hi octane fuel

How will higher octane fuel affect your a/f ratio?
Going to a higher octane fuel...will it run richer or leaner?

Since higher octane fuel generally has less energy per gallon, I'd say it would make for a leaner A/F ratio.

Thanks...I think you might be correct.After a lot of timing and distributor work i still occasionally got a bit of detonation so i tried Petro Can 94.What a huge difference.There must also have been some detonation that wasn't audible because acceleration is noticably stronger and smoother with no audible pinging....but idle and cruise A/F did go leaner.

Since that is an incorrect statement I'd say any conclusions derived from it would be incorrect as well.

Gasoline has the same amount of energy per gallon, regardless of octane.

All octane levels do is affect detonation and/or preignition.

Assuming nothing different other than octane levels you will see nothing changed.

i

Not having any experience in this, would your statement apply if higher octane was achieved by adding ethanol to the mix?

higher ethanol was my thought as well which would be less power.
I agree higher octane doesn't make more power,it is just more stabile?I hadn't adjusted my a/f for some time...maybe a bit of dirt in the carb or whatever?Would a detonation free engine be a more complete burn and give a lean reading?
..Time for one of the experts to weigh in?

The specific gravity of the fuel effects tuning through a carburetor, not octane.

Gasoline is a blend and gas blends can vary in energy BTU/gal as much as 4% in the summer/winter blends. Of course the octane doesn't reduce the BTU content, but without any additives such as TEL, the blend needed to get gas to a higher octane generally does.

& ethanol has less BTU energy than gas but E10 here is 89 octane & non ethanol is 87. that's all I know (or can add) on the subject. the non E10 is 10 to 20 cents more a gallon but even with that I avoid E10 like the plague (& I got a gas hog).

,Somewhat The oxygenated additive fuels like Q16 do need different jetting than C16, usually 5 to 10% richer to go as fast as they can on that fuel

Yes and yes. What we call gasoline is a pretty complex mixture of components. Sometimes the effect of one component or property is not straight forward.... But I think we agree the basic point about density is generally correct.

The Octane listed on the pump is just the Anti-knock number, not a component of the fuel. This is why octane can exceed 100, its not a percentage. In the USA the number is the Anti-knock Index (AKI) which is the average of two octane tests. These are tests against a mix of two fuels; one with octane ratings of 0 (heptane) and the other 100 (isooctane). see Chevron's Motor Gas Tech linked here

Oxygenated blends tend to be less dense and therefore lower BTU. But if the engine can use the oxygenates well, it may produce more power.

A wide band oxygen sensor is an indirect method of measuring AFR. If there is a question about density, then it is better use the lambda number rather than an AFR number. The AFR is based on an assumed stoich, and the stoich varies with composition and density. Suppliers of racing fuels publish stoich and density.
examples:
VP Master Fuel Table
Sunoco Race Fuels Comparison Chart

Street fuels are more difficult to deal with because their properties vary so much. As pointed out, most regions of the country have winter and summer blends. Automotive fuels throughout the country also have regional requirements due to climate as well as regulation. The suppliers need to stay within those requirements, but that can allow quite some latititude.

Thanks for a good answer. Cab was also pointing in the right direction.

So the answer to the first two questions is that higher octane fuel will change your Air/Fuel ratio either up or down, and it can make the mixture either richer or leaner.

Higher octane fuel either has more, less or the same energy per gallon.

Trace knock happens and can affect power output. Changing to a higher octane fuel with everything else being equal can cause
increase in power due to elimination of trace knock.

Gasoline DOES NOT have the same amount of energy per gallon regardless of octane. It can vary, as gasoline is a common name for a range of mixtures of molecules.

It is true that the specific gravity of the particular fuel governs the amount of fuel metered by the carburetor. While this will have an effect on the mass Air/Fuel ratio, the effect on whether the mixture is richer or leaner can go either way.

Ethanol has less energy content per gallon, but a higher octane number. My rule of thumb is that 10% ethanol results in a 5% loss in mileage. As I have been using oxygenated fuel since 1991, I have had no problems with it. For about 85-90% of the country's population, oxygenated fuel is all the EPA allows to be sold except for race gas which is generally for off-highway use. All the urinating and crying about alcohol gives me a headache. It's funny that most of you have no objection to putting ethanol into your bodies! I think I'll go have a beer.

Racing gas companies have had no problem using ethanol in their fuels. Indy cars run on pure ethanol, switched from methanol to look "green".

Truth is, every different fuel needs a specific tune, and that's why Oxygen sensors are so valuable. The post talking about Lambda is right.

Now on to detonation. The absence of detonation does not mean it was a cleaner burn. Not in the least. For example, one way to reduce detonation is to make the mixture rich. All the absence of detonation means is that whatever was in the cylinder didn't self-ignite. Octane ratings of fuels measure the knock resistance of a particular fuel in a specific test.

So it all boils down to fuel chemistry, and since Fuel is a product manufactured by different companies and facilities using highly variable feedstocks, there aren't any successful generalizations. As Cab alluded to, each fuel has its own best air/fuel ratio. We're lucky that what comes out of the pump is pretty consistent.

R.

Good stuff Dog.

There was an EPA study of gasoline at retail outlets several years ago showing a pretty large range of 107,000 to 116,000 BTU per gallon across the 50 States.

The automakers for years trucked in Chevron Techron 87 AKI to their Detroit labs for emissions tests because it was consistently high in BTU per gallon.

My fuzzy memory is that pure Octane is around 121,000 BTU per gallon.
It used to be a standard that pure Octane fuel was used in scientific engine tests.
While costly this was a good thing for consistency and "replicating" experiments.
It is not real science unless original test tesults are replicated.
That is always worth remembering.
Science is not theories or computer models.
Science at heart is solely successfully replicated real world tests.

I would bet that no one on Moparts has run as many BTU/calorie tests on the original Parr Calorimeter machine with its "simulated perfect insulator" water jacket system as I have, even though I have not done one since 1990.