EFI or Carburetor? Consider This Before You BuyTuesday, June 4, 2019

Carburetors offer the purest form of fuel induction, pulling fuel and air into the intake manifold where they are mixed. Some racers, especially those on tight budgets prefer the ease of performance control that a carb offers.

Speaking of tight budgets – carburetors are economical, another reason for their popularity today. You can get more bang for the buck with a carbureted engine vs. EFI, which is a main factor for many hot rodders, and racing divisions that only allow carbureted engines in competition. What about POWER? How does a carburetor compare against fuel injection? According experts, very favorably. 

Comparing a carb to EFI, the carb typically produces more power, vs. many EFI applications, because it shears the fuel into droplets of mist spray, allowing the fuel to stay suspended in the air and mix inside the manifold. This atomization  cuts down the chance of the fuel dropping out of suspension, for a more efficient combustion.

EFI, in comparison, dispenses fuel under pressure, and the fuel is less easily atomized into the air. Reversion pulses inside a typical EFI intake plenum can allow the fuel to separate, which will reduce combustion efficiency.

The key to optimal carburetor efficiency for a given engine configuration comes down to airflow capability and atomization of fuel into the combustion chamber. A carburetor that is too big, will be lean at wide-open throttle (WOT), and rich at idle. This means the fuel isn’t being pulled through the boosters due to not enough vacuum at WOT, and the fuel is not breaking at idle.

Bigger isn’t always better when it comes to determining CFM for your carburetor. Components such as camshafts, converters, heads and intake should be mapped out already (per manufacturer’s specs), and can have an impact on the type of carburetor you choose. You also have to be realistic about the engine and what your use is. Rather than determine your carb by CFM rating, instead look at the type of boosters, venturi sizes and butterfly sizes when determining the carburetor for your application.

Engine CID x Max RPM/3456 is the formula to help them select a carburetor. Let’s take a Small Block 350 Chevy for our example at 6000 rpm (redline).

350 x (6000/3456) =

350 x 1.736 =

= 607.63

A 600 CFM carburetor would more than suffice. Keep in mind that the formula figures 100% volumetric efficiency, which most engines are not. Use the chart provided, or various online "Carb Calculators" available, to help you decide.

Carburetor tuning is done the old-fashioned way – with a few wrenches, a screwdriver, and a few specialized tools. The old school guys used spark plugs to monitor their engine tuning. Reading spark plug markings will tell you what is happening inside the combustion chamber and if it's lean or rich.

Although EFI can monitor some of these things for you, there’s no substitute for having the knowledge yourself, and once you get the hang of it, you can trust your own judgement rather than hoping a computer was programed correctly.