How to Choose your next Carburator.

 By Chris Samuel

 

OK, Here is the answer to the "What Carburetor should I use?", question
that has popped up regularly;  well it may not be the answer but it
is my answer.  Carburetors are Carbs, and a Four Barrel Carburetor
is a 4BBL from this point on.

Basics:

A Carb is a simple, yet complex fuel metering device that uses the difference
in pressure above and below the venturi (s) to pull fuel into the air stream
through a series of metering orifices.  It doesn’t matter what type
or brand the Carb is or even what size they all work basically the same
way.

If you are using the common Carb Manufacturer’s formula to determine
the Cubic Feet per Minute (CFM) requirement for your engine:

    CFM = Cubic Inches x RPM / 2 / 1728

    Where:

     CFM: Cubic Feet per Minute, of air flow.

    Cubic Inches: the Engine Size.

    RPM: The RPM that you want to know the flow at.

    Generally this will be the maximum RPM that you intend the engine to
    see.

     2: corrects for the 4 stroke nature of our engines.

     1728: a constant that turns Cubic Inches in to Cubic Feet

You must accept that the results are intended to select a Carb for a 100%
Volumetrically Efficient engine.  Volumetric Efficiency (VE) is the
measure of the engines breathing efficiency.  Race engines can reach
VE’s that exceed 100% while most street engines are between 65 an 80% with
some well tuned street engines in the 90% VE range.

You can not directly compare the CFM rating between 2BBL’s and 4BBL’s
as the test pressure that is used to measure the CFM is different between
the two.  The 4BBL is measured at a test pressure of 1.5 inches of
Mercury (Hg) of pressure drop, while the 2BBL is rated at 3 inches HG. 
To convert the 2BBL into 4BBL use the following formula:

    4BBL Flow = 2BBL Flow / 1.414.

    So that 500 CFM 2BBL Carb actually flows 353.606 CFM on the 4BBL scale

     

Size:

The size of a Carb for a given engine combination is "always" a compromise! 
The 300 to 460 cubic inch class of engines can use Carb’s that range from
300 to 2400 Cubic Feet per Minute (CFM).  The rather simple formula
listed above will give you a vary safe CFM rating for a given size engine. 
This formula (and others like it) are "vary" conservative by design; Holley
and Carter, Et-al recognize that a Carb that is too "small" has a better
chance of working out of the box then one that is to "big".  What
size is right then?  Between 300 & 460 Ci. on the street a Carb
that is rated at 500 to 1000 CFM is going to work.  1000 CFM on a
300 Ci engine won’t work!  Yes I can hear you!, and your right it
won’t work if you just take the Carb out of the box and bolt it onto your
stock engine.  It can work if you have the engine combination that
produce a demand that "matches the Carb".  For example the 780 and
800+ CFM Rochester’s were used quite successfully on engines as small as
302 CI.  In general though that stock 300 cube engine will be much
happier with the 500 CFM Carb.  If this engine has a good set of full
length headers, mild camshaft and a good intake manifold a Carb in the
650 to 750 CFM range will produce better power in the mid to top ranges
of engine operation.

So there is a trend here; On the street use a smaller CFM rating on
smaller displacements and the inverse on big displacement engines. 
There is a further trend use a smaller carb in applications where your
demand for power is in the lower half of the engine operation range. 
There are exceptions to these trends.  Many times changing a Carb
to a larger one will yield more power everywhere in the power band. 
This brings us to the ultimate rule:

"Give the engine what it wants, not what you, your buddies, or some
Magazine think it wants

 and you will be rewarded with power."

How do you find out what the engine wants?  That is simply a matter
of trial and error.  As a general starting point I use the following
guideline,  This is simply the results of my experience and nothing
more.

Engine Displacement -VS- CFM:

    289 – 302 Stock  500 CFM Mild  600/750 CFM Race  750/UP
    CFM

    351 – 400 Stock  600 CFM Mild  650/750 CFM Race  800/UP
    CFM

    400 – 460 Stock  650 CFM Mild  750/850 CFM Race  900/UP
    CFM

    460 – UP Stock  750 CFM Mild  750/900 CFM Race  1000/UP
    CFM

How much will it cost to do this "Trial and Error" approach, well… I have
spent literally thousands.

Brand:

What Brand should I use?

What Brand do you like?  Are you going to work on it?

If you know how to make Brand "X" work in your application then use
that one.

My experience has lead me to make these general recommendations.

If you are playing on the street any of them will work with the difficulty
in tuning them becoming a factor.  In my humble opinion the Rochester
is the most complex and parts can be hard to find.  The Holley comes
in a close second but parts are everywhere, and so are "experts"; so it
is easier from that stand point.  The simplest are the Carter’s, parts
are available almost everywhere

Off road poses another set of problems.

Carbs like to work in a stable environment; that is to say they don’t
like to be tilted in any direction and they don’t like to be shaken. 
The Holley’s are more sensitive to this due to their float bowl design
and therefore I do not as a rule recommend them, they can be made to work,
but it takes just that, plus time, plus money.  The Rochester’s and
Carter’s on the other hand are much more tolerant of the Off Road environment,
and so get my recommendation for this application; with the Carter being
my personal favorite.

If your going racing, on or off road, your are going to be ether limited
to a specific Carb, or size, or both: and or none.  In all of these
cases get the best Carb that you can afford and get it even if you can’t! 
Contact Barry Grant or Braswell (there are others) and have it built! 
You’ll be ahead of the game and competition!

Conclusions:

Out of the box.

I have had the best luck with Carter’s and then Holleys and lastly
Rochester’s for just bolting them on and having them work.  By work
I mean that the engine started and ran "OK" with the Air Fuel mixture being
close enough that the engine was not going to melt down.  No Carb
is ever correct right out of the box, period!

For making power.

The Holley is the hands down winner, this is simply a result of more
people spending more time tweaking the Holley than all of the others combined. 
This is not to say that the Holley is the only power producer, in the hands
of someone who knows how, all of them can be made to make power.

If you are new to aftermarket Carbs how do you pick one?  Setting
my opinions and everyone else’s aside start by getting all of the Manufacturers
information that you can lay your hands on and read it until you think
that you understand your options.  Pick a size that is the same as
or just slightly larger then what ever the factory used for a stock application,
and start learning.  Give yourself plenty of time, with out some type
of dynamometer it may take you a year after you get the engine driveable
to get a carb optimized for your particular engine, and right foot (or
you might get lucky and nail it on your first try!).  To shorten this
process buy or build an O2 Sensor(s), or two if you have dual exhaust these
devices are simply invaluable in Carb tuning.  The price of an O2
sensor should be included in your Carb budget if maximum performance is
your goal I.E.: Power and Economy.

For those of you who have read this far, I’ll share a secret; There
is no such thing as too big a Carb.  What happens is the signal from
the engine to the Carb is effectively reduced as the Carb gets bigger.

If the signal gets too small the Carb can’t hear it so to speak, When
the engine is running at low RPM’s the engine is whispering, as the RPM’s
increase the engine’s voice rises louder until it is singing loudly. 
The Carb interprets not hearing anything as no signal, and shuts off the
gas!  The louder the song the more gas the Carb sends.  The trick
then is to have a Carb with super hearing!  These Carbs are expensive
to get and do not come out of any of the big three’s boxes regardless of
what they say and perhaps more importantly what the magazines say! 
I have known people that have the ability to make huge Carbs work on small
engines, and they have consistently been the ones that cross the finish
line first, (or their customers do!) but generally most people never do
get it right.

"Predator" Carb’s deserve a mention. They have a following and the basic
design has been around for something like 30 years.  I think that
the first time that I saw one it was called a "Kendig" which should date
me.  I have little experience with this design which is unlike any
conventional design.  The Predator is a variable ventury design, in
that the internal size changes with the engine demand.  I have heard
good reports by those that I know that have run them.  I have also
heard that they can cause fuel distribution problems on intake manifolds
that have been refined specifically for the Holley flange Carb.  When
I have more experience with this Carb I will add to this section. 
Until then this is it.

I should note that Edelbrock Carbs are exact functional clones of the
originals, while their advertising touts them to be vastly superior in
reality there just is no practical difference in my experience.

.  For one of the best general books on Carb’s get yourself a copy
of: David Vizard’s  "How to Build Horsepower Vol. 2, Carburetors &
Intake Manifolds" ISBN 1-884080-14-3, SA Design.

By C. Samuel Mountain Machine Company Portland, Oregon.

All of the above is my opinion, it is based on fire, smoke, broken
parts, fast times, tight corners, deep mud, good beer , better bourbon,
and all of the costs that go with each of them.  I retain all of the
rights of my opinions in their entirety. If you infringe on any of my rights
be certain that I will assert my legal right to justice.  If you listen
to my opinion (whether you agree or disagree) and then act on it in any
way you are solely responsible for those actions and the results there
of.

 

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