switching fuel

[akaja]

Let just say that if I want to put better fuel in my car like from REGULAR TO PREMIUM.

Do I need any kind of special gas treatment or cleaning?

[JoshM]

No, just wasting money if you're stock.

[akaja]

No, just wasting money if you're stock.

Okay, what about the other way around from premium to regular?

[kouzie]

Okay, what about the other way around from premium to regular?

I never did on my 91 GTS.......
Don't know if other used it or not..

[johnl]

I can't see why you'd need to clean or flush anything either way.

It's a common misunderstanding that 'premium' pump fuels give more power, they don't. What they do is allow the use of a higher compression ratio (or greater supercharge boost) without the fuel 'knocking'.

Knocking is the premature spontaneous self ignition of the compressed air / fuel mixture that can occur before the mix is ignited by the spark. This is caused by the heating of the air / fuel mix as it's compressed (more compression = more heating, possibly to the flash point of the fuel). This causes a loss of power and can seriously damage the engine.

It's the higher compression ratio that gives the improved performance, not the fuel itself. If you have a relatively low compression engine that works happily (without knocking) on lower octane fuel (i.e. 'regular', having a lower flash point) then using higher octane fuel (i.e. 'premium', having a higher flash point) may even see a very small decrease in power because lower octane fuel typically has a very slightly higher calorific value per unit of volume than does higher octane fuel.

The higher compression ratio creates a more efficient fuel burn that creates more power (and economy) despite the lower calorific value of the 'premium' fuel.

[AbsurdNY]

I have heard you can advance your ignition timing slightly and use a higher octane so the motor does not ping.

[92-cb7]

So its safe to say pumping premium in a stock car is pointless?

[JoshM]

So its safe to say pumping premium in a stock car is pointless?

Stock CB, yes.
H-series, boosted, or extremely built n/a you'll need higher octane.

[92-cb7]

Stock CB, yes.
H-series, boosted, or extremely built n/a you'll need higher octane.

wouldn't there be some benefits seeing as premium has less ethanol?

[mozzandherb]

I have a question. I'm wondering if it would be a bad idea to go from 94 to 91 octane? The reason I ask is because my car was tuned with 94 octane, but with the gas prices and the limited amount of gas stations that have 94 octane, I would like to have my car re-tuned with 91, but I'm wondering if this would be a bad idea.

[deevergote]

If it is tuned properly for it, it should be fine. However, seeing as you have a boosted H22 (JDM, I assume...) you're looking at the potential for problems. You have high compression AND turbo...

If you get it carefully tuned on 91, it should be ok still. You may want to lower your boost levels a little to be safer.

[bcjammerx]

wouldn't there be some benefits seeing as premium has less ethanol?

who says it has less? I've never seen that. But even IF that were true...the answer is still no. 10% eth MAY lower mpg by...up to 1mpg...MAYBE. Seems that's what I read...somewhere

[92-cb7]

who says it has less? I've never seen that. But even IF that were true...the answer is still no. 10% eth MAY lower mpg by...up to 1mpg...MAYBE. Seems that's what I read...somewhere

Same thing Ive read on some gas pumps

[johnl]

I have heard you can advance your ignition timing slightly and use a higher octane so the motor does not ping.

That will be retard the ignition timing, not advance it.

If you have detonation of fuel (knock, ping, caused by auto ignition of fuel), then this may be occuring before the spark occurs, but also possibly simultaneously or a little later. Whichever, this added source of ignition (i.e. in addition to the spark) will cause an additional source of simultaneous pressure rise within the combustion chamber, so overall the pressure rise will be too fast.

This means that the rising piston will encounter a wave of much higher pressure before it has reached top dead centre, resulting in an impact loading on the piston that attempts to violently push the piston back down the bore (against the direction of crank rotation).

Retarding the ignition timing delays the pressure rise within the combustion chamber, so knock should be reduced or eliminated. Note that this is still not an ideal solution to the problem as there may still be multiple flame fronts within the combustion chamber (from the auto ignition source of the detonation and the spark plug) instead of one as the designer intends, and this may affect combustion efficiency, but at least the point of max cylinder pressure will occur late enough after TDC to avoid the damaging affects of detonation.

Retarding the ignition timing (either manually or automatically by means of a knock sensor etc) is a band aid that prevents engine damage, the problem can only be properly cured by using fuel with a high enough octane rating.

[deevergote]

That will be retard the ignition timing, not advance it.

If you have detonation of fuel (knock, ping, caused by auto ignition of fuel), then this may be occuring before the spark occurs, but also possibly simultaneously or a little later. Whichever, this added source of ignition (i.e. in addition to the spark) will cause an additional source of simultaneous pressure rise within the combustion chamber, so overall the pressure rise will be too fast.

This means that the rising piston will encounter a wave of much higher pressure before it has reached top dead centre, resulting in an impact loading on the piston that attempts to violently push the piston back down the bore (against the direction of crank rotation).

Retarding the ignition timing delays the pressure rise within the combustion chamber, so knock should be reduced or eliminated. Note that this is still not an ideal solution to the problem as there may still be multiple flame fronts within the combustion chamber (from the auto ignition source of the detonation and the spark plug) instead of one as the designer intends, and this may affect combustion efficiency, but at least the point of max cylinder pressure will occur late enough after TDC to avoid the damaging affects of detonation.

Retarding the ignition timing (either manually or automatically by means of a knock sensor etc) is a band aid that prevents engine damage, the problem can only be properly cured by using fuel with a high enough octane rating.

No, he was referring to advancing the timing, which could potentially cause detonation. He said that the higher octane fuel will help prevent this detonation beyond the abilities of a lower octane. The fuel is the knock suppressant in question here, not adjusting the timing to prevent knock. He was talking about an ignition setting that normally WOULD knock with lower octane fuel.