# Fuel Octane

## Main Question or Discussion Point

I have had a pressing question haunting my mind for quite some time, and I need an expert opinion.
My car calls for premium fuel (2007 Honda Civic Si), yet states (in the owners manual) not to use gas below the octane level 87. Now I purchase my gas from Sinclair and the octane levels read:
Super: 88
Regular: 85

I don't have a problem using premium gas, other than the fact that the price is escalating rapidly. I would prefer to use a gas with a lower octane level, however I don't want to use a 88 octane gas. So, my question is: If I wanted to mix the 88 with the 91 and achieve a overall octane rating of 89, how would I do this?

Parameters: 12 gallon tank

I'm not highly aware of the properties and characteristics of mixing octane levels, but if I were to do the math I would do this:

For every 1 gallon of 88 octane there is 88% iso-octane and 12% heptane.
For every 1 gallon of 91 octane there is 91% iso-octane and 9% heptane.

(8 gallons of 88 octane + 4 gallons of 91 octane)/12 gallons = 89% iso-octane and 11% heptane.

Is this correct?
Also, is there a large difference between 1%-2% iso-octane, relative to a car engine?

Thanks.

## Answers and Replies

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russ_watters
Mentor
You are correct, but why don't you just use the 88?

FredGarvin
You are wasting money by buying the higher octane fuel. Unless your owner's manual specifically state to buy the higher octane, it is not doing anything for you.

djeitnstine
Gold Member
You are correct, but why don't you just use the 88?
I second that

You are correct, but why don't you just use the 88?
Its mostly personal preference. The owner's manual states to not use anything less than 87 octane, and 88 is rather close....but I'm just weird that way, I guess. I could try it and document the characteristics that it presents and see if it has any adverse effects...hmmm

You are wasting money by buying the higher octane fuel. Unless your owner's manual specifically state to buy the higher octane, it is not doing anything for you.
Could you explain what you mean in greater detail?
So you are saying I could use a fuel with an octane level of 50 (if it did exist)?

The reason, I believe that lower octane ratings are bad, is because greater levels of heptane exists, meaning that it has a higher possibility of premature combustion. Thus, "knocking."

It seems also that the difference between 1% octane seems significant, because of the closeness of the different fuel ratings.

Maybe I will use the 88....

Thanks.

Most modern engines have knock sensors that will help protect your engine by retarding the timing when knocking is detected. You will suffer a momentary decrease in power (you might never even notice) if the timing retards but are not risking any damage using the 88 octane. I would mix the 88 and the 85 if the money is worth more than your time.

FredGarvin
Last edited by a moderator:
I had heard rumors of this, however I have never heard it from creditable sources. I suppose that we all are try to take care of our cars and spending money on a "better" gas seems like the best way to prolong it's life, however that is not the case. It's like we feel guilty for following the recommendations that the manufacture suggests (at least I did).
I found it interesting in USA Today article where Porche designed its cars to run on regular gas, because of the lack of a "premium" fuel in regions where their cars are sold. Somehow this gave me greater belief that running a lower octane in my car would be suitable.

Thanks.

brewnog
Gold Member
There's a difference between the fuel's octane number (which doesn't have anything to do with the amount of octane in the fuel anyway) and the 'quality' of the fuel. You could have a fuel with a very high octane number, full of crap which will cause excessive fuel system wear. It would just be very knock-resistant. Similarly, you could have a high quality low octane fuel, which (other than being knock-prone) has all the right additives, in all the right amounts, and no nasties.

You can take care of your car by using higher quality fuel; but just looking at the octane number won't really give you a meaningful measure of this.

After the first few thousand miles on the engine, I would use whatever it calls for in the manual. Most engines, especially of the normally-aspirated variety, are probably not going to be able to fully use higher octane fuel, which is why you will see black exhaust (from unburnt hydrocarbons), or so I have been told, though I am hardly an expert in the subject.

I know that my Saab's manual calls for high octane gas for maximum performance, but will reduce the turbo pressure to accommodate for lower octane gas (but when I put low octane gas in the car, it feels sluggish, like driving a Honda Accord).

Bottom line: I would use whatever octane of gas the manual suggests. Using lower octane gas can cause knocking and rob you of performance, and using higher octane gas is probably going to be a waste of money, since it is probably not going to give you a performance boost.