I've been trying to use higher grades of gas thinking it's better for the Xterra. Will it run okay on regular and will that cause any problems later?

If theres not a supercharger, then regular is all you need.

There's nothing special about higher-octane gas if your vehicle isn't tuned to take advantage of it and/or require it.

There have been many tall tales about putting higher octane gas in your car, but studies have shown that you won't even get better fuel economy. Put in the grade that your manufactorer recommends, and you will never go wrong.

It definitly makes a difference if you have a supercharger. I put in two tanks of regular about 6 months ago when gas prices started to hike and it ran like shit. It hesitated and got horrible gas milage. I switched back to 91 octane and problem solved. This week I discovered that Sinclair has 93. 93 it is!
In a supercharged engine it matters. Otherwise I dont think it does.

Yeah, you definatly need to put in what the manufactorer recomends as I said in my post.

Think of it this way...the only thing the octane number does is tell you how fast the gas will burn. That's pretty much it.

The higher the octane, the slower it burns...

The slower it burns, the sooner the spark has to fire to get it going in time to push the piston with the explosion. (Timing)

If an engine is designed to adjust the timing to account for this, then higher octane fuel may improve performance.

In the X - the regular grade gas is pretty much the hot ticket.

If you drive all over the country...what is considered to be "regular" will change, mostly based upon the altitude of the region.

Regular gas at sea level areas (NJ) is around 87 octane...Regular gas in Colorado is more like 85 octane.

Thinner air has fewer molecules in the same space...so, to compensate, the gas needs to burn a bit faster for the same timing settings to work.

In other words, if you are going up into the mountains, and the engine starts to feel weaker, putting in a higher octane gas will make it worse, not better.

Thats what I said.

Supercharged 2002 Nissn X

When i first gotmy X i hadn't read the manual yet and was putting in 87. I got about 9-10 miles to the gallon. Then i came to the chapter about gas and how they recommend 91 i switched immediatley.

The truck runs smoother, sounds better and now i run at 12-14 mpg.

On both my '02 and '03, I have been running premium (91) - it's not an SC, I would just get a knocking noise when I started it up, and was told by the dealer that is was an "octane knock"?? So, I run premium all the time, after they adjusted something......

I haven't put cheap gas in it, I just bend over at the tank, no matter what...

Your timing is off. Look at my post above where it talks about what knocking is.

You are using an expensive timing correction rental to compensate. (Premium)

Have the timing corrected, and use regular instead of renting the correction.

I'm sure there will be those who will miss watching you bend over though.

Shit, that would be nice!! I paid $2.33 on Sunday before going out with RMXC. All it left was money for Pringles.

I'll mention it when I go in for my 45K work....

$2.33!? holy cow, You get dinner and a movie before they stuck it to you?

It's $1.78 down here in San Antonio at Shell for their Power V stuff. I thought I was being robbed.

Of course, this was in Frisco - a conoco exploded last thursday, so there's one less place to get gas - i'm sure a very lucrative opportunity for all others.

But yeah, we've been running between 2.02 and 2.40 around here for premium.

Dinner and a movie? Hah. At least I got to air my tires back up for free.... :rolleyes:

Gas prices high? Really?

Hey TJ, nice way to put that - I remember seeing those lower 85 octane grades back when I lived in Wyoming, but never put 2 and 2 together.

Octane is also key in engine compression in that the higher the octane level, the more you can fit in a tighter space before it spontaneously combusts. This is important in engines that are designed for higher octane fuel, as low octane fuel will spontaneously combust under compression before the engine has ignited it - causing a knock, or explosion in the cylinder while the cylinder is still compressing.. Modern engines sense this (using knock sensors) and advance or retard the timing of the spark ignition to 'sync' with the fuels combustion level.

A good read on the subject can be found HERE.