Turbo Set-ups

There are various different configurations one can employ to supply an engine with compressed air, and the matching amount of fuel to that air.  These configurations can be divided into two main groups:

1. Fuel is delivered before the compressor (draw through).
2. Fuel is delivered after the compressor (blow through).

Regardless of how the fuel is delivered, whether it be fuel injection, via a carburetor, or from a LPG mixer, the fuel has to enter the airflow in one of the two ways listed above.

When the fuel is delivered before the compressor, such as a carburetor mounted to the front of the turbo, it is said to be in a draw through configuration, as the air/fuel mixture is being drawn through the turbocharger.

When the fuel delivery system is mounted in between the engine, and the turbocharger, the configuration is said to be a blow through, as the turbo is blowing through the fuel delivery system.  Although blow through usually refers to a carbureted fuel delivery system, technically any fuel delivery system can be in a blow through configuration.

There are pros and cons of each configuration.  Typically, and draw through is a simpler proposition in the fuel delivery side of things, as the original fuel delivery device can be moved from the factory fitted position to in front of the selected turbo compressor inlet.  Plumbing to connect the compressor outlet to the inlet manifold must then be constructed.  A draw through configuration is shown in the diagram below.

The benefits of the draw through configuration are primarily the ease of implementing fuel delivery system and the inherent charge cooling from the vaporization of the fuel.  

A blow through configuration on the other hand is more complicated from a fuel delivery viewpoint, as boost pressure present within the system will try and force the fuel back through the metering device(s).  Typically, the pressure of the fuel is raised so that a constant level of fuel pressure is used above the level of boost pressure.  This is done with a boost reference fuel pressure regulator.

The benefits of a blow through configuration are many, including the option of placing an intercooler between the compressor and the fuel delivery device to increase air density and reduce temperatures.  The other main benefit is the ability to retain the factory fuel delivery device (EFI or carby) in its original location, giving factory like throttle response and other characteristics.