it lower power output. In other words you need to spend more gas to accelerate to the speed and your MPG goes down. So looks like you will spend more money per mile if you use low grade gas.

what do you think?

The higher the octane ratio, the more efficient the burn is in the cylinder = more power. The higher the octane rating the less prone the fuel mixture is to pre-ignite and actually more fuel burns. I wouldn't say there is more power (energy) in a 93 octane vs the same amount of gasoline in a 87 rating. It is just that the antiknock, computer controlled engine is able to put more of the 93 octane's potential energy into kinetic.

By retarding the timing, you'll also reduce the engine output, which means gas consumption will increase. For the ~$ .15 per gallon difference (or $3.00 per 20 gal), it really doesn't make sense to use the lower 87 octane unless you can't get the 91+ premium.

High octane fuels are only more resistant to detonation than lower octane fuels. There is nothing in an octane rating that measures the specific energy of a fuel. It is in fact quite possible for a manufacturer to make a higher octane fuel with a lower specific energy than a low octane fuel (as is the case with most ethanol blended gas).

The only concern that an octane rating has on power is when you run a fuel with too low an octane. This will lead to detonation, causing the ecu to retard the engine timing thus reducing power.

Now of course all that changes if you are designing an engine, but since audi already did that...just stick with their rating.

yes you will increase gas consumption if you are running your engine at full compression (actual not maximum). Keep in mind that if you live in a nice flat, hot, high altitude area, you could possibly run 89 octane gas without ever experiencing detonation because you would never reach full compression. You would actually get the same mileage with the lower gas.

The physics and chemistry are pretty complex. It's generally best to just run the recomended octane.

I probably could have just typed that last line.

The controll on the burn is by allowing the plug to start it and not simply compression. You are correct that octane is a rating on pre detonation and not energy within the fuel. That's why I said I can not say for sure that 93 would contain more energy than 87. However, most premium blended fuels (with a higher octane rating) will have a higher carbon composition therby actually containg more power or energy.

an 87 octane rating of higher carbon content fuel with an additive to increase the octane rating would yeild more power and better fuel consumption than an ethanol based 93 rating...at sea level or better would it not?

blanket statements like that are what get honda owners running 92 octane. Didn't mean to sound like I was disagreeing with everything; just wanted to provide a little more info.

would I be able to get it for my Audi?

and if I had more money I'd be rich.

My head hurts. I'm going to bed.

The higher the octane the higher the resistance to burn. Therefore a higher octane fuel will not ignite as easily, and better resist 'knock' or 'pinging'.

The benefit is that with the higher octane (resistance to burning) you can create higher cylinder pressures, and advance the ignition timing, this creates higher output from the engine.

If the engine however is not designed to take advantage of this, there will be a decrease in power from using 'high octane' fuels, as the engine has a harder time igniting the fuel, and the fuel burns late/slower.

Ambient air pressure, temperature, and of course the engine itself play a role. Where I live (high altitude) I use regular 89 and the owners before me used that in my 94CS since new, never an issue. However if I was in a lower place like Vancouver BC I would go with the higher octane fuel (and I do when I drive over there once a year to visit relatives).

Depending on where you live, the octane issue is differant. AKI vs Ron V Mon etc.. In the USA AKI is what is on the pump in the U.S. So in the States Audi recomends @ least a 91 AKI but says if in a crunch, no less than a 87 AKI. Some apparenrly have felt slight pinging in using 87 AKI. Although some like me have seen no adverse afects from 87. If you experience pinging, it is not good. Up your octane immeditely.@ least to 89. The bottom ine is if you'r car has adverse affects of lower grade gas, put in better. I however, like many have seen no adverse affects in pinging or MPG by usuing 87 and going agfainst the recommended 91