Regular vs. Premium Gasoline

avelanchefan

I always tell people that if their car does not require premium, and they use it they will get better gas mileage. Only because their wallets are lighter because of the price!:rolleyes:

billbillw

Strictly talking about street cars, if your car doesn't say to use premium only (at least 91 octane), then putting anything above 87 octane is a waste of money. If your car says to use 91 and you don't, you are asking for trouble. You will at the very least, have reduced horsepower, and at worst, cause engine damage from ping/spark knock.

This of coure doesn't apply to cars that have been modified with a performance chip (in fuel injected cars) or if they have their timing advanced (in carburated cars).

Jstas

Hmmm...if your car is designed for 87 octane, use it. Gone are the days of carb's and carbon build-up from unburnt fuel in the combustion chamber. With modern electronics, fuel miserly fuel injection systems and fast burn heads, carbon build up is negligible. I've pulled apart engines with 200,000+ miles on them and the pistons look as shiny as when they were new.

The only reason anyone ever had to use an octane higher than what was recommended was because high levels of carbon build up in the combustion chamber would actually cause the compression level to increase which would cause knocking or pinging with a low octane fuel. The two options were to either change to a higher octane to adapt to the new higher CR or tear the engine apart and clean it with a rebuild. The first option was vastly cheaper.

Now though, engines are designed to run within specified parameters. The computer modulates air intake, fuel pressure and ignition timing thousands of times a second to maintain a beneficial although not stoichiometrically efficient air/fuel ratio of about 12.1:1 and that is to help with emissions. If the engine is out of specified parameters, it will resort to running in a reduced performance mode to keep from generating too much heat and destroying the engine. Either that or to keep the emissions down. Running a higher octane than what your engine was designed for can trigger this "limp home" mode. You will experience reduced power, reduced fuel mileage and since you will be running very "fat" in the A/F mixture, you will get that carbon buildup which will foul spark plugs and cause even worse running

The reasons you need to use the right octane are not only what I already said but to prevent things like preignition which was already mentioned. Preignition is otherwise known as detonation and it occurs when something happens to cause the air/fuel mixture to ignite before the compression stroke is complete. The pre-ignited a/f charges flame front impacts the upwardly moving piston and makes a bunch more work for the engine to do to over come that excess pressure. The knocking sound you hear is the super-heated, compressed air expending and impacting the moving piston. That impact has enough energy in it to eventually break the engine. Any time you hear someone say that their engine is "pinging" or "knocking" they are referring to detonation.

In engines with some sort of forced induction from a supercharger, turbocharger or even nitrous oxide, octane rating is extremely important. This is because these engines experience cylinder pressures 2-3 times as high as a naturally aspirated engine tuned for a Honduh Accord and whatnot. The effects of detonation are catastrophic. My truck has a supercharger on it that pushes 10 pounds of boost. I MUST run 92 octane or better otherwise the computer backs so much timing out that Ford can not even be sure the engine will start. Unlike a few others that have already posted, I can run any octane above 92 all the way up to 112 octane fuel I think it is. After that point, you are no longer running unleaded gasoline.

That brings us to Xylene and Toluene that Face mentioned. Toluene is an octane booster for leaded fuels that withstands huge amounts of pressure. It saw widespread use in Formula 1 back in the 80's when it was run at it's maximum effectiveness of 86%. You can't run much more than that because the engine will not fire on anything more. You need at least 14% fuel otherwise there is not enough spark energy to ignite the toluene. The same idea is seen in Top Fuel dragsters which cannot run more than 97% nitro in the fuel mixture otherwise, the nitro will not ignite and the engine will not start. You need at least 3% methanol to fire the engine. If it wasn't needed, the teams would run 100% nitro because that 3% that is missing equates to another 400-500 horsepower I think it was. Or something crazy like that.

Toulene is an additive though, used to boost octane and you won't see it in DOT approved fuels anymore. It is a major component in racing fuels, especially fuels like VP Racing Fuels C16 which is for supercharged and turbocharged engines running compression ratios up to 17:1. It smells like model airplane glue and has a similar make up because it's used as the solvent in model airplane glue. It's also what burns when you light model airplane glue on fire. Anyhow, toluene is for leaded fuel and Xylene is often used as a replacement and you will see very small amounts in gasoline because they are used as solvents for some of the paraffin components of gasoline. Both though find much more widespread usage in industrial sectors as a cleaning agent, in manufacturing as a solvent and even in plastic production. Not so much in cars anymore though.


By the way, Marvel Mystery Oil is pretty much kerosene with some heavy fuel oils and solvents like toluene and Xylene in it. If you run it with the Marvel Mystery Oil like Face said, you will be able to use it as a straight fuel but most of the funky deposits are from the heavy oils and fuel oil which is essentially a version of diesel fuel and machine oil. They have higher concentrations of the paraffins and such and the gasoline engines do not have enough cylinder pressure to make a flame front hot enough to burn them all off.

Diesel engines are usually running compression ratios well about 16:1 with boost pressures in the 20-40 PSI range. Formula 1 cars are also running very high cylinder pressures. While they aren't boosted anymore, they are still around 15:1 and even up to 20:1 for a compression ratio. They were running a less insane compression ratio in the 80's but like CART cars were running, they were limited to around 38-42 PSI of boost on the turbo charged engines.

BTW, alot of this info can be found by just searching online or even just hitting up Wikipedia. While it may be considered dubious, a bunch of the information is quite accurate. Especially science based stuff that is footnoted and has bibliography credits.

steveinaz

Oddly enough, My Suzuki Forenza only calls for 87 octane. Usually high-compression "4-bangers" need high octane, but 87 works great. I use the same in my Honda Magna motorcycle. Many people confuse octane rating with quality--octane rating has nothing to do with the fuel quality.

Jstas

steveinaz wrote: »

octane rating has nothing to do with the fuel quality.

DING! Fries are done!

Honestly fuel brand doesn't matter much either because it all comes from the same refineries. Fuel companies only get to take from the refinery the amount of product equal to what they put in to the refinery. It's all the same. The only differences are the additives, detergents and octane booster combinations they use.

The problems with off-brand stations is that they don't always keep things in tip top shape. Tanks get old, sometimes leak. Filters aren't replaced as often as they should be and just general maintenance isn't the best. Plus equipment is old. Then again, I've seen some brand name stations that are just as bad. But it's not the refinery that is dumping out shoddy product, it's the pumping station facilities that have shoddy equipment and storage facilities and lack scruples.

steveinaz

Ed Zachry Jstas. I do like Chevron fuels, but it's a little pricey.

madmax

I get slightly better mileage with my Scion with the 93 octane. (way more testing done here than just noticing what my last fillup was...) In any case when I compared the price of 87 per mile to the cost of 93 per mile I was going further with the 93 for the same dollar amount.

The key here is how the ignition timing is controled, limits set in the advance curve, shifting style and a whole bunch of other factors I don't know anything about. Different designs will give different results so there is no real answer. I would never use a lower octane gas than is called for unless I absolutely knew for sure the load on the engine was going to be light and well controlled by someone who understands the issue.
madmax

heiney9

Premium because my car is chipped which changes all the fuel and timing maps compared to stock. The chip I use is pretty aggressive. If I use a lower octane fuel the knock sensors on each cylinder will **** the timing to stop pre-detonation of the fuel which causes pinging and could result in damage.

There is a limit to the range the knock sensors can prevent pre-detonation. I also find that using higher octane (with the chip installed) gives a bit better power and mileage. Because it doesn't run as lean.

I use 93 in the summer when it's hot out and I can run 91 in the winter when it's colder out.

H9

RuSsMaN

My truck runs on 87 octane Shell. I run on 80 proof James Beam.

Face

heiney9 wrote: »

There is a limit to the range the knock sensors can prevent pre-detonation.

I experienced this on my old Buick. I melted 3 ring lands, one so bad it lodged the top ring into the cylinder head.

heiney9 wrote: »

I also find that using higher octane (with the chip installed) gives a bit better power and mileage. Because it doesn't run as lean.

I believe you have this backwards.

Remember, lean is mean, rich is a ****.

heiney9

Face wrote: »

I experienced this on my old Buick. I melted 3 ring lands, one so bad it lodged the top ring into the cylinder head.

I believe you have this backwards.

Remember, lean is mean, rich is a ****.

OK, it runs leaner to lean is not good either. You want it to run as close to optimal as possible. Many "factory" ECU's have fuel and timing maps that ensure long term reliability, gas mileage, and reduced emissions. Sometimes this means leaning or richening the "mix" through the whole power band. For lack of a better word there are more tradeoffs from "factory" settings.

If I run 89 or less in the summer I get slight pre-detonation (pinging) lesser throttle response and poorer mileage (even if it is just 2-3 mpg).

Face

Correct. You want to run on the ragged edge of detonation to make the most power. But of course, there are always exceptions.

exalted512

Ive always heard its bad to run high octane in engines such as marine outboard motors, riding lawn mowers and motorcylces, any truth to that?
-Cody

WilliamM2

exalted512 wrote: »

Ive always heard its bad to run high octane in engines such as marine outboard motors, riding lawn mowers and motorcylces, any truth to that?
-Cody

Both my Motorcycle and outboard call for premium in the manuals, if that's what you mean by high octane.

exalted512

Right, high octane as in above 87. Guess that solves that!
-Cody