Efi Vs Carbs

Karl Anderson, via CarCraft.com: In the Oct. '08 issue, your Swap Swamp answer to a reader states that because of emission regulations, he can't replace his throttle-body injection (TBI) with a carb "even though that would significantly improve power." Doesn't electronic fuel injection (EFI) produce more horsepower? Does port EFI produce more horsepower than a carburetor? In terms of horsepower developed, please rate them in order: carb vs. TBI vs. port EFI.

Jeff Smith: There's a lot of confusion around these different types of intake systems, Karl. All things being equal, It should not matter which way you introduce fuel and air into the engine as long as equal amounts of air and fuel are delivered to each cylinder. But these three systems actually differentiate the three popular forms of normally aspirated induction. The devil, as they say, is in the details.

Let's start by stating that the above is a huge assumption because each of these induction systems changes the way air and fuel are introduced into the cylinders. Some systems do a great job of balancing the air/fuel ratio between all eight cylinders on a V-8. while others are far less efficient at that same job. Let's start with some basics. A carbureted intake manifold's job is to deliver air mixed with fuel from the carburetor and send that mixture to all eight cylinders. This is often called a wet-flow design because the manifold runners direct air mixed with fuel. This is a challenge, since fuel is heavier than air and the two

For detailed specs on this engine build go to knfilters.com/cc components tend to separate when the mixture has to change direction, causing air/fuel separation. Poorly designed single-plane manifolds at high speeds can often suffer from fuel actually collecting in puddles on the manifold floor that then travel in tiny streams into certain cylinders, causing air/fuel ratio difficulties. On the plus side, carburetors introduce the fuel much farther upstream than multipoint fuel injection, using this early introduction to help reduce inlet air temperature because the fuel tends to pull heat out of the incoming air. This is especially prevalent with alcohol fuels such as ethanol and methanol. Basically, if the carb and manifold are properly designed, they are difficult to beat in terms of overall WOT power. However, carburetors suffer from a lack of feedback that keeps the air/fuel ratio lean during part-throttle conditions. This hurts emissions and fuel economy.

When the OEs converted to electronics in the late '80s, the performance option was to go with multipoint port fuel injection (MPFI), which places a single injector in each manifold runner just upstream of the intake port. Electronic feedback control over the air/fuel ratio using oxygen sensors in the exhaust allowed specific control over the air/fuel ratio for each cylinder, which improved part-throttle performance. For the most part, these early systems didn't flow a lot of air, especially with examples such as the '88 Tuned Port Injection (TPI) systems from GM in the Corvette and Camaro or the '87 Ford mass airflow-controlled systems used in the Mustang. This spawned a new generation of after-market EFI throttle bodies and manifolds that flow much more air and are therefore better at making horsepower. Port EFI is the most efficient at maintaining a given air/fuel ratio across the engine, but because the injector is located much farther downstream, it lacks the time to cool the inlet mixture like a carburetor. To boil this down, if the manifold is exactly the same between port EFI and a carburetor and the air/fuel ratio is the same between the two, the carburetor will enjoy a slight advantage (perhaps 1 to 2 percent) over port EFI due to the latent heat advantage of introducing the fuel farther upstream.

The original TBI systems you mention are basically electronic two-barrel

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