car recommends premium so why does it get the bes

[lexus114]

My car recommends premium so why does it get the best gas mileage on the 89 octane? (I asked this before, but I cant find it now) This is really odd to me because the power is down some, but I get almost a mile to the gallon more? And I have tried this on several occasions during various times of the year as well.Not only do I go by the read out, but figuring it manually as well (I usually use Premium) Anyone able to shine some light on this subject for me? All I can figure is the ecm adjusts the timing to a point where it is just right for maximum fuel millage, but as I mentioned before I really do not like running a motor on retard timing. And with the RX, it really makes a difference in power. So, I guess I need to decide if I want the extra power, or the mile to the gallon more.

 

[NMBurb02]

Do you perhaps change your driving style due to the decreased performance? A little lighter on the throttle?

 

[rjundi]

What is the MPG difference?

 

[lexus114]

Originally Posted By: NMBurb02
Do you perhaps change your driving style due to the decreased performance? A little lighter on the throttle?


No, I believe I drive it the same way

 

[lexus114]

Originally Posted By: rjundi
What is the MPG difference?



1mpg more on the 89 octane

 

[The_Jeff]

Maybe 89 has no ethanol and 93 has 10%.

I'd love to see a car with a 2nd gas tank that it switches over to when you're getting on it. Or maybe toluene injection. EPA would have a fit of course.

 

[Burt]

More octane theorectically = advanced timing = better fuel economy, however, more octane generally = less BTU per gallon. E.g. putting more butane in the mix raises octane but lowers BTU per gallon.

Ethanol has more octane and lowers mpg.

Any extreme example is natural gas is very high octane, but you won't get as many mpg because is it is much less energy dense.

If your engine is not significantly retarding timing for knock on 89 octane and/or your driving style, then it may get lower mpg on higher octane.

I had a Maxima that got the highest MPG on 87 octane and more power with 93.

 

[lexus114]

Originally Posted By: eljefino
Maybe 89 has no ethanol and 93 has 10%.

I'd love to see a car with a 2nd gas tank that it switches over to when you're getting on it. Or maybe toluene injection. EPA would have a fit of course.



eljefino, Our pumps blend 89 and we have ethanol in 87 and Premium.....un friggin fortunately.

 

[UNC017]

What's your altitude? It only takes a couple thousand feet above sea level for a car nominally designed for 93 octane to become perfectly happy on mid-grade or even lower. In fact they often don't even SELL 93 as premium (91 gets the label) in mountain states.

There are competing factors at work so it depends on where the "crossover point" is. Higher octane fuels have a little less energy per gallon because the octane boosters are all lower energy/gallon than gasoline (ethanol and methanol are good examples). But the competing factor is the fact that the engine management system can optimize better on higher octane IF the ambient air pressures and temperatures make premium necessary. At higher density altitudes, the engine management system may reach its maximum just fine on a lower octane fuel.

 

[lexus114]

Originally Posted By: Burt
More octane theorectically = advanced timing = better fuel economy, however, more octane generally = less BTU per gallon. E.g. putting more butane in the mix raises octane but lowers BTU per gallon.

Ethanol has more octane and lowers mpg.

Any extreme example is natural gas is very high octane, but you won't get as many mpg because is it is much less energy dense.

If your engine is not retarding timing, for knock on 89 octane and your driving style, then it will not get better mpg on higher octane.



I think it is Burt due to the power loss.

 

[lexus114]

Originally Posted By: 440Magnum
What's your altitude? It only takes a couple thousand feet above sea level for a car nominally designed for 93 octane to become perfectly happy on mid-grade or even lower. In fact they often don't even SELL 93 as premium (91 gets the label) in mountain states.

There are competing factors at work so it depends on where the "crossover point" is. Higher octane fuels have a little less energy per gallon because the octane boosters are all lower energy/gallon than gasoline (ethanol and methanol are good examples). But the competing factor is the fact that the engine management system can optimize better on higher octane IF the ambient air pressures and temperatures make premium necessary. At higher density altitudes, the engine management system may reach its maximum just fine on a lower octane fuel.



Pretty much my thoughts as well.^^

 

[expat]

If you are typically driving with a light foot, you may not be inducing Knock.
(which will signal the ECU to retard the timing)
In which case the BTU advantage of Reg gas might win you a little MPG.

I know with my Spitfire, I can happily run around on Reg PROVIDING I don't get on the gas too much.

 

[d00df00d]

Is your premium gas 91 or 93 octane? If it's 93, have you tried 91?

 

[lexus114]

Originally Posted By: d00df00d
Is your premium gas 91 or 93 octane? If it's 93, have you tried 91?



d00df00d, it is usually 92-93 around me here. Hardly ever see 91. Sunoco might have 91octane yet, but I usually dont get my gas there. I`ll have to check it out.

 

[d00df00d]

I've often heard that 93 is less energy dense than 91, even neglecting ethanol content.

It's also true that at some point, the lower energy density of some high-octane fuels offsets the power gains from a leaner mixture and more advanced timing; usually that crossover point is right around the maximum octane that is recommended for your car.

You said your owner's manual recommends "premium" -- try to look for a number. If you find one, I bet it's 91.

 

[lexus114]

Originally Posted By: d00df00d
I've often heard that 93 is less energy dense than 91, even neglecting ethanol content.

It's also true that at some point, the lower energy density of some high-octane fuels offsets the power gains from a leaner mixture and more advanced timing; usually that crossover point is right around the maximum octane that is recommended for your car.

You said your owner's manual recommends "premium" -- try to look for a number. If you find one, I bet it's 91.



Yup,say`s 91 octane or higher. With 91 probably being the "magic" number.

 

[SteveSRT8]

Doodfood is correct, there is always a point of diminishing returns. Most modern cars can quickly retard timing to compensate for poor fuel but can only advance the timing to a pre set spec.

My car specifically states 91 MINIMUM, and when it was new my wife ran an entire tank of 87 through it and it never made a peep. No knocking, pinging, or anything at all. You would only have noticed if you got it nice and hot and really held your foot down for a long time, you could feel the power ebb and flow as the computer pulled the timing back.

 

[Towncivilian]

Interesting discussion. I have ran 93 in my Pathfinder which recommends 91 for "best performance" for a long time now, perhaps I will try 91 to see if there is any difference with fuel economy. I would imagine any performance difference will be unnoticeable.

 

[hypervish]

I run 93 in my RX as well, it too calls for 91 or higher.

I haven't calculated the mpg on anything lower than 91, but I definitely feel a loss of power on the butt dyno when running regular. Throttle is much more responsive on higher octane!

 

[lexus114]

Originally Posted By: hypervish
I run 93 in my RX as well, it too calls for 91 or higher.

I haven't calculated the mpg on anything lower than 91, but I definitely feel a loss of power on the butt dyno when running regular. Throttle is much more responsive on higher octane!



I see your using Quaker State Ultimate Durability 0W-20? Mine is burning a little on Pennzoil Ultra 5W-20. I smell it in the morning, and the tailpipe has a slight burnt oil residue in it. But I havent had to add any yet.