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Tuning a Carbureted Street Rod Engine – Part 1 of 2

 
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Picking the Correct Carb

By: Henry P. Olsen

A question often asked is how big of a carburetor can be put on my engine, what is too big and what will be lost if the carburetor is too big for the engine's needs? On a recent trip to the SEMA show in Las Vegas, I asked several of the leading suppliers of high performance carburetors and intake manifolds these questions, many of the answers were quite insightful.

What A Carburetor Does

The carburetor and intake manifold work together as a team to supply a mixture of air and air to the engine so the engine can convert this air/fuel mixture into energy to power. This air/fuel mixture will vary from 14.7 to 12 parts of air to 1 part of fuel depending on the power demands of the engine, the carburetor must mix the fuel into the air in a fine atomized mixture if the engine is expected to be able to convert this mixture into power. The carburetor by the use of a power valve or metering rods will vary the air/fuel mixture from about 14.7 to 1 a/f at low engine power demand state such as cruising down the highway at 65 mph for fuel economy to a high power condition like wide open throttle where the engine needs a a/f mixture of 12/1 in order to supply maximum power. A smaller airflow carburetor will do a good job of mixing the fuel and air together at normal/low power driving conditions and also at higher power demands of wide open throttle, a larger airflow carburetor can supply more of the air/fuel mixture to the engine, but this may be at the expense of low rpm performance/power and fuel mileage. As the carburetors airflow is increased in an attempt to increase high rpm power the carburetor will lose begin to it's ability to properly mix the air and fuel together properly at lower engine rpm and power demands, the question for you to decide is what do you want from your engine high rpm horsepower, low rpm torque or a combination of both.

Having the proper fuel pressure and volume that can maintain the correct fuel level in the carburetor, as well as a ignition system with the correct ignition advance curve that will cause the combustion cylinder pressure to be at the ideal 12 degrees after top dead center are also items that must be correct if you want to get all the power out of the fuel the engine is burning. This proper tuning of the ignition advance curve is one of the most ignored and yet cheapest method of increasing engine performance, even if the box your distributor came in said "ready to run" the advance curve that it comes with probably is not correct for your engine and the fuel you are using.

The correct carburetor is a very important part of a complete engine package consisting of a intake manifold, cylinder heads, camshaft, exhaust system and compression ratio that all are designed to work with each other to provide power in a defined rpm operating range. A standard dual plane intake manifold will be best for engine speeds from idle to 5000 rpm, a single plane intake manifold will provide more power at higher engine speeds at the expense of lower engine speed engine power and drivability. The newer race inspired air-gap intake manifolds help cool the air charge temperature in the attempt to create more engine power, but this lower temperature can cause the fuel/air mixture to separate at lower engine speeds causing a whole host of drivability problems. When you are using an air-gap or other race inspired intake manifold, I suggest using a high performance carburetor designed with a tuner in mind such as a Mighty or Race Demon carburetor that is available from Barry Grant Inc. Almost every camshaft, carburetor, intake manifold or cylinder head manufacture will have a tech line that you can call for advice on which of their product will work best for you and your engines needs, using this tech advise can help you get the correct product the first time and keep you from wasting time and money in your search for more power. First before you call a tech line know how you will use your engine and what the rpm range the engine will normally be operated at, if your normal driving speed is 65 mph what rpm is the engine turning, how much does the car weigh, if a automatic transmission is used what is the torque converter stall speed, is the vehicle used to tow a trailer or carry heavy loads, is the vehicle used mainly at sea level or at higher altitudes. All this information is needed in order to make the correct decision for almost any performance engine component.

A good carburetor for a "stock" to "mild" V-8 engine can be anything from a q-jet to a afb style that is sold by Edelbrock as the Performer series to a modular style carb such as any of the Demon series of carburetors from Barry Grant, Inc. or Holley carburetor, all of these carburetors can be tuned to run well on a normal engine as long as you select a lower cfm carburetor. In my experience, almost any carburetor can be tuned to run well as long as you stay below 650 cfm, many of the larger air flow carburetors can have a host of stumbles, hesitations and other drivability problems due to the larger venturi necessary to increase air flow not always supplying the correct air/fuel mixture at part throttle driving conditions. For me, the notable exception of this 650 cfm rule is the Demon series of carburetors from Barry Grant Inc., which have been designed to avoid the stumbles, and hesitations that are common on many other types of carburetors. A larger cfm carburetor will often help a engine to create more power at the higher rpm ranges at the expense of low rpm power and drivability, a race designed carburetor can be used to help overcome some of these problems, but the carburetor still must not be too big for the engines needs.

The formula for determining the what the air flow or cfm demands of an engine is that I have read in many of the performance carburetor books is, the engine cubic inch displacement times the maximum rpm divided by 3456. This formula would mean a 350 cubic inch engine turning a maximum of 6000 rpm would need a 600 cfm carburetor, this is a good starting point, but there can be a lot of variables that are added in such as the intake manifold, the camshaft, cylinder heads and so on. One rule of thumb told to me by one of the excellent people that work for Barry Grant is the correct cfm carburetor will cause the engine to have one inch of vacuum at wide open throttle with the engine at peak rpm. If the engines vacuum is below one inch, the air speed will be too slow thus not mixing the fuel and air together properly, if the engines vacuum is above one inch the carburetor is too small and acting as a restriction to the engines demand for the air fuel mixture.

The March 2002 issue of Kit Car has a very good article by Ron Ceridono titled Tune-Up Tricks which can be a good reference on how to tune in your fuel and ignition systems, or if you have internet access try www.automotiveu.com.

Source

Ole’s Carburetor & Electric Inc.

120 El Camino Real

San Bruno , CA 94066

650.589.7377

olescarbs@sanbrunocable.com

 

John Bishop

Hot Rod Tuning

808 Burlway, #2

Burlingame , CA 94010            

650.343.4860

jfb396@aol.com    

 

OTC/SPX Corporation

655 Eisenhower Dr .

Owatonna , MN 55060                       

800.533.6127    

www.otctools.com        

 

Barry Grant Inc.

1450 McDonald Road

Dahlonega , GA   30533                               

706.864.2206

www.barrygrant.com

 

 
 
 
 
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