Im just curious as to know if there is such a thing as too high octane for your car. Im interested in blending some xylene or tolune with out 98ron fuel and wonder if you can damage your engine if you go to high in octane.
how high is too high octane
- Thread starter miggles
- Start date
higher octane is just more controllable detonation i think. for instance, if you use 87 octane with a turbo, youre more prone to detonation due to unpredictability. reason why they reccomend premium in turbo cars.
correct me if im wrong
correct me if im wrong
The higher the octane the slower the burn and unless the motor "needs" it you'll drop power.
Lots of Boost Compression or timing and high octane make power
Tipping a can of high octane fuel in a low comp low boost low timed motor is a total waste of money and will perform worse.
You'll always make the most power with a fuel that has just a high enough octane rating to prevent detonation.
Lots of Boost Compression or timing and high octane make power
Tipping a can of high octane fuel in a low comp low boost low timed motor is a total waste of money and will perform worse.
You'll always make the most power with a fuel that has just a high enough octane rating to prevent detonation.
I'm curious, does higher octane place higher demands on the ignition system? Since it is essentially harder to ignite, does it need a stronger spark?
Usually as boost or comp go up plug gap comes down but there's a point where spark energy has to go up Mike.
:withstupi
Also the higher octane fuels will contain things that your stock O2 sensor is not going to like.
Also the higher octane fuels will contain things that your stock O2 sensor is not going to like.
For instance... water has such a high octane that it won't even burn. Use it and you will be making no power...... :bigthumb:
IJ. said:
Right, but aside from the pressure of the mixture and the size of the gap, does just changing the octane of the fuel have any effect on the power necessary to make a spark and ignite the mixture?
Adjuster
Supramania Contributor
I've seen a demo where low octane fuel vapor was put into a clear tube about 10' long and then ignited on one end. You hear the POP sound, and low octane fuel vapor was consumed so fast you really can't see the flame front burn across the tube.
Then high octane fuel vapor is put into the tube after blowing out the remains of the first test, and it's lit off.
WOW< almost amazing how "slowly" the flame front burns down the tube, you can see it clearly consume the vapor, and it's not nearly as loud, more of a whoosh than a pop like before.
So, imagine that your engine is a series of short tubes that vary in length depending on piston location at any given moment. When the flame front is created by the spark, it has to burn in a controlled manner. This controlled burn quickly raises the pressure, and forces the piston down, making the tube longer in effect, and as the piston reaches the bottem of the stroke, all the fuel should be burned in theory.
If it burns too fast, and it's "burned" out before the end of the stroke, and you lose some power potential.
Too slow, and it's still burning as the piston comes back up. (you might percieve this as a loss in power, but since the exhaust valve is opening up at this point, the unburned fuel is pushed out into the exhaust manifold, and consumed eventually. Either in the Cat, or as a huge fireball/backfire out your tail pipe.)
The problem occurs when the octane is so low, and the flame front is so fast, it raises the pressure in the combustion chamber so quickly that remaining fuel is ignited by the now even more compressed and very hot air/fuel. (Similar effect to a diesel motor where compression is used to create the heat needed to light off the fuel injected into the combustion chamber at just the right moment.)
This is generally called detonation. ("Ping" or a ringing sound from the engine under load generally.)
A very rapid chain of events can happen with low octane fuel.
1) Uncontrolled ignition of the fuel at the wrong timing point. This can happen as the pressure rises, and lights off the fuel prior to your spark plug being "fired". (May or may not have a "Ping" noise. Depends if secondary flame fronts are created or not.)
2) Upon the normal advanced ignition event, the fuel is burning so fast, the pressure spikes up, and you get secondary flame fronts started in other areas of the combustion chamber, and those flame fronts, hit each other, and you hear a "Ping" noise. The ping is not metal hitting metal, but pressure spikes hammering your engine from the inside out.
3) This flame front collision creates tremendous pressures, and that disturbs the boundry layer that keeps flames from direct contact with your piston crown, and other "stuff" in the combustion chamber. (Sounds weird, but think of this as a force field if you will, and it protects soft metals like alluminum from super heated burning gas that would quickly melt it.)
4) The disrupted boundry layer exposes metal to superheated burning fuel, and that metal is then quickly heated up to melting point. In the case of alluminum pistons, they turn soft first, deform and start to cause problems as the ring lands compress, then "grip" the rings, and your engine starts to destroy it's self at that point. If it lasts that long. (Typical broken piston ring lands are a symtom of this IMHO.)
5) Worst case, on the next intake stroke, the air fuel mixture is lit off by some glowing white hot metal, and then the intake valve closes, and you try and compress burning fuel.... This PRE-IGNITION generally takes out the motor at that point creating new holes for rods to poke out of, or valves get bent and broken, and a host of other problems occur. (Remember, much of your moving parts are superheated at this point, and not nearly as strong as they should be due to the heat effects on metal strength/hardness.)
So there you have it.
Slow burning high octane fuels where you risk the loss of power on the extreme end, or low octane fuels that burn quickly, but are fickle on force fed motors, and suddenly can turn and bite you in the ass at a moments notice. (Only takes a few seconds at the most, then it's over.)
Then high octane fuel vapor is put into the tube after blowing out the remains of the first test, and it's lit off.
WOW< almost amazing how "slowly" the flame front burns down the tube, you can see it clearly consume the vapor, and it's not nearly as loud, more of a whoosh than a pop like before.
So, imagine that your engine is a series of short tubes that vary in length depending on piston location at any given moment. When the flame front is created by the spark, it has to burn in a controlled manner. This controlled burn quickly raises the pressure, and forces the piston down, making the tube longer in effect, and as the piston reaches the bottem of the stroke, all the fuel should be burned in theory.
If it burns too fast, and it's "burned" out before the end of the stroke, and you lose some power potential.
Too slow, and it's still burning as the piston comes back up. (you might percieve this as a loss in power, but since the exhaust valve is opening up at this point, the unburned fuel is pushed out into the exhaust manifold, and consumed eventually. Either in the Cat, or as a huge fireball/backfire out your tail pipe.)
The problem occurs when the octane is so low, and the flame front is so fast, it raises the pressure in the combustion chamber so quickly that remaining fuel is ignited by the now even more compressed and very hot air/fuel. (Similar effect to a diesel motor where compression is used to create the heat needed to light off the fuel injected into the combustion chamber at just the right moment.)
This is generally called detonation. ("Ping" or a ringing sound from the engine under load generally.)
A very rapid chain of events can happen with low octane fuel.
1) Uncontrolled ignition of the fuel at the wrong timing point. This can happen as the pressure rises, and lights off the fuel prior to your spark plug being "fired". (May or may not have a "Ping" noise. Depends if secondary flame fronts are created or not.)
2) Upon the normal advanced ignition event, the fuel is burning so fast, the pressure spikes up, and you get secondary flame fronts started in other areas of the combustion chamber, and those flame fronts, hit each other, and you hear a "Ping" noise. The ping is not metal hitting metal, but pressure spikes hammering your engine from the inside out.
3) This flame front collision creates tremendous pressures, and that disturbs the boundry layer that keeps flames from direct contact with your piston crown, and other "stuff" in the combustion chamber. (Sounds weird, but think of this as a force field if you will, and it protects soft metals like alluminum from super heated burning gas that would quickly melt it.)
4) The disrupted boundry layer exposes metal to superheated burning fuel, and that metal is then quickly heated up to melting point. In the case of alluminum pistons, they turn soft first, deform and start to cause problems as the ring lands compress, then "grip" the rings, and your engine starts to destroy it's self at that point. If it lasts that long. (Typical broken piston ring lands are a symtom of this IMHO.)
5) Worst case, on the next intake stroke, the air fuel mixture is lit off by some glowing white hot metal, and then the intake valve closes, and you try and compress burning fuel.... This PRE-IGNITION generally takes out the motor at that point creating new holes for rods to poke out of, or valves get bent and broken, and a host of other problems occur. (Remember, much of your moving parts are superheated at this point, and not nearly as strong as they should be due to the heat effects on metal strength/hardness.)
So there you have it.
Slow burning high octane fuels where you risk the loss of power on the extreme end, or low octane fuels that burn quickly, but are fickle on force fed motors, and suddenly can turn and bite you in the ass at a moments notice. (Only takes a few seconds at the most, then it's over.)
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Adjuster
Supramania Contributor
As far as plug gap is concerned, as the pressures go up, the spark energy needed to jump a given space increases.
So, if your 8.5:1 CR engine can jump a .40 gap at an effective CR of say 15:1 that gap might be as low as .28 or less to get the same spark energy across the space. The rest of the time, when your running low compression, it's still sparking fine, but your spark distance is slightly lower. As long as you light off the fuel/air mix, who cares if the gap is less? It's making sure you can light off the mix at 25psi that matters... Not when your driving at 75mph and no boost. (At least that's my opinion.)
So, if your 8.5:1 CR engine can jump a .40 gap at an effective CR of say 15:1 that gap might be as low as .28 or less to get the same spark energy across the space. The rest of the time, when your running low compression, it's still sparking fine, but your spark distance is slightly lower. As long as you light off the fuel/air mix, who cares if the gap is less? It's making sure you can light off the mix at 25psi that matters... Not when your driving at 75mph and no boost. (At least that's my opinion.)