I just bought a 2004 Nissan Xterra with 44,000 miles . It's not the Supercharged model and the manual says you can use 87 gas.

With the price of gas I want to use 87 but need to know if it's safe and are there any disadvantages to using it?

Use 87. You know why. Beause the manual says too.

Well to take things from a chemistry standpoint...the difference btwn octance ratings is the length/makeup of the hydrocarbon chains, b/c thats just what fuels are n-ane hydrocarbon groups that when pressurized and/or ignited they combust. A 93 octane would have a longer chain, and therefore a higher heat of combustion, which means a longer lasting fuel in the chamber. Longer lasting in the chamber = cleaner burning (this is why its good for S/C and T/C engines, is b/c it maximizes the fuel/air burn ratios). SORRY I'm done now just wanted to throw that out for anyone that was wondering!

87.

There's no advantage to the more expensive fuel .. it's no 'better' than 87 - it's just a matter of what the engine was designed for. Some engines, like my S2000, use a higher compression ratio and require higher octane fuel or they will "ping".

Don't be fooled into buying 89 octane either. It's a marketing tool used because when presented with 2 choices, most people choose the less expensive but when presented with 3 choices, many will choose the middle one.

Use 87 - it's what the manual recommends and if there was any problem using it, well...the manual wouldn't recommend it!

LOL

How about that octane is an 8 carbon chain, and the entire rating system used to be based upon the time it took it to burn...and ZERO to do with cleanliness as presented above...or even the actual octane itsef, which is not used in modern fuel like it used to be, etc.

In fact, diesel fuel has longer chains than gasoline, but gasoline burns hotter, not colder than the longer chain molecules as presented above.



ALL the octane rating does for you is give the time it takes to burn the gas....the higher the octane the SLOWER it burns.

If your timing is adjusted to ignite the gas with the spark from the spark plug at the earlier time required to allow a nice long full burn, then there can be les damage from higher octane fuel.

Bottom line - use 87 octane in a non-SC X, because if you don't, you'll just be blowing unburned hydrocarbons out your exhaust.



The exceptions would be if you were driving up into the mountains, like the "14-ers" in CO, etc...where you would use LOWER OCTANE at the higher altitude. (That's why "regular gas" at the pump in Denver, etc...is lower octane than "Regular" in New Jersey or Kansas, etc...so you don't have to KNOW that)



The reason that higher octane CAN produce more power is NOT because of ANY of what was (clearly) stated above.

It has LESS energy content than lower octane, and in fact, gasoline in general has a lower energy content than even diesel fuel.

The fact is, without additional COMPRESSION, higher octane does NOTHING for you at all.

Its a trade off between the power GAINED by burning the fuel under pressure (Like deisel is burned at REALLY high pressure), vs the power LOST by using a fuel with a lower energy content.



So - If I burn a slow burning high octane gasoline...I am giving the piston a nice slow push...if I up the compression though, WHOOSH! more ooomph from the juice.

On the flip side, to use diesel again as a long chain fuel, that doesn't burn as hot, and takes longer to combust...and add compression....WHOOSH! It ignites JUST from the compression, don't even NEED a spark plug once its running, etc.

So - A HIGH energy content fuel, like a LONG Chain, Cold Burning, Slow Burning Diesel...with LOTS of compression, gives more fuel efficiency per gallon burned.

A lower energy content fuel, like 93 octane gasoline, can produce more power than a higher energy content fuel, IF sufficiently compressed.

87 octane gasoline doesn't require as much compression to acheive combustion efficiency...so, at a lower compression ratio, it makes more power than the lower energy content 93 octane can.

If I change the ignition timing...I can use 93 octane in a engine designed for 87...I just have to retard the timing...so it fires the spark earlier, to give the gas more time to burn.

If I increase the compression ratio on my 87 octane designed engine, it will burn too fast...and I'd have to advance the timing, to give less burn time...or put in 93 octane, which WOULD have burned slower, but, with the higher compression, can now provide a real increase in power.

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A visualization of this timing advance/retarding:

You are pushing a kid on a swing....

You push, he swings away, and then, reaches the end of his upswing, and, swings back to you....as he comes back, you wait for him to finish swinging up at you, and then, as he's done travelling at you, and starts to swing away again, you give another push.

OK - this kid's a piston...and you're push is the exploding fuel propelling him.

OK -

The kid is coming back to you, you don't wait for him to completely stop, you push him away while he's still coming towards you a little bit...he almost falls out of the swing, as you push him too soon (Ping). He gives you an annoyed look...you pinged him

Try again...this time, you see him coming back to you, and you rush forward and push the swing while its still swinging towards you hard...you knock the kid off of the swing, onto the ground....and now he's really pissed (That's Knock)

You feel badly now...

OK, now the kid is swinging in towards you, and you wait until he's stopped coming in, and has started swinging away from you...you wait a bit more, and try to push him, but only get off a weak push, as he's already too far away to get a good push off :rolleyes: (You're too retarded) The kid's getting fed up with your timing problems.

With fresh resolve, and knowing his Mom is going to be talking to you about this....you try again...

He swings in, you catch him just as he crests, and push strongly along with his direction of travel...the kid says "WHHEEEE!!!" and you have gotten the timing down.

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That's all its about - high octane burns slower, so you need to give it more time, and/or, more compression, so it has more power, as without the extra compression, because high octane gas has a lower energy content, it only makes SENSE when you can up the compression...which speeds up the burn, etc.

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Hope that clarifies a few of those pesky octane issues.

My hats off to you sir

Thats a great explanation, I feel smarter for having read it, lol.

Don't be a dick chia.

How many of you read your manuals?

This is what it says for the 2000 and my 2007

FUEL RECOMMENDATION
Your vehicle is designed to operate on
unleaded gasoline with an octane rating
of at least 87 AKI (Anti-Knock Index)
(Research octane number 91).

RON is the number you see on the pumps not the AKI.

However I use 87 with no ill effects and that was with a bad KS and also using a KS bypass.

AFAIK the US uses AKI and not RON at pumps. The labels are normally as follows (R+M)/2 Method. I have yet to see pump labeled with RON in any US gas station.

Agree - in fact, the (R+M)/2 is the average of the Research Octane Number and the Motor Octane Number.

IIRC, they do label pumps differently in Europe, but in the US, I've only seen the (R+M)/2 number posted on pumps.

ed

I did not know this. Thanks for posting.

Speaking of has anyone tried going from regular to super and noticed a difference? I know my honda and scion both responded noticeably to super. Not in the butt dyno either but my scion used to get a mpg increase from the super (enough to pay for the cost difference and then some).

Yeah the 93 gives ya a little more boost but sure doesn't increase your mpg. With regular I was seeing about 150-157 mpg now I see about 138-144 depending on the drive. Speeds up a lot better though.

Wow 150 MPG - is that some sort of special Jersey only gas???

Holy shit! 138-157 MPG??? How much do you want for your motor?

ETA: damn...AZ_Ted beat me to it!

lol sorry what I meant to say is that i get 150-157 on a half a tank with regular 87. My engine bidding will start at $8,000.