Wednesday, December 17, 2014

What is the correct size carburetor for my engine?

After my recent post regarding the replacement of the carburetor on the 1968 GTO I thought it would be a good time to share some information on the process I use to determine what is the best size of carburetor, commonly referred to as CFM  (cfm = Cubic Feet per Minute of air flow), for a particular engine. I am by no means an expert on carburetion, so the following is only a "shade tree mechanic's opinion," but I do know one of the most common mistakes a classic car owner makes is to put too large of a carburetor on their engine. A carburetor that is too large for the engine can actually reduce the performance of the motor.

There are a number of formulas on the internet that will help get you into the ball park of correct carburetor size. I use one of the more simple formulas and I find it to be one of the most accurate. You will still need to plug into the equation some of the specific knowledge you have about your engine and be realistic in your assessment especially when it comes to Volumetric Efficiency.

A few basic terms that are helpful to know:
  • Almost all carburetors are rated in size by CFM or Cubic Feet of Air Flow Per Minute at Full Throttle.
  • Volumetric Efficiency of the engine is a key factor in any formula to determine the engine's CFM requirements. This is the one that requires some knowledge of the engine and any modifications that have been made or added to the engine. This is not as difficult as it may sound and with a little thought you can probably get it within 40 to 60 cfm. Being that a car engine is basically a big air pump, the figure you are attempting to find is how much air/fuel mixture your engine can efficiently handle at peak RPM. For example, a stock 350 engine with a factory single snorkel air cleaner runs at about 75% efficiency. So, if you install a 650 cfm carburetor and everything else stays factory stock the engine can only handle about 525 CFM at peak RPMs. At this point you have over carbureted your engine and have very likely reduced both low end torque and HP.

Volumetric Efficiency is very important if you are looking to enhance the power and overall performance of your engine. The good thing is there are a number of modifications that can be made to the engine to increase the Volumetric Efficiency at a modest cost.

The following are a few examples of modifications that will increase Volumetric Efficiency.

Air filters: Original snorkel type air filters are very restrictive. Some can reduce the CFM by 100 points or more. By simply changing to an open element filter of approximately 14 inch diameter by 2 inch high you can gain 100 - 125 CFM of air flow.

Intake Manifold: Installing a quality performance manifold such as an Edelbrock or Weiland will significantly improve efficiency of the engine and bring out the true performance of the carburetor.

Headers: What we are attempting to improve by increasing Volumetric Efficiency is the passage of air/fuel gases from the carburetor completely through the engine including the exiting of those gases via the exhaust system. Factory exhaust manifolds are often known to be poorly designed and very restrictive in removing exhaust gasses. A good set of headers will greatly reduce the restrictions built into the factory exhaust manifolds and often increase flow in the 50% range.

This is where the modest cost typically ends, your personal desire for more power and budget begins.

Heads / Valves: A good set of performance heads / valve setup is also a big step in increasing efficiency however this is a area where pricing and budget becomes an issue. So this becomes an area of just how much modification and HP you are willing to buy.

Cam: A performance cam can be very helpful in increasing Volumetric Efficiency as they keep the valves open longer and allow more gases to flow in and out of the cylinders.
Cams are not excessively expensive, however the installation can be unless you are capable of doing the install yourself.

The following is the formula I typically use. I will be using the engine in my 1968 GTO as the sample engine for the formula as it is not stock and most of the enhancements I have mentioned above are in this engine. I will also show the results of the formula as if the engine was factory stock. The change in the two formula's recommended CFM size of carburetor is due to the increase in Volumetric Efficiency.

The GTO engine currently contains the following and I rate it at 90% Volumetric Efficiency.

400 Cu. In. displacement
5200 RPM red line
750 CFM Carb
14 "  X  3"  Open air filter
Edelbrook Pontiac Performance manifold
"16" code factory large valve heads
Pertronix Flame Thrower electronic ignition
Crane mid range performance camshaft
10.75 - 1  Compression
Hedman Headers  to 2.5" exhaust and Flowmaster Super 44, very high flow through mufflers



{   Engine as currently modified and with a 90% Volumetric Efficiency rating. }

                               400  X  2  =  800  X  .9ve  =  720 CFM  

Note: If the formula indicates a CFM carb that is not produced always go slightly higher. In my case it was to a 750 CFM.

As a comparison, this is the data based on the GTO engine in factory stock configuration and a 75% Volumetric Efficiency rating.

                              400  X  2  =  800  X  .75ve  =  600  CFM  


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