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I lost the post that discribes the proper way to determine the correct vacuum advance unit. I think that it was originaly posted by Duke? Would anyone be able to email me a copy?
Tom S
Tom S
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Re: vacuum advances
The basic rule of thumb is easy.
Select a vacuum can that provides approximately 16 degrees maximum crankshaft advance at AT LEAST 2" LESS than idle manifold vacuum at the prevailing best TOTAL idle timing, which is in the range of 20-30 degrees. SHP engines will be at the mid to high end, medium performance at the low to mid part of this range.
From 1964 to the emission area the OE spec vacuum can is near ideal for most Corvette engines; '63 SHP/FI engines should use the '64-up SB SHP/FI can.
Within the NAPA/Echlin product line the following three parts will handle just about any application. The code in parentheses is the number stamped on the vacuum can mounting bracket.
VC680(B1) start@8", 16@16"
VC1765(B20) start@6", 16@12"
VC1810(B28) start@4", 16@8"
Typical idle vacuums (with total idle timing at or near optimim) are at follows:
30-30 cam 10"@900
Duntov and LT-1 cam 12"@900
L-79 cam 14" at 800
L-72 cam 14"@900
Medium performance BB or SB hydraulic lifter cam 16-18"@500-600
Duke
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