Does octane evaporate?

 

I really don't know why people think they will run faster on a 1/4 tank. So there's less weight, but what about the quality of the remaining gas and the potential of the car to use that remaining gas?

 

No. The octane number is just a scale from 0 to 100 that relates to gasoline's tendancy to knock or ping. Gasoline on the other hand, like any other liquid, will evaporate.

Btw, nice pic with the SEMA girls, that was a fun day.

 

Maybe it is tied to the DTE starting to flash (i.e. the ECU has made the determination you are about to start walking)

 

READING THIS MAKES ME FEEL LIKE I'M GOING INSANE!

 

If any of you guys fill up your tank and can feel a difference let us know.. This is driving me nuts, my car felt soo good last night

 

I notice this everytime I fill-up. I use the same gas station and octane (92) everytime. I was also wondering if the ECU was adjusting something.

 

No actually octane DOES evaporate. That is what happens when you have stale gas from storing a car or whatever. The gas is still there but it doesn't work. The octane is actually what is most prone in gasoline to disappear first. Your car will probably be fastest if you run it empty and then fill about 1/4 to 1/2 of the way back up with some good fresh gas thus saving a little weight at the same time.

 

The big one is AC on vs AC off. Oh what a difference! Open them windows and let it rock!

 

I think afr0puff and enthuziast are on to something here. I wasn't aware of any tricks like this being performed on "common" cars (but definitely high-end Porsches, BMWs, etc.), but it does make sense. There are two "bad" things I can see happening when the tank gets low:

1) There is no longer enough gasoline to immerse the fuel pump. It is not uncommon for newer fuel pump designs to use the surrounding gasoline to remove heat from the pump. Supposedly, some manufacturers recommended never going below 1/4 tank with a fuel injected car...maybe because the fuel pump would overheat? Nissan may be limiting the speed of the pump to keep it from overheating when it is no longer immersed.

2) If gas is splashing around in the tank due to a low level, there may be points at which the pump will draw air instead of fuel. This would effectively cut down from the maximum fuel pressure normally delivered by the pump. Reduced fuel pressure means a lean condition would occur (which would be bad for emissions) since the engine now has more air than fuel. I would guess they may be limiting the throttle position to compensate and allow less air through, thus returning the intake charge back to a stoichiometric ratio.

These engines have dozens of sensors, so they could be doing a number of different tricks. I, myself, haven't noticed this behavior but I don't pay much attention either. I've only driven until the gauge says I have 80 miles left and no lower...I don't want to risk pushing a 3200lb car in busy traffic!

 

having only filled up my first time tonight, i can't say for the z.

but, my previous car (toyota 4runner) would experience severe performance degradeation once the fuel tank dropped below one quarter full. at 1/8th tank, you could floor it and the engine would never reach high rpm. i had always assumed this was either a fault in the fuel pump or an intentional feature of the ecu to stop me from running out of gas.

seeing people report the same behavior here in other cars, i find it hard to believe that the engineers at nissan would have not corrected this if it was a fuel pump issue. this leads me to conclude that it is a feature, designed to save my äss when i'm half way between vegas and l.a. and forgot to fill up in the last town.

 

until a Nissan engineer (or someone of similar stature) confirms this, it is hard to say that this is more than urban legend. My car was at 1/16 tank tonight and running same as always.

 

i'm fairly observant and have a good sense of feel but EVERYTIME we fill up the z it feels faster & more powerful. This is before I read the thread about how less weight(1/4) will make the z quicker. So since it theortically makes sense I just convinced myself to believe it's faster when I'm low on gas. Only problem is it's NOT CONVINCING ME because it just feels sluggish compared to filling it full(call it psychological and so forth-i'm aware of that).

I'm not a mechanic but it feels like the more fuel inside the tank the more pressure? pushing something?

 

archman what you say sounds logical=)

 

Wow, what a coincidence that this thread happened to have been created... I just filled up tonight from about a gallon left. The car definitely felt measurably quicker/more responsive; at least I know I'm not the only one who detects this behavior.

 

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Mr. Potato Head
quote:

Originally posted by hfm
No. The octane number is just a scale from 0 to 100 that relates to gasoline's tendancy to knock or ping. Gasoline on the other hand, like any other liquid, will evaporate.


Btw, nice pic with the SEMA girls, that was a fun day.



No actually octane DOES evaporate. That is what happens when you have stale gas from storing a car or whatever. The gas is still there but it doesn't work. The octane is actually what is most prone in gasoline to disappear first. Your car will probably be fastest if you run it empty and then fill about 1/4 to 1/2 of the way back up with some good fresh gas thus saving a little weight at the same time.

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I agree with your post, but wanted to make one clarification. Octane by itself is a chemical molecule, however octane when referred to gasoline is simply a measurement of that gasoline's tendency to knock, it is not a molecule in gasoline that evaporates. The Octane rating is assigned to gasoline by comparing it?fs knock properties to the properties of the chemical molecule Octane (given a rating of 100) and N-Heptane (given a rating of 0).

Gasoline is made of more than 500 hydrocarbons, composed approximately of 50-70% single-bonded carbon and hydrogen (easily combusted - i.e. higher octane rating characteristics) and the rest of heavier molecules (denser). So, when gasoline sits in a gas tank, the lighter molecules of gasoline do evaporate more easily (those are the ones that have better octane rating properties), while the denser molecules remain. In summary, gasoline degrades and the molecules that helped it the most in achieving its stated octane rating left the gas tank. This does impact engines that require high octane, since if the gasoline degrades past the engine?fs design parameters it will not function as good as before. That?fs where a new tank of gasoline helps.

Just wanted to add this clarification since so many octane this and that products are out there in the market taking money away from people.

 

Well my Z is near the empty line (although no light yet) and it's dragging.

Damn, gotta go pump some $1.99 gas. COME ON ARNOLD DO SOMETHING!!!!!!!!!!

 

I knew my 3 years of UCLA biochemistry wasn't a total waste.

 

Ok then what do you call those molecules?

 

Hydrocarbons.

The thing about this is that when someone says "octane" when they speak about gasoline, they are referencing the octane number and not the chemical molecule of octane that maybe a component of gasoline.

You wouldn't evaporate the octane rating out of gasoline. Rather, like ether stated, some of the components of gasoline may degrade reducing the effective octane rating.