Kevin is on the correct track.

But there is more to it than just pure compression ratio.

A turbo has a higher effective compression ratio than a non-turbo because it pressurises - spelling is messed up - increases the pressureat theintake and thus the pressure in the cylinder is higher before the piston starts moving up.More gas and air in the cylinder equals higherpressures and temperatures thus more power if you do it right. In the old days we actually misted water intothe gas and air to help cool the cylinders - but that is a different story.

The timing also affects the requirements for octane rating. Advance the timing too far and you'll burn a hole in the piston in a few seconds at high loads. Retard it too much and you have very little power. Higher octanes allow for more advance without the dreaded pre-ignition and subsequence holes in the piston. Audi actually has a microphone attached to the left and right cylinder bank that listens for engine knock and adjusts the timeing accordenly.

Contrary to popular belief, higher octain gas is harder to burn. It requires a higher temperature to ignite.

[sm=imsorry.gif]I got carried away....

 

Yes I did read your whole quote,I lost you along the way there but I think you have some good points.I really didn't get desciptive but will look in to this a little more since I'm basing what I say off what I recall of issues I've ran into.I also know from maintaining work vehicle thatrunning cheap 87 fuel (Mobil,Shell) gasoline makes your engine ping and run choppy regardless of whatcold or hot plugs you run.I wasnn't refering to theMAF intake I was refering to the intake manifold.Your going to have clogged fuel filters/injectors up from a low octane fuel.This is where I disagree with you.I think thehigher the octane the higher the tempature of the burn off andcleaner leaving no carbon in your cylinder heads just over heating.There only going to be over heating In the heads if you run octane booster, cam2, or NOS.I know from experience on my gsxr1000 &jetta.The exhaust turn in to a rainbow from the high tempature and over heating.The thing I'm hung up on though is flames shooting from the exhaust from unburned fuel.But I think thats just from tuning on the jets/too much fuel in a low idle.I think your best bet is to just run the recommend fuel for the vehicle mines 91 and thats what I run or 93 if thats all they have never a lower grade fuel.Your going to lose more mpg with running the wrong plugs dirty filters and injectors or underinflatted tires etc.,than 20 cents a gallon.In conclusion F EXXON with there two record profit years they better be building a fuel solution with there 41+ BILLION Dollar years.



gas prices suck but its only money!
[/quote]

You've got the right idea but wrong reason.

Due the the mass air flow sensor being before the intake manifold, the fuel never touches it. We have fuel injection so the fuel is put directly into the intake manifold right before the valves to each cylinder. The mass air flow sensor gets dirty simply from impurities that get past the air filter.

Also, the quality of fuel doesn't have anything to do with the octane rating. The rating is there as a measure of how explosive the fuel is. The higher the rating, the slower the burn (less explosive)....in this order as an example....87, then 89, 91, 93...lastly diesel.(this is why a diesel engine will self destruct if you put gas in it rather than diesel fuel.) Therefore, if you use the higher octane rating...you timing will advance more meaning better gas mileage and better performance (automatic timing advance and retard on our cars). If you put the lower octane gas in, the timing will automatically retard and therefore worse performance and worse gas mileage. As always, there is a limit. Don't go putting in 100 octane thinking your car will turn into a rocket ship.

That said you can only put so low of an octane rating in before the timing cannot retard enough to save the engine from damage. This is not a danger with our cars because the compression ration ratio is not terribly high. However, if you have a turbo or supercharged engine, you do not want to put in low octane gas as the compression is raised so much by the forced air. This especially goes for those of us with chipped forced air engines. That chip in most cases raises the pressure of air allowed to the cylinders (psi or boost) and therefore the compression. IF YOU GET YOUR TURBO CAR CHIPPED DO NOT RUN 87 OCTANE FUEL!!! I would not even run 89 octane fuel in that case. Ask the installer or shop about it. They have a minimum octane for a reason.


This idea does not work with older cars (about 95 and older). Those cars had fixed timing. This means that they were designed for only one grade of fuel. If you put low octane gas into an old high compression engine then the engine would start to ping upon an applied load. This can bend connecting rods (very very bad!) and burn holes in valves and piston heads.

--------------------------------------
Lastly, if you want to treat your car right and not harm the injectors, lines or regulators then simply don't buy gas at the cheap stations. They add alchohol to their fuel to lower costs. This can harm you seals and rubber (minimaly though...not any real danger on cars after 1996). The more costly stations don't add as much alchohol and they also add some cleaners to their fuel that help to keep your injectors and lines clean.



Hmmmm.........................I wonder if anyone will read this explanation????????
[/quote]

 

I stick to 93 in chipped mode but go to 91 if I switch to stock, which is... almost never!!!
The difference in $ is not worth the risk... Like someone said above, if you drive an a6 which may have been 45k new, get the good stuff for a few cents gallon more... but it is just my opinion.
it is a better idea for yuor saving and the environment to commute if you need to save $.

 

You seem to have caught the gist of it. I gotta clarify in two spots though.

-- In reference to the low octane and fouled lines and filter...The octane rating is a rating based purely (PURELY) on the burnability of the fuel. It does not have anything to do with the cleanliness of the fuel. The cleanliness of the fuel is determined by the condition of the delivery truck, the lines, the holding tank(s) and lastly the amount of alchohol added.

-- In reference to the octane rating vs. the temperature of the burn. You are correct here but for a slightly different reason. Its not the octane rating in itself that causes the higher temperature. Its the use of that higher rating. The advantage of the higher octane rating is that it will withstand higher pressures before it spontaniously combusts. We can now add more pressure via longer connecting rods, shaved heads, turbo's and superchargers. This higher pressure means higher temperature air into the cylinder. You start with hotter air before the combustion and you end with a hotter burn! Therefore its not the fuel that causes the hotter burn but rather its the fuel that makes the hotter burn even possible. With this hotter burning method available, more power can be made.

 

Thats a head full but very informative (I think though that for *members in the US.) WE SHOULDNT BE COMPLAINING ABOUT GAS PRICES !!!

 

This may sound crazy but I popped my hod o check out my oil level and noticed leaves / and and stuff around the engine (Gum too) wen to the gas station ad tried to blast air & blow the stuff out ..
go most of it but still some in there ? What to do ? Pay a Mech to get it all off when I next service ?

Or is it Nothing to worry about ?

Also I am at 81K is it time for changing the belts (drive) or not

Thanks

 

i run either chevron mid/premium (because they both have cleaning additives) or shell v-power, i could care less about the performance and mileage obtained in higher octanes, i just want to keep my engine clean. how much is everyone paying to fill up at the pump? i just paid 69 and some change to fill up on basically empty.

 

"Altitude, temperature, and humidity are all factors that can slightly affect your car's octane requirements. As a basic rule, hot, dry ambient air requires a higher octane rating than cool, damp ambient air, and higher altitudes allow a lower octane rating. "

In Calgary, with altitude of 1100-1200 m, very dry, there is no higher than 91.

Vancouver - see level, humid!!! - 94.

There are people I know with imports (BMW, Infinity) that use (very happy) 87 only with gas additives (LubeCorp). I have not found any proof for consistent savings using the mentioned additives.

 

Bought a used 2000 A6, and have no idea what gas was ran through the engine before. Any reason to think a fuel cleaner might be needed. I am unfamiliar with Audi engines and whether gas treatments are problematic.

QUOTE=frankinstyn;815304]You've got the right idea but wrong reason.

Due the the mass air flow sensor being before the intake manifold, the fuel never touches it. We have fuel injection so the fuel is put directly into the intake manifold right before the valves to each cylinder. The mass air flow sensor gets dirty simply from impurities that get past the air filter.

Also, the quality of fuel doesn't have anything to do with the octane rating. The rating is there as a measure of how explosive the fuel is. The higher the rating, the slower the burn (less explosive)....in this order as an example....87, then 89, 91, 93...lastly diesel.(this is why a diesel engine will self destruct if you put gas in it rather than diesel fuel.) Therefore, if you use the higher octane rating...you timing will advance more meaning better gas mileage and better performance (automatic timing advance and retard on our cars). If you put the lower octane gas in, the timing will automatically retard and therefore worse performance and worse gas mileage. As always, there is a limit. Don't go putting in 100 octane thinking your car will turn into a rocket ship.

That said you can only put so low of an octane rating in before the timing cannot retard enough to save the engine from damage. This is not a danger with our cars because the compression ration ratio is not terribly high. However, if you have a turbo or supercharged engine, you do not want to put in low octane gas as the compression is raised so much by the forced air. This especially goes for those of us with chipped forced air engines. That chip in most cases raises the pressure of air allowed to the cylinders (psi or boost) and therefore the compression. IF YOU GET YOUR TURBO CAR CHIPPED DO NOT RUN 87 OCTANE FUEL!!! I would not even run 89 octane fuel in that case. Ask the installer or shop about it. They have a minimum octane for a reason.


This idea does not work with older cars (about 95 and older). Those cars had fixed timing. This means that they were designed for only one grade of fuel. If you put low octane gas into an old high compression engine then the engine would start to ping upon an applied load. This can bend connecting rods (very very bad!) and burn holes in valves and piston heads.

--------------------------------------
Lastly, if you want to treat your car right and not harm the injectors, lines or regulators then simply don't buy gas at the cheap stations. They add alchohol to their fuel to lower costs. This can harm you seals and rubber (minimaly though...not any real danger on cars after 1996). The more costly stations don't add as much alchohol and they also add some cleaners to their fuel that help to keep your injectors and lines clean.



Hmmmm.........................I wonder if anyone will read this explanation????????[/QUOTE]

 

I have an Audi A6 with the 3.0 AVK engine. My Audi Owners Manual tells me that 91 Octane is "Recommended" but that the minimum "Required" is 87 Octane. So for the last 100K miles I have used 87 Octane from major fuel distributors, no very cheap stuff. The Audi A6 has crappy fuel economy anyway.
I did hook up my scanner early on in the cars life and the timing advance seems to be the same with 87 as with 91 under hard acceleration. No signals from the knock sensors.
It seem to me that with modern engine management systems any fuel in the standard range would work just fine with perhaps a small reduction in power under hard acceleration as the timing is retarded a bit if the knock sensors light up.