Thread: Is everyone using 91 octane?
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Old 05-15-2008, 09:13 PM
180OUT 180OUT is offline
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Join Date: Mar 2007
Location: San Antonio, Texas
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[quote]
Quote:
Originally Posted by Joe Papa Sr View Post
I think, technically, and it may matter in this discussion, the uneven flame front you mention is detonation, not preignition. They are different phenomenons, and both damaging to engine. Preignition is caused from mostly super heated carbon deposits, inducing the chamber to fire before the spark plug fires. It is also known as "deiseling".
I think you're right. I can't find Vizard's book right now so I can't quote him directly---he could have been/probably was talking about detonation. (BTW the 2400 rpm phase Vizard describes is the only time I've read about that particular phenomenon. He's a well regarded authority on tuning so I think he knows his stuff, however.)


Nonetheless, the phenomenon I described, whether preignition or detonation, was pronounced enough to be troublesome and caused me to seriously consider replacing the high comp pistons. The problem was effectively eliminated by the addition of the MSD6A ignition. In addition I could run an "all in" advance by about 3000 rpm. The mod (from an old Owner article) consisted of replacing the light advance spring in the MarelliPlex with a even lighter secondary spring from a Chrysler Hemi carburetor. Worked great.
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'70, 1750GTV, 2nd series
'62, Lancia Flaminia Zagato3c, 2nd series
Last edited by 180OUT; 05-15-2008 at 09:17 PM.
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