I was wondering why don't they use higher octane fuel. Wouldn't that allow more power (from a smaller engine) and fuel efficency? It would clean up the combustion chambers leading to reduced emissions, and allow higher compression ratios.

Japan runs something like 100 octane, so why do we have 87? The highest I have seen is 94.

Anyone care to comment on this?

 

Octane costs money. Costs more to boost it that high and the engines can be designed to run on lower octane without losing power.

 

So many people misunderstand the true meaning of "Octane Rating"........
It is simply a quantification of how difficult a fuel is to ignite. The higher the octane, the harder it is to ignite it. That is why low-octane fuel is recommended for such a great percentage of vehicles on the road; they have relatively low compression ratios and thus need lower octane fuel for it to be able to be consumed properly by the flame during the ignition process and subsequent power stroke. High compression ratio engines on the other hand, need higher octane fuel since the heat of compression would light the fuel prior to the spark occurring (pre-ignition) or the secondary compression that occurs after the spark lights the a/f mixture in the combustion chamber and the expanding burning fuel further compresses the portion not yet burning to the point that it spontaneously ignites itself elswhere in the combustion chamber (detonation). Both are deleterious to engine longevity. If you can remember when they used to advertise on TV for high octane gas there was tiny print at the bottom of the screen that said "for engines that benefit from higher octane fuels".
I can't remember the exact number, but diesel fuel is of significantly lower octane than gasoline, thus its ability to self-ignite due to the very high compression ratios common to diesel engines.
I am sure there are plenty of thread explaining this better than me elswhere in these forums, but they will affirm that high octane fuel in a 8.5:1 compression ratio engine is a complete waste of money.

 

Moved to proper forum.

 

I thought I read somewhere once that using higher octane than recommended by the OEM can mean hot spots within the cylinder and premature wear somehow related to that. But I read recently than modern engines with 10.5:1 compression are OK for 87 octane. I have a 68-71 385 series 429 rebuilt but it has 10.5:1 compression so I'm kind of on the borderline of what to use I guess. I've always used just the 87 but I was thinking about going to 89 for a while and see if it seems to run better.

 

The only think you didn't cover was the Octane rating being mainly an average these days with the anti knock index. That doesn't change any of your key points though. You pretty well rocked that explanation. The only point I would add is they measure Diesel with a Cetane number and there is no comparison to Octane rating. If you search hard enough you will find a chart or two that give an approximate Octane rating for Diesel, but as far as I know it has never been studied that way. Although, general consensus indicates a little diesel in your gas will help with ping, it will for sure raise the boiling point and help with vapor lock.

You can use a higher octane than recommended. All it will do is thin your pocket book faster. Now, running a lower than recommended octane will cause preignition and that will destroy your engine in short order. If you haven't noticed any issues, you should be fine. You're at the top end of what compression is acceptable for standard pump gas. It would depend on how much spark advance you run I would say. As far as running a higher octane, you won't realize any benefits from it. No more power or fuel economy.

 

Running a higher octane than you need can acutally degrade power. Slow the burn down too much.

 

Just an experience of mine.
Years ago I drove my parents Buick LeSabre home from Florida to just north of Detroit.
Bought gas and my dad saw that the station had 85 octane cheaper and my dad wanting to save money told me to fill with 85.
I did and wished I hadn't. That car had the Buick 350 in it with about 8.5 compression, and it knocked like crazy when I tried to accelerate, so I had to get tricky and maintain steady throttle position, back off on it when going up hills, and pick up great speed going downhill to use to get up the next hill. And it used gas like crazy. All this to keep from ruining the engine.
When I got to half a tank, I filled it with the highest octane I could find, probably 94, and it started to work better, no knocking and better fuel economy.
Bottom line is 87 was what it needed, a full tank of 94 didn't make it perform any better, but anything less than 87 was awful. That half tank of 94 probably brought it up to 89-90.
If your engine doesn't knock on 87, anything higher is a waste of money.
The composition of the gas can make a difference. I once filled my Aviator with Sunoco 91 and right away noticed higher gas mileage on my dashboard indicator vs other brands of 91, which is what was recommended.

 

Octane is like rungs on a ladder. You only need enough to prevent pre-ignition.
Higher octane fuels do NOT burn slower. They just resist pre-ignition.
Once ignitied, flame speed is the same as 87. That has been proven over and over on dyno tests varying timing. If high octane fuel burned slower, it would require more timing. Dyno tests prove otherwise.
Higher octane does not make more power unless you are experiencing fuel management or ignition issues.
Run what the owners manual says.