How important is it to only go with 91 octane?, can one get away with 89 as a minimum.

 

What's the price difference between 89 and 91 octane where you are? Let's do the math on this one... Suppose you drives 12,000 miles a year and get 20 MPG on the average. If you saves $0.10/gallon by using 89 octane, you ends up saving a whopping $60 a year. Personally, I don't think that's worth going against what the manufacture recommends.

 

Valid point, but will it hurt the engine?

 

*Probably* not. Assuming the knock sensors and ECU are working correctly, the engine's timing *should* be dialed back by the ECU if the lower octane gas is making the engine ping. But regardless, I would rather not trust the ECU and avoid doing any WOT runs if I have lower octane gas in the tank.

 

If you use lower octane fuel you’ll lose about 5HP and actually waste a little more gas, cuz the ECU compensates by sending more fuel to burn efficiently.

 

Thanks for the logic and peace of mind.

 

A little of topic but I was at Pep Boys today and saw this thing called Octane fuel boost 106. It said it was like puting octane 106 in your car. Now what I want to know is does this thng actually add power to your car? What about the Nos octane bost?

 

On a Sedan, you won't hurt the engine. It'll retard timing to prevent problems, causing a slight drop in power and fuel economy. On a Coupe, the timings are too aggressive to run anything less than 91 for extended periods of time.

On a side note, you won't save any money. Note that I said you'll get a slight drop in power and fuel economy. This effectively cancels out any dollar savings at the pump by making you buy gas more often.

But, if you're out in the boonies and Bubba Joe's Gas-n-Stuff only has 89 (or maybe even 87) available, it's okay to put it in your tank.

2003.5 G35 Sedan Desert Platinum/Graphite Premium/Sport/Aero/Nav/Winter

 

I thought BUbba only lived in appalachia

 

Hmmm... lotta misinformation here.....

First of all, assuming that the knock sensor is working properly, there will be no damage from running low octane fuel (even 87 octane) on a G35 Coupe. The knock sensor works by retarding timing in STEPS. In other words, if knock it detected, the ignition timing is retard XX amount of degrees. If knock is still detected, after the initial timing retard, then it is retarded XX degrees. This process can repeat multiple times, until a specific timing point is reached.

But the funny part about all of this is that a G35 Coupe engine is very unlikely to experience detonation at all (even with low octane fuel), unless it is under HEAVY load. For example, driving up a mountain road, under full throttle thru several gears. Only is situations like that should detonation even be a concern. And even then, the knock sensor should protect the engine from any possible damage.

Also, it's a myth that lower octane fuel causes poor fuel economy. The engine doesn't "pump" more fuel into the cylinders because of the octane of the fuel. Nothing could be further from the truth. But yes, you can indeed lose a few horsepower because of retarded ignition timing. There is no correlation between the two, however.

That said, I'd still stick with the recommended octane..... The cost difference is negligible and the performance is worth it.

 

It's the retarding of the timing that reduces fuel economy, not the octane rating itself.. though since the engine is calibrated for 91, anything lower can cause knocking and indirectly reduce fuel economy. The engine may not pump more gas into the cylinder, but with retarded timing, that same amount of fuel produces less power and does less work, causing you to either A) travel slower (travelling fewer miles per amount of fuel consumed), or B) press the pedal farther down (consuming more fuel). It's a pretty small difference, but then so is the difference in the price of gas.

And the owner's manual specifically states that 91 is the minimum required octane for the coupe, and the minimum recommended octane for the sedan. We've had at least one coupe owner on this forum that tried to run a tank of 87 through, and about 3/4's the way through the tank, the engine started running horribly... filling up with high octane eliminated his problem. The coupe can take it for short durations, but not on a daily basis. I can't say if this person drove like a granny or Andretti during the time the low octane was in his tank, though. I have a hard time running through a whole tank of gas without pushing my car hard a few times, though, and I imagine others have that same difficulty. [img]/w3timages/icons/smile.gif[/img]

2003.5 G35 Sedan Desert Platinum/Graphite Premium/Sport/Aero/Nav/Winter

 

ryoken,

Think about this for a moment-

Do you honestly believe that the engineers that designed the engine management system for the G35 coupe would design a system that would run horribly on 87 octane fuel?

In other words, don't you think that Infiniti (as well as other manufacturers) recognize that there will ALWAYS be people who will fill up on the cheapest gas available (especially during fuel price hikes)???

Believe me, they know for a fact that people WILL definitely fill up with 87 octane, regardless of instructions in the owners manual.

This is exactly why the coupe (and many other "performace" oriented cars) are DESIGNED to run on high octane fuel, but will run fine on the lower octane stuff as well.

If not, Infiniti would have nothing but trouble with the cheapskates running low cost fuel.

Car and Driver did an interesting article a while back on octane ratings vs. performance. They tested several cars ( I specifically remember a new M3 and a new Mustang GT were two of them) and did dyno runs with both low and high octane fuels to compare power output.

Out of all the cars tested (some, like the Mustang were recommended to run 87 octane, but were tested with higher octane as well), none suffered any driveability problems when run on low octane fuel.

I'll see if I can find the article and let you know the month/year of the issue.... it's an interesting read.

 

Just making sure I'm understanding this here:
My impression has always been that higher octane fuels are able to sustain more pressure without pressure-induced combustion, thus reducing misfires. Now, is the compression greater on the coupe, thus requiring a higher octane in order to run perfectly on all cylinders?
I don't care about short-term dyno results, what I'm concerned about is the life of my engine. It's long been said that higher octane fuels reduce residue, thus improving engine performance and lifespan. Are there any longitunal studies of a few engines running solely on one gas or another? That's the only way to answer this question... just wondering, thanks
G

2003.5 Black w/ Willow
Premium / Sport / Aero / Winter / Navi / Tint
Indianapolis, IN

 

Yes, that's exactly it.... lower octane fuel self-combusts at lower cylinder pressures than higher octane fuel. On a high compression engine with aggresive timing, low octane fuel can certainly combust from heat/pressure before the spark plug fires. That's the cause of detonation. It all happens so quickly though..... sometimes, there are two flame fronts-- one from the spark plug firing and one from the fuel self igniting.... It's important to remember that the "explosion" of the fuel during the power stroke of the piston actually "spreads" across the cylinder. It is not just an instantaneous explosion.... That's why they call it a "front".

The actual compression of the engine isn't what requires the higher octane fuel.... it's more because of the factory ignition timing. If we were still back in the days of distributors, we could take an otherwise identicle G35 Coupe, retard the timing by turning the distributor, and never have to worry about detonation again, regardless of the compression level of the engine.

I'm not sure where you heard that higher octane fuels reduce residue any differently than lower octane fuels. All comparable fuels have detergents added in them and the octane level should not make a difference.

 

Ok here's a question, What effect does running 94 octane have on the engine. I usually get Sunoco 94 but never thought about the engine timing on the other side of the spectrum.