401 octane requirements
- Thread starter kbalser
- Start date
Are you rebuilding it completely stock, or are you putting in a different camshaft? The actual static compression and valve timing affect octane requirements. You should know the actual static compression ratio (don't rely on advertised specs). Something to read,
It is doubtful you will detonate on 91, but i would not try 87. The Nailheads had a more radical stock cam (more duration and late closing intake) then most because of their limited head flow. That in turn reduces dynamic (or cranking) compression. The other factor at play here is that the static compression of the stock Nailhead actually measures at 9 or 9.5 to 1 instead of the advertised 10.25 to 1 compression. The dynamic (or cranking) compression is a derivative of the static compression. A lower static compression will yield a lower dynamic compression for the same cam.
I would build it as you are and limit vacuum advance if you hear detonation. You could also reduce initial advance if it continues to detonate. Nailheads like a total advance of 32* combining initial and centrifugal advance (vacuum advance disconnected and plugged). A good starting curve would be 18-20* of centrifugal advance at the crank all in by 2500 RPM with 12-14* of initial advance. You can adjust the initial timing depending how it reacts to the curve.
Thicker head gaskets could actually increase the probability of detonation. There are some quench areas between the head and piston that are reduced in effect with thicker head gaskets. There is less turbulence produced to cool hot spots when the distance between the head and piston is increased in these areas.
I would build it as you are and limit vacuum advance if you hear detonation. You could also reduce initial advance if it continues to detonate. Nailheads like a total advance of 32* combining initial and centrifugal advance (vacuum advance disconnected and plugged). A good starting curve would be 18-20* of centrifugal advance at the crank all in by 2500 RPM with 12-14* of initial advance. You can adjust the initial timing depending how it reacts to the curve.
Thicker head gaskets could actually increase the probability of detonation. There are some quench areas between the head and piston that are reduced in effect with thicker head gaskets. There is less turbulence produced to cool hot spots when the distance between the head and piston is increased in these areas.
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