Steve Laifman wrote:
>
> The advice on removing vacuum advance, which should be about 10 degrees
> distributor (20 degrees crank) is only valid for setting the idle
> advance, and checking the total counterweight advance. Then hook it back
> up. It is only active when the special ignition port (not manifold) tube
> is used and that only activates under light load, when the engine can
> easily take that advance. When power is applied, the vacuum advance
> goes away.
> --
It has been pointed out to me that certain high performance camshafts
can radically alter ported vacuum, and manifold vacuum conditions, and
that the above recommendation may not be useable with such cams, as the
ported vacuum does not drop low enough.
The particular cam reported was a high performance dual-profile cam. The
port vacuum could not get low enough to get the adjustable vacuum
advance lower than 12 inches-Hg, which was the soonest the Crane
adjustable canister would stop vacuum advance. Under very HEAVY
acceleration, it does allow the vacuum advance to be inactive, but not
under mild performance demands, which is where you want it moving.
Other issues associated with very high performance cams is the
difficulty finding a Holley Power Enriching Valve, as the earliest
opening one found by Curtis Fisher was about 10.5 in-Hg.
Similar effects can be found in cars that are "over-carburetted" with
too much flow capacity for the engine's breathing (volumetric
efficiency) capacity. The vacuum operated secondary (if equipped) would
require work on the diaphragm spring, as well. A lot of racing cars
avoid these issues with centrifugal only distributors, at the expense of
economy and smoothness of operation. Go to far and they barely idle at
1,250 rpm (vs 400-450), and then shake like a wet dog. But have the most
power at wide-open-throttle (but only there) in the extreme cases. This
are special application motors.
There is no comment that can be made about any part of a car that is
"universally" true", as the exceptions do exist with design and usage needs.
Steve
--
Steve Laifman < Find out what is most >
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