The Successful Plumber

I have never been able to use 93 octane in my C7 LT1.

Those of you that have used both 91 and 93 octane in their C7 LT1, is there any noticeable differences, smoother idle, better MPG, increased performance?

The main thing I was interested in was idle smoothness. The C7 LT1 has a rough idle, not like the idle of a high lift and duration cam shaft. So I was wondering if the rough idle was caused by the use of 91 octane instead of 93 as recommended.

dbj216

I live in the Colorado mountains near Denver. 93 octane fuel is not available around here that I have found, so I use the 91. It has something to do with the altitude where we live. The owner's manual says to use 93 octane fuel for maximum performance. I trust the anti-knock sensors will protect me. My 2017 C7 LT2 seems to run just fine on 91 at my higher altitude. I'm sure owner's have made errors and accidently filled up with 87 octane fuel. I doubt this error is fatal to the engine.

David

Vetteman Jack

I have used both and really have not noticed a difference between the two octane levels. My car runs just fine on 91 octane, which is about the highest we can find here.

Dads2kconvertible

Your lower air pressure at higher elevation results in lower octane requirements.

aklim

Wouldn't the better way be to put away what you feel and check the knock counts between the 2 fuels?

The Successful Plumber

The main thing I was interested in was idle smoothness not knocking. The C7 LT1 has a rough idle, not like the idle of a high lift and duration cam shaft. So I was wondering if the rough idle was caused by the use of 91 octane instead of 93 as recommended.

aklim

​​​​​​All things being equal, 93 should have more knock resistance than 91. Not much else is different so not sure how that affects idle quality. Besides, the ECM should compensate as much as it can.

449er

93 octane would be the best way to go but your car's ecm will adjust accordingly for 91 octane

The Successful Plumber

I thought the ECM might advance the timing and help smooth out the idle using the recommended 93 octane.

aklim

It might adjust but the adjustment might also be so slight your butt dyno misses it. Kinda why I suggested getting it scanned. Data can tell you what is happening and someone here might be able to better interpret the data and we can learn from it

mbaessler

As far as idle goes, to me they set it to low, but I assume that is for gas mileage reasons. If you press the gas peddle and add 50-100 rpm it smooths right out. I asked if I could get to adjusted just a little higher but most respondents on this forum said it's programed into the ECM and can't do much about it. I agree it is pretty rough almost like it wants to stumble.

The Successful Plumber

The way my C7 LT1 idles is similar to the other three that I have driven. There’s nothing wrong with mine.

When my car was new I took my 88 year old father in law for a ride and he immediately commented on the rough idle. It rough idles at sea level with 91 and now at 5000 plus feet where the 91 should now should have enrich the air fuel ratio to where it’s more like run 93 octane. I guess it’s just characteristic of the way these LT1’s idle.

aklim

Rough is a subjective thing. Compared to a dragster, it is silky smooth. Compared to an MB sedan, it is rough.

The Successful Plumber

True.
My ‘04 Chevy 2500 6.0 gas idled rough. My ‘22 Ford F-250 7.3 gas idles very smoothly.

I’ve found that running fuel injector cleaner though them and clean the mass airflow sensor and throttle body helps a little.

Bill Dearborn

The octane level won't make a difference at idle. The engine isn't under any severe load and the engine is producing between 5 and 10 HP, just enough power to keep the engine turning and powering the belt-driven accessories. The octane level affects the engine when it is under full load where engine knocking can start to happen.

Bill

Exia

No difference for idle or street driving for 91 vs 93. If you will be doing high performance driving 93 should be used but for street driving and the occasional highway pull, you won't notice a difference.

aklim

Different engine, different purpose, different design so idle quality might be different.

Yep. Definitely will. You put the stuff in, the mind imagines an improvement and you will see it, feel it, etc. Google Placebo Effect. Many people have put all kinds of go fast goodies on the car, felt a huge difference until they get to the dyno or racetrack and are surprised at the results. I avidly distrust my own senses since they are capable of being influenced by all kinds of things I might imagine and go for data.

vader86

The octane rating here would make no difference for your idle.

Besides at your DA in Wyoming, the engine does not need the 93 octane, thus why local stations don't have it. If you put in 93 and don't need it, you'll actually get slightly worse gas mileage.

robert miller

May get a few haters here, But from watching a show on T V this last weekend. They did a show on 89 then 91 then 93 then 110 & 116 racing gas. With that said wasnt per say no diff in it at all as long as you are not at a track the only diff I saw from it would be when it would start knocking from the lower octane in it. But for a daily driver no diff at all. Robert

aklim

What car? What compression? On lower compression, sure. On a modified car with high compression? Good luck with that. I have seen increased knock counts on lower octane gas on higher compression motors on hot days and no difference on low compression cars. How did they test the difference? Scanner or butt dyno?