Premium gas? Facts & Myths

[JohnnyDaJackal]

As my title says, I'm trying to find out the facts and myths between premium gas vs regular gas and how 'bad' it is for a car. I remember reading in Motor Trend or C&D that they tested cars that required premium gas with both grades of octane and noted negligible performance gains with the particular cars. From what I understand if a car says use premium you should use it not so much for the performance but for the engine that has been tuned for it. I've also read about people reporting 'engine knocking', 'worse gas mileage', and premature of engine parts.

Reason I ask is because I've gotten into an argument with a friend over my cousin's new MB C300 sport and if he really needed to use premium gas or not. My friend was trying to tell him that premium is only for performance, and unless your pushing your car it's not needed. I was making the argument that the car is tuned for the premium, but my friend was trying to say the computer will adjust to the current gas.

And to reserve the situation. I know people who will put premium gas in cars that require regular, claiming it cleans the engine. I'm wondering if it's a complete waste of money, and if it has any negative effects on the car as well.

Finally, I see mid grade all the time, but have yet to see a car that requires it. I'm wondering what that's about? Is that for special applications or something?

I feel this area is a little 'gray' so I wanted some opinions, facts and experiences on the matter.

[smprince1]

I am not a mechanic, but I believe the octane rating refers more to the rate at which the fuel burns, and has little or nothing to do with performance or cleaning of the engine. At certain compression ratios and temperatures, certain fuel will burn without a spark, thus causing uncontrolled ignition (knocking and pinging?).

I believe the cars are tuned for a certain octane fuel, with the design compression ratio and ignition timing optimized all for the right combination.

I would simply say, use the octane rating recommended by the engine manufacturer. Anyting else seems like a waste of money and/or potenially dangerous to your engine parts.

[todd@bsaw]

There was a very good comparison test on Top Gear that showed the differences.
If I could find it, I'd post it, but I can't. Sorry.

[rjstaaf]

The computer in most cars can compensate for lower grade fuel but the cost is in performance. It will adjust the timing to prevent knocking. If the car is tuned for premium you have to use it unless you are willing to give up some performance for the lower cost of regular.

Using premium in a car that calls for regular is a complete waste of money. The government mandates a certain level of cleaners in ALL gasoline. Some brands of premium do have more than the minimum but at what cost? You could probably buy a bottle of Techron cheaper than using premium just for it's cleaning abilities.

Over time when carbon builds up in the fuel system or for a car that is out of tune, they may ping or knock on regular even if they are designed for it. Mid grade can be used to compensate for a car that calls for regular but pings or knocks on it.

There have been threads on these issues here on Autopia and I am sure you can find more by using the search feature here...

[Yal]

With regard to the mercedes, all mercedes benz cars are tuned for European 95 RON. The exception are the AMG's which require European 98 RON. The equivalent of 95 RON here in the USA is 91 gas. Unfortunately most gas stations in the USA have eliminated 91 octane in favor of 93 octane. I can only find it in Sunoco stations here on the East coast. USA 93 gas is actually European 98 RON and technically is unnecessary unless you drive an AMG. But, because 93 gas is widely available that is what MBUSA recommends, but if you find 91 you can use it without performance lost. If you are under warranty just stick with 93

[SpoiledMan]

Originally Posted by rjstaaf

Using premium in a car that calls for regular is a complete waste of money.

Not hard and fast true. Most every J series Honda engine that's used in Honda marked cars requires only 87 octane while their Acura counterparts require 91 octane. The Honda cars are usually rated lower for power but the numbers become quite similar when dyno tested with 91 octane. I'm sure that there are other examples as well.

[red4.8]

Found this:
Premium vs. Regular Gas: How to Decide
Nov 14 '01 (Updated Nov 16 '01)

The Bottom Line Most cars don't need premium, but a few do -- and not necessarily the ones you'd expect. And premium's not likely to "clean" your engine, either.

While many people claim that their car runs better on premium, this costly fuel is really only needed in a few circumstances. However, when it is needed, it must be used to spare your engine from costly damage. On the other hand, if you burn premium when your car doesn’t need it, the damage won’t be to your engine but only to your pocketbook.

It’s All In The Numbers

The AKI (anti-knock index -- also known as "octane rating") of the fuel is an average of two different methods of computing the fuel's ability to resist engine knock. Regular unleaded gas in the U.S. has an anti-knock index of 87, while mid-grade is usually 89, and premium typically is 91 - 93. The higher the number, the more anti-knock protection the fuel offers.

Engine knock occurs when the fuel in the cylinder ignites by itself before the spark plug ignites it. The technical name for this condition is "pre-ignition." It sounds like marbles rattling around in a can, and it generally gets worse the harder you press on the accelerator pedal.

Knocking can indeed damage an engine because what is in effect happening is the "explosion" of the fuel/air mixture is trying to push the piston downward in the cylinder before it's gotten to the top of its stroke and is free to move downward. The engine is actually working against itself to a degree, and there is a lot of mechanical stress placed on certain engine parts, such as the pistons. In extreme cases, knocking can burn holes in the pistons and create other forms of engine damage.

Oil companies sell higher AKI fuels (also known as premium) to address this problem. A fuel with a higher AKI actually burns more SLOWLY than fuel with a lower AKI. This is because the fuel is less volatile and requires more heat input before it begins to ignite.

(For this same reason, high AKI fuels will actually make your car HARDER to start on a very cold day. In extreme cold -- below 0 degrees F -- you want a more volatile fuel which ignites easily so you can get the engine going without extensive cranking. So if extreme cold is expected, buy 87 AKI gas unless your owner’s manual advises against it.)

Who Needs Premium?

Basically, all premium fuel does is resist knock. So the short answer to the above question of “Who Needs Premium?” is simply any vehicle whose engine knocks without it. Consider these guidelines as you decide whether you really need premium or are in the majority who do not:

1. If your vehicle makes the above-described knocking sound when you accelerate, try the next higher grade when you refuel, and then see if the knocking goes away. If it does, stick with that grade. If not, bump it up another grade the next time you get gas. Generally speaking, high mileage vehicles with a lot of carbon deposits in the engine are likely candidates for needing premium or mid-grade gas, since these deposits reduce the volume of the engine's combustion chamber and make knocking more likely. In addition, the carbon deposits themselves get hot and may act to ignite the fuel if lower AKI fuel is used. High mileage engines do not always have carbon deposits; those that have been driven mainly in town and done a lot of idling are most likely to be “carboned up.” My Ford F-150 has 210,000 miles on it and does not knock on 87 AKI regular unleaded, so that’s what I use.

2. If your owner's manual calls for higher AKI fuel, use what the Good Book says. Usually this will be specified by manufacturers of high compression, high performance engines. If lower AKI fuel were used in these engines, performance would suffer because the engine's computer system would have to retard the ignition timing (reducing horsepower and fuel economy) to keep the engine from knocking.

If, in such an engine, the computer could not retard timing enough to keep it from knocking, the engine itself could be damaged. This would not be likely to happen on one tank of fuel, however. So if you lend your BMW to your son or daughter who has a hot date, don’t run for the siphon hose if he or she returns it with a full tank of regular unleaded. Just drive more easily than normal so the engine's maximum performance isn't needed, and there should be no problem.

But I Want to Keep My Engine Clean!

The higher AKI of premium gasoline does not in itself make it any cleaner than regular gas. Oil companies, however, like to advertise that their premium fuels are ‘specially formulated” to clean fuel injectors, restore lost power, and, hey, maybe even improve your sex life. But the truth of the matter is that any “cleaners” that could be added to gasoline itself are probably not going to be present in sufficient quantities to do much good.

A better strategy to keep your fuel injectors clean is to buy your gas from reputable stations that have a relatively brisk business. In this way the gasoline is liable to be freer of water and other contaminants. It’s also not a bad idea to avoid buying gas from stations while their tanks are being filled by a big tanker truck. It’s possible that the filling process could stir up gunk that’s settled to the bottom of the station’s storage tanks and make it more likely that debris could wind up in your own tank. (Gas stations have filters that are supposed to prevent this, so this is a minor point for those who want to be extra careful; it’s not a really big deal.)

“But I Just Think Premium Makes My Car Happy”

If you enjoy using premium because you think it makes your car “happy,” then go right ahead and use it. President Bush has asked Americans to spend money to support the economy, so you can view it as your small patriotic contribution. And if you own stock in an oil company, so much the better. Using fuel with a higher AKI than needed will in no way harm an engine. Also, your engine won’t get “a taste” for the good stuff and rebel against you if you fall on hard times and have to use cheap gas.

[TrueDetailer]

i think it has to do with the compression ratio of your motor. Use what your car calls for.

[01TrublUGT]

'I can give you the short and skinny on it.\r\n\r\nOctane ratings are merely the fuels resistance to detonation, detonation is when the fuel ignites without a spark, basicly once the fuel is heated or compressed beyond its rated threshold it will autoignite. How is this bad?? Well if the piston is still on its upstroke, or at top dead center and the fuel ignites the \"explosion\" will drive the piston down. Now if the ignition occurs at the right time it will simply force the piston back down and create power. However if ignition occurs at top dead center then it will slam the piston down, which will slam the rod down, which will put severe stress on the crank. If ignition happens when the piston is on the upstroke it will basicly be like the piston just hit a brick wall, forcing it to go in the opposite direction as all of the other cylinders are making it go. So if 6 people were tied together and 5 decided to go left, and one was determined to go right what would happen? Either the link holding them together would break, or the one that wanted to go right would get yanked to the left.\r\n\r\nCan cars that are tuned from the factory for premium fuel safely run lower octane fuel, and can the computer compensate for it?? Theoreticly yes to a degree. You have to understand how the computer will compensate.\r\n\r\nMost any motor that is set up to run premium fuel(and even some that aren\'t) will have knock sensors, these sensors will pick up detonation and reduce timming to save the engine. So the computer is not saying \"Hey the guy is using 87 fuel, lets adjust everything to correct for it\", the computer is saying \"Holy crap, I am seeing detonation, I need to retard timming or else this motor is history\" So the computer DOES NOT adjust for the lower octane fuel, the computer basicly goes into safe mode to save the motor.\r\n\r\nFor the Honda and Acura, yes when running the Acura on 87 will reduce its power down to the levels of the Honda motor, the only reason why is because the computer has kicked into safe mode because it sensed detonation. The difference between the Honda and Acura motor from the factory is the Acura motor has a more aggressive tune which increases the power at the cost of needing to run premium fuel.\r\n\r\nReal world here. Lets say your car is made to run on 91 octane, you believe the computer will adjust if you run 87. you fill it up with 87 and go putting around town noticing very little difference. You get on the freeway and need to pass or get up to speed on the onramp. No detonation so far, however you get into the throttle, at 3500RPM the motor detonates, that is enough to destroy a piston. Like I said the computer does not adjust from sensing what is in the tank, it adjusts ONCE THE PROBLEM OCCURS. What happens if you are doing 60 when it happens, the damage is enough to lock up the motor, in turn the tires lock up, you wreck. Bring the car to the dealer, they test the fuel......see where I am going. Any time a motor that is made to run premium incurs damage the first thing they will do is check the fuel.\r\n\r\nMy car requires 93, all I run is 93. The difference between filling up with 93 and 87 is about $2.50 a tank. I think that $2.50 per fill up is worth it.'

[Accumulator]

Originally Posted by red4.8

While many people claim that their car runs better on premium, this costly fuel is really only needed in a few circumstances. However, when it is needed, it must be used to spare your engine from costly damage. On the other hand, if you burn premium when your car doesn’t need it, the damage won’t be to your engine but only to your pocketbook.

-AND-

Originally Posted by 01TrublUGT

...the computer does not adjust from sensing what is in the tank, it adjusts ONCE THE PROBLEM OCCURS...

Yep, that's why I too *never* try to pinch pennies on this stuff. I only run regular in vehicles that're designed to use it.

[rjstaaf]

Originally Posted by 01TrublUGT

snip...

Like I said the computer does not adjust from sensing what is in the tank, it adjusts ONCE THE PROBLEM OCCURS. What happens if you are doing 60 when it happens, the damage is enough to lock up the motor, in turn the tires lock up, you wreck. Bring the car to the dealer, they test the fuel......see where I am going. Any time a motor that is made to run premium incurs damage the first thing they will do is check the fuel.\r\n\r\nMy car requires 93, all I run is 93. The difference between filling up with 93 and 87 is about $2.50 a tank. I think that $2.50 per fill up is worth it.'

I certainly wasn't suggesting in my earlier post that the computer could somehow detect what octane you were running in the tank. Of course the computer tries to compensate when the engine starts pinging and knocking at which time it can likely be too late...

[grease]

Originally Posted by 01TrublUGT

'I can give you the short and skinny on it.\r\n\r\nOctane ratings are merely the fuels resistance to detonation, detonation is when the fuel ignites without a spark, basicly once the fuel is heated or compressed beyond its rated threshold it will autoignite. How is this bad?? Well if the piston is still on its upstroke, or at top dead center and the fuel ignites the \"explosion\" will drive the piston down. Now if the ignition occurs at the right time it will simply force the piston back down and create power. However if ignition occurs at top dead center then it will slam the piston down, which will slam the rod down, which will put severe stress on the crank. If ignition happens when the piston is on the upstroke it will basicly be like the piston just hit a brick wall, forcing it to go in the opposite direction as all of the other cylinders are making it go. So if 6 people were tied together and 5 decided to go left, and one was determined to go right what would happen? Either the link holding them together would break, or the one that wanted to go right would get yanked to the left.\r\n\r\nCan cars that are tuned from the factory for premium fuel safely run lower octane fuel, and can the computer compensate for it?? Theoreticly yes to a degree. You have to understand how the computer will compensate.\r\n\r\nMost any motor that is set up to run premium fuel(and even some that aren\'t) will have knock sensors, these sensors will pick up detonation and reduce timming to save the engine. So the computer is not saying \"Hey the guy is using 87 fuel, lets adjust everything to correct for it\", the computer is saying \"Holy crap, I am seeing detonation, I need to retard timming or else this motor is history\" So the computer DOES NOT adjust for the lower octane fuel, the computer basicly goes into safe mode to save the motor.\r\n\r\nFor the Honda and Acura, yes when running the Acura on 87 will reduce its power down to the levels of the Honda motor, the only reason why is because the computer has kicked into safe mode because it sensed detonation. The difference between the Honda and Acura motor from the factory is the Acura motor has a more aggressive tune which increases the power at the cost of needing to run premium fuel.\r\n\r\nReal world here. Lets say your car is made to run on 91 octane, you believe the computer will adjust if you run 87. you fill it up with 87 and go putting around town noticing very little difference. You get on the freeway and need to pass or get up to speed on the onramp. No detonation so far, however you get into the throttle, at 3500RPM the motor detonates, that is enough to destroy a piston. Like I said the computer does not adjust from sensing what is in the tank, it adjusts ONCE THE PROBLEM OCCURS. What happens if you are doing 60 when it happens, the damage is enough to lock up the motor, in turn the tires lock up, you wreck. Bring the car to the dealer, they test the fuel......see where I am going. Any time a motor that is made to run premium incurs damage the first thing they will do is check the fuel.\r\n\r\nMy car requires 93, all I run is 93. The difference between filling up with 93 and 87 is about $2.50 a tank. I think that $2.50 per fill up is worth it.'

Paragraph's illegal in your neck of the woods? JAYSUS!

While all these arguments are valid, what you are forgetting is that alot of newer computers can adjust well beyond what you are saying they can. Lots of the newer Cadillacs can run 87 and lower, not just in safe mode, but as normal operations, tho the suggested is 91. Others can as well. I know my car can basically run perfectly fine on 83 octane (mathematically theoretical). It will also run normally on 87, and further adjust timing on 93. Most newer up level cars will allow a spread of over 15 pts octane, 10 degrees timing...The biggest reason for this is that lots of areas only supply 87...I know some gas in my own area has been tested less then 87.

I know someone who's been running 87 octane in an SLK32 for over 2 years now, and engine is as good as new, dealer certified.