10.5 Compression Ratio on 91 octane???

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Sep 29, 2009 | 02:32 PM

Z06FRC

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Staging Lane

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From: Bradford, Canada

10.5 Compression Ratio on 91 octane???

I'm going to be doing a h/c swap on my 99 vette. I'm replacing the stock heads with cnc ported LS6 heads, which should raise the compression ratio from 10-1 to 10.5-1. Now in Canada every gas station only carries 91, but only Sunoco carries 93. Can I still run 91 octane with 10.5, or do I need to start feeding it 93 octane? I ask my tuner and he doesn't recommend running 91 with 10.5 compression. But my car usually runs on Shell 91 octane.

Sep 29, 2009 | 02:34 PM

novaflash2002

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From: Billings, Mt

ls2s have 10.9:1 and they run on 91. i have a buddy in his gto, that had 12.2:1 on 91. your tunner will have to adjust yor timing tables accordingly

Sep 29, 2009 | 02:38 PM

WS6TransAm01

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From: East Brunswick, NJ

you can make it run on 87 if you like... just has to be tuned for it as novaflash said.

Sep 29, 2009 | 03:11 PM

02*C5

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From: Az

You have the same issues that a lot of people run into the Western U.S. The only available gas on the pump is 91 octane. I was running 11.5:1 and I had to do was adjust timing. Good Luck!

Sep 29, 2009 | 05:13 PM

Z06FRC

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Staging Lane

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From: Bradford, Canada

Quote:

Originally Posted by WS6TransAm01

you can make it run on 87 if you like... just has to be tuned for it as novaflash said.

No......, but thanks. I rather run on 91 octane.

Sep 30, 2009 | 08:51 AM

Chrome355z

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From: Shelbyville, IN

Higher octane just burns slower, in effect it's not much different then pulling timing. (in a "very" broad interpretation. But it makes it easier for ppl to understand.

Sep 30, 2009 | 11:19 PM

hammertime

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From: Smithton, IL

That static compression with a larger camshaft should be fine. It's you dynamic compression that will determine the minimum octane required. DCR will depend on the cam you run and how it is installed.

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Sep 30, 2009 | 11:32 PM

hammertime

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From: Smithton, IL

Quote:

Originally Posted by Chrome355z

Higher octane just burns slower, in effect it's not much different then pulling timing. (in a "very" broad interpretation. But it makes it easier for ppl to understand.

Pehaps a little too broad....

There is an ideal crankshaft angle where maximum cylinder pressure translates into the most power available. Ignition timing is nearly always shown as advanced because of the time the combustion event takes from the moment of ignition, past TDC to the point of maximum power transfered, and continuing as the piston moves down the bore.

Lowering the octane does change the burn rate, which will require you to remove timing (assuming it was optimized at the higher octane). Pretty soon, you are trying to hit a moving target.