Basic rule for vacuum can selection:

THE VACUUM ADVANCE CONTROL SHOULD PROVIDE FULL ADVANCE AT NOT LESS THAN 2" LESS THAN PREVAILING IDLE VACUUM 
AT NORMAL IDLE SPEED WITH APPROXIMATELY 24-32 DEGREES TOTAL IDLE TIMING.

One of the following three NAPA/Echlin vacuum cans should be appropriate for all OE engines, 
including those converted from ported to full time vacuum advance, using the above rule. 
Vacuum cans for modified engines (such as non-OE cams that alter idle vacuum) can be selected using the basic rule.

VC680 (stamped "B1") 0@8". 16@16"
VC1765 (stamped "B20") 0@6", 16@12"
VC1810 (stamped "B28") 0@4", 16@8"

If you're at the ragged edge of this rule a small increase in idle speed - 
on the order of 50-100 revs - will usually achieve the 2" difference since vacuum increases with increasing idle speed.

The correct total vacuum advance for most pre-emission Corvette engines is about 16 degrees, 
and any ported vacuum signal lines (such as SHP big blocks and '63 FI) should be converted to full vacuum advance, 
and on some of these applications, a new can (VC1810 for '63 FI and VC1765 for L-71) 
must be selected to meet the above criteria. Also, 
'63 SHP does not meet the above rule and should be converted to '64-'65 SHP/FI specification - VC1810. 

A "more aggressive" vacuum advance than necessary 
(significantly more than 2" difference between idle vacuum and full vacuum advance) 
is okay as long as there is no detonation. 
A too aggressive vacuum advance may cause transient detonation, such as on upshifts or part throttle acceleration.

For example, L-79 was originally equipped with a Delco can equivalent to VC1810, 
which is more aggressive than necessary, but since L-79 pulls enough idle vacuum (14"-15")
to keep VC1765 pulled to the stop at idle, VC1765 can be considered as optional on L-79, 
especially if there is transient or part throttle detonation.

A "not sufficiently aggressive" vacuum can - one that does not keep the plunger pulled 
to the stop at idle can cause high coolant temperatures due to insufficient total idle timing and variation 
in idle timing that can lead to idle instability, poor idle quality, and even stalling!

Using the basic rule, one of the three above mentioned vacuum cans should provide full 
advance in the range to at 2"-4" less typical idle vacuum which is the ideal range. 

These same Dana Controls manufactured vacuum cans are also marketed under other brand names/part numbers 
such as Delco, Borg Warner, and others. 
The alphanumeric code stamped on the mounting bracket is the code that denotes the specifications.


I am never going to talk about vacuum cans again. EVER!

Duke