I thought it about time to write an article on an advanced topic for you experienced rodders out there. Please note that I have adopted the following number standards to indicate the technical level of the article: 101 for Beginners or for General Information, 201 for Novice, 301 for Intermediate, and 401 for Advanced. As the technical levels advance, the terminology and concepts will ***ume an increased level of understanding. So you want to get more power out of your early to mid model engine? Unfortunately, many of these types of engines do not produce much horsepower or torque. Equally unfortunate is that there are not many performance parts available for these engines to increase power, which brings me to the purpose of this article. Believe it or not, you can actually turbo charge a normally aspirated, carbureted engine. It is not the easiest thing to do, but it will allow you to get significantly more power out of your vintage engine. Important disclaimer: Anytime you increase the pressure of the air provided to your engine above ambient pressure, you run the risk of damage to the engine ***embly. Part I: Turbocharger Definition: Simply stated, turbochargers are p***ive compressor devices that consist of a radial-flow compressor and turbine mounted on a common shaft, and use the energy in an engines exhaust to provide pressurized air to the cylinders. Understanding The Turbocharger Design: Figure 1: Typical turbocharger schematic (click on thumbnails for larger image) In Figure 1 above, ambient air is drawn through the air filter before entering the compressor (1). The air is then compressed, which raises the airs density (m*** / unit volume) (2). Many turbocharged engines have a charge air cooler (aka intercooler) (3) that cools the compressed air to further increase its density and to increase resistance to detonation. After p***ing through the carburetor and intake manifold plenum runners (4), the air enters the engines cylinders, which contain a fixed volume. Since the air is at elevated density, each cylinder can draw in an increased m*** flow rate of air. Higher air m*** flow rate allows a higher fuel flow rate (with similar air/fuel ratio). Combusting more fuel results in more power being produced for a given displacement. After the fuel is burned in the cylinder it is exhausted during the cylinders exhaust stroke in to the exhaust manifold (5). The high temperature gas then continues on to the turbine (6). The turbine creates backpressure on the engine which means engine exhaust pressure is higher than atmospheric pressure. A pressure and temperature drop occurs (expansion) across the turbine (7), which harnesses the exhaust gas energy to provide the power necessary to drive the compressor Understanding The Turbocharger Components: Now that we understand the typical turbocharger schematic, we need to understand the basic components of the turbocharger itself. As noted in the definition, a turbocharger consists of a radial-flow compressor and turbine mounted on a common shaft. The turbine housing is physically connected between an engines exhaust manifold and exhaust system. The turbine uses the energy in the exhaust gas to drive a compressor wheel mounted in the compressor housing. As the exhaust gas drives the compressor wheel, it draws in outside air, compresses it, and forces the compressed air through the carburetor and intake manifold plenum runners to the cylinders at a pressure above ambient atmospheric pressure. This increase in pressure above ambient atmospheric pressure is referred to as boost, and is described in psi. Figure 2: Cutaway of a typical turbocharger Figure 3: Compressor and turbine wheels on common shaft Figure 4: Compressor housing showing 64mm compressor inducer and fresh air inlet Figure 5: Turbine housing showing 65mm turbine exducer and exhaust outlet Figure 6: Compressor housing compressed air outlet Figure 7: Exhaust manifold flange on turbine housing Blow-Off Valve (BOV): Pressure relief device on the intake tract to prevent the turbos compressor wheel from going into surge. The BOV is located between the compressor discharge and the carburetor, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads ***ociated with it. Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract to the external atmosphere to relieve the pressure, helping to eliminate the phenomenon of surge. Wastegate: On the exhaust side, a wastegate provides a means to control the boost pressure of the engine. Some commercial diesel applications do not use a wastegate. This type of system is called a free-floating turbocharger. However, the vast majority of gasoline performance applications require a Wastegates. There are two primary types of wastegate configurations: Internal Wastegates and External Wastegates. Both internal and external wastegates provide a means to byp*** the exhaust flow from the turbine wheel. Byp***ing this energy reduces the power driving the turbine wheel to match the power required for a given boost level. Similar to the BOV, the Wastegates uses boost pressure and spring force to regulate the flow byp***ing the turbine. Note: When turbocharging a carbureted engine, a maximum boost pressure of 15psi should never be exceeded. In most cases, boost pressures should be limited to 10psi. Be sure to adjust the wastegate spring to appropriately limit boost pressure. Carbureted Turbocharging Designs: Technically, there are two methods for turbocharging a carbureted engine: Draw-Through and Blow-Through. As the names imply, the draw-through design uses a post-carburetor turbo to draw air through the carburetor, while the blow-through design uses a pre-carburetor turbo to force air through the carburetor. Please note that this article will only address the blow-through design. Blow-Through Design: In a blow-through arrangement, the turbo forces pressurized air through the carburetor. This presents two problems. First, a carburetor does not understand pressure. Remember, a carburetor supplies fuel by creating a pressure drop across the venturi. The outcome is an inversely proportional disparity between the air and fuel mixture ratios. For example, if the carburetor sees a 2psi pressure drop across the venturi, only the corresponding level of fuel will be supplied, regardless of the turbo boost pressure. The practical result is the engine running very rich at no/low boost psi, and running very lean at maximum boost psi. The second problem you will encounter is that when you force pressurized air into the carburetor, it will cause the atomized fuel to flow in ways that it is not supposed to. The increased air pressure will try to crush the fuel float, as well as attempt to blow the air/fuel mixture out the throttle shaft, or back into the fuel line.
Part II: Turbocharging A Carbureted Engine: Carburetor Type And Size: Typically, the same size carburetor can be used for a turbo installation. This is often true since the maximum turbo boost pressures are limited to a relatively low level (e.g. <15psi), and the fuel mixtures can be adequately modified with different jetting. Sealing The Carburetor: Carburetors are not designed to handle air pressure above ambient atmospheric pressure. As such, the entire carburetor must be sealed so it does not leak the pressurized air-fuel mix. The most likely areas to leak are the throttle shaft, carburetor body plug(s), and inspection window(s). If possible, the throttle cable bearing should be replaced with a sealed bearing. Carburetor body plugs and inspection windows can be sealed with epoxy. Fuel floats should be filled with foam so they don't crush under the increased pressure. Some manufacturers sell floats designed to withstand the increased pressure. The specific steps to sealing a carburetor depend entirely on the type and model of the carburetor being utilized. Please note that leaking pressurized air/fuel mixture in the engine compartment can be extremely hazardous. Turbo boost should initially be limited to very low psi to verify proper sealing and carburetor integrity. The book Turbocharging by Hugh MacInnes covers many of the carburetor modifications you can implement to have a successful blow-through turbo setup. Fuel Pressure: The existing fuel delivery system will need to be modified in order to cope with the increased air pressure provide by the turbo. Every pound of boost that the turbo provides is another pound of boost that the fuel pump must overcome to deliver fuel into the carburetor. Mechanical fuel pumps will need to be removed, and the opening in the engine block covered with a fuel block-off plate. A high volume electric fuel pump (see Figure 8), high flow fuel filter, and an adjustable or boost sensing fuel pressure regulator will need to be installed (see Figure 9). The boost sensing fuel pressure regulator will allow for increases in fuel pressure as the boost pressure increases. Figure 8: Holley Black electric fuel pump Figure 9: Holley adjustable fuel pressure regulator Fuel Enrichment: This is critical. A carburetor does not recognize or take into account boost pressure when it supplies fuel, which results in mixture problems as discussed in the Blow-through design section. Because of this, certain modifications must be made so the carburetor supplies the correct amount of fuel to correspond with changing boost pressures. One option is to install the maximum size fuel jet in the carburetor, so that at maximum boost pressure the carburetor supplies enough fuel. The problem with this method is that the engine will run extremely rich at boost pressures below the maximum. A second and better option is to use a carburetor with a power valve, and reference the valve to the boost pressure. An example of this is to use a 4bbl carburetor with vacuum actuated secondaries. In this case the, secondaries are jetted and plumbed so that they open during boost. A separate alternative is to use an external controller that can overcome the inability of the carburetor to understand boost pressure. A controller is an advanced electronics box that has the capability to meter the fuel flow based on how much boost the turbo is providing. The controller is then able to adjust the fuel curve to more closely match the air requirements. This allows the carburetor to be jetted as it would under normally aspirated conditions. Several manufacturers offer air/fuel controllers. There you have it. Now go throw a hairdryer on something and go fast! Cheers. <!-- / message --><!-- controls -->
You need to modify the carb for blow-thru like you stated. To handle the fuel supply side you need a really decent electric fuel pump and you should use a boost referenced regulator like the ones Aeromotive and Magna Fuel sell. These regs. increase fuel as boost rises. I had a blow-thru supercharger and a turbo on my tub so I no they will work great.
An interesting article. Too bad you plagurized a bunch of it, and changed a few words around. http://www.dune-buggy.com/turbo/carbureted.htm Good info nonetheless. It just doesn't do well to act like you wrote something technical when you didn't.
Holley Blue or Black electric pumps really won't build enough pressure for a blow-thru system capable of more than 7 psi boost. I have had good luck using the Aeromotive A1000 pumps with a boost reference byp*** style regulator.
I've seen modifications for older fuel pumps to make them boost referenced. They drill a hole in the upper casting and place a vacuum hose there from manifold pressure
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