My wife has a 2007 Pont. Grand Prix GXP. (8 cylinder) I've seen some posts about octane ratings and wanted to ask some thoughts on a matter. Along with my own car I usually fill her tank when its about 1/2 empty. When she 1st got the car I was using 93 octane (the pumps in my area usually have,,87,,89 or 93). Seems shes getting really crappy gas milage as the DIC shows 13.5 miles per gallon,, so what I do is ocassionally put 87, 89 or 93 octane in, so that at any one point the octane in the tank is a mix. My question is this,,I know she can run the 87 octane and what it will do is basically reduce her h.p.,,I was wondering if by changing the octane rating as often as I do,,that being at just about every fill-up,,am I doing any harm as the cars computer might be confused as to whats going on with the mixture of gas with different octane ratings being used,,and one other thing,,if you mix say a half tank of 87 octane, and fill it the rest of the way with 93 octane,,is the octane rating in the tank now 90,,to me it would stand to reason that it would be,, just curious if thats the way it would work out. Thanks. BTW even thou I know I can run lower octane in my car I use the 93,,,its not worth the few pennys saved to me to use a lower number.

 

I may be wrong here but in general octane rating has nothing to do with your MPG unless you are running the wrong gas.

Octane rating is just another word for how fast the fuel burns. The higher the octane the slower the burn. If the car is designed to run on 87 octane fuel running a higher octane could cause an incomplete fuel burn.

It is always best to run the octane rating that manufacture recommends unless you are running an aftermarket tune.

 

This is not a situation where the computer “learns” the best settings over time. If it hears knock it retards timing until the knock goes away. There’s nothing more to it than that. So your mix of octanes isn’t going to confuse it.

 

[QUOTE=TMyers;1564233972]I may be wrong here but in general octane rating has nothing to do with your MPG unless you are running the wrong gas.

I believe if you use a lower octane rating it will reduce your h.p.,,not your m.p.g.,,someone can correct me if I'm wrong,, I was more concerned about the use of different octane ratings that I use and not the fact she gets crappy milage.

 

You will lose power because of detonation and the computer pulling timing. The octane rating determins how well it burns in resistance to detonation. Using a lower octane than recommended is silly in my opinion. Have you done the math between the price difference of 87-93? It doesn't equate to a whole lot per tank. Let's say a $.40 difference between 87 octane and 93. At roughly 18 gallons, you're looking at $7.20 per tank difference. Seeing as how you fill up at half a tank then you would see a difference of $3.60 per tank. Even filling up the entire tank once a week, would only come out to about a $375 difference over a year between 87 and 93.

In regards to the computer. There were a few theories way back when about computers learning the octane but those have all just about been debunked. The computer will react to whatever octane it's burning. You put in 87 and you will lose power and the engine may run like crap. You put in 93 and you will run on full power and have a strong engine. Mixing 87, 89, 91, and 93 averages out to 91.5 given equal parts of each(which will be hard to do since you constantly switch). Why not just run 91 then anyway? 93 preferred since it's available. I really don't understand your motivation.

 

actually, quite awhile back there used to be a practice of mixing part low and part high octane gas and actually coming out with a higher number (higher than the highest octane put in the tank). I don't understand the math or science but supposedly it was true.

 

Actually octane=the amount of anti-knock compounds added to the gas, and has absolutely nothing to do with the volatility/burn rate. Lower octane gas allows more premature detonation(pinging) especially at higher combustion ratios. The car responds accordingly which reduces horsepower.

Detergent levels are also interesting, and really do prevent carbon from accumulating. See these or do a search to learn more.

http://forums.corvetteforum.com/show...ghlight=octane

http://www.toptiergas.com/

 

Again what I was trying to say is what is the recommended octane for the vehicle she is driving. If it is 87 you will see no improvement when running 93 octane. The vehicle will not advance the timing enough to take advantage of the higher octane.

Conversely if the recommended octane is 91 octane running anything less is leaving power on the table.

 

Not possible (unless Torco were part of the mix).

It's just simple averaging:
9 x 87 = 783
9 x 93 = 837

783 + 837 = 1620

1620 ÷ 18(gal.) = 90 octane

Running a higher octane than the engine requires will reduce gas mileage and produce no beneficial effect. Since the car is designed to run on 93, there is no point to this exercise.

 

This was true of leaded gas. The effect of tetraethyl lead on octane is not linear - the first bit of TEL adds a lot of octane. So when you mixed high and low octane the result was higher than the average. Not true of unleaded.

 

ah ha! thx!

 

both right! thanx!

 

Isn't it amazing how a post can produce so many replies,,her car is suppossed to run on 93,,I posted the original question to ask about my habit of changing the octane rating so often. BTW I did read in another post a question as to what octane the guys here are running in their vettes,,,for me it is ,and always will be 93,,its not worth the few pennys saved for me to do anything else.

 

Work out your gas mileage by hand to see how it compares to the DIC. I know in my GP it is off a little...

 

Ok now that you have cleared that up running less than 93 is leaving power on the table. That means you will lose MPG.

 

Quick question - since the C6/Z06 has dual gas tanks and I mix 91 octane with 100 - how can I ensure that they will mix properly? Or even with any gas additive for that matter?

 

I was gonna call BS on this post, but I went and read up on it and it's true. All this time, I thought that the octane rating described burn rate. It doesn't at all, it describes the fuel's resistance to knock.

http://en.wikipedia.org/wiki/Octane_rating

Thanks!

 

I disagree. What do the anti-knocking compounds do? How is the gas resistant to knock? It burns slower which prevents the detonation/pinging.
The slower burn rate would explain why when using higher octane gas in a non-computer controlled engine requires the ignition timing to be advanced to allow a more complete burn of the fuel.

 

You can disagree all you want, you're entitled to your opinion. But the facts are that the octane rating of a fuel has absolutely no correlation to its' burn rate/flame speed. In fact, a slower burning fuel is more prone to detonation and is the opposite of what you want. Rocket Brand has a 118 octane fuel that is formulated to burn very fast for use in big bore high compression 500+ cu in 10,000 RPM drag race motors that will also make more HP in a regular 9:1 stock class cars because they can reduce the timing advance (which reduces the negative work on the piston) and still have peak cylinder pressure occur at the optimum 11-15 degrees ATDC. The higher octane/slower burn is a common misconception like the higher octane/higher energy myth. Here's some more reading for you with some good links:
http://forums.corvetteforum.com/show...9&postcount=31
http://forums.corvetteforum.com/show...7&postcount=32

 

This is also a good point. If you have a supercharged GTP as my mom does it will effectively create a need for higher octane fuel under boost. I have told her and told her to run premium but she almost never has the sc kicked in to be under any amount of boost and she has run it on midgrade for almost 30k. The engine won't let itself get harmed by the lower octane it will just adjust the timing via the knock sensor. Performance may not be 100% but in some instances that really isn't of importance. If you want full power potential from the car then by all means run premium if not then mix it or run straight midgrade. Following the manufacturers recommendations is usually the best bet, but some people like to pinch a penny here and there and make themselves feel like they are saving money.