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Gasoline
- Thread starter npoe1
- Start date
Hail The Brain Slug
Diamond Member
Higher octane = burns more quickly = possible preignition because engines meant for 87 octane don't have the correct timings for 91 octane.
DisgruntledVirus
Lifer
performance cars require higher octane.
For example the Civic Si, and Mazdaspeed3 both say anything other then premium fuel can damage the engine. As XabanakFanatik stated, engines are designed with their timings for a certain fuel burn time. Lengthen or shorten that time by a certain amount could throw off the engine and cause damage to it.
The higher the octane the slower the combustion (and the more energy you get out of it).
The main factor when determining which octane rating an engine gets is its compression ratio.
A high compression ratio usually calls for 91-93 octane while a low compression ratio calls for 87 octane. (This rule of thumb excludes FI engines)
On any engine developed in the last decade, the engine will automatically adjust timing if an improper octane rating is used. I wouldn't make a habit out of running 87 where 91 is required but it won't hurt anything if you do it by accident a couple times.
Running premium on an engine which runs on regular is a waste of money and has no benefit.
Simply put - Engineers decide what fuel type will be run on a certain engine and design and build it accordingly.
Mr.IncrediblyBored
Lifer
It depends entirely on the car and the type of engine. Your owners manual will tell you what type of gas to use. Hell, it usually says the octane type right inside the fuel door next to the gas cap. At best your just pissing money away.
Zenmervolt
Elite member
There are many VW engines that work best with higher octane. The turbocharged engines, for example.
Also, Ford has a few engines that want higher octane (Shelby GT500 comes to mind), though yes, most engines from Ford are designed to run on the less expensive regular gasoline.
You cannot use a simple blanket statement like that.
ZV
Zenmervolt
Elite member
Depends on the engine. 9:1 and above used to be high, but with more modern materials and computer-aided design of the combustion chamber to eliminate hot spots there are now engines that have 10:1 compression ratios and can still run on regular gasoline.
It also depends on whether or not an engine uses forced induction (turbocharging or supercharging). My 951 has only an 8:1 compression ratio, but runs 15 PSI of positive manifold pressure at full boost and requires premium because of this.
ZV
StageLeft
No Lifer
For this argument, whatever your manual says. If your car is designed to run optimally on 87, 91 shouldn't hurt it but it is a TOTAL and uttery waste of money. There is no benefit to it. I think I've seen numbers that indicate in some cases a very minimal loss of fuel economy with 91 in an engine designed for 87, this because it does have a very slight bit less actual useable energy in it.
exdeath
Lifer
Octane rating is auto-ignition resistance. The higher the octane the harder the fuel is to ignite, thus allowing the use of more boost or higher compression which would other wise heat up the mixture enough to cause it to ignite unintentionally before the optimal time. This is known as detonation or pinging or knocking, and is very very bad on supercharged or turbocharged engine which already has a hotter than normal intake temperature and density.
Most cars use knock sensors that are basically speakers that listen to the engine block for the distinct frequency range associated with knock and the ECU can retard timing or enrich the mixture to keep combustion temperatures below the knock threshold. Using lower than recommended octane in these vehicles will result in reduced power and fuel economy if they are designed to accommodate premium. This is why people incorrectly believe that higher octane has more power when they go to the proper octane and performance resumes to factory spec.
Use of higher octane than the engine is built for on the other hand will not provide any gains, unless the engine is mechanically built to take advantage of it (more compression, more boost, etc) or the computer is reprogrammed to allow for more timing allowed by the higher knock threshold.
Not all cars have knock sensors however.
The supercharged Cobra and GT500 have the knock sensor boss eliminated in the valley to make room for the supercharger intercooler and lower intake and don't have knock sensors.
There was also concerns with sensor sensitivity due to the supercharger noise confusing the knock sensor, thus the sensor was eliminated entirely.
Paying attention to timing and proper fuel octane is critical with these cars when you start messing with them.
Proper piston and combustion chamber design, head gasket design, and quench clearance (how close the outer ring of the piston crown gets to the head at top dead center, accounting for rod stretch at high RPM), play a key role in how much boost or compression you can get away with for a given fuel octane before knock occurs.
Most cars use knock sensors that are basically speakers that listen to the engine block for the distinct frequency range associated with knock and the ECU can retard timing or enrich the mixture to keep combustion temperatures below the knock threshold. Using lower than recommended octane in these vehicles will result in reduced power and fuel economy if they are designed to accommodate premium. This is why people incorrectly believe that higher octane has more power when they go to the proper octane and performance resumes to factory spec.
Use of higher octane than the engine is built for on the other hand will not provide any gains, unless the engine is mechanically built to take advantage of it (more compression, more boost, etc) or the computer is reprogrammed to allow for more timing allowed by the higher knock threshold.
Not all cars have knock sensors however.
The supercharged Cobra and GT500 have the knock sensor boss eliminated in the valley to make room for the supercharger intercooler and lower intake and don't have knock sensors.
There was also concerns with sensor sensitivity due to the supercharger noise confusing the knock sensor, thus the sensor was eliminated entirely.
Paying attention to timing and proper fuel octane is critical with these cars when you start messing with them.
Proper piston and combustion chamber design, head gasket design, and quench clearance (how close the outer ring of the piston crown gets to the head at top dead center, accounting for rod stretch at high RPM), play a key role in how much boost or compression you can get away with for a given fuel octane before knock occurs.
If your car is designed to run on regular, and you run premium, it CAN cause issues.
I remember when the Explorers went OBD II in 1996, in the winter people started coming in left and right with problems starting them in the morning. Either they ran rough or had long crank times/multiple attempts before they'd start.
Problem ended up being that people were using premium regardless of what the manual said to use. It took a long time to convince some of them that premium wasn't better and was actually causing the problem.
Just one example, this woudn't be the case with all cars.
I remember when the Explorers went OBD II in 1996, in the winter people started coming in left and right with problems starting them in the morning. Either they ran rough or had long crank times/multiple attempts before they'd start.
Problem ended up being that people were using premium regardless of what the manual said to use. It took a long time to convince some of them that premium wasn't better and was actually causing the problem.
Just one example, this woudn't be the case with all cars.
Hail The Brain Slug
Diamond Member
Whoops, yeah. I got it backwards.
GoatMonkey
Golden Member
And as you sap the performance, you also cut fuel economy and haven't gained anything by using the cheaper gas in a car that should get premium. So, just use the grade it was designed for.
People spend $30k+ on a car that requires premium gas and then won't spring for the extra $150 a year to make it run the way it should. :roll:
I can't believe that nobody mentioned diesel or E85 when talking about octane. Or direct injection.
Direct injection is a great idea that has started taking off recently. It pretty much puts the fuel injector right in the cylinder instead of mixing the fuel with air before it goes through an intake valve. Then during the compression stroke as the piston is moving up and the air is being compressed you don't have to worry about early ignition, because there is no fuel mixed with the air yet. Then at the last possible instant, the direct injection system sprays the fuel into the compressed air, and then the spark plug fires and sets off the explosion. This wait till the last instant approach allows for a higher compression ratio on lower octane gas.
In a car with a diesel engine, there is no spark plug. Diesel fuel is made to auto-ignite at a certain compression ratio.
Then you have E85. It has an octane rating of 105. You know all those cars you see driving around with E85 logos on them? None of them have the compression ratio to take advantage of 105 octane. All those cars do is run it. At a big loss in fuel economy compared to regular gas. And if they ran E85 full time, the engine would probably fail significantly sooner than if they ran on regular gas. Ethanol doesn't give lubrication inside the cylinder the way gasoline does.
They could make an engine that runs well on E85 with forced induction. Saab did it. Probably not worth the trouble due to the inefficiencies of making E85. Still, it's worth a mention when talking about octane.
First of all, you have to understand what the octane rating means.
the octane rating is a measure of how much compression a certain fuel can take before igniting. All fuels will ignite if you pressurize it and / or raise its temperature to a certain point; however, some fuels can take a higher pressure or temperature before igniting. The octane rating is a measurement of this - so the higher an octane rating, the more pressure / temperature the fuel can be exposed to before igniting.
In your car's engine, you don't simply inject fuel into the cylinders then ignite it immediately. All engines go through a compression stroke before the gas is ignited, which pressurizes the fuel. This is measured by the engine's compression ratio, which generally ranges from 8.0:1 to 12.0:1. The higher the compression ratio, the more pressure is exerted onto the gas.
Now, if you use a fuel that has ignites at a lower pressure / temperature than that exerted by your engine's compression stroke (ie. a fuel with a lower octane rating than recommended), the gasoline will auto-ignite before the stroke is completed. This condition is called 'knocking' or 'pinging' and causes excessive wear and damage to the engine.
What happens if you use a higher octane rating than recommended? Since the octane rating is primary to prevent knocking, there are no advantages to using a higher rating than recommended.
To envision this, imagine you diving into a swimming pool. You know you will decend 6 ft into the pool when you dive. So if the pool is only 4ft deep (too low an octane rating), you will hit your head on the bottom of the pool and die. If the pool is 8ft deep (the recommended octane rating), it will be just right, you won't hit the bottom of the pool. Now, what if the pool was 12ft deep (higher than recommended octane rating)? You would still not hit your head on the bottom of the pool, and the extra 4ft over the 8ft pool has no effect; it would be no different than a dive that you've done in the 8ft pool.
the octane rating is a measure of how much compression a certain fuel can take before igniting. All fuels will ignite if you pressurize it and / or raise its temperature to a certain point; however, some fuels can take a higher pressure or temperature before igniting. The octane rating is a measurement of this - so the higher an octane rating, the more pressure / temperature the fuel can be exposed to before igniting.
In your car's engine, you don't simply inject fuel into the cylinders then ignite it immediately. All engines go through a compression stroke before the gas is ignited, which pressurizes the fuel. This is measured by the engine's compression ratio, which generally ranges from 8.0:1 to 12.0:1. The higher the compression ratio, the more pressure is exerted onto the gas.
Now, if you use a fuel that has ignites at a lower pressure / temperature than that exerted by your engine's compression stroke (ie. a fuel with a lower octane rating than recommended), the gasoline will auto-ignite before the stroke is completed. This condition is called 'knocking' or 'pinging' and causes excessive wear and damage to the engine.
What happens if you use a higher octane rating than recommended? Since the octane rating is primary to prevent knocking, there are no advantages to using a higher rating than recommended.
To envision this, imagine you diving into a swimming pool. You know you will decend 6 ft into the pool when you dive. So if the pool is only 4ft deep (too low an octane rating), you will hit your head on the bottom of the pool and die. If the pool is 8ft deep (the recommended octane rating), it will be just right, you won't hit the bottom of the pool. Now, what if the pool was 12ft deep (higher than recommended octane rating)? You would still not hit your head on the bottom of the pool, and the extra 4ft over the 8ft pool has no effect; it would be no different than a dive that you've done in the 8ft pool.
Well, I have a V6 Fusion with a compression engine of 10.0:1 (according to manual). I always have used premium on all my cars, but lately I'm more curious about cars so I was reading the manual and bang! 87 Octanes.
Now I have to accept the new idea that 87 octanes is no lower quality than 91+.
Now I have to accept the new idea that 87 octanes is no lower quality than 91+.
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