Lowest octane that doesn't ping gives the most power without damage.
Higher octane may allow more timing advance and higher compression, but if you don't have either of those as tuning options you get squat.

My 911 runs kinda bad for a few seconds after I start it. It pops and pings, then it starts running great and idles right where it should. It's an old car. I think some V Power by itself would do something for it. I agree about how the corn alcohol is not helping it in any way though. I'm not really sure octane booster really boosts octane either. I just don't like engines to ping at all. I want them to run with power for the very rare oppertunities like when a Lotus Espirit trys to pass me on I-30 just out of Memphis. :rolleyes:

Carbon build up can bump compression ratio, the other nasty common source of ping is OIL. Suck some oil in from valve guides or vent system and that drops octane rating a bunch.

Ethanol is actually beneficial, it is a better anti-knock agent than the old fashioned lead additives. the only real problem is making sure that all rubber fuel lines are ethanol resistant.it will rot non-resistant lines real fast!
In Australia we only have 93, 96 and 100 octane,(lead additives were made illegal about 20 years back,) for a while there, older cars knocked and pinged and were not nice to drive without expensive synthetic lead replacements ( a bottle of metholated spirits per tank was a cheap fast alternative, Metholated spirits in this country is actually Ethel alcohol, but they keep the Methyl on the label to sort of frighten off the drunks)) The introduction of ethanol in petrol about 5 years back, actually removed the need for the additives.no more knocking.
Buying a 93 blend with 10% added ethanol actually boosts the octane rating to about 95.
(I'm amazed that any performance car can run on 89 octane:confused: What compression ratio's are US cars?? 8 to 1? have to be low to run on that low an octane.


Great article here on lead/ethanol as octane booster
long read, but worth it
http://www4.hmc.edu:8001/Chemistry/Pb/resources/secret%20history%20of%20lead.pdf

Thought this post (and a couple of others) from The Straight Dope might be of interest:

Q. In this time of high gasoline prices, the Teeming Millions need your guidance (well, at least I do). What is the difference between premium and regular gas, and is this difference worth the extra money? I normally put premium gas into my car because I don't mind paying two or three extra dollars at the pump. Am I being scammed by the gas stations, or is the benefit to my car worth it?

— Jeff, via e-mail

Not to introduce a radical concept, Jeff, but have you tried reading your owner's manual? If it says to use premium, my advice is to use premium. If it says regular, use regular. The fact that your note indicates no acquaintance with such matters suggests that you may in fact be a scam victim, assuming by this you mean "someone who believes what he hears in commercials." I have a hard time working up much outrage over this deception, since discovering the facts requires so little effort. If you don't mind paying the extra money for no reason, don't expect the oil companies to suffer any pangs accepting it.

In most of the U.S., regular gas has an octane rating of 87, midgrade gas is 89, and premium is 91 or 92. (Octane ratings are lower in the mountain west due to the effects of thin air on internal combustion.) Contrary to widespread belief, the octane rating doesn't indicate how much power the fuel delivers; all grades of gasoline contain roughly the same amount of heat energy. Rather, a higher octane rating means the fuel is less likely to cause your engine to knock or ping. Knock, also known as detonation, occurs when part of the fuel-air mixture in one or more of your car's cylinders ignites spontaneously due to compression, independent of the combustion initiated by the spark plug. (The ideal gas law tells us that a gas heats up when compressed.) Instead of a controlled burn, you get what amounts to an explosion--not a good thing for your engine. To avoid this, high-octane gas is formulated to burn slower than regular, making it less likely to ignite without benefit of spark.

The majority of cars are designed to run on regular gas, and that's what the manuals tell the owners to use. Higher-performance cars often require midgrade or premium gas because their engines are designed for higher compression (higher compression = more power), and regular gas may cause knock. If your car needs high-octane gas, the manual will say so.

Using high-octane gas in a car designed for regular accomplishes little except more rapid combustion of your money. Some refuse to believe this, claiming, for example, that premium gives the family Toyota better mileage or more power. These people are in dreamland. Others say premium is purer or contains detergents that will cleanse your engine of uncouth deposits. Likewise misguided thinking--government regulations require detergents in all grades of gasoline. (BP Amoco, I notice, asserts that its premium gasoline contains more detergents than legally required; if you think that's worth 20 extra cents a gallon, be my guest.) Some automotive types claim that using premium in a car designed for regular will make the engine dirtier--something about deposits on the back side of the intake valves. I've also heard that slower-burning high-octane gas produces less power when used in ordinary cars. Believe what you like; the point is, don't assume "premium" means "better."

Occasionally you get some genius who takes the opposite tack--he spends an extra 10 or 20 grand buying a high-performance car, then decides he's going to save three bucks per tankful using regular instead of premium as specified. He figures as long as the engine doesn't knock he's OK. Wrong, carbon monoxide brain. Car engines nowadays contain knock sensors that detect detonation and automatically retard the spark to compensate. The delay means maximum gas expansion occurs when the piston is farther along in its downstroke and thus there's more room in the cylinder head. This reduces peak cylinder pressure, eliminating knock but also giving you less power and poorer mileage.

You may ask: Don't knock sensors make it hard to tell when an old car needs higher-octane gas? Years ago, when your beater started pinging on grades or under acceleration, that was the sign that carbon had built up in the cylinders, increasing compression, and it was time to switch to high-test. Now the knock sensors compensate, which seemingly might conceal the problem. Don't fret--today's fuel injection systems precisely meter the fuel-air mix, resulting in fewer unburned hydrocarbons and less carbon buildup. If you're still concerned, I'd say it makes more sense to spend $6 on a bottle of carbon clean-out juice than an extra $150 a year on high-priced gas.

— Cecil Adams

Source: The Straight Dope: What's the difference between premium and regular gas?

Q. Will higher octane (than called for in the manual) give me better gas mileage?

A. Octane is a measure of resistance to preignition. If your engine is not preigniting (pinging or knocking) then higher octane gas can't solve a problem you don't have.

An engine designed with high compression will need high octane fuel, and get better gas mileage than the same engine with lower compression, but that's because of the compression ratio, not the gas.

I look at it like giving a performance beverage like Gatorade to a couch potato. His low performance in a 1/4 mile foot race is not due to dehydration and electrolyte imbalance, so the Gatorade won't help him.
Source(s):
Automotive Consultant and Buyer's Advocate for 18 years, and Automotive Writer. Gasoline .. Does the Octane Rating make a difference ? - Yahoo! Answers

And, finally, here is a link to a 13 page document presented at a recent World Congress of Petroleum Engineers that seeks to answer the same question: Octane Number and Automotive Gasoline Engine Performance

The conclusion? Higher compression demands higher octane which will prevent pre-ignition; however, the energy derived from all gasolines is, for all intents and purposes, the same (not including some parts of the Dark Continent where gas can contain some 20 percent water).

ethanol is extremely hydroscopic which is very bad
ethanol contains less energy which lowers fuel mileage and overall consumption which is bad
modern gas with ethanol wont' store long as it deteriorates quickly.
an engine not designed for ethanol won't even run at 20%
I believe that the octane rating already accounts for the ethanol that's in it.
Ethanol in an engine that is designed around it would be very good. Ethanol in our old engines sucks

Gasohol -
 

Hey Pete - Amen Brother, From the effects of gasohol I've seen personally and with all the related performance and storage issues that I've read about, if we were still in the cold war era I'd be convinced that the production of gasohol was a communist plot to screw up the economy....;)

Michael

We have ethanol for one reason.. politics and our mid western corn lobby, that's it, no other reason

I supose it all boils down to compression ratios/octane, Is this why the Euro models are so popular in the States? all cars here are Euro spec.
A majority of cars in Australia are Japanese, Japanese engines (at the performance end) run high compressions , I have installed a 1JZVvti (2.5 litre single turbo, standard 10.5 /1 comp) in a 260Z actually ran better on an ethanol blend.
In another direction LPG- liquid petroleum gas (I think you just call it propane in US) has octane ratings of up to 115, This fuel will even make Rover V8 engines get up and go , A 4.6litre with 12/1 compression on gas in my TR8 of a few years back would frighten most 928's
Definately be running ethanol blend in the 3UZ engine (10.5/1 comp )that's going into my 928.(longer term,this may even end up LPG powered)
We have emision laws here, but not as mad as in the US, LPG is the easiest way to get around them,- it's near emission free.
LPG- Love it!
I added a link to a lead/ethanol story to my previous post, worth a read.

Is Wiki wrong in stating Australia's regular is 91, premium 95 and 98 being "reasonably common"? According to the article, United Petroleum is the only company that currently offers 100 and it's limited to only 67 gas stations throughout the entire country.

There are multiple rating systems for octane. You guys use RON, we use AKI (R+M)/2). Your 91 regular would actually be slightly less than our 87 regular. Your 98 is equal to our 93. We don't have 95 which would be equal your 100; however, we have places that offer 100 to 110 (110 has lead). This is racing fuel sold at our gas stations and there are dozens of stations in my area that offer it.

The 100 octane is only sold in major cities, have to drop down in country areas, Totally illegal here to sell fuel with lead, Aviation gas is about 110,race gas I think is 105.both sort of ilegal on the road, (and expensive!)
Ethanol for some reason is added after the fuel is octane rated, so it picks up a few octane points, some sort of incentive to make people use it.
LPG (propane)can vary depending on brand/ location, octane rating can vary between 110 and sometines up to 120,does not seem to have to comply with anything but a minimum standard, some gas stations take their supply from companies that supply cooking/heating gas, that's where the higher octanes come in. good variety of jettable gas carbs and injection systems available .
I would be removing the 928 tank/battery well and cutting out the floor tomorrow to make space for a tank, but the government has just cranked up the tax on it, and our prices are on a world pricing system, -if Europe has a cold winter, we pay what they are pay(to heat their houses) Price per litre a month ago was 61 cents, 89 today:confused:have to wait till it warms up in Europe to see what the price will steady at.

Euro spec cars are popular here because they are usually hot rodded and have more hp then US spec, They won't meet US emmisions though. Whick is ok becuase it's usually older cars that are exempt now. I haven't seen US/Euro spec in anything newly manufacured, that seems to be a thing of the past.

Ethonol can produce great results when the motor was made for it in the first place. I believe out Indy race series is using straight ethanol. The problem is the crap blend we are getting now, for the reasons stated earlier. You may actually be using a better grade of ethanol than us too. From what I understand corn may not me the most efficient source.

Ethanol here is mostly made from sugar cane waste, but regardless of what it's made from, should be ethanol at the end shouldn't it??
this could be of use to people having a hard time passing smog in California, Few years ago had a car fail an emission test, drained the tank, put in about 10 litres of a 3 parts ehtanol to about 2 parts petrol, made the car totally gutless, and it stank of the ethanol, but it passed.;)

I don't know if its illegal, but its not at all unusual to see cars getting gas at the airport. What is unusual are cars that actually need it. ;)

But SoCal has some crazy cars on the road.

That has never occurred to me.-thanks.