irenesbob

That's a typo, right? You meant "saleshole" I assume. lol

q5q7

off topic from OP's original question
As I searched the BMW,Mercedes,Porsche forums, I see people recommend Shell,BP, Exxon, Mobil etc, but never see people recommend 7 eleven, Citgo etc. So It seems there is still a somewhat consensus there.

On another note, how tuning affects which gas is best is beyond me Those who are able to explain please do.

in2dwww

The higher the octane number, the more volatile the fuel. The more volatile, combined with the higher compression of the engine under load, the more power you net. Some of the power is lost between the flywheel and the wheels. Some say you lose 20% horepower on an AWD car and 15% on a RWD setup.

The octane rating and volatility are no different than 80 proof and 100 proof vodka.

The lower octane relates to retarded ignition, meaning its HARDER for the engine to ignite the fuel at or near normal compression. Just as it is harder to get drunk on 40 proof than 50 proof, although not by much.

The decline in volatility means the engine is having to compensate just like anyone would with cheap well drink specials at the local watering hole.

That's the best way I can describe it.

Jatmobil

Not sure if it has been mentioned already but you can "blend" 89 and 93 using a 1/1 ratio to get 91. We have SUNOCOs here in NJ and they sell 91 so that gets done for me at the pump automatically, but for those looking for 91 and its not available, fill up half tank with 89 and then half with 93.

nobbyv

It is unlikely that when using 89 octane your engines have been pushed hard enough to begin knocking (and thus have the timing retarded). Because an engine running with retarded ignition timing will DEFINITELY affect both mileage and power. The question is if the average driver ever pushes their car that hard. On a track, or maybe a steep climb in a desert, I would suspect that knock would be common. If you do neither, maybe a moot point.

Also, note that octane rating will have no effect on carbon deposits. Carbon deposits are directly related to the quantity (and quality) of detergents used in the chosen gasoline, no matter the octane rating.

What exactly about it is bull****? Are you claiming knowledge that Top Tier designated gas does not contain higher levels of detergents?

Can you clarify what you mean here? Additives are a general classification meaning "things added to gasoline that aren't gasoline". Anti-knock agents make up the largest percentage of additives. That's how higher octane gasoline is created.

In addition, manufacturers add things like detergents and anti-oxidants in varying amounts. V-power is a brand name for Shell's own blend of anti-knock and detergent additives, for example.

No gasoline of assured octane rating will cause knock under typical operating scenarios when the manufacturer-recommended octane rating is used. I'm curious under what conditions you've detected this knock.

snagitseven

No, he's claiming that the higher level of detergents ("cleaners") like Shell uses causes engine problems for example, as shown in his photo. Then he when on to write about engine performance:

"Additives are definitely another big cause. Not all additives though, just cleaners. Cleaners will not burn, so they reduce the AKI. Dyno a car with just gas, and then dyno it again with a cleaner in the tank. You'll lose 3-5% of your horsepower due to a less energetic burn in the cylinders".

What I find hard to believe that with all the research Shell has done and the many years their product has been #1 on the market, one would think there would be public exposure to the ill effects he has claimed. I can't find any, only positives re Shell V-Power.

nobbyv

Ah, I think I understand a bit better now what he's trying to say. And yes, anytime you take a fixed volume of fuel and replace some of that fuel with something that isn't fuel (additives), you're going to have a less energetic combustion. That's why you shouldn't use a HIGHER octane rating than what the manufacturer claims; you're likely to get WORSE gas mileage with no benefit.

However, I still don't see what that has to do with knock: a gasoline tested at 93 octane will have been verified to prevent knock under all typical operating conditions for that octane. Octane should really be thought of as a test result. Whatever else may or may not be in the gasoline is immaterial: the fuel has been certified to resist knock up to XXXX conditions if it has a particular octane rating. There can be liquified unobtainium in the gas, for all the lab cares when testing it. All they do is verify that it resists knock up to the level for that particular octane rating.

So to say "Shell 93 causes knock" isn't correct. BY definition, if it's certified as 93 octane, it DOESN'T knock. No offense to Mike_L, but I'll take the word of independent labs performing testing according to ASTM protocols.

I also cringe when I hear someone use the phrase "water down their fuels". NO ONE literally waters down fuel. It's impossible. Gasoline and water don't mix. If you had water in your engine, you would hydrolock and your engine would be destroyed.

newmoon

Actually, the higher the octane number, the less likely the fuel is to ignite under compression, so I would say it is less volatile. If it does ignite under compression without the spark from the spark plug, this is bad because it can create another flame front inside the cylinder which can collide with the flame front from the spark plug causing shock waves and engine knocking.

High performance engines like the Audi's have design features to get more power out of the fuel, such as higher compression ratios and forced induction. However, these features also make the fuel more likely to self-ignite and cause knocking, thus the manufacturer needs to specify that we use higher octane gas to prevent this.

Tuning pushes the engine even further, such as by increasing boost or advancing the ignition timing. Once again, doing these things makes it more likely for the fuel to self-ignite, so high octane gas would be required and it will probably be even more sensitive to variations in the octane caused by cleaning additives, as mentioned by Mike_L.

q5q7

to see if i understand it correctly..

the higher the octane, the less prone to knock because higher octane harder to ignite.

additive reduces octane level + shell has most additive => shell prone to knock

However. I assume Shell 93's octane rating is 93 AFTER adding the additives not BEFORE adding the additives.

nobbyv

Bolded isn't correct. Antiknock is an additive used to raise the octane number of fuel. Other additives (detergents, etc.) have no effect on octane rating, since as you said (correctly) "Shell 93's octane rating is 93 AFTER adding the additives not BEFORE adding the additives".