The
following is a partial list of Holley Carburetors
and their specific information. This list covers
some of the more widely used Holleys for Ford
engines. 4150 carb types use a rear metering
block with jets similar to the front metering
blocks. 4160 carb types use a rear metering plate
with passages for fuel flow instead of a metering
block, thus making the overall length of the 4160
carbs shorter than 4150 carbs and requiring
shorter fuel feed tubes between bowls. One reason
often given for this change in design was to
lower the cost of manufacture as well as
eliminate the use of rear jets and metering
block. The larger sized float bowls, sometimes
called "Cathedral" bowls, have the
float suspended from the center for better fuel
levels under hard cornering. The smaller sized
float bowls have the float suspended from the
side. Most 600 CFM Holleys use the smaller bowls
while the 725, 780, and highter CFM carbs often
use the Cathedral bowls. The CFM ratings refer to
the cubic feet per minute of air flow.
Without going into specific carburetor dynamics,
just keep in mind that many engine variables
determine what size of carburetor is best for
your application. A regular Ford 302 will run
just fine with a Holley 600 CFM while the Boss
302 uses a 780 Holley because of much different
engine dynamics, design, and use. The stock Ford
Autolite 4 barrel carburetor is rated at 470 CFM
so you be the judge. Most stock 302's of the day
were 2 barrel engines anyway so just going to a
Ford 4 barrel carb will show significant HP
increases. The rear two "barrels" of
the carburetor are opened during operation in two
ways, vacuum secondary and mechanical secondary.
Vacuum secondary carburetors use a vacuum
diagphragm to open using engine vacuum signals to
let the engine "tell" the carburetor it
needs more fuel under acceleration. Mechanical
secondary carburetors use a mechanical linkage
that opens the rear barrels as you move the
throttle for more fuel. For most street use, it's
recommended that you use vacuum secondary
carburetors.
The power valve is a vacuum operated valve that
allows fuel into the delivery circuit off idle.
The rating of the power valve, such as 8.5 or
10.5, is the engine vacuum where, when it drops
below that, engine vacuum is not enough to hold
the power valve closed and fuel will be allowed
to mix with the idle circuit. When camshafts have
lower than normal engine vacuum at idle, this can
cause an overly rich condition thus the reason
for different power valve ratings. Stock
camshafts have higher engine vacuum at idle than
a racing camshaft would. Now everyone has their
favorite brand of oil, gas, car, and carburetor.
I myself have used and worked on Holleys since
1971. They are easy to work on and calibrate.
Their worst trait is that they don't like sitting
for too long. The metering block gaskets will
sometimes dry out and shrink causing idle
problems. Just make sure that if you have someone
work on your Holley, that they know what they are
doing. At any rate, here is just a basic look at
some different Holley carburetors and their
specifications should you decide to go this route
with your engine.
Special Note: DO NOT USE the
Holley rebuild kits that use the big thick nut
with the needle and seat. They can strip the
threads in your float bowl. Use only rebuild kits
that use the Holley style needle and seat
adjusting nut and screw. Just a personal warning.
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