running 100 octane
running 100 octane would it be worth it? i live in california next to willow springs raceway. they have 100 octane unleaded gas. to tune for this you just up the timing to pull out more horsepower? any input please
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Joined: Mar 2003
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From: San Diego
No it would not be worth it. Higher octane fuel takes more energy to combust and lowers combustion chamber temperatures. You will always make the most horsepower with the lowest octane fuel you can safely run.
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Joined: Nov 2001
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From: Lake Zurich, IL
^^^ because they can add more timing ... high compression cars need higher octane because the air charge has a higher likelihood of pre-ignition, so by using a higher octane gas, you delay this
A car running 91 octane may get some knock and the knock learn factor would then lean more towards the low octane timing tables... a car running 93 octane may not generate as much knock so it has a learn factor closer to the higher octane timing tables...
If you could run the car on 87 octane (i'm not suggesting doing this) without detonation and the same timing, in theory you would make more power because the fuel and air charge is easier to ignite
A car that runs fine on 93 octane (meaning it's 100% high octane tables, proper AFR etc...) will likely actually slow down if it runs 100 octane (may not be drastic, but since the air/fuel charge is harder to ignite , the flame front won't begin at the proper time... so instead of beginning at say 28* before TDC, it may not actually begin until 27* TDC (even though you commanded 28* of spark advance)
A car running 91 octane may get some knock and the knock learn factor would then lean more towards the low octane timing tables... a car running 93 octane may not generate as much knock so it has a learn factor closer to the higher octane timing tables...
If you could run the car on 87 octane (i'm not suggesting doing this) without detonation and the same timing, in theory you would make more power because the fuel and air charge is easier to ignite
A car that runs fine on 93 octane (meaning it's 100% high octane tables, proper AFR etc...) will likely actually slow down if it runs 100 octane (may not be drastic, but since the air/fuel charge is harder to ignite , the flame front won't begin at the proper time... so instead of beginning at say 28* before TDC, it may not actually begin until 27* TDC (even though you commanded 28* of spark advance)
Originally Posted by horist
^^^ because they can add more timing ... high compression cars need higher octane because the air charge has a higher likelihood of pre-ignition, so by using a higher octane gas, you delay this
A car running 91 octane may get some knock and the knock learn factor would then lean more towards the low octane timing tables... a car running 93 octane may not generate as much knock so it has a learn factor closer to the higher octane timing tables...
If you could run the car on 87 octane (i'm not suggesting doing this) without detonation and the same timing, in theory you would make more power because the fuel and air charge is easier to ignite
A car that runs fine on 93 octane (meaning it's 100% high octane tables, proper AFR etc...) will likely actually slow down if it runs 100 octane (may not be drastic, but since the air/fuel charge is harder to ignite , the flame front won't begin at the proper time... so instead of beginning at say 28* before TDC, it may not actually begin until 27* TDC (even though you commanded 28* of spark advance)
A car running 91 octane may get some knock and the knock learn factor would then lean more towards the low octane timing tables... a car running 93 octane may not generate as much knock so it has a learn factor closer to the higher octane timing tables...
If you could run the car on 87 octane (i'm not suggesting doing this) without detonation and the same timing, in theory you would make more power because the fuel and air charge is easier to ignite
A car that runs fine on 93 octane (meaning it's 100% high octane tables, proper AFR etc...) will likely actually slow down if it runs 100 octane (may not be drastic, but since the air/fuel charge is harder to ignite , the flame front won't begin at the proper time... so instead of beginning at say 28* before TDC, it may not actually begin until 27* TDC (even though you commanded 28* of spark advance)
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Joined: Mar 2003
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From: San Diego
I run about that same mix... 3:1 91/100, comes out to about 94
I agree, in a stock motor with stock timing 93/94 octane would pretty much insure you wouldn't get any knock and would always be on the high octane timing table. Anything higher would just be a waste. I think with 91 you would not suffer any performance loss or see any performance gain going to a higher octane except when ambient conditions are pushing it (real hot weather or really extreme driving).
Now, start modifying the engine, higher compression, nitrous, boost, your needs will change and you will have to modify your combustion setup appropiately. How you modify that really depends on your specific combo. Trial and error is really the best way. If you bump up the compression you WILL have to play around with timing and octane to figure out what the engine wants. Its a give and take scenario, and in my case since I drive a lot on the street running 100 octane and aggressive timing isn't really a viable option, at least not at the moment. So I run a 94 octane blend and mild timing. I'm giving up a bit, but saving some money. The car can still hang with cars that have dyno'd well over 400 at the wheel, but there is more in it.
So basically, its not what octane do I want to run, its what octane do I NEED to run And does my wallet support my engines appetite.
I agree, in a stock motor with stock timing 93/94 octane would pretty much insure you wouldn't get any knock and would always be on the high octane timing table. Anything higher would just be a waste. I think with 91 you would not suffer any performance loss or see any performance gain going to a higher octane except when ambient conditions are pushing it (real hot weather or really extreme driving).
Now, start modifying the engine, higher compression, nitrous, boost, your needs will change and you will have to modify your combustion setup appropiately. How you modify that really depends on your specific combo. Trial and error is really the best way. If you bump up the compression you WILL have to play around with timing and octane to figure out what the engine wants. Its a give and take scenario, and in my case since I drive a lot on the street running 100 octane and aggressive timing isn't really a viable option, at least not at the moment. So I run a 94 octane blend and mild timing. I'm giving up a bit, but saving some money. The car can still hang with cars that have dyno'd well over 400 at the wheel, but there is more in it.
So basically, its not what octane do I want to run, its what octane do I NEED to run
And does my wallet support my engines appetite. 
