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Discussion Starter #1
This is only for N/A applications.

How do Compression an Octane rating correlate, for so long it been if you get to 10 or more 87 wasn't used/suggested now multiple new cars are claim 10-11:1 compression on 87.

Have we been useing either too low of compression or too high an octane for years, or are Knock Sensor now that standard?
 

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It's not just a matter of octane versus compression ratio. No, not nearly so simple.

There's the amount of advance in ignition and the curve as relaters to load and engine speed. Lower gears where motor can rev quicker as load is less, motor can stand more advance all else being equal. High gear, slower advance rate and / or less total is needed or tollerated as the engine can't gain the revs as quick because it sees a heavier load. Engine temperature and even the design of the combustion chamber itself both play into it as well ... as does air/fuel ratio at any given moment.

Back in the old days, we depended on preset advance curves using springs and bob weights in the distributor and vacum for light loads or coasting and these curves inter related.

Go way back, and drivers / riders manually adjusted advance as they drove along.

Now we have computors that can monitor many imputs and adjust the advance rate and air/fuel ratios for us in milliseconds according to needs. The knock sensors help to let them know when to cut back a bit.

It's why you see the high copmpression ratios burning 87 octane. I can recall when 10:1 required 96 or 100 octane and Sunoco had the pumps with the dial where you could blend your own favorite level for that hot rod 396/375 Chevelle or Chevy II with 194 I-6 for driving to work.
 

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to expand on the previous reply, part of the reason newer vehicles can tolerate higher compression is certainly due, in-part, to knock sensors removing "premium fuel levels" of spark advance whenever knock or detonation is detected when using non-premium fuel is used in such a vehicle (that was probably intended for premium fuel)

and then there are those vehicles are actually have high compression and are designed to run on 87 octane fuel.....many of these have (among other things) very well designed cylinder heads that provide the optimum amount of intake charge swirl and (usually) a very symmetrical combustion chamber, as well as correctly routed coolant passages to minimize combustion chamber hot spots.......these attributes contribute to creating a very even, homogeneous distribution of fuel within the aircharge mixture which can help curb detonation by minimizing uncontrolled combustion reactions
 

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My audi has 10.7:1 CR stock and runs on 87 with spark retard, the manual states 91 Octane is "suggested" (136K miles on it)
My dad's Jeep had 8.8:1 CR stock and needed 89 or it would ping even though it was rated for 87. (brand new)


As mentioned, it's the quality of the combustion chamber that determines the octane need, and the spark retard which allows a lower octane to be used at a loss of power. If your goal is power, you don't want to have the spark retarded. Your better off with a lower CR and the right amount of advance than a higher CR and spark retard.
 

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Discussion Starter #5
I knew it was a matter of temperature, why I excluded boosted application. What I'm take form this is if have a properly designed combustion chamber then 10.5-11.0 is achievable on 87.

I also understand about ignition retard/advance, just seems to me to be stupid (for lack of a better word) to quote peak power at anything but peak cylinder pressures or as close as can get with commonly available fuel.

There must be a formula for determining octane needed.
 

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The new Camaro V6 has an 11.7:1 compression ratio and runs 87 octane. That's nearly 12:1 compression... on 87. It's getting sillier every year.
 

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I knew it was a matter of temperature, why I excluded boosted application. What I'm take form this is if have a properly designed combustion chamber then 10.5-11.0 is achievable on 87.

I also understand about ignition retard/advance, just seems to me to be stupid (for lack of a better word) to quote peak power at anything but peak cylinder pressures or as close as can get with commonly available fuel.

There must be a formula for determining octane needed.
such a formula would have to have the person using it input a qualitative measure of swirl and combustion chamber efficiency, since they are dominant factors.....i doubt such a formula exists that encompasses all possible scenarios...there are just too many variables.....sure you could probably find guidelines, and 'rules of thumb' that apply to one particular engine, but to generate a formula that applies all the time would be quite a feat, I would say
 

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Ghost. 11.7 to 1 CR. on low test!!! Where did you find that?
 

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Remember that he 3.6 in the new Camaro has direct injection. Basically the fuel is getting shot into the cylinder at just the right time, so there is much less need to have a fuel that is more resilient to detonation.
 
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