LDB

With high gas prices, there have been lots of octane questions lately. Yes, you can use regular or mid grade without grave risk to your engine, but…… This post summarizes the “buts”. For those who haven’t seen my posts, I’m an oil company engineer with one of the majors, but I keep my brand out of it. The Forum is a good place to share knowledge, so I’m simply trying to share where I know the facts.

First, understand how your engine compensates. There is a knock sensor which detects when your engine starts knocking. It can eliminate knock by retarding the spark. So when you use regular or mid grade, your engine knocks momentarily, the sensor senses the knock, and it eliminates the knock by retarding the spark, probably before you even notice that it’s knocking. Do not kid yourself into thinking nothing has happened. Trust me on this. If you are running regular or mid grade, your spark is running retarded.

So what, you ask? There are three effects. First, you have slightly less power. It is not a huge drop, and the Vette has so much power, you probably won’t feel the loss in the seat of your pants. But if you measured acceleration time with a stopwatch, you will have lost a few tenths of a second in 0-60 time. Second, your gas mileage will be slightly poorer. This will be even more difficult to see than the acceleration time because so many thing affect mileage. But if you ran an average over many tankfulls, you’d lose half a mile to a mile per gallon. Third, you will frequently have mini-knock episodes that you probably won’t even notice. That’s because the engine computer assumes you will normally run premium, so it keeps trying to run the spark advance back to normal, then encounters knock, and has to retard it again. This is not a serious engine damage risk because the computer is so quick at detecting and correcting. But it is some slight risk. Do the sum of these three issues equate to 30 cents per gallon? That’s for you to decide. The important thing is that you know the facts, and don’t go around telling people that you use regular and there’s no difference. Huge difference? No. Difference? Yes. 30 cents per gallon worth of difference? Your decision, not mine.

Incidentally, while there is a performance penalty for octane below 91, there is no benefit of octane over 91. That’s because there’s an optimum spark advance. Performance is degraded in either direction from optimum, both in the less and in the more directions. The Vette is designed for 91, which means that it can get to optimum advance at that octane. Once there, it does not keep advancing the spark, because that would actually lose performance. Thus, octane over 91 is wasted. Slight fib there. As an engine ages, deposits very slightly increase the required octane to avoid knock, perhaps by a number at very high mileage. But other than that, the only circumstances where greater than 91 octane would be of benefit would be if you modified your engine, such as raising its compression ratio or adding a supercharger. So all you California people who complain about not being able to buy 93 octane shouldn’t worry.

Indy-hp

I've read most of your prior posts. Good info and timely. Thanks for posting.

AFVETTE

Unless you track your car, the only penalty would be a slight loss of HP/TQ if the knock sensors detect any pre-detonation and then pulls some timing out.

In everyday driving you'll never miss the 10 or so HP lost. But I'm sure you'll get the usual responses wondering how you can afford a $50K car and can't find a few extra dollars to feed it the best fuel.

Bottom line, you won't harm the car.

Tom

KENZ51

Thanks for the info!

RJMan

LDB -

Thanks for the great post. You're one of the reasons why this forum rocks!

I've been one of those that have been feeding my Vette 93 octane, because it is readily available. Next fill-up I'm going with 91 octane and save a few bucks.

zuldane

Great post, and very informative as usual. Thanks for the information!

millpond

Thanks for the great post!...After reading it, I plan to continue feeding the Vette what it was designed for.

ZeusC6

A couple of questions I have, maybe you can respond.
First, what efeects does ethanol have on our engines?

Second, does it matter where you buy your gas? I use BJ's Wholesale Club because of membership discounts per gallon, but is it OK to use versus the majors?

Thanks in advance!

UNKNOWN

ty cool

Michrider

Assuming that the knock sensors and ignition retard can completely eliminate engine damaging spark knock is not true.

You can get away with a little less octane than desirable, and the computer will compensate. But put 87 in an 11 to 1 CR motor like the LS7 and it will rattle to death. The reason is some spark knock has nothing to do with timing, it's the mixture self igniting (preignition) and the flame front hammers the top of the piston. The owner's manual cautions that engine damage can occur from severe spark knock.

I've heard it tons of times in plenty of engines, all with knock sensors. Every pickup I've owned with a 5.3L V-8 will spark knock like crazy if I don't blow the carbon out once in a while. I tried a K & N filter in my 04 Z LS6 once. The motor had ran flawlessly on 93, until the K & N and the first time I went wide open. It sounded like shaking marbles in a tin can, and a big puff of dark soot out the back. Just that filter making it a hair leaner put the engine into severe spark knock.

87 is for lawn mowers and grocery getters. In the vette, buy the best.

GrayC5

Excellent information and very well written! Thank you very much for posting this!

antoniopaolo

Excellent information and much appreciated. thanks.
antoniopaolo

VetterGetIt

Good post and thanks.

harrisraw

Ditto. Its funny, my GF was asking me a question about using 91 versus lower grades of fuel this past weekend. I just sent her your posting to explain it further.

I don't know if she'll understand it all (any), but thanks for what I feel to be is a thorough, but not verbose explanation of the subject!

VET4LES

Good information.

RedHot

Great stuff.

LS1LT1

Ok that is the part that concerns me. I've always sought out 93 octane (though I passed on the Sunoco Ultra 94 when they still had for some of the very reasons above) because I've felt that it could potentially help my (mildly modified) LS1 Z28 and my stock LS2 C6 perform a little better. I have these fears that if fuel sits around to long in storage, or has some other contaminants (ie: water) and such in it that it would still be better to start with a 93 versus a 91 octane so one would at least have some more padding or window of degradation (ie: 93 getting reduced to a 91, 91 getting reduced to an 89 etc.) built in.
I generally only use Amoco Ultimate 93 or Mobil 93 in my performance cars for this reason but would consider 91 to save a few bucks, though they're actually all virtually the same price in my area anyway LOL.

HyperX

The octane mix is added as the trucker fills up the gas station reservoirs, right? so how do we know for sure that the truck driver did his job.

Scuba Ghost

If you fill up and put in 15 gallons you are talking about a whole $1.50. I'm not bragging but I can afford the $1.50 to keep the Vette running the way God intended her to run.

rothchilds

no doubt, it's like people who drive across town because gas is .05 cheaper, but they don't calculate the fact that they burn a gallon going to and from, thus negating their savings