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Octane Question. (Read 349 times)
gerald.hughes
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Octane Question.
06/30/13 at 08:20:11
 
I was taught that the only reason for octane in gas was to retard ignition, specifically pre-ignition in a cylinder.  I was also taught that if the compression ratio was 10:1 or lower in the cylinder, then gas with an octane rating of 87 would be sufficient.  

Lately, I have been looking at bikes with compression ratios below 10:1 where the manufacturer is recommending  91 octane for the engines.  Can anyone explain the reason behind this to me?

Thanks
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Re: Octane Question.
Reply #1 - 06/30/13 at 09:34:51
 
I suppose some might recommend it at 9.5 compression...,10:1 is not set in stone...
... but,... most performance vehicles today will be fuel injected, and will have an automatic timing retard if the CPU senses engine knock... so the vehicle won't suffer any damage with regular... it might take a slight performance hit under load... climbing a steep grade with 2-up or something...

No matter what the compression is,... if it don't knock under load, higher octane isn't needed...
JMHO...
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Re: Octane Question.
Reply #2 - 06/30/13 at 10:58:29
 
Octane does not retard ignition.  A higher octane rating simply makes the mixture less susceptible to detonation,... it resists exploding all at once and tends to make the mixture burn more evenly.

The higher the octane rating, the less energy is in the fuel.  As long as the motor is not knocking, you should always use the lowest octane rating you can.
 

The higher octane is there simply to allow a higher compression without detonation.  This is why high-performance cars (Corvettes, WRXs, M3 BMWs, etc.) call for premium,... not because it has more energy, but because these high-compression motors need it to suppress detonation.

Putting 89 or 91 or higher octane in a Savage is like putting a drag chute on your bicycle,.. just because high-speed vehicles use it doesn't mean it'll make you go faster.
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Re: Octane Question.
Reply #3 - 06/30/13 at 11:15:32
 
Gyrobob,

I am a little confused.  Could you please explain the difference to me between, "retard ignition", and "makes the mixture less susceptible to detonation."?
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Re: Octane Question.
Reply #4 - 06/30/13 at 11:40:57
 
Your statement is a little bit true. Premium burns a little slower. Would you or I be able to tell... Probably not, would it retard the timing enough to tell? Don't know.
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Re: Octane Question.
Reply #5 - 06/30/13 at 12:18:19
 
Since the flame front travels in milliseconds that would be no. Fuel cannot advance or retard timing. Unless you claim that pre-ignition advances the timing. High octane fuel has a resistance to knock.

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/question90.htm

http://www.streetrodstuff.com/Articles/Engine/Detonation/index.php

Detonation
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. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition

Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.
Detonation
Unburned end gas, under increasing pressure and heat (from the normal progressive burning process and hot combustion chamber metals) spontaneously combusts, ignited solely by the intense heat and pressure. The remaining fuel in the end gas simply lacks sufficient octane rating to withstand this combination of heat and pressure.

Detonation causes a very high, very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, then all of a sudden you would see a very sharp spike when the detonation occurred. That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of detonation is commonly called spark knock. This noise changes only slightly between iron and aluminum. This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to 6400 hertz and they will pick up that spark knock. Incidentally, the knocking or pinging sound is not the result of "two flame fronts meeting" as is often stated. Although this clash does generate a spike the noise you sense comes from the vibration of the engine structure reacting to the pressure spike.

One thing to understand is that detonation is not necessarily destructive. Many engines run under light levels of detonation, even moderate levels. Some engines can sustain very long periods of heavy detonation without incurring any damage. If you've driven a car that has a lot of spark advance on the freeway, you'll hear it pinging. It can run that way for thousands and thousands of miles. Detonation is not necessarily destructive. It's not an optimum situation but it is not a guaranteed instant failure. The higher the specific output (HP/in3) of the engine, the greater the sensitivity to detonation. An engine that is making 0.5 HP/in3 or less can sustain moderate levels of detonation without any damage; but an engine that is making 1.5 HP/in3, if it detonates, it will probably be damaged fairly quickly, here I mean within minutes.

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Re: Octane Question.
Reply #6 - 06/30/13 at 13:22:34
 
As far as the LS650 goes 87 Octane is all you need, anything higher is basically just wasting money  Huh
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Re: Octane Question.
Reply #7 - 06/30/13 at 15:02:08
 
It also depends on whether the octane is rated by the research or motor method, or a combination of both. The techies can explain this better than me. The handbook recommends a minimum of 87 octane Research plus motor divided by two method. If you can only get ethanol blended petrol don't go higher than E10. If the pump says 87 you should be ok. If you get audible pinging noises you can go up to 91.
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Re: Octane Question.
Reply #8 - 06/30/13 at 15:31:37
 
Just a quick question, What Octane would work best for a 97mm piston with a 10.5:1 ratio?
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Re: Octane Question.
Reply #9 - 06/30/13 at 16:30:35
 
paulmarshall wrote on 06/30/13 at 15:31:37:
Just a quick question, What Octane would work best for a 97mm piston with a 10.5:1 ratio?


You have to run premium.....91 or higher.  The Savage spark advance cannot react to knock and retard the timing....so you have to buy the best you can and hope it is enough.
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Re: Octane Question.
Reply #10 - 06/30/13 at 16:35:12
 
Dave wrote on 06/30/13 at 16:30:35:
paulmarshall wrote on 06/30/13 at 15:31:37:
Just a quick question, What Octane would work best for a 97mm piston with a 10.5:1 ratio?


You have to run premium.....91 or higher.  The Savage spark advance cannot react to knock and retard the timing....so you have to buy the best you can and hope it is enough.

The best in NZ is 98 Octane.
We have 91,95 and 98.
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Re: Octane Question.
Reply #11 - 06/30/13 at 16:39:19
 
paulmarshall wrote on 06/30/13 at 16:35:12:
Dave wrote on 06/30/13 at 16:30:35:
paulmarshall wrote on 06/30/13 at 15:31:37:
Just a quick question, What Octane would work best for a 97mm piston with a 10.5:1 ratio?


You have to run premium.....91 or higher.  The Savage spark advance cannot react to knock and retard the timing....so you have to buy the best you can and hope it is enough.

The best in NZ is 98 Octane.
We have 91,95 and 98.


I have no idea how the NZ octane number compare to the US ones.  If you have a choice between regular, mid grade and premium....use premium.  Use the highest grade pump gas that is available at normal retail stations.....not race fuel.
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Re: Octane Question.
Reply #12 - 06/30/13 at 16:42:55
 
I believe their 91 is the same as our 87.

New Zealand: 91 RON "Regular" and 95 RON "Premium" are both widely available. 98 RON is available instead of 95 RON at some service stations in larger urban areas.
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Re: Octane Question.
Reply #13 - 06/30/13 at 16:44:18
 
Different grading system NZ fuels are roughly 87, 89, 91-93 U.S.A. method.

Savage will safely burn USA spec 85 octane. Some parts of the mid-west used to sell 85, 87, 89, my 1998 model was quite happy running the 85 octane "paint brush cleaner" as fuel. About E12-15 range.

Want to make your bike really happy? make friends with a crop duster who buys his av-gas in drums. It is hi octane LEADED fuel, real gasoline, not the modern on road imitation stuff that ruins older engines.
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Re: Octane Question.
Reply #14 - 06/30/13 at 16:45:25
 
Dave wrote on 06/30/13 at 16:39:19:
paulmarshall wrote on 06/30/13 at 16:35:12:
Dave wrote on 06/30/13 at 16:30:35:
paulmarshall wrote on 06/30/13 at 15:31:37:
Just a quick question, What Octane would work best for a 97mm piston with a 10.5:1 ratio?


You have to run premium.....91 or higher.  The Savage spark advance cannot react to knock and retard the timing....so you have to buy the best you can and hope it is enough.

The best in NZ is 98 Octane.
We have 91,95 and 98.


I have no idea how the NZ octane number compare to the US ones.  If you have a choice between regular, mid grade and premium....use premium.  Use the highest grade pump gas that is available at normal retail stations.....not race fuel.

OK I will use the highest Octane. If I had a stock piston would I still be wanting to use the higher octane?
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