Just Another.... question about octane

goodnigh

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There is a gas station nearby that sells 100 octane out of the pump. The best I currently buy is 91. My mechanic tells me I should put half a tank in the Mustang to see what my car "is supposed to run like". Would it be worth it ($$) to try the 100 stuff? Would probably need to change the timing. The engine is an M-code closed chamber.

mike

 

72HCODE

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if your car is not pinging on 91 then you could try the 100 it will give it a little more if the engine is leaning out a little in the RPM range. the big benefit would be the ability to add more timing to the engine before detonation(ping) meaning you could turn up your vacuum advance more or add more engine timing.

is it worth it? no not really, i threw some 106 in my tank once it reduced a pinging issue i had and gave me a little more power since the gas wasn't pre-igniting in the combustion chamber, but that was more due to tuning problems i had in the engine itself.

higher octane means higher temperature burn level so if the 91 is on the edge of pre-ignition(detonation,ping) on the compression stroke then 100 would stop that from occuring and give the engine more power because it the pistons would not be fighting against the gas igniting at the wrong time. pre-ignition/detonation/ping, is like a hammer slamming against the top of the pistion as it is trying to come up on the compression stroke just before ignition and the power stroke, so if the pistion doesn't have to fight against this extra force(if it exists) then you gain back lost power.

for the street its better to tune to the cheap available fuels, in my area: i think 100 is in the 5.30$ range a gallon 106 was like over 6 last time they had 108 and 110 as well but those were pure race fuels and like over 8$ a gallon. there is just no way your going to fill a 20 gallon tank on that stuff and tune your car to drive on it all the time.

 

Bill73Ragtop

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It may not be worth it as a usual practice, but if you can afford it for a trial basis (as the mechanic suggested) you may find it interesting. When I had my 71 M-code 351c back in the late 90's - early 2001 - 3; I typically ran 93 octane with octane booster. For the most part, it ran just fine. Once in a while (like once a year) I would splurge on a tank of 100 octane. The M-code responded very well. It ran a few degrees cooler temp-wise, had more 'snap' to it, and just seemed to run better. This was without changes to the timing, etc.

Of course, the price back then was structured somewhat more lenient to the practice. 94 octane was maybe $1.90/gal. 100 octane was $3.25/gal. Sure would like to go back to those prices.

 

droptop73

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Octane only rates the resistance to detonation. If you do not notice detonation it won't make any difference in performance unless you are going from a fuel with ethanol to one without. Ethanol is the bigger performance issue today it can make an engine from our era run lean without some carb tuning. Ethanol only has 70% of the heat energy of gasoline which means to maintain the same performance the carb needs to be jetted slightly richer (e-10 = 3% richer).

 

goodnigh

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Octane only rates the resistance to detonation. If you do not notice detonation it won't make any difference in performance unless you are going from a fuel with ethanol to one without. Ethanol is the bigger performance issue today it can make an engine from our era run lean without some carb tuning. Ethanol only has 70% of the heat energy of gasoline which means to maintain the same performance the carb needs to be jetted slightly richer (e-10 = 3% richer).
The gas in CA has ethanol around 10% I think. So the pure 100 octane I would notice. Local people say it has a strange odor, smells like gas ??

So when the Eagle flies I will have to splurge on a tank.

Thanks, that was good advice.

mike

 

71 'stang

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turtle5353

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I was always told you only need enough octane to quench detonation. Any more is kind of a waste. But it does smell really good. Sunoco by my house sells 114 octane for the dirt bike race track down the road and I splurged and put 5 gal on my tank. Close to $8 / gal. Didn't notice much of a change but did have great smell the next few tankfuls.

 
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