Skid Row Joe

You're golden following the owners manual. Some choose to disregard it and go their own way. Your LT-1 was built for octane as low as 87. You're good.

kennyjames21

Cripes dude, the manual said the engine will run on 87 octane, but then adds the important part about the fact it may start knocking and cause engine damage, lol. In other words 87 octane fuel is not a good idea...

PureJoy

Have you checked the oil yet????????/

kennyjames21

Nope, he's not good. If the engine starts knocking and is damaged from using 87 octane fuel, GM will say the engine was damaged due to improper use. They'll say they warned him to stop using 87 if the engine began knocking - good luck beating GM in court.

PatternDayTrader

Are we talking about the same thing ?
I'm addressing the altitude/octane issue.
The theory posited by you, seems to be that because barometric pressure is lower at higher altitudes, less octane is required, but once we remove barometric pressure from the discussion (forced induction), your saying it doesn't matter ?
Did you read the quote I copy pasted earlier, or the link with its attached data sources ? Data sources that came from actual studies of the issue, and not hearsay ?
In case you didn't, heres another article questioning the premise of the altitude/octane claim

https://durangoherald.com/articles/96803

Heres a nifty quote from a Ford owners manual that also speaks directly to the issue at hand.

OCTANE RECOMMENDATIONS
Regular unleaded gasoline with a
pump (R+M)/2 octane rating of 87 is
recommended. Some stations offer
fuels posted as Regular with an
octane rating below 87, particularly
in high altitude areas. Fuels with octane levels below 87 are not
recommended. Premium fuel with an octane rating of 93 or higher will
provide improved performance and is recommended for severe duty or
high performance usage.


And another article questioning the same issue, also written in Colorado.

https://www.denverpost.com/2006/10/1...octane-levels/

And another that includes input from a GM fuel specialist, and questions the validity of the same study that I wouldn't pay the 30 bucks for.

https://www.usatoday.com/story/money...hanol/2369579/


PS - I don't know what it takes to fix the copy/paste font issue.

Skid Row Joe

Sorry, but the Owners Manual does not state that. That's your personal take or opinions. Not GM's.

kennyjames21

The owners manual clearly states that using 87 octane fuel can cause engine knock and then result in engine damage.

The 'my take' part is where I use common sense to state that GM will use that to avoid warranty claims for anyone who is foolish enough to use 87 octane fuel with regularity in these engines. I wouldn't expect you to understand that, being someone who can't read that engine oil must be changed at least yearly as stated in the same manual when you think it's OK to go years (and was it 20k miles...) without changing it...

But you keep going years/20k miles between oil changes and run your car on 87 octane if you want - it's your car, but good luck with warranty fixes with anything related to those two moves, and good luck selling that car to anyone who sees your posts, lol.

LDB

We’ve been debating two issues. First is the question of whether a normally aspirated engine can get by with less octane at high altitude. Second is the question of whether a supercharged engine maintains power as altitude increases. You’ve been saying no yes and I’ve been saying yes no.

I’ll take the second question first, as I think it is easier. That answer would only be yes if the supercharger had extra fat built into it like an airplane engine supercharger does. Extra fat for a supercharger means that even at full throttle at sea level, the internal bypass would still be partially open thus giving room for it to gradually close to maintain MAP as altitude increased. But in most cases, a car supercharger is designed just big enough, with no extra fat, because putting in extra fat would reduce sea level mileage. In the no fat situation like most car superchargers, then at full throttle at sea level the internal bypass valve in the supercharger is fully closed, and as altitude increases, both the full throttle MAP and engine power decrease just like they do in a normal aspirated engine. They of course remain higher than a normally aspirated engine, but they are lower than the supercharged engine was at sea level.

As to the first question, my answer was correct basis prevailing understanding as of my retirement in 2010. It is possible that since then, in the ever-increasing quest for better mileage, manufacturers have put ever more spark advance into engines at part throttle. If they have done that, then rather than risk of knocking/detonation being greatest at full throttle with substantially lower risk at part throttle, it has moved closer to being the same at all throttle settings. That in turn would mean it is no longer reasonable to supply lower octane fuel to high altitude areas. Your references indicate that is what has happened. I’ll have to look further into it before deciding whether I agree. The references you cite are better than generic internet sources, with the Ford manual quote being the best in my eyes, but newspaper stuff on technical topics isn’t exactly rock solid.

V Vette

LOL, too many opinions and contradictions here.... Maybe I should say like most (when they don't have an answer) "Its a matter of what you like".

Red86Cfour

Speak for yourself

V Vette

The issue was NOT price but is it good for our engine, correct?...Uggh..

Patman

No, the engine was not built for 87. It was built to use premium, it's just that 87 can be used in a pinch, but like LDB described above, it can still cause damage because the computer only pulls out the timing once it starts hearing detonation and then it still tries to keep dialing back in more spark advance, it's an endless circle until you put in the correct fuel. Owners that continuously run 87 octane, especially in hot weather, and taking a huge risk of engine damage. Don't come crying to us when you put a hole in one of your pistons on a 100 degree Texas day.

PatternDayTrader

Yeah I wasn't happy about the newspaper articles, but since the only formal study of the altitude/octane issue is the one on 84-86 model cars, I'm stuck citing materials that question that study, as opposed to citing a competing study. Although the sources in the .gov link were scientific, I didn't read them in detail. Reason being, they are not making the proper distinctions, in spite of coming up with the proper conclusions. Even if we set aside modern fuel injection and timing controls, the core issue here is that during the 84-86 time frame, the highest compression vehicle would have been maybe 9 to 1. This all by itself, explains why an older car could be more tolerant of lower octane fuel.

Regardless of all that. Its worthwhile to explore these issues here in a public format. Now the OP and any other reader can decide for themselves how they want to proceed, and regardless of what someone may decide, they have been properly warned.

ZZ06

I have seen this first when I left Hurlburt Field FL, and got stationed at Cannon in Clovis NM. Literally went from sea level to 4300ft. While I was learning to tune my own car, I started to understand how MAP and altitude correlated. This was just one of many adjustments that I saw. My timing also needed to change as the car was experiencing a little knock. 27 degrees at 6500rpm with 91 instead of 93 didn't suit well.

GaryPitts

I run 89 in mine. I'm older and do not drive aggressively. I can tell no difference. I enjoy saving the $5 per tank

Kingtal0n

gasoline is terrible. lets just get that out of the way. 93 is better than 87 but both of em are total crap for performance to begin with.

V Vette

Great but are your driving slow in the left lane too??? LOL

LDB

Like you, I was not able to find any references with a clear test of altitude versus octane on a recent car having fuel injection and knock sensors. Plenty of references question whether the historical relationship of needing less octane at altitude still applies with recent model cars, but I didn’t find a single article that actually did the tests that would be required to know for sure. That kind of makes me suspicious. Some reporter comes out with a big scandal story (ie, late model cars are being damaged by 85 octane at high altitude), yet they don’t do the obvious, simple, and cheap tests to determine whether this is true. That makes me think some of those reporters must have tried the tests, didn’t get the scandalous results they were looking for, so didn’t mention the lack of those results in their stories.

What simple and cheap tests do I mean? Take a car designed for 87 that has knock sensors and see if it pulls timing when run on 85 at altitude. You could run a similar test on a Vette, but since it’s designed for 91, you’d have to run two tests. You would know in advance that it would pull more timing on 85 at altitude than it would on 87 at altitude. But the question for a car designed for 91 would be whether it pulled more timing with 85 at altitude than it did with 87 at sea level.

Lacking the simple test data above, I would lean toward sticking with the historical info that says less octane is needed at altitude. It would be so easy to disprove the historical info that it seems to me somebody would have done so if the historical info was indeed wrong. That said, I admit that I don’t have the data to prove the historical info still holds either.

Kingtal0n

The best thing you can do for the sole gasoline injected engine is add water/methanol injection.
Even a regular engine from a random car will benefit. The water cleans the carbon up and the methanol increases octane dramatically.
I think 50/50 mix raises 93 to something like 110 or 114 octane.
I usually just use 100% distilled water for the cleaning benefits and EGT drop it provides, while running 15-20psi of boost on 93 octane.
EGT drops from 1400~*F (never held it down long enough to find out how high it would have gone) to about 1250*F with the right amount of water only.
it doesn't take much. And it beats upgrading the entire fuel system for E85 and dealing with that stuff (corrosive and often contains biomass/jelly like substance)

Renfield

Sorry, not sorry. It absolutely does state that, and the post quoting it is in this thread.