Reading the owners manual, 93 octane recommended. Rare in Colorado, checked the 93 octane websites, can only find 2 in Aurora. But then, reading up on octane, the consensus seems to be that due to the altitude, 91 is fine. Can't compress high enough to take advantage of 93. Anyone?

Back to the Indy 500. Pace Car - C7 driven by Earnhardt, Jr.

 

91+ is acceptable.

 

Better be ! I live in Calif. and can only find 91 out here !

 

Read your car's owners manual. It has complete instructions.

 

91 octane gasoline in the higher altitude of Denver will pretty much function the same as 93 octane at sea level. Notice also that the unleaded regular fuel in Denver is 85 octane vs 87 elsewhere.

 

I did, Hence the question.

pg 212
"Recommended Fuel
(LT1 6.2L V8 Engine)
Do not use any fuel labeled E85 or
FlexFuel. Do not use gasoline with
ethanol levels greater than 15% by
volume.
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."

Why would I not want the very best out of my car? But, the real question here was about octane at altitude. I'm at 5280 feet (you know, Mile High City). The word "altitude" does not appear anywhere in the 367 pages of my owners manual. So, I was reaching out to those with altitude experience, tuning older vehicles, etc.

So, I beg to differ sir. It does not have complete instructions, it was written for sea level cars.

I'm your Huckleberry. :-)

 

See post #10. If the OP's car is literally just a Z51 without supercharging, the rest of this post is irrelevant. It applies only to Vettes like Z06 and ZR1 with supercharged engines.

I can only answer half your question. Maybe somebody else can answer the other half. The half I know is that once a turbo or super charger reaches what’s called critical altitude, the turbocharger waste gate or supercharger internal bypass is fully closed at full throttle, meaning it’s putting up as much boost as it can, and further increases in altitude will cause MAP (manifold absolute pressure), power, and octane requirement to fall off just like a normal engine. Turbo or super charged airplane engines have a ton of fat built in, so their critical altitude is usually in the 20,000-30,000 foot range, and they require full octane until they get above that altitude. A car obviously wouldn’t have anywhere near that much fat, but I don’t know whether the Z51 supercharger’s full throttle MAP starts dropping off right away like a regular engine, or whether it has a bit of fat that would let it maintain full throttle MAP up to a few thousand feet. If there’s little or no fat, the lower octane gas in Denver should be ok. But if the Z51 supercharger has a bit of fat, it may not be.

One last warning. If you can’t find a spec sheet with full throttle MAP versus altitude, it can be confusing to try to figure it out by testing with your car. It’s MAP that matters for octane requirement and power, not boost, and the gauge on your car’s instrument panel is probably boost. MAP is absolute pressure while boost normally means differential pressure above the atmospheric pressure where you are. Atmospheric pressure in Houston at sea level is about 15psi, while in Denver it’s about 12psi. MAP is equal to that pressure plus boost. So let’s say you run a full throttle test in Denver and another in Houston, and you get 15psi boost in both places. In that case, the Houston MAP would be 30psi while Denver MAP is only 27psi. That would indicate that the Z51 supercharger has little or no fat, and using lower octane gas in Denver is ok. But if the Houston full throttle boost is 15psi and the Denver boost is 18psi, then both MAP’s would be 30psi. That would indicate there is fat in the Z51 supercharger, and you are potentially in trouble using lower octane gas in Denver.

 

What is a Z51 supercharger?

 

OK only has 91 and it runs fine

 

I assumed from the OP’s comment in post #1 about “can’t compress enough at high altitude” that he was talking about the supercharged engine in Z06 or ZR1, but I used his terminology. Z51 is only a modest performance package with dry sump and various gearing, tire, wheel, and brake upgrades. If the OP’s car does not have a supercharged engine then I guess I’d wonder why he posted, since there have been plenty of threads about octane versus altitude in non-boosted engines. Unless his car has a supercharged engine, my post #7 is irrelevant.

If the OP is indeed talking about the normally aspirated engine, then he should be ok with lower octane gas at high altitude. The reason it’s ok is that engine power and octane requirement are related to MAP, not throttle opening, and with a normally aspirated engine, MAP can never be higher than the atmospheric pressure at your location, which in Denver is only about 12psi versus about 15psi at sea level. So when you floor it in Denver, your MAP is only about 12psi. If you floored it in Houston, your MAP would be about 15psi. To get a MAP of 12psi in Houston, your gas pedal would only be about 80% of the way to the floor. So if you were living in Houston and never pushed the throttle any more than 80% of the way to the floor, you wouldn’t need max octane either.

 

Sorry on two counts. It a straight Z51 with no mods, no supercharger. I did use the words "Can't compress high enough..." which I see in retrospect was a bad choice. I had been googling outside the site, and found a statement to the effect "can't achieve the 11.5:1 compression at altitude". Whether that is true or not I don't know. That brought me here.

On the second count, I apologize for opening a new thread. Perhaps I'm just not adept with this search engine. I tried "octane' and got 496 hits. Perusing through a bunch, I couldn't find anything pertinent, so I tried again with with "octane and altitude". The engine said it threw away extraneous words.

Regards

 

The issue of compression ratio at altitude depends on how you look at it. It’s related to the second paragraph of post #10. The compression ratio in Denver is still 11.5, but the resulting pressure inside the cylinder is lower because the starting pressure is lower. With max MAP in Houston being 15psi, the max cylinder pressure prior to the fuel igniting is 11.5 times 15 equals 172psi. With max MAP in Denver being only 12psi due to the lower atmospheric pressure in Denver, max cylinder pressure prior to fuel igniting in Denver is 11.5 times 12 equals only 138 psi. So same compression ratio of 11.5 both places, but resulting pressure in Denver lower due to lower starting pressure. To get a max pre-ignition pressure of 138psi in Houston, compression ratio would only need to be 138 divided by 15 equals 9.2.

 

No need to be sorry - you had a not-unreasonable question and got an answer. Besides, reopening an old thread is sometimes frowned upon - depends on some unwritten rule regarding age of thread, pertinence to current issue, and phase of the moon. Welcome to the Forum.

 

In Arizona we too only have access to 91 on a regular basis. There is one location in north Scottsdale that carries 95, so when i'm in the area I blend the two.

 

Plenty of access to 93 octane here in C. Florida. Love it.

 

91 octane is the Max in a number of states who aren't at 5000+ feet! Works fine but as you say, being higher is like me NOT using full WOT so detonation chances are less!

 

IMO, no need to apologize for not having any luck on the search engine and wind up asking a question that has been discussed on other threads. The main reason for these forums is to share info and experiences and, fortunately, new people are joining all the time. Unfortunately, they are often chided by 'forum *****' who spend their time commenting that the poster is wasting their time!

 

My 2016 manual calls for 91 Octane. I understand that the 2019s say 93 octane. I also only found 2 stations in Colorado... Aurora for 93. Just from doing some light reading, I agree, the 91 at altitude will act as a 93 (some weird atmospheric pressure math that lost me). I'm just gonna stick with Shell 91. I think you'll be fine.

My question is, what changes to the motor were made in 2019 that require 93 (per OM)? I can see the Z06/ZR1 (LT4/LT5), but for us on NA LT1?

 

FYI, the 2017 Owner's Manual clearly states 93 as well. Maybe that was the changeover year and not 2019. Basically states that the LT1 should use 93 and octanes as low as 87, anything under 93 will result in reduced acceleration and fuel economy.

 

My 2014 Manual states 91 or higher only. "If the octane is less than 91, damage to the engine may occur and may void the vehicle warranty."

I was at a remote location the other day and all they had was 90. The car seems to run fine but I'll be sure to fill up with 91 = at next fill up.