Premium Gas

[guppy]

My friend just got a new car that only takes premium gas (takes = the manual says you can only use premium gas). I’ve always thought that premium gas was rip-off and that the difference between the regular and premium was negligible (like 6 octane). Then I think, “You know absolutely nothing about gas and/or cars and you have no idea what the hell an octane is”. So, what’s the deal with premium gas, is it worth it or is a scam?

[control-z]

Partly scam IMHO. Premium fuel may be required to get the manufacturer’s advertised horsepower and performance. However, your friend should be able to put lower octane fuel in his car, and the computer will compensate by changing the engine parameters. If the engine knocks or doesn’t run well, stay with premium.

[Johnny_L.A]

@control-z

the computer will compensate by changing the engine parameters.

If you have a computer. The manual for my '66 MGB says to use at least 97 octane fuel, and recommends 100 octane. Obviously that’s at least impractical, if not impossible. I haven’t driven it as much as I’d like, but it seems to be running well on premium fuel. My first '66 MGB did need premium fuel, at least on hot days. In the winter it would run on regular, but in the summer it would ping on lower grade fuel.

Related question: Someone told me recently that premium gas is a waste of money. He said it’s cheaper to use bottled octane booster in regular gas. True, or false?

[gotpasswords]

That should be simple to figure out. Just multiply the number of gallons a bottle of octane boost treats by the price difference at the pump for 87 vs 91 octane fuel.

On a quick look, Amsoil sells their boost online, and it works out to 59¢ per gallon treated, not including tax or shipping. The price difference at the pump is almost always 20¢, (around here, at least) so the bottled stuff is very costly compared to just pressing the 91 button on the pump.

[Thudlow_Boink]

The Master speaks.

[guppy]

@Thudlow Boink

The Master speaks.

Thanks! I searched the forum but forgot about the straight dope articles! :smack:

[David_Simmons]

As Cecil says, if the manual says use premium, then use premium. The danger is that lower octane fuel with result in detonation which can knock small pieces of metal out of the cylinder head. You apparently have a high compression engine and they need a high octane fuel.

[Shagnasty]

I once had a BMW and there are several in the family now. They most definitely need premium fuel. I once filled up and put regular in by mistake and I realized that something was very wrong just from driving it before I realized what I had done. It ran but power and performance were very noticebly reduced immediately.

[Balthisar]

Same thing with Shagnasty’s BMW’s as my Lincoln. The anti-knock sensors lets me put in 87, but the manual “recommends” 91. Previous years required 91 as apparently they didn’t have anti-knock sensors. Horsepower’s reduced only slightly; the V8 performs quite well for my needs at the reduced level, and I’m a foot-to-the-floor type of driver. Fuel economy is reduced as well, but the math still works out in my favor. I think I’ve got all of the calculations detailed in another post here somewhere.

You could always try mid-grade if the 87 doesn’t work out.

(Of course I say 87 because that’s the base fuel I’m used to. Coming across the country last year, it seemed that 86 was the base fuel in the higher altitudes. Fuel-air mixture and all that).

[Absolute]

I wrote this for a BMW message board I frequent. I’m sure I got something wrong, but I think it is correct in general.

@Absolute

Okay, here’s the deal with high-octane gas. It seems like there is a lot of misinformation out there.

In high-compression engines, you can have problems with detonation. This happens when, after the spark plug fires, the burning gas in the combustion chamber creates enough additional pressure and heat (beyond that produced by the movement of the piston) for some of the gas that has not yet burned to explode (instead of burn smoothly). The explosion produces a shockwave that bounces around the combustion chamber, putting a lot of stress on your engine components, and making a loud pinging noise.

The solution to this is to use high-octane gas. Contrary to popular belief, premium gas is actually less volatile, and contains less energy, than regular. The difference is that it requires more heat and pressure to explode, and thus can run in higher-compression engines without detonating.

If you put regular gas in your BMW, the engine detects detonation, and retards the timing. This means that the spark plug fires later than it would normally, when the maximum pressure in the cylinder has already been reached, so that there is less chance that the additional pressure from the burning fuel will cause the unburned fuel to detonate. However, this means that some of the energy in the expanding fuel is wasted, since the piston is already moving down and away from it. Normally, the spark plug fires just before maximum compression is reached (at the top of the piston stroke), so that the pressure from the expanding fuel begins to affect the piston just at the moment it begins to move downwards.

The wasted energy comes right out of your fuel mileage, which eats away some (if not all) of the $0.20 / gallon you save by not using premium.

Personally, I prefer to let the engine operate as its designers intended. It will certainly run on regular, but is engineered to use premium. If BMW wanted to build a car that used regular, they wouldn’t have used such a high compression ratio. Chances are, you’ll be okay with regular, but why risk it?

Incidentally, premium is more expensive because it takes more steps in the refining process to decrease the volatility.

And of course, running premium gas in an engine that does not need it is useless.

[Rick]

@Absolute

I wrote this for a BMW message board I frequent. I’m sure I got something wrong, but I think it is correct in general.

Actually you got a bunch wrong.

@Absolute

Okay, here’s the deal with high-octane gas. It seems like there is a lot of misinformation out there.

You got that right.

In high-compression engines, you can have problems with detonation. This happens when, after the spark plug fires, the burning gas in the combustion chamber creates enough additional pressure and heat (beyond that produced by the movement of the piston) for some of the gas that has not yet burned to explode (instead of burn smoothly). The explosion produces a shockwave that bounces around the combustion chamber, putting a lot of stress on your engine components, and making a loud pinging noise.

You get 1/2 credit. There are two types of detonation pre-combustion and post combustion. You correctly described post combustion. Pre-combustion detention is caused when either the fuel air charge is heated past its self combustion temp, or there is something inside the combustion chamber that is glowing red hot and acts like a glow plug (sometimes a carbon deposit can do this, or the wrong spark plugs)

The solution to this is to use high-octane gas. Contrary to popular belief, premium gas is actually less volatile, and contains less energy, than regular. The difference is that it requires more heat and pressure to explode, and thus can run in higher-compression engines without detonating.

Volatility has nothing zero, zip to do with octane rating. As far as the amount of energy contained in a given amount of fuel is concerned may I quote our perfect master:

@Cecil Adams

Contrary to widespread belief, the octane rating doesn’t indicate how much power the fuel delivers; all grades of gasoline contain roughly the same amount of heat energy.

If you put regular gas in your BMW, the engine detects detonation, and retards the timing. This means that the spark plug fires later than it would normally,

Up to here you are doing just fine.

when the maximum pressure in the cylinder has already been reached,

This is incorrect. The spark plug typically fies quite a ways before Top Dead Center (TDC) for the piston, so that maximum pressure is reached at 5-10 degrees after TDC. When the computer detects knock, it retards the timing so that the piston fires closer to TDC, but I doubt under any conditions would it fire after top dead center as your post would tend to indicate.

so that there is less chance that the additional pressure from the burning fuel will cause the unburned fuel to detonate. However, this means that some of the energy in the expanding fuel is wasted, since the piston is already moving down and away from it. Normally, the spark plug fires just before maximum compression is reached (at the top of the piston stroke), so that the pressure from the expanding fuel begins to affect the piston just at the moment it begins to move downwards.

Well you came back strong here.

The wasted energy comes right out of your fuel mileage, which eats away some (if not all) of the $0.20 / gallon you save by not using premium.

Personally, I prefer to let the engine operate as its designers intended. It will certainly run on regular, but is engineered to use premium. If BMW wanted to build a car that used regular, they wouldn’t have used such a high compression ratio. Chances are, you’ll be okay with regular, but why risk it?

With this I agree completely.

Incidentally, premium is more expensive because it takes more steps in the refining process to decrease the volatility.

No, no, no. Octane rating has nothing to do with volatility. Regular and premium have very similar volatility. Here is some info from Chevron

Volatility

Driveability describes how an engine starts, warms up and runs. It is the assessment of a vehicle’s response to the accelerator, relative to what the driver expects. Driveability problems include: hard starting, backfiring, rough idle, poor throttle response and stalling (at idle, under load or when decelerating).

The key gasoline characteristic for good driveability is volatility — the gasoline’s tendency to vaporize. Volatility is important because liquids and solids don’t burn; only vapors burn. When a liquid appears to be burning, actually it is the invisible vapor above the surface that is burning. This rule holds true in the combustion chamber of an engine; gasoline must be vaporized before it can burn. In cold weather, gasoline is blended to vaporize easily. This allows an engine to start quickly and run smoothly until it is warm. In warm weather, gasoline is blended to vaporize less easily in the vehicle to prevent vapor lock or other hot-fuel handling problems and minimize evaporation, which contributes to air pollution.

And of course, running premium gas in an engine that does not need it is useless.

Agreed, and Chevron has this to say about this:

**Using gasoline with an antiknock rating higher than that required to prevent knock or to prevent spark retardation by the knock sensor will not improve a vehicle’s performance.

**

BTW Chevron has this to say about knock sensor equipped engines

It is difficult for a driver to know whether a gasoline has the antiknock performance the engine requires when the engine is equipped with a knock sensor system. These systems, which temporarily retard spark timing to eliminate knocking, are installed on many late-model engines (see Chapter 5, Gasoline Engines). Retarding the spark reduces power and acceleration. The knock sensor responds so quickly that the driver never notices the knock. Loss of power and acceleration will be the only clues that the antiknock quality of the gasoline does not meet the vehicle’s octane requirement.

From here (page 3 to be exact)

[GusNSpot]

If the fuel burns, it is, a)pre-ignition, b)normal ignition, or c)late ignition.

If the fuel explodes, it is detonation, either pre or post TDC.

A lot of small aircraft engines can be totally ruined in less than 30 seconds with sever detonation.

YMMV