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This gets tougher than you would think. It isn't so muchas the compression ratio but the final pressure of the air/fuel mixture and things like hot-spots in the combustion chamber that determine whether you're going to get a ping or not. I have something like 11:1 compression and got rid of the ping using cooler than stock spark plugs and a little octane booster with 93 octane gas/petrol. It would have been wiser to aim for something in the 10.5:1 kind of range to get away easily on the 93 octane. Also the ignition timing will have to be adjusted for the octane/compression.
There is some good info on th subject at:
http://www.badasscars.com/techtips.html
How does your mechanic say he is going to get to the sub 9:1 compression ratio? Are you planning on new heads? Is he talking dished pistons? I don't know much about the 289 and the heads available but if it was me I'd be aiming at aroun 10.5:1 max, 10:1 min and then play it by ear on the timing and use the 93 octane gas you can get regularly. Also, if yu are using heads that didn't have hardened valve seats now is your chance to get them installed, just a little security against the unleaded.

69 Sportsroof, 351C, 4spd toploader, Restomod BOSS 429 clone - the one and only (?) 1969 BOSS 351

http://home.swbell.net/boss351/mustang/boss1.jpg
http://stangnet.cardomain.com/user/adamg69
 
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