Octane level

Well, since their owner's manual says that you can use 87 octane fuel without damaging the engine.....you do the math.

The back of the owners manual says to use 91 octane and only use 87 temporarily.

The ignition system uses electronic spark timing advance and a knock sensor to set ignition timing at the optimum level for the fuel being used. Higher octane fuel allows the timing to be more advanced for more HP and fuel economy. Lower octane means more retarded timing. From a financial viewpoint, it's close to a wash so I use premium.

How is it close to a wash?

What do you get for your 20 cents/gallon more for premium?

This has been debated to death. Independent engineers have demonstrated that although some engines will indeed run on lower octane fuels than recommended by the manufacturer, the engine timing settings are electronically retarded to compensate for the lower octane fuel. Doing so increases engine heat/wear which translates into lower MPG. Therefore, not only are you creating additional wear and tear on the engine, you are sacrificing MPG as well, and the difference in fuel mileage alone will usually offset the difference in cost of fuel, not to mention the extra wear and tear on the engine.

In summary, if you want to burn regular fuel, buy an automobile that was designed for it; Lexus was not. If you want to ignore the obvious to save 20 cents on the gallon, you'll pay the difference in lower MPG and additional wear and tear. There is no free ride. IOW, penny wise and dollar foolish.

s

This has been debated to death. Independent engineers have demonstrated that although some engines will indeed run on lower octane fuels than recommended by the manufacturer, the engine timing settings are electronically retarded to compensate for the lower octane fuel. Doing so increases engine heat/wear which translates into lower MPG. Therefore, not only are you creating additional wear and tear on the engine, you are sacrificing MPG as well, and the difference in fuel mileage alone will usually offset the difference in cost of fuel, not to mention the extra wear and tear on the engine.

In summary, if you want to burn regular fuel, buy an automobile that was designed for it; Lexus was not. If you want to ignore the obvious to save 20 cents on the gallon, you'll pay the difference in lower MPG and additional wear and tear. There is no free ride. IOW, penny wise and dollar foolish.

s

I still have a hard time understanding why this engine is low on power so requires the use of premium? The 0-60 and 1/4 mile times are anemic as indicated by its advertised HP.

I see cars like the Infiniti, Acura, and other performance cars requiring premium as their engines have much more HP. Heck, even the Avalon and Hemi equipped cars put out better performance with their engines and do not require premium.

Ken

through what mechanism?

(I thought advanced timing increased the heat...)

Lets here from an auto instructor,me. Using a lower octane WILL NOT increase engine wear or heat! (where the hell did you get that from) The higher octane is for performance only. You cannot damage today's engines with all the electronics that controls timing and crank position by using lower grade fuel.Unless you like to race,and lay rubber at traffic light, use 87 or even 89 octane. I use 87 in both my Lexus cars and they both run like new, one has 82,000 miles, the other just turned 30,000.

That's what I'm asking him. Where the hell did he get that idea?

The fact is, he *wants* to believe that if he pays more, he gets more. With gas that you're putting into a Lexus, that simply isn't true. But he's made up this whole imaginary world where retarding the timing increases the heat and wear in his engine.

We'll use my LS 400 and Chicago northwest suburban gas prices for an example. For example, premium is $2.55/gallon, and regular is $2.35/gallon.

I get 20 mpg mixed highway and suburban driving using premium, so at $2.55/gallon for premium, I get 20 miles per $2.55, or 7.84 cents per mile for gas cost.

I get about 1 mpg less on regular, so if I get 19 mpg at $2.35/gallon for regular, that is 19 divided by 235 cents or 8.08 cents per mile for gas cost.

In this example, on paper, the premium is actually lower cost per miles but in the real world, 1 mpg difference or 2 tenths of a cent difference is not very significant, so to me, it is a wash.

If the owner's manual recommends premium, that means that the ignition timing will advance enough to take advantage of premium. On the other hand, if the owner's manual recommends regular, then using premium is a waste of money because the ECU will not advance ignition timing enough to make it necessary to use premium.

You may want to consider remedial training if you teach what you preach. :)

I did have a detailed explanation complete with lab results from a couple of years ago, but I am unable to locate it. However, a quick Google search came up with the following:

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"Knock, also known as detonation, occurs when part of the fuel-air mixture in one or more of your car's cylinders ignites spontaneously due to compression, independent of the combustion initiated by the spark plug. (The ideal gas law tells us that a gas heats up when compressed.) Instead of a controlled burn, you get what amounts to an explosion--not a good thing for your engine. To avoid this, high-octane gas is formulated to burn slower than regular, making it less likely to ignite without benefit of spark."

"Occasionally you get some genius who takes the opposite tack--he spends an extra 10 or 20 grand buying a high-performance car, then decides he's going to save three bucks per tankful using regular instead of premium as specified. He figures as long as the engine doesn't knock he's OK. Wrong, carbon monoxide brain. Car engines nowadays contain knock sensors that detect detonation and automatically retard the spark to compensate. The delay means maximum gas expansion occurs when the piston is farther along in its downstroke and thus there's more room in the cylinder head. This reduces peak cylinder pressure, eliminating knock but also giving you less power and poorer mileage."

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"For the 4th and 5th gen Maxima, it is important that you use a premium grade gas (i.e. 91 or greater octane). Using premium gas is not for increasing performance (not directly) or for giving you extra horsepower. What it does for you is to prevent detonation or pre-ignition. Detonation means that the fuel and air mixutre ignite at the wrong time due to the high compression and heat. This is VERY bad for the engine. The effets of detonation is cumulative, meaning the damage adds up. Higher octane gas will be more resistant to detonation and allow your engine to run as intended." "The knock sensor was designed to detect detonation/pinging/knock in you engine. When it detects a ping, the ECU will retard (pull back) the ignition timing to prevent further knocking. You may be asking then why can't I use

87 octane gas since there is this knock sensor. Two main reason: 1) performance will be impacted when the timing is retarded and 2) When the knock sensor does its thing, that means there has already been some knocking that occured. Remember the effects of detonation/knocking is cumulative. It is possible that using low grade gas may not cause any problems, but remember it does not take that many detonations to break your engine."

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I just posted the first two that I found. There are many more if you want to spend the time looking.

In summary, given the fact that miles-per-dollar, premium typically offsets the price difference, it seems there is a class of Lexus owners who twist logic to satisfy their penny wise-dollar foolish mentality. Given the evidence, it's asinine to detune your vehicle and risk accelerated engine damage for what appears to be a near wash in difference of cost.

Thanks for all the input. Basically what it comes down to is don't buy a lexus if you cant afford the gas.

LOL!! Bingo!!!!!!!

s

Short answer, compression ratio dictates octane, not horsepower.

s

As evidenced by your reply, I find it remarkable how large a part ignorance plays in making a man satisfied with himself.

s

Stealth, you're contradicting yourself. First you say buy 91 octane because it's ultimately cheaper, or at least no more expensive than 87 octane, because you get better mileage. Then, you agree with a guy who says that if you can afford a Lexus, you can afford to pay more for the fuel.

Which is it? You can't have it both ways.

holy crap, Elmo P Shagnasty drives a lexus AND works in prepress?!?!

...I think he's following me, or vice-versa.

additional

Well I tried a tank of 93 octane in my 2004 ES for the first time and liked it. More peppy and the funky tranny doesn't bother me nearly as much.

I'm going to use 91 or 93 octane in the future. I agree that it makes sense that you should use the fuel that the car was designed to run on. Also, it helps knowing that the cost is about the same since you make up the higher price per gallon in improved gas mileage.

Thanks for all those who provided guidance on this question.