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Phil,
I gather that you are hooked up to manifold vacuum, not "ported vacuum" (just upstream from closed throttle plate) ? Normally, with ported vacuum, vacuum advance only cuts in at part throttle and does not affect timing at idle or have much effect at full throttle. I think this is due to the high velocity air rushing past the partially cracked throttle. At closed throttle (idle) the port is far enough upstream so that it sees low vacuum, and at wide-open throttle, the velocity right adjacent to the port and resulting vacuum is also low.
It seems to me your approach of running vacuum advance of manifold vacuum is rather novel. This is the first I've ever heard of anyone doing it intentionally. I'm still trying to wrap my little mind around this dizzy timing business. I was quite startled to read about Volvo vacuum retard just the other day.
Actually I was running my 260Z with dual Weber DGVs with the vac advance hooked up to manifold instead of ported vacuum for quite some time. When I corrected the connection to ported vacuum, I noticed a slight improvement in performance and driveability. Dizzy is stock Nissan with mucho miles. I need to do some testing to see where its at with vacuum and mechanical advance. Maybe I'll just take it out at a local dizzy specialist outfit and have 'em test it and dial it in on their test machine.
I am most impressed with your stats: 10.6:1 CR, 296-degree cam, and NO PING !
On my 260Z I got all of 8.56:1 CR, stock cam, stock dizzy, and most annoying ping. I do have compression test results around 200 psi. Probably a bigger cam would help get the compression down some and help with the ping. Also, here in North Calif max octane available is down to 91.
Later,
Al
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