Higher Octane ?

[KayGT]

Not being a rocket scientist...i came to the conclusion that 94 Octane gas for $2.70 burns alot faster and i get much lower millege than 93 Ocatne for $2.50 which burns faster and gives lower millege than 92 octane....Did i come to the right conclusion being that higher octane combusts faster(cleaner too?).....

 

im starting to think that 94 isn't worth that little extra boost....Is Sunoco 94 healthier for my engine then lets say 93 VPower from Shell?

 

i dont want educated guesses or opinions...i want the facts...sorry if im demanding.

[IwannaSportSedan]

Higher octane ratings means that the fuel is more resistant to pre-detonation, in effect, harder to ignite.

 

If the fuel is harder to ignite, it takes more of it to burn the way your ECU wants it to. Your ECU is dumping more fuel into the mixture (making it slightly richer, and allowing more timing advance and not cutting boost pressure) which is good for power, once the fuel is ignited, but it will drain a tank of fuel faster.

 

Lower octane fuels are more likely to pre-detonate (knock, ping, etc.) when a lot of it is mixed with a lot of pressurized and heated air touching a hot engine. So, to prevent that, the ECU senses that, and dials back the advance, opens the wastegate a bit sooner, and dials back the fuel mixture a bit, all of which make less power, and help the fuel supply last longer.

[PhilT]

Is the actual energy per ounce (Specific Calorific Value) the same though ?

 

This dictates how much power you can extract from a certain quantity of fuel.

[KayGT]

is higher octane healthier for the engine....im talkin from 92 and up.... is 94 better for the longevity of my car than 92? or fuel makes little difference in the life of the car.

[2005garnetGT]

it needs over 91, thats it, and yes, higher octane does have less energy

[V-Power]

V-Power to the rescue! Yes, Shell branded fuel tops Sunoco. In the industry "three tier" bracket, Shell ranks in the top tier. I can assure you Sunoco is not in the top tier. But of course, I am a Shell employee - hence, my user name on this site. For $2.50/gal, I'd rather fill my tank with piss rather than Sunoco's premium grade. In all honesty, Sunoco gas will not harm your engine. But it's proven our detergent (additive) is top grade.

[deneb]

I would think piss can be had for a lot less than 2.50/gallon...

[rob_m]

I would think piss can be had for a lot less than 2.50/gallon...

 

unless you drink lots of expensive beer!

[Mblock66]

I would think piss can be had for a lot less than 2.50/gallon...

 

HAHAHAHAHAHAHAHA ROTF

[KayGT]

that beer would have to be imported from another planet.........Im having a hard time believing that sunoco's 94 octane SUCKS....honestly like you said...i kno shell's vpower is great....but dont u think u'r being a lil biased?

[AWDpower]

Higher octane gas has a lower energy content. The additives that enhance octane displace gasoline, so the more additives, the less gas. All things being equal, you would get less power and mileage with higher octane gas.

 

But all things are not equal. Knock sensor equipped engines have the ability to alter engine timing to control pre-ignition. If you use a lower octane, the lower the threshold of pre-ignition. When the ECU senses excessive knock, it retards ignition timing, and is not making the most of the energy in the fuel. With higher octane fuel, even with it's lower energy content, the ECU can advance ignition timing and make better use of the fuel with more power.

 

Of course this all depends on the engine and ECU. If the engine can max out the timing on a lower octane gas, then higher octane won't do any good. But if the programing of the ECU is more aggressive, and takes advantage of the higher octane, you can make more power and get better gas mileage.

 

As far as a stock LGT, 13.5 psi with a 8.2 compression ratio is pretty lightly stressed. But crank up the boost, and the difference in octane will play a much bigger role.

 

Jason K.

[wukindada]

I have used only shell 93oct since the car was purchased:) No problems, well worth the $$

[red beast]

so basically octane boosters suck!

 

a $5.00 bottle raises a tank 1 point to make your car burn more gas.

 

a lose lose situation

[John M]

The ECU will reward you for the extra octane by being more agressive with the timing, meaning more power. It won't affect the logevity of the engine or your gas mileage, except because you'll be enjoying it so much you'll be WOT a lot more.

[nooks44]

so the higher the better performance? and faster fuel burning?

[AWDpower]

so the higher the better performance? and faster fuel burning?

 

The higher the octane, the better the potential performance. It depends if the ECU is retarding timing already, and how much advance the ECU will allow. I'm pretty new to Subarus, but turbo cars in general respond well to increased octane.

 

Higher octane fuel is slower burning. It is a more controlled burn, which allows the ECU to fire of the air fuel mixture earlier, when the piston is closer to the top of it's stroke. You make more power because the combustion event is started earlier, and has more time to do it's work. You also could get better gas mileage, if the same amount of fuel can do it's work for longer at part throttle. Lower octane fuel is more likely to "explode" rather then burn smoothly, which is not a good thing for your engine. To compensate, the ECU fires the air fuel mixture later, when the piston is farther along on it's stroke. The cylinder pressure is lower, so the lower octane gas will burn more smoothly. The ECU monitors knock, and retards ignition timing until it falls under acceptable limits. Think if you were pedaling a bicycle. More timing advance is like if you were alowed to push down after the pedal reaches the top. retarding the timing is like waiting until the pedal is farther down before you push.

 

One side effect of retarded timing is higher exhaust gas temperatures. When the ECU retards timing to make knock go away, the cumbustion process takes place later. This means the process is closer to (or still occuring) when the exhaust valve opens. This means higher EGT's.

 

Jason K.