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Engine Induction (carburetor and intake manifolds)
What size carb do I need for my XXX cubic inch engine?
What intake do you recommend?
Those are both very common questions, and good ones, for people working on and tuning their cars, assembling a new hot rod, or going racing.
The first thing you need when sorting out what parts to buy,
is to ask yourself what are you planning to do with it?
Is this a street driven, original car or stock engine?
Will you be racing it, and if so, what type? If you’re racing, the
answer may be the simplest, as some sanctions like Wissota, IMCA or even non-sanctioned classes have specific carb rules. For example, many of the Wissota
classes are mandated to run a Holley 4412, 500 CFM two barrel carb.
After you’ve decided what it is you want to use your car/motor to do, the next step would be choosing the type you would like. This is often personal preference, but
you may be limited to running what will work for your intake manifold if you already have one. If you’re running a stock GM manifold with a quadrajet bolt pattern,
that will limit your choices unless you want to run an adapter, which may or may not be possible depending on hood clearance, track rules, etc.
So now we know what type we need, but what size?
With the huge variations in size, how does one know what to pick out?
Contrary to what many people believe, a bigger carb will not always make more power. A motor can only take so much fuel, and trying to force too much
through it will, in reality, slow it down.
A basic equation to figure size is CFM = (RPM times CID) / 3456
where CFM is the final carb size. RPM is the maximum RPM’s
you expect to turn. You should be careful to be realistic with this.
Even if you build a lower end that will handle 7500 RPM’s, the chances of running that on a street driven car are practically nill. CID is the cubic inch size of your
motor when complete, so this should take into account boring, stroking, etc. that has been done. 3456 is a constant that will always be used to calculate this.
Let’s use the example of a street driven Chevy 383 stroker motor.
Since this is street driven, we’ll use 5000 RPM’s as max, x 383 /3456= 554. This assumes you are running 100% volumetric efficiency. Even the best tuned motor
with regular tune ups will rarely accomplish this. Lists of common engine’s typical efficiencies are available online. There are cases where
you can have/will need more than 100% efficiency but those are rare cases where there is a supercharger, turbocharger, etc. on the engine. Now back to the
equation, we take our 554 CFM and multiply that by our volumetric efficiency (80%) and
we end up with 443 CFM. This may seem counter productive to what you have seen before, but in this real world example, a 450-500 CFM carb will run great,
provide decent mileage, give good performance, and have adequate capacity to supply our
sample motor. Many, many engines are overcarbed for the street.
As important, if not more, as the size of the carb is the tuning of it.
The spectrum of this article will not be covering tuning, but if you are putting a different carb on and don’t have the knowledge yourself, you should find someone
well versed in tuning and reading spark plugs to get whatever carb you choose up
and running to the best of it’s ability. An AFR (Air fuel ratio) gauge is another great tool for tuning and if you don’t have experience reading plugs, can be invaluable
to you, as well as a seasoned carb tuner.
If you are looking to have your carb rebuilt or modified, we recommend you go to http://www.kineticperformance.net and call the owner. In our opinion, they put out
the best modified carbs on the market and offer great service at very reasonable prices. They also offer stock rebuilds that can’t be beat. Dayton
is one of the most knowledgeable carb guys out there and will help you tune it to it’s best performance.
So you’re choosing an intake and don’t know what to use. Again, this can be answered by deciding on what it’s use will be. If you’re planning street driven and RPM’
s under 6500, a dual plane intake will best suit your needs. If your max RPM’s are going to
be over 6500, and lower end power isn’t as important, a single plane will suit your needs. There are other factors that can make both of these rules incorrect, but in
general, this is correct. If this is a street driven vehicle, chances are good you’ll want a dual plane style, and some other important features that you will want to
consider would be carb mounting flange (to match what you want to run for your carb) and whether or not it has an EGR valve mount and heat crossover. For
emissions standards, the EGR valve is important. If you’ll be driving in cool and cold weather, the crossover is important to help your motor run it’s smoothest while
it is warming up. Material is generally aluminum if it’s aftermarket, which helps with heat dissipation but cast iron may be needed if your rules dictate. In most
instances, aluminum is superior unless rules won’t allow it.
In closing, you will need to consider many factors when selecting the induction system for your motor. Try to look at the entire motor as a working assembly that
needs certain parts to work their best, and not just pick a cool shiny one or the latest, greatest, gee-whiz part that promises to make XX more horsepower than the
next one. Correct motor building isn’t that simple and needs to be carefully considered before the time comes to put everything together.
For More Information Contact:
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