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I've heard about studies stating that 89 octane is the best grade fuel you should put in your vehicle to get better gas milage. Is this true?

You should always put what is required into your vehicle according to the owner's manual. Some vehicles run better at higher octane levels, and they will "recommend" a certain level to fuel it with. However, for a vehicle that normal runs (and runs normally) on 87-89, putting 93 in will not increase power, fuel efficiency, or anything else for that matter.

Just your credit card bill. :)

Yeah, I have heard that "chemically" 89 octane contains more energy. But if you have a high compression engine (above 10), the owners manual should recommend you should use 92-93.

I was running 87 octane in my Saturn-my son is still driving it- after the head was shaved and it was at 10.5:1. I put the car in 5th and at 25 mph with the whole family in the car there was no pinging. Of course, the head also had the Sommender Singh grooves in the combustion chamber and Power Lynz in the intake ports that helped with combustion efficiency.

Octane rating is a rating of how much the fuel can resist detonation so, the lowest possible octane you can run, without detonation, should give the best FE since it would ignite the easiest. My $.02 worth.

Hi All:

___Usually, the higher the octane, the lower the BTU content. It has been a while but I thought I had read that in the EPA’s multitude of RFG docs.

___The Acura MDX recommends 91 and I have been using 87 in it since I first filled up. 30 + mpg segments are the norm in that beast. If you are not into the throttle or running high loads, the KS will not pull back the timing and you will see no difference between the 3 grades of fuel.

___Good Luck

___Wayne

I would say go with what the book says. If it says use 87 then use it.

But beware. Auto makers are using some tricks. Lets take my Civic. The R-18 is spec'ed to run on 87. But it has a rather healthy compression ratio of 9.5:1. In the old days that would have been a compression that would have required higher octane. As a matter of fact super unleaded 91 to 93 is needed to be ran in a motorcycle I'm wanting to buy that has a 9.5:1 compression ratio. So what gives?

Well Honda got tricky. They put oil nozzles that spray up into the piston dome to control heat. Thus no preignition or knock. It also has some trickery in the ECU that Honda does not wish to talk about. The R-18 is a good engine for turbo charging. With no exhaust manifold to speak of a bracket and turbo right off the head is a sweet thing. But the aftermarket folks are having a devil of a time with Hondas basic ecu code. I don't think Honda data is even going to mess with it. Thus I believe everything for lean burn is in the ECU. Just turned off. Well I know it is.

If I run a higher octane fuel in it. Results are mixed. It seems to favor it in the winter months and hates it in the summer months. With the SG you can see it advance its base mark on the ignition timing over the course of a a hour or so of driving for the use of say 91 octane. Run a few tanks thru it. Then go back to 87 and over the course of a hour or so the baseline timing will retard. Why it doesn't like it in the heat of the summer I do not know. I figure its some of Hondas trickery in the ECU. I think its elements of lean burn that they massaged to make the semi atkins head and intake work. you can see the timing advance and retard at steady loads as if its looking for something thats not there. Thus causing bad mpg and sluggish performance.

So best bet would be to run what they say to run in your car. Because you might hurt the performance in certain applications trying to fake out motther care maker. :)

I'd think the answer is generally the lowest octane that the engine does not knock.

Most of the recent models take 87.

Octane is a measurement of thermal resistance, not BTU/caloric content. That would be Cetane.

As Delta said, most modern, non-supercharged vehicles only require 87 Octane. As psyshack said, most ECUs retard the timing to accomplish this on high compression engines.

For example, my Yaris' 1NZ-FE engine has a compression ratio of 10.5:1, which is decently high, but the recommended fuel is 87 Octane.

Hi Brian:

___It is not a measure of BTU content but there is a difference. The higher Octane reduces flame front propagation speed but Octane increasing additives such as Ethanol is a BTU reducer. This is a broad statement as E10 based RFG from 87 – 91 Octane contain the same % of Ethanol. Somewhere on the EPA’s site in one of the many RFG doc’s, there was a very good report and IIRC, the BTU content was lower as Octane increased in the samples tested. I do not know if this is the same today as it was back in 99 – 05 but it was an interesting data point none-the-less.

___Good Luck

___Wayne

Well, my car is rated for 91+ octane, so putting a lower grade of fuel in it retards the timing so much that my mileage drops and it actually increases my cost/mile. So, I begrudgingly keep feeding it 93 octane.

Well, my car is rated for 91+ octane, so putting a lower grade of fuel in it retards the timing so much that my mileage drops and it actually increases my cost/mile. So, I begrudgingly keep feeding it 93 octane.

That is one of those times when you can't do much about it. Sometimes you get the bear and sometimes the bear gets you.:)

Hi JCP123:

___The timing should only be retarded on knock and not before. You can see this on your ScanGauge II with a total fill of 91 almost to empty and then with a total fill of 87 afterwards. I bet you will not see any difference “if” you drive like a hypermiler. If your foot is in it, of course it will retard and all kind of bad things will happen. Just keep track of how advance changes during accels at higher and higher RPM’s under both fuels.

___Good Luck

___Wayne