Thought it might be helpful to post what I've collected on vacuum secondary diaphragm opening rates.
Below are whatever springs I had loose lined up on bench. The free heights and
installed hieght in a Holley Quick Change vacuum pod were measured with a vernier caliper.
- VacuumSecondarySprings-Heights.jpg (20.49 KiB) Viewed 1910 times
Taking the above height information and adding it to the Holley graph* reveals how changing the springs will effect opening point and rate relative to each other. For example, white and purple have similar opening points, but white will then fully open much quicker or with less velocity in the barrels. So this allows some tailoring of both opening point and how quickly the secondaries then continue open. Not sure why the black spring is non-linear. Maybe they all are, but the softer ones to a lesser degree.
- VacuumSecondarySprings-ForceGraph-revised2.png (68.07 KiB) Viewed 1910 times
Secondary Spring Part Numbers and Opening Points
Holley Secondary Springs available for purchase.
I can't speak to how universal those opening points are. My own experience has been that WOT secondaries are not effecting power until higher in the rpm curve. However I suspect that may have to do with other factors specific to my carb and engine. A test for whether the secondaries opened is to attach a flag (paper clip or wire tie) on the stem.
Diaphragms
Last but not least, it helps to have the matching diaphragm for the carb.
holley04-Secondary-diaphrams.JPG
Secondary Diaphrams part numbers for Holley List numbers.
Probably from an older catalog
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*In Urich's Holley Carburetor Handbook: 4150 & 4160 Selection, Tuning & Repair book and also in Urich & Fisher's Holley Carburetors and Manifolds and another version in Vizzard's latest Holley book. The Vizzard and Urich versions appear identical besides color and a minor difference in the purple and green spring rates.
