[badboyzbadboyz]

Hi,


I was reading through another forum and came across what I thought was an interesting thread, so I thought I would pass it on. I was always under the impression that the higher the octane level in the fuel the better ..... its a long read ....

Quote: Originally Posted by

“I only use super/premium/performance/ high octane fuel in my Civic DX. I can feel the power…”

This is probably one of the biggest misconceptions in the automotive world, as we know it. Yet, I hear this all the time. For example a close friend of mine who drives a 1996 Pontiac Bonneville tells me that he only uses Chevron 92 octane fuel because “It performs so much better, and my fuel economy is way higher.” This opinion or misconception is not an isolated view to the “gear-heads” that are or think they are car savvy. And the oil companies don’t help the matter by calling their high-octane fuels “Performance” or “Premium,” or whatever they choose. Here is why.

The first thing we have to know is obviously, what is octane and why does it matter? Octane can be simply defined in one sentence: Octane is a rating of a fuels resistance to ignite (evaporative qualities). WAIT A MINUTE! Resistance? That’s right, resistance. To put it simply, the higher the octane, the harder it is to get to ignite. Higher-octane fuel resists burning. Lower-octane fuel does not. Lower-octane fuel is actually more explosive, move volatile, and creates a much bigger explosion then its higher-octane counter-part.

When you go to the pump to fill your car, you generally have three options. Regular, mid-grade, and super. You also see that there is a rating system. It is a sticker posted to the pump next to the octane rating of the fuel that you choose. It generally says something like this. Regular for example: 87 Octane minimum, measured by using the (R+M)/2 rating. What does this mean? The R in the equation stands for Research Octane Rating, known as RON. This is a test that tries to simulate variables that your car would go through on a given day. It is a test using a single cylinder, four-stroke engine that is idling at 600rpm. The fuel temperature going into the engine is not controlled. It is allowed to vary and fluctuate with the temperature of the conditions at the time. The intake air temperature is varied with barometric pressure to simulate the air being sucked into the car from the outside. The timing is set at 13° BTDC. This is done to covert everything to a SAE standard day, which is 60° F, 0% Humidity, and 29.92 inches barometric pressure.

The M in the equation stands for Motor Octane Rating, known as MON. When testing MON, the fuel is heated to 300° F and the intake air is heated to 100° F. The test engine is a single cylinder 4-cycle engine that is run at 900 rpm. Ignition timing is varied with compression ratio. Engine load is varied during test. If you are deciding what fuel to use on a high-compression, high-performance engine, this is the test you would want to use. The RON will always be higher than the MON.

So what does this all mean when you go to the pump? It means that to extract the most from the car that you are driving, you have to use the appropriate octane fuel or the car is not going to perform up to its potential. Either the fuel will burn too slowly, and in many cases not burn completely, or it will burn too fast and out of control. The latter is called Detonation. Normal combustion will take place at a pretty steady rate (for a given rpm and load), when a large amount of the charge burns extremely fast and uncontrollably, it is know as detonation. Detonation can destroy an engine in a matter of seconds. There are a few causes of detonation. Usually it is attributed to too much heat, but that is only part of the problem. Pressure and advanced ignition timing both play a role in it too. Usually when detonation takes place, the detonation occurs on the intake side of the chamber, which is the coolest side of the chamber. This happens because detonation did not occur until the pressure got excessive, which was after the spark. By the time that happened, the charge near the exhaust valve has already been burned. Most of the time, detonation will occur after normal combustion has started. Apart from destroying pistons and spark plugs, light detonation can cause all sorts of other problems, like fatiguing cranks and rods quickly and pounding bearings to death, so avoid detonation at all costs.

Another problem is Pre-ignition (for cars using octane that is too low). When the charge lights off before the spark, it's called pre-ignition. This can happen with or without detonation, but usually will cause detonation in a high performance engine. Hot spots in the combustion chamber are the usual cause of pre-ignition. This is very unpredictable and can lead to detonation, because it will act just like too much ignition timing, but it won’t be controllable.

So this is what happens in cars that use octane ratings that are too low, but what about vise-versa? Generally, the problem with using an octane rating that is too high for your car causes problems that, while not generally as severe as going too low, are still detrimental to the performance and efficiency of your engine. One of the most common problems is lost power. Yes, you read that right, if you use an octane of fuel that is too high for your car, you will lose power. This goes for all the stock or lightly modded Civics out there. Remember when we discussed that octane measures the fuels resistance to burn? Well, this is where that really comes into play in a low compression engine, such as the engines found in 90% of all streetcars. As the fuel is injected into the combustion chamber and the compression stroke begins, cylinder pressure is created. This is vital to combustion as cylinder pressure is what translates into power. When that pressure is not high enough (low-compression), the fuel only partially ignites. This means that there is leftover fuel that has to be expelled through the exhaust valves and expelled through the exhaust. This is bad for several reasons. The first is that this un-burnt fuel leaves deposits and grime all over the cylinder and valves. This robs you of power, fuel economy, and wears engine components pre-maturely. There is also a vital part of any streetcar, the emissions control devices. The catalyst for example often is destroyed by being clogged with carbon and un-burnt fuel resulting from using too high-octane. This results in costly repairs, and guess what? The loss of power and lower fuel economy. Not to mention, you had to pay $.20 more per gallon just to lower your cars potential and performance. So you guys that think that adding 100 octane race gas to your Civic Ex at the track is going to improve your times, you are running in the wrong direction unless you have some kind of forced induction such as Nitrous, Turbo, or Super Chargers.

So the next question people ask is, “Why then do high-power, ultra-fast cars use high octane fuel?” Well the answer is simple. Higher-octane fuel contains more POTENTIAL energy but requires the higher heat generated by higher compression ratio engines to properly condition the fuel to RELEASE that higher potential energy. Here is the general rule for choosing octane if you have a question of what it should be. If your compression is 9.5:1 or lower, 87 octane should be plenty. As your compression rises, so should the octane that you choose.

Remember that there are variables to everything in life and this is no exception. If you have dramatically advanced ignition timing for example, high-octane may be needed. But for the vast majority of the people on the street, for optimal performance in your streetcar, use the octane best suited for your car, and remember, high-octane does not mean more power!

[stryder]

This isn't ground-breaking (unless you belived octane was related to power) but the point on MINIs are they do have higher compression, 10.5:1 on the MC. (MCS is lower to start with, but then has the supercharger)

So essentially the 2nd to last paragraph proves Premium is the better choice, because of the higher octane, if you run lower octane, the engine will retard its timing to prevent detonation, and thus you have poor performance.

The article is only relevant for the people who put premium into cars that say use regular gas.

[Paul]

What Stryder said.

[Z1GZAG N]

Another thing to keep in mind is that most all cars built today, have knock sensors. You might think you're ok running 87 in your Mini or other performance car because you don't her any detonation (Pinging). But, what is really happening is the computer is getting a detonation signal from the knock sensor and has backed the timing down. So, it may not be causing any detonation (Pinging) but with less timing, the motor is no longer running efficiently and thus is making less power which means worse fuel mileage.

One other thing, especially with the supercharged car, you may not hear detonation (Pinging), but it may still be happening to some extent and in a supercharged car, even a small amount of detonation (Pinging) can cause burnt pistons. $$$

With premium 91-93 octane fuel typically costing .20 cents more a gallon, at 13 gallons for a fill up from empty, you're talking about $2.60 more a fill up. If you fill up twice a week, you're only talking $270.00 bucks a year.

Compared to paying for a new motor, $270.00 a year is cheap insurance.

[Vizilo]

I have just purchased a MCS Converible as a "sister" to my Saab 9 3 Turbo. I am embarrased to say that I generally use regular gas even though super is recommended for the Saab. I do not notice a difference in performance or fuel economy.

I don't know much about cars. Would using regular gas in the Mini be better or worse than using it for a turbo engine?

You do make a good point. You spend CDN$40K on a car. You shouldn't worry about spending another $4 to $5 per tank of gas if you can save on potential repairs. I'll try to do better with the Mini

[quentinberg007]

Quote: Originally Posted by Vizilo

I have just purchased a MCS Converible as a "sister" to my Saab 9 3 Turbo. I am embarrased to say that I generally use regular gas even though super is recommended for the Saab. I do not notice a difference in performance or fuel economy.

I don't know much about cars. Would using regular gas in the Mini be better or worse than using it for a turbo engine?

You do make a good point. You spend CDN$40K on a car. You shouldn't worry about spending another $4 to $5 per tank of gas if you can save on potential repairs. I'll try to do better with the Mini

Seriously, you have supercharged cars (the turbocharged saab is technically supercharged). You need to use the fuel that the owners manual says. My Impreza is naturally aspirated with a compression ratio of 10.1:1. This can easily handle 87 octane gasoline (as per the owners manual). My g/f's MCS is supercharged. The final compression ratio of the air fuel mixture in the cylinder at top dead center is much higher than my 10.1:1 in my RS, due to the supercharger. That is why is makes the same power (165 hp) out of a 1.6L as compared to my 2.5L in my Subaru. Now, if you put 87 octane in the MCS, the air-fuel mixture would auto ignite at a compression ratio that is well below the top dead center compression ratio. i.e. The piston is still sweeping upward and the fuel auto-ignites before the spark plug sends the spark to the mixture. So, you get early detonation of the fuel when the piston is still much earlier in the upswing motion than it was designed to be. You have the inertial forces of the piston going upward and the explosion force of the fuel and air forcing downward. That is bad times, indeed. Seriously, if you want to destroy your engine, keep putting 87 in it. I hope, for you 9-3's sake, that it has a low compression engine with low boost.

~~Quentin

[beken]

This has been discussed adnauseum somewhere else.

Our MINIs were designed with high compression engines and designed to use higher octane fuel. At 1.6cc, it's amazing how much power they wring out of it.

A Civic DX was designed to use regular gas. Higher octane gas would do it no good. But neither would it do bad other than waste a lot of money. Of course, if the guy chipped his Civic to take advantage of the higher octane, that might be a different story. I also have another car that uses regular gas and putting even mid-grade in it causes cold start problems. So use the fuel your car was designed for.

Having said that, and the fact that this thread came up, implies that someone might be asking if using regular 87octane gas would be fine for our MINIs. The computers in our MINIs would adjust to allow them to run fine. However, you will probably notice a decrease in performance and gas mileage as the car is not running at its optimal level of tune. As Stryder has already said, the article was written more for those with cars that are designed to use regular gas.