the only premium available anywhere east of the Mississippi.
Wow, I agree, that's a huge change in position. So, if 91 or 93 octane isn't available, what does a driver do? If these engines don't have enough control margin to use any available rating of octane, that seems like really thin ice to develop a product for. Reduced performance, sure, but the control systems HAVE the capability of managing octane limitations (timing, throttle position, fuel delivery, etc.)
Well, with that in mind, I retract my previous post. That science is still valid, but that statement in the owner's manual is pretty direct.
So, if 91 or 93 octane isn't available, what does a driver do?
If you had to use 87 because that was all that was available, if you drive carefully by avoiding hard accelerations and avoiding lugging the engine in higher gears the engine will never know the difference.
Yes, most probably. The sky is not falling, but as Patman articulated, it would be interesting to know why the change was made in the manual guidance.
The most logical explanation would be that based upon 5 years of experience with the LT1/4, they've seen occasional issues with the use of lower than 91 octane, as well as concerns about reduced performance and fuel economy with fuel below 93 octane. I can't imagine the change was made arbitrarily, because often manuals aren't changed even when they should be.
"Do not use any fuel labeled E85 or FlexFuel. Do not use gasoline with
ethanol levels greater than 15% by volume.
For the LT1 6.2L engine, premium unleaded gasoline meeting ASTM specification D4814 with a posted octane rating of 93 is highly recommended for best performance and fuel economy. Unleaded gasoline with an octane rated as low as 87 can be used. Using unleaded gasoline rated below 93 octane, however, will lead to reduced acceleration and fuel economy. If knocking occurs, use a gasoline rated at 93 octane as soon as possible, otherwise, the engine could be damaged. If heavy knocking is heard when using gasoline with a 93 octane rating, the engine needs service."
Right you are, and I hadn't noticed the change. It was always 91 or greater previously. The '18 manual does go on to say on page 206:
If unavailable,
unleaded gasoline with a posted
octane rating of 91 may be used,
but with reduced performance and
fuel economy. If the octane is less
than 91, the engine could be
damaged and the repairs would not
be covered by the vehicle warranty.
That's quite a significant change in my view. The problem for many people in the western US is that 93 octane is not easy to find, particularly in CA. 93 is usually the only premium available anywhere east of the Mississippi.
If 91 octane reduces performance, I wonder by how much? How much of the 460hp 465 lb-ft does it lose? Also, how bad does the mpg suffer by? Because GM is selling the car in states where 91 octane is the max but never disclosed that on the window sticker.
Around here 91 octane is available (and I can't remember what the ethanol content is, if any, but I presume plenty) via Shell stations. So getting 91 octane has never been the concern. The concern that I have is that the owners manual specifically stipulates 93 octane is recommended or preferred for optimal performance.
Being OCD, and the fact that I got my car with only 4 miles on it, I wanted to put in the fuel that's best for my car - which in this case, as stated in the owners manual is 93 octane. That is completely problematic for this part of the country. 93 octane is simply not simple to find.
When out of town (where I can't make it back to home base) I would choose a top tier gas station such as Shell and put in the highest octane (upto 93) possible with the least amount of ethanol. When I'm near my home base, I simply put in the 93 octane fuel that I ended up special ordering from Sunoco. 93 octane from Sunoco
Wow, I agree, that's a huge change in position. So, if 91 or 93 octane isn't available, what does a driver do? If these engines don't have enough control margin to use any available rating of octane, that seems like really thin ice to develop a product for. Reduced performance, sure, but the control systems HAVE the capability of managing octane limitations (timing, throttle position, fuel delivery, etc.)
Well, with that in mind, I retract my previous post. That science is still valid, but that statement in the owner's manual is pretty direct.
Thanks for sharing, and have a good one,
Mike
If you can't find the proper octane, just purchase some octane booster and keep a can in the car at all times
Also, if you are driving at higher altitudes the engine (except a Z06) can't pull as much manifold pressure and can get by on lower octane. I've been at towns in the mountains where the highest octane sold was 89 and sometimes only 87. But remember to take it easy on the throttle when you take that tank of low octane down to lower elevations.
Last edited by Gearhead Jim; 02-20-2018 at 11:08 PM.
02-19-2018, 09:13 PM
