They filled my SS with 87 at the dealer when I bought it. I ran it down to 1/4 tank and changed over to 93 octane for a while and when gas started to spike agan I tried 87 and have stayed there since. If there is a power loss it can't be more than a modicum and there wasn't much noticable milage loss until they started to change over here in the Tampa Bay area to 10% ethanol. Now I can't find 100% gas anywhere. Record high gasoline prices and decreased milage , bummer

 

In a car designed for 91 octane fuel, 87 oct means less spark advance due to knock sensor, less spark advance = less efficency, less power, less mileage. At $4.10 a gallon, how much less mileage does it take to over-ride the 20 cents more for premium? Not much. Lets say you get 1 mpg less in city driving at 20 city, that is roughly 5% less mileage, 5% of $4.10 is 20.5 cents, so you break even on costs, and have ****ty performance to boot. (this is a turbo after all). So why bother?

 

Lets take a look at what octane does to gasoline, octane is an additive that causes the gasoline to be LESS volatile. Sounds strange, why does gas that is harder to burn cost more? It's marketing, higher octane fuel is not better gas, it is actually worst from the context of it's flash point.

Why do we need higher octane in some engines, well, the higher the cylinder compression ratio along with forced induction requires fuel with a lower flash point so that the entire air fuel charge in the cylinder can be ignited all at one time and on command. Higher octane fuel does not pre-ignite on the compression stroke therefore all of the energy of the charge is delivered to the piston at one time thus causing the most bang for the buck.

E85 may have a higher octane rating but so what. It's energy density is very low (double digit percentage) compared to gasoline. This means in order to generate the same amount of power from an engine, the fuel charge has to increase along with the air charge thus consuming more cylinder space and reducing the compression ratio thus removing the need for a higher octane rating.

E85 is nothing more than a political passifier to the energy crunch. Politicians blab it because that is what they think we want to hear because we the people are looking for quick answers to oil prices.

 

Another thought, .20x15= $3.00 if you can not afford that small of a difference in gas then one has other problems.

I get better mileage with 93 vs 87, it averages about 2MPG better with 93.

For what it is worth, I put 93 in the HHR and my 9-3. In the HHR I SAVE an extra $100 a year factoring in the difference in gas mileage. Since the 9-3 gets lower mileage than the HHR but factoring in the same MPG difference between gas grades I save over $200 a year in the 9-3.

HHR Example:

Driving 15K a year @ 22MPG using regular gas you would use roughly 682 gallons of gas @ 4.00/gallon = $2728.

Driving 15K a year @ 24MPG using premium gas you would use roughly 625 gallons of gas @ 4.20/gallon = $2625.

$103 savings per year.

SAAB Example:
Driving 15K a year @ 17MPG using regular gas you would use roughly 789 gallons of gas @ 4.00/gallon = $3528.

Driving 15K a year @ 19MPG using premium gas you would use roughly 882 gallons of gas @ 4.20/gallon = $3315.

Savings $213 per year.

If maintaining a constant 2MPG improvement using premium gas, you would have to see 38-40MPG to break even.

 

The amount of spark advance is a product of many variables from throttle request to the environmental properties the engine is operating in. For example, regardless of the fuel octane being use, timing is not pulled until the inlet air temperature exceeds 81 degrees.

I agree with what you said but only under some operating conditions will loos of timing advance have an effect.

 

Wow.... Oh, just wow!!

There is no such thing as an octane sensor. Most cars today have a knock sensor that detects knock. If the ECM algorithms regularly detect knock, it will compute what is known as the Knock Learn Factor, which will average down between the High and Low Octane timing tables, IOW, lower timing. Lower timing will result in a very small change in mileage and a more noticeable change in power.

Octane, or more accurately, iso-octane is the PRIMARY ingredient of gasoline, not an additive.

Volitity has little to do with it. Higher octanes actually are slightly harder to ignite, but also have a slightly slower flame propogation rate, allowing earlier ignition in the engine cycle.

There is nothing in any calibration file that I've looked at that would hold off retard until IAT exceeds 81*

Engines won't run quieter, generate more carbon.... etc., etc.

Where are some of you getting your information from??

 

I run 87 in my SS. I only notice a slight decrease in power at heavy loads and throttle probably because the knock sensor may be detecting something that I can't hear, then it retards timing slightly until it's happy. JoeR is right, there is no "octane sensor" in the system and regular fuel does NOT burn any cleaner than higher grades..........higher octane just slows flame propagation as stated. I am allowed to use company gas where I work, it's 87 and I'll continue to run it because at normal throttle/loads, my engine runs fine.

 

RTFM, right from GM regarding the carbon build up.

 

I remember a few years back when one of the TV stations ran a test of fuel samples from numerous gas stations in all grades and on the most part there was no difference in the fuel from the different pumps. meaning the suppliers were putting the same fuel in all the tanks and nobody noticed until the tests were done...

 

Well JoeR, I am a senior controls engineer at GM Powertrain and I have designed and coded many engine controllers in my time and there certainty are octane sensors (don't think so? google it). If you have access to an instrument like the Dashhawk for example, you can view the octane sensor's output.

In the context of this discussion, volatility has everything to do with it and you described the meaning very well in the words that followed.

You did not reference what cal file you looked (nor did I) but I was simply offing an example, the example I used is from the IAT table for an LS2 engine.

So I'll ask you the same, where do you come up with this stuff?