Thread: Octane-Compression Ratio

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Could someone chart octane and with steel heads what compression ratio would be within the range of that octane.As a example:

87 octane pump gas and you could run up to a certain compression ratio

89 octane pump gas and you could run up to a certain compression ratio

91 octane pump gas and you could run up to a certain compression ratio

91 octane pump gas with additive and you could run up to a certain compression ratio.


Thanks

The best answer I can muster is:

with 89 I stop at 9-1 with iron heads

With 91 I stop at 9.5-1

With Aluminum heads add 1 point of compression

This gives an easy to live with, no death rattle, street combo.

Having said that, keep in mind flame travel plays a part. Try to use a flat top piston if possible.

Good Luck

From 9.1 to 9.4-89 octane???.Yepper I'm splitting hairs ant I???.

I am thinking there is a formula,but for the life on me I can't remember it.

over compression mistakes can be fixed by retarded timing but performance really suffers .. my 4.3 v6 in my pinto is a bit high on comp i think due to it being an original computor equiped fuel injection motor .. it really suffers from slow timing

It is not static compression ratio that guides what octane to use but dynamic compression ratio. My 383 has 10.6:1 static compression but I can run 92 octane just fine because I run aluminum heads and my dynamic compression ratio is 8.46. You can run .5 more dynamic compression if you have aluminum heads. Engine temperature is also a factor. A hotter running engine will not tolerate as much compression. Cam timing, quench, and connecting rod length all play a part.

Not to confuse a confusing subject, there are different way of calculating octane as well. Research Method (R) is the theoritcal value that the octane could be; M Method (i can't remember what the 'M' stands for, but it is the actual real world (measured?) value and most race fuel is rated on this scale) and the average at the fuel pump - there is a sticker on the side of the pump for the octane method: R + M / 2
If in a discussion of octane values/compression ratios be sure to get which octane rating method is being used.

There is a lot of information on this link:

http://www.chevytalk.org/fusionbb/sh...php?tid/92966/

I am using 91 in a 10.4:1 Alum head 383, pay attention to both squish and dynamic comp ratio