Curtis,
Vacuum secondaries do not open the air flow to the secondaries until the
motor load, as measured by the air flow, requires them. This prevents
the motor from suffering the "sog-and-bog" of too much venturii, and
reduced air flow rates, as well as gas mileage. Slow moving air and poor
main jet flow is the result
On vacuum secondaries, flow is controlled by a spring in the secondary
vacuum diaphragm housing. Holley sells an inexpensive tune-up kit, with
multiple strength spring coils of different colors.
When used with a 260 engine, F4B carb, and the 1848-1 carb, the "plain"
colored spring is recommended, and it works just fine. On light
acceleration there is no stumble, and on full throttle there is no
hesitation. Measured air-fuel ratio can stay optimized, regardless of
throttle opening, or load. (Edelbrock Fuel-Air exhaust sensor system).
A mechanical secondary opens with throttle position, regardless of load,
and leads to "bog and slog". This is acceptable in a race driven
vehicle, when the throttle is only in one of two positions, shut and
open. It is sog and stumble on normal use.
On full throttle, a correctly tuned vacuum secondary will deliver all
the power that a mechanical secondary will, without the partial throttle
problems.
The spring kit contains many springs, is cheap, and the springs are easy
to change to a different stiffness - depending on tuning. Unless you
are running larger than 289, and have large valves and tuned ports, it
is very unlikely that you will flow enough to require a 600 CFM carb,
unless you are racing. It will exhibit all the problems of
overcarbutation, or be leaned out so severely that performance could be
uneven and not crisp.
This is what Holley, race tuners, and personal experience have concluded.
Good luck,
Steve
65Tiger@comcast.net wrote:
>Carb gurus,
>
>After years of adjusting, rebuilding and tweaking my Holley 1850, I am ready
>for a change. I'm thinking of going to a carburetor with mechanical
>secondaries. The primary reason is that the vacuum secondaries are coming in
>too late and I'd like to have more power sooner. I realize there are ways to
>adjust the 1850 to come in sooner but I'm beyond frustrated with this
>carburetor for several reasons.
>Holley's website states that mechanical secondaries are suitable for light
>cars. Do any owners out there have mechanical secondaries and a
>recommendation for a good 600 cfm carb? I'm considering Demon and Holley.
>My goals are a daily driver, good performance, low and midrange HP,
>reliability, economy is not important, a choke is not required (I will not
>accept an electric choke), a good idle is desirable. The motor is a crate
>302, F4B intake, B303 cam, GT40 heads, 1.7 rocker arms, CAT headers.
>
>Recommendations?
>Curtis Fisher
>B9470844
>
>
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Steve Laifman
Editor - TigersUnited.com
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