Octane rating is the gas' resistance to detonation, the higher the rating, the higher the resistance. Running a lower octane than recommended could cause knock/engine ping. I'm commenting on this from pure memory so feel free to correct any false statements. Also, octane to some degree is a measure of oil refining, lower octane could cause pre-mature sludge deposits. For any high-powered vehicle, especially turbo'd ones octane matters greatly. I see you have a regular 3, high octane does nothing for us except *maybe* a slight loss of low-end due to the increased detonation resistance. Running race gas (100+ octane) would benefit a Speed GREATLY where as for us it does nothing. Trust me, I've run 100 octane in my car and it made no difference.
I can tell a huge difference in octane rating in my speed. much smoother throttle response and even shifting. I always use 93, but when im low on fuel I have stopped at a gas station near my job that only has 91 and I can always tell the difference. The post above me is correct about higher octance ratings and knocking.
I read in a thread the other day where it said the speed3 can run any ocatae rating. the ecu can re-learn and adjust boost/timing at a loss of HP for lower octanes. so you can run 87, but you will loose hp.
Advocating that anyone run 87 octane in a Speed is just sad. You'll lose engine smoothness (what little there is) and have you noticed the speed3 says "min. 91", min means minimum. As in, don't go lower or face the reaper!
Anything is possible, heck it might run on water for a few minutes if you add a gallon in to a half full tank, doesnt make it correct.
To get the advertised performance and longevity of the engine, run the higher octane. Higer octane on a car that needs 87 wont mean better performance, but lower octane on a car that needs 91+ does mean lower performance.
Yes both cars will run on the other, but the car has to make changes to timing to allow it to do it safely. The MS3 will pull timing (or is it advance) to reduce the chance of predetonation.
The need for higher octane is due to the increased compression of the air inside the engine. When air is compressed, from the turbo,it heats up and then is further compressed in the cylinder once mixed with gas. Hot highly compressed air/fuel can cause fuel to detonate prior to the spark from your spark plug. By raising the octane level of the fuel, it will ignite at a higher temperature and HOPEFULLY not ignite prior to the spark plug doing its job.
Using a lower octane increases the chance and therefor the ECU will detect the detonation and change the timing to save your engine, performance will be decreased due to this.
[quote author=Eternal link=topic=153340.msg3279300#msg3279300 date=1250266193]...but the car has to make changes to timing to allow it to do it safely. The MS3 will pull timing (or is it advance) to reduce the chance of predetonation.[/quote]
The car does not necessarily HAVE to pull timing to run safely. It will only pull timing if knock is detected; though if you run lower octane there is a good possibility of that happening on a MS3.
[quote author=Eternal link=topic=153340.msg3279300#msg3279300 date=1250266193]The need for higher octane is due to the increased compression of the air inside the engine. When air is compressed, from the turbo,it heats up and then is
further compressed in the cylinder once mixed with gas.[/quote]
Errr.... yes and no. I'll deal with this in a bit.
[quote author=Eternal link=topic=153340.msg3279300#msg3279300 date=1250266193]Hot highly compressed air/fuel can cause fuel to detonate prior to the spark from your spark plug. By raising the octane level of the fuel, it will ignite at a higher temperature and HOPEFULLY not ignite prior to the spark plug doing its job.
WRONG. What you are referring too, when saying the mixture ignites BEFORE the spark plug is pre-ignition. Pre-ignition is a VASTLY different animal than knock (a.k.a detonation). Pre-ignition is often caused by a glowing ember which ignites the mixture early in the compression stroke. The reason this is so catastrophic is because the engine attemps to compress the already expanding mixture and cylinder pressures and temperatures go SKY high. NO amount of octane can prevent the mixture from igniting if there is a ignition source like a glowing ember.
Knock or detonation, on the other hand, is caused by parts of the mixture unable to resist the increasing pressure and temperature during the combustion event. The key part of this is that it occurs AFTER the spark plug is fired. The mixture is unable to resist this heat and pressure (which is rising immensely) and detonates. As such, two flame fronts then exist inside the combustion chamber which can cause bad things.
The MS3 does not have to run high octane DIRECTLY because of the boost and/or compression ratio but because of the timing curves with which Mazda programmed the ECU. Mazda easily could have reduced timing and made the engine run on 87 Octane. However, doing so would have reduced engine output (as occurs when the engine pulls timing if it detects knock).
I think we are describing the same thing, but knock happens prior to the spark, and is due to compression causing the heat to rise and the mixture to detonate prior. Tis can be called knock, predetonation, preignition, ITS ALL THE SAME THING. It does not happen after the spark.
"As long as this process works as described above, the engine runs smoothly. But occasionally the pressure of the piston itself will cause the air and gas mixture to ignite prematurely during the compression cycle, creating a smaller, less powerful explosion. This is called preignition and it's the cause of engine knock, the erratic rattling or pinging sound you may occasionally hear underneath your car's hood. A little bit of engine knock isn't necessarily bad for your engine, but it's not desirable, either. It means that your engine isn't running as efficiently as it could be, and left unchecked, it could eventually cause damage. Engine knock reduces your car's performance, too, so you definitely want to avoid it. How, you may ask? Well, low-octane gas is more likely to ignite under the pressure of the piston alone, so it's also more likely to produce engine knock."
"Engine knocking is the premature fuel combustion that can result in power loss of the engine. Engine knocking is compression detonation or pre-ignition of fuel in the power stroke of the engine. Engine knocking is normal for 4-stroke bike. "
There are about 10 other websites that say this, but wikipedia has the exact opposite answer.
[quote author=Eternal link=topic=153340.msg3280278#msg3280278 date=1250308848]
I think we are describing the same thing, but knock happens prior to the spark, and is due to compression causing the heat to rise and the mixture to detonate prior. Tis can be called knock, predetonation, preignition, ITS ALL THE SAME THING. It does not happen after the spark.[/quote]
WRONG. WRONG. WRONG. WRONG. I hate to be so bold, but that source is flatly wrong.
Knock, also known as detonation is NOT the same thing as Pre-ignition. They are VERY different animals.
"Knocking (also called knock, detonation, spark knock or pinging) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front."
"Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires."
"Confusion and a lot of questions exist as to detonation and pre-ignition. Sometimes you hear mistaken terms like "pre-detonation". Detonation is one phenomenon that is abnormal combustion. Pre-ignition is another phenomenon that is abnormal combustion.
"Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug.
"Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing."
"Another condition that is sometimes confused with detonation is "preignition." This occurs when a point within the combustion chamber becomes so hot that it becomes a source of ignition and causes the fuel to ignite before the spark plug fires."
"For completeness, it is worth mentioning that "detonation" refers to abnormal explosion(s) AFTER the normal ignition. If spontaneous ignition occurs before the spark plug fires, that's a different and far more dangerous condition: "preignition."
Here's WHY they are VASTLY DIFFERENT ANIMALS.
When pre-ignition occurs, it often occurs VERY EARLY in the compression stroke. Why? Because as the air/fuel mixture is compressed it becomes HARDER to ignite. The energy needed to 'spark' the mixture goes up with the pressure inside the combustion chamber. So, since it usually happens early in the compression stroke, the air/fuel mixture begins to expand and the engine attempts to compress this already expanding mixture which leads to INSANELY high pressures inside the cylinder. Generally, a pre-ignition event will cause catastrophic engine damage in a single event. The pressures are ridiculously high. FURTHERMORE, pre-ignition is not something normally detectable by a "knock sensor." I'll explain why in a bit.
When you have knock or detonation, the combustion process starts normally; as in, by the spark plug. Yet, for various reason, a pocket of air/fuel SELF ignites -- normally due to the rising pressure/heat caused during combustion. The rising pressure/heat becomes greater than the fuel can withstand and... boom. When the pocket self ignites, it creates a SECOND flame front. This second flame fronts cause a very sudden, very high, but very short spike in cylinder pressure. There is literally a shock wave that bounces against the insides of the combustion chamber.
IT IS THIS SHOCKWAVE that causes the engine block to literally "ping." It is akin to striking the block with a hammer. It is this "ping" that the knock sensor detects. During a pre-ignition event, there is no "sudden" rise in cylinder pressure and thus no "ping" and thus nothing for the knock sensor to detect. The pressure rises more gradually (relative to knock) but builds to enormous pressures.
Bottom line: Detonation/Knock and Pre-ignition are VASTLY different animals and should NOT be confused.
I dont understand why there's such a big debate/war going on about what octane to use... If it's recommended and in this case, "required" by the manufacturer... then just use it.
Synthetic oil is universally recommended and used for this car even though Mazda have said nothing about it, yet no one argues about it. And it is more expensive than the conventional stuff.
Synthetic tranny fluid is also universally recommended and used for this car... again, it's more expensive and no one complains about it.
91 is the bare minimum for the car, so why risk it by going to 87? The car wasn't built to be conservative...
It's like hearing an athlete saying "I'll be just as fast and strong going on junk food alone"
Yes I do realize this was the most technically challenged post for this thread, but that's how I understand it
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