Just purchased a 2006 M35 RWD (non-Sport) model. Car came filled with gas, but wondering what group recommends in octane rating to fill up with.

Infiniti M35 manual says to use 91 octane, but is there really a diffence noticed in the lower octanes?

I live in Texas and octane choices are 87, 89, and 91.

Thanks.

"Gasoline retailers and refiners like high-test because it's more profitable than regular-grade gas is. The retailer paid about 8 cents more for the premium you pay 20 cents more for — though that margin can swing wildly. Refiners make a few cents a gallon more on premium than on regular when they sell to wholesale distributors.

As long as it's clean

Profit is meaningless to the modern engine, which, regardless of what's specified in the owner's manual, hardly cares what you use — as long as it's clean.

Today's engines use highly evolved versions of a device called a knock sensor to adjust settings automatically for low-octane gas. And more engine control computers have adequate memory to allow separate sets of instructions for various octanes. The engine control computers keep pushing to maximize performance on whatever grade of fuel is used.

Extreme pressure inside the cylinders causes knock, which is the sound of the pistons literally rattling inside the cylinders. Too much too long can damage the engine. A little now and then won't.

The only modern engines that should really need premium are those with superchargers, which force-feed fuel into the cylinders. "You're driving along and just tramp the gas and the knock sensor cannot sense the knock fast enough in some cases," because the supercharger boosts pressure so fast, says Bob Furey, chemist and fuels specialist at General Motors.

Burning regular when the owner's manual specifies premium won't void the warranty, nor damage the engine, even the most finicky automakers say. "You're giving up perhaps just a little bit of performance that a customer wouldn't really even notice, it's so slight," says Furey.

Automakers say they don't test premium engines on regular to check the difference, but some auto engineers estimate that power declines roughly 5%.

"We can't guarantee the vehicle will perform as specified if other than premium fuel is used," says Mercedes-Benz spokeswoman Michelle Murad. All U.S. Mercedes engines specify premium.

All Porsche engines are designed for premium, too, but it's not available everywhere. "Our cars must be able to drive all over the world, and so we are able to run on regular," says Jakob Neusser, director of powertrain development at Porsche's research and development center in Weissach, Germany. "You don't have to feel that a mechanical problem or anything else will happen" using regular gas, even in the highest-performance, regular-production Porsches.

Premium, in fact, sometimes is worse fuel than regular. It resists knock because it's harder to ignite than lower-octane fuels. As a result, some engines won't start as quickly or run as smoothly on premium, notes Gibbs, the SAE fuel expert.

High-test does have a potential fuel economy benefit. It is slightly denser than lower-octane gas, meaning there's a little more energy in a gallon. But the small difference is hard to measure in real-world use, and that same density can contribute to undesirable buildup of waste products inside the engine.

No data show that engines designed strictly for regular run better or longer on premium.

The Federal Trade Commission, in a consumer notice, emphasizes: "(I)n most cases, using a higher-octane gasoline than your owner's manual recommends offers absolutely no benefit. It won't make your car perform better, go faster, get better mileage or run cleaner."

There is "no way of taking advantage of premium in a regular-grade car," says Furey.

"There is no gain. You're wasting money," insists Jim Blenkarn, in charge of powertrains at Nissan in the USA.

"No customer should ever be deluded into thinking there's any value in buying a higher grade of octane than we specify," says Toyota's Paul Williamsen, technical expert and trainer.

But premium retains a mystique.

Even Mayne, the sensible Subaru owner who has switched to regular, says she'll buy premium when her neighborhood station has a special price. "It's my perception that I might get better gas mileage or that it might be better for my engine," she says.

"I would stop driving rather than use a lower grade of gasoline," says Andrew Martschenko of Boston, who drives a 2003 Nissan Maxima. Nissan says premium is "recommended" for that engine — automaker code for regular is OK, but you'll only get the advertised power on premium."

I have a '06 M35X and over the last year I switched back and forth between 87 octane and 91 octane. I don't notice any performance difference between the 2 octanes. I do get about 1 mpg better "in city" gas mileage using 87 octane (19 mpg vs 18 mpg).

The M35 manual says 87 or 89 would be fine, but that you might get better performance with 91. Whether this is true or not is unclear. So, the choice of which gas to use is yours ...

The M45 requires 91. Don't skimp here. You will not be happy with the outcome on your engine.

And, as always, remember that the cost difference is generally less than $15 to $20 per month for most drivers and distances. Is that a big deal for you? If so, then buying an expensive luxury car is also questionable. This is not meant to be a dig or insulting in the least, by the way. Some people truly have to budget carefully - particularly in this day and age, but then they are also better off making decisions to get better gas mileage cars.

Z

Madison avenue knows all about "placebo effect." For an M35, the placebo effect is the only significant advantage of hi octane fuel.

Oh yeah, your M doesn't know from placebo effect. You do...

"Whilst part of what we perceive comes from our senses from the object before us, another part (and it may be the larger part) always comes out of our own mind."

William James, 1890 The Principles of Psychology

antzrus wrote:Madison avenue knows all about "placebo effect." For an M35, the placebo effect is the only significant advantage of hi octane fuel.

Oh yeah, your M doesn't know from placebo effect. You do...

"Whilst part of what we perceive comes from our senses from the object before us, another part (and it may be the larger part) always comes out of our own mind."

William James, 1890 The Principles of Psychology

"Occupation Clinical Psychologist"

I saw that coming!

bredtime wrote:
"Occupation Clinical Psychologist"

I saw that coming!

heheheeheee...

I have to agree, if you are skimping on 10-20cents a gallon I think you'll be happier with a car that is less expensive to maintain. Im not trying to put anyone down, as I only have a salary to buy a used M(my personal finance sidenote is you should have at least 3x gross salary to the cost of your vehicle)

I have the M45 and I just recently discovered without a doubt I can tell the difference between a cheap 91 octane and something like Shell or Chevron 91 octane. Perhaps the 89 octane Shell/Chevron gas would be better than a cheap 91 octane from another store, but I am not going to try it(see first statement above)

I've done a little experimenting with my M35. I've found little to no difference between 89 and 93 (we don't have 91 in Atlanta) I get about 20 MPG mixed driving and about 24 MPG hiway at about 70MPH. However when I had to use 87 last year when we had the gas shortage (4 tanks) and that's all I could find I noticed the car was more sluggish and my mixed driving MPG fell to 18 mpg.

The difference between 87 and 93 in Atlanta is about $.30 a gallon. If you drive 15,000 miles a year and get 18 MPG you're only saving about $250 a year. Since the street price on a new M35 is about $42K thats pretty insignificant, so why not follow the manufacturers recommendations?

I switched to 87 after a year or so of 91 and saw absolutely no difference either in performance or gas mileage. The only place I noticed the difference was in my wallet!

The M45 is a totally different story I've heard but for the 35 it's just not an issue.

I sometimes get pinging when I use 87 gas, so I always put 91 in my M35. I would also swear that using 91 is slightly faster than, but I suppose I could just be imagining it.

Actually octane [reaction] varies by altitude and barometric pressure.

ATL being 1000'+ AMSL allows the blenders/ marketeers to post 93 when the exact same product would only be a 91 at sealevel.

MPG is a function of BTU per gallon not octane and E10 is lower than conventional..............why you get 3.5-5% less MPG with the same published octane.

Ethanol [10%] allows real garbage gasoline to be dolled up to a higher octane by 2 real octane points!

I don't think its "skimping", I look at it as being logical. If the car runs the same (for the type driving a person does) and there's no harm being done to the engine, then it seems logical to use the lower octane gas, save 20 - 30 cents/gal and get better gas mileage (perhaps). A co-worker has a G37 and the feedback is that the lower octane gas will damage the engine. In that case, you'd be crazy to spend that kind of money on the car, but then risk damage so save a couple of bucks on gas. But the M35 appears to be a different story...

83wildcat wrote:If the car runs the same (for the type driving a person does) and there's no harm being done to the engine, then it seems logical to use the lower octane gas, save 20 - 30 cents/gal and get better gas mileage (perhaps).

You might get the same gas mileage with 87 as with 89 or 91, but certainly not better gas mileage! (In an M35, of course).

The other point to mention is that it is sometimes possible to get bad gas from a station ... and this can cause the effective octane to be a point or two below the rated value. Yeah, the M35 engine management will compensate for this, but the result will be sluggish performance and "silent" pinging - not loud enough to be perceptible, but hurting your engine in minute ways.

Remember that the anti-knock systems in cars works by detecting actual pinging and knock in the engine when running ... not some measurement of the energy content and quality of the gasoline!

Again, it comes down to personal choice. If 87 works for any M35 owner and they are happy with it, fine. Just keep in mind that the cost difference between 87 and 91 is about $15 to $20 per month for typical drivers.

Z

I drive 70 miles a day which is probably on the high side for most commuters. That's 1400 miles/month. At 20mpg (I usually get about 22) I use 70gallons/month. At a price difference of 20 cents between regular and premium, that's only an extra $14/month.

I have found no difference bewteen 89 and 91 octane in my 08 M35x. I have noticed that the engine revs higher at the shift points when I have used 87 octane. I found this annoying, so I usually stick with 89. There was no difference with mileage for any octane.