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 Higher Compression for 91/94 Octane? 
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Post Higher Compression for 91/94 Octane?
Just wondering how high compression people are running and still safe on our 91/94 octane? I know there are tones of builds in the states that have run safely up to almost 13:1 on 93 octane. Not sure how different our grade fuel is compared to theirs? Figuring when i have the time to do a proper matched h/c topend wondering if 12:3 is too high as that will be the max i can go if its feasible.

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Sat Nov 10, 2012 7:53 am
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Post Re: Higher Compression for 91/94 Octane?
Vetty wrote:
Just wondering how high compression people are running and still safe on our 91/94 octane? I know there are tones of builds in the states that have run safely up to almost 13:1 on 93 octane. Not sure how different our grade fuel is compared to theirs? Figuring when i have the time to do a proper matched h/c topend wondering if 12:3 is too high as that will be the max i can go if its feasible.


Im at 12:1 with the LT1. I cannot run 91 octane unless i back the timing down. 94 octane works perfectly, and thats with my radical cam. You'll be fine ;)

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Sat Nov 10, 2012 8:59 am
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Post Re: Higher Compression for 91/94 Octane?
Go read up on dynamic compression ratio. That is as important as static but is dictated by cam timing.

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Sat Nov 10, 2012 9:18 am
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Post Re: Higher Compression for 91/94 Octane?
Here is the link I found helpful when I ran into detonation from too high compression ratio http://www.empirenet.com/pkelley2/DynamicCR.html

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Sat Nov 10, 2012 3:25 pm
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Post Re: Higher Compression for 91/94 Octane?
Agreed.
Big duration cams bleed lits of compression when you dont need it and timing(advanced)is high.


Generally speaking, high compression will be ok as long as timing is low and cam duration is high.

FYI I was running 12.5:1 on iron heads using 89 octane..268/272@.050 though..lol(BBC)


Sat Nov 10, 2012 6:24 pm
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Post Re: Higher Compression for 91/94 Octane?
Thanks for the link... looks like I'm going to have do some quick math and get the cam events of the one i have in mind. What is Leroy running for cr in his monster Dan?

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Sat Nov 10, 2012 8:01 pm
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Post Re: Higher Compression for 91/94 Octane?
His is 12.25 with lots of duration on the cam. Keep the dcr below 8.5 for pump gas.

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Sat Nov 10, 2012 8:30 pm
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Post Re: Higher Compression for 91/94 Octane?
My current Gen 1 SBC has a 10.25:1 SCR and 8.3:1 DCR on an iron head. That's with a relatively short cam (224°/230° @ .050").
While it'll run on less than 94 octane, with 91 octane, it has a tendency to run-on ever so slightly when turning off the engine (just the slightest kick back). With Ultra 94 there's no indication of detonation or run on. The spark is timed at 14° initial with 32° total (not including vacuum advance). I was running 16° initial but have dialed it back a little until the new combination is sorted out.
With an aluminum headed LS, I would expect at least another full point of SCR is do-able given a short cam as above. As has been mentioned, a cam with longer duration allows for (and actually requires) a higher SCR.

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Sun Nov 11, 2012 10:06 am
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Post Re: Higher Compression for 91/94 Octane?
Well had some time yesterday to do some poking around trying to find my dcr with 11.8-12.3scr.

Used this calculator to try and unfortunatly most cam cards i can find are .006 valve timing lift vs at .050 which its wanting. Most cams that ive found were in the 63-69* closed abdc at .006. The cam builder im talking wont release any info untill $$$ is in his hands. How do i convert it to .050? I tried plugging in roughly 65* + 15* ontop of that which calculator needs and got 8.1dcr but i have no idea if thats right. http://www.kb-silvolite.com/calc.php?action=comp

54
5
.039
4.030
4.030
0
3.48
6.0
80*

This calculator here needs our altitude which only thing i could find was through wiki and was 347ft and boost pressure which i have no idea what it is??? http://www.wallaceracing.com/dynamic-cr.php

Using this calculator changing everything to above leaving the ring gaps alone and advance/retard since i have no idea what they are. If i add roughly a 278/280 duration on a 108lsa or check marked off 65-69* closed abdc im getting 9.0-9.3 dcr. http://www.projectpontiac.com/ppsite15/compression-ratio-calculator

Which one should i follow?

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Mon Nov 12, 2012 6:37 pm
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Post Re: Higher Compression for 91/94 Octane?
I use the compression ratio calculator (SCR and DCR) that's attached to the website posted earlier.
http://www.empirenet.com/pkelley2/DynamicCR.html
It uses advertised (.006") duration as input values.
If you're creative when using it, it's easy to swap in various values for duration, cam timing, rod length etc and observe the various effects it has on DCR.
There's no set formula for deriving advertised duration from that at 050" or vice versa. Cams have different acceleration rates and as such the values aren't necessarily linear. Keep in mind too that duration specs will be based on a given lifter ratio too. That will skew the numbers slightly.

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Mon Nov 12, 2012 6:50 pm
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Post Re: Higher Compression for 91/94 Octane?
If you are buying heads and cam from the same place then they will sell you something that is gonna work together and they should ask you bottom end specs to verify compression ratio

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Mon Nov 12, 2012 10:19 pm
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Post Re: Higher Compression for 91/94 Octane?
http://www.rbracing-rsr.com/compstaticcalc.html

Try this tool.

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