I gathered tons on information on boosting my ride before I chose my current setup, here is a tidbit of what I learned. Maybe I should create a new thread and post all my articles into it. Anybody be interested in that?..
Considering boosting your ride? June 2006
I have been looking into supercharging options over the last few months, and have settled on an ATI Procharger for my project. For me this will bring the performance and fun factor I am looking for while keeping its true identity under the hood.
I’m sure you’re aware there have been tons of articles on the subject of boost and how to make it, so I will use some of that info here. No sense writing the same thing over and over.
How It Works
To understand how a supercharger works, you have to think of the engine as a giant air pump. The motor’s four-stroke cycle first allows the pistons to draw air in through the carburetor, the intake manifold and the intake valves. The air is then compressed by the piston while all the valves are closed and ignited by the spark plug. The resulting explosion forces the piston down in the power stroke. The final stage of the four-stroke cycle is when the piston comes back up and forces the spent gases out of the exhaust valve.
In theory, a denser air charge mixed with additional fuel will create a more powerful combustion explosion inside the cylinder, thus creating more downward force on the piston which translates into horsepower. Increasing this air charge is where the supercharger comes in. The mechanically driven supercharger force-feeds the combustion chamber a denser air/fuel mixture. This mechanically increases the compression ratio by forcing a boosted (pressurized) air/fuel mixture in the combustion chamber. The denser charge will increase dynamic cylinder pressure upon ignition to create more power.
The most common types
Mechanically driven superchargers can be classified into two distinct types: basic air pumps and true compressors. While that cool-looking monster 6-71 may look like a giant air compressor, it’s actually just an air pump. The 6-71 is a classic Roots-type blower which dates back to the 1880s when the Roots brothers designed it as an air conveyor for mine shafts. General Motors later adopted the design in the 1930s as a supercharger for its GMC diesel engines, using it mainly to pump out exhaust gases. In general, Roots blowers have a two- or three-lobe rotor design, depending on the size of the case. They can move mass quantities of air which stacks up inside the intake manifold to create positive pressure, or boost. Not only do they look cool sticking through the hood, but they inherently have higher discharge temperatures than other common designs.
The second type of supercharger utilizes a more efficient design which actually compresses the air as it's drawn through the unit. Some examples of centrifugal blowers are those from ATI Procharger, Vortech Engineering and, Paxton. Turbos also fall into this category.
A centrifugal supercharger is basically belt-driven turbocharger whose inlet air is drawn to the center of the blower where the high impeller speed converts this air velocity into pressure. The high turbine speeds are generated by internally overdriving the input shaft which is also overdriven by the external drive pulley. Turbine speeds can approach 40,000 to 50,000 rpm on these types of blowers, with normal boost in the 6 to 10 pound range for street engines.
The mechanical cost
As with everything in life, nothing is free. All belt-driven superchargers need horsepower to run--sometimes as much as 20 percent of engine’s total. The good news is that the additional horsepower created by the blower will more than make up for power loss. The other concern with supercharging deals with the law of physics. High school physics taught us that when air is compressed, heat is created. The more efficiently a supercharger compresses air, the less heat added to the intake manifold. And we all know the cooler the intake charge, the denser the air and the more horsepower available.
Blower efficiency is measured by the discharge air temperature at a given pressure. For example, Blower A might have a discharge air temp of 180 degrees at 6 pounds of boost, while Blower B might have a discharge temp of 195 degrees. Blower speed also comes into play here. The higher the speed, the more heat generated. Supercharger manufacturers refer to a blower's efficiency in terms of adiabatic efficiency: how well it pressurizes air into an engine. For example, a Roots blower typically has an adiabatic efficiency of 50 percent, while a centrifugal blower can see up to 75 percent. Thus, higher boost levels do not necessarily mean more power. Intercoolers lower intake temperatures by pushing the compressed air through a radiator-type device--but at the cost of boost pressure, some more than others. Intercooler design is another factor. You see most centrifugal supercharges that are intercooled using a bar and plate design, while the turbocharger crowd uses a lot of radiator style intercoolers.
Other Considerations
Other areas I took into consideration included the internals of the engine, the camshaft profile, the flow characteristics of the heads, and the ability needed to alter the engine timing as boost levels increase. I also think exhaust upgrades are another critical element to making the most out of your combination.
Here is some information straight from Procharger’s website on blow through carburetor applications.
The blow through carburetor supercharger application is probably the most misunderstood forced induction application. However, by sticking to fundamentals, amazing results of more than 1700 hp are possible. Carburetors meter fuel based on airflow through them, or more directly, air velocity. To deal with high boost air flows, many aftermarket carburetor builders make modifications to increase the signal to the carburetor so it is able to deal with the additional fuel flow required when the engine is under boost. Modifications include annular discharge boosters, power valve channel restriction modifications, accelerator pump pullover enrichment mods, air bleed sizing, large needle and seat assemblies and several other tricks to provide a flat fuel curve under boost and still provide acceptable non-boosted drivability. Another necessary modification is solid nitrophyl floats as the brass floats can collapse under boost pressure.
To ensure adequate fuel delivery, the carburetor must see an extra pound of fuel pressure for every pound of boost to get fuel across the needle and seat. The fuel pump must be able to deliver adequate volume at a much higher pressure than before supercharging. With the typical street/performance system this can be anywhere from 14-24 psi of fuel pressure at max boost, or even more. The trouble is that most carb specific performance fuel pumps start to bypass internally anywhere from 12-28 psi. Many start significantly dumping volume well before they meet their bypass pressure setting. When the bypass valve opens fuel flow effectively stops and the bowls run out of fuel, dangerously leaning out the engine and losing power.
One fix is to run a high volume pump designed for EFI use. These pumps are capable of producing the required volume at higher pressure. The pumps that incorporate an internal relief generally don’t start to bypass until they reach over 70 psi. This allows the regulator to do its thing without the pump sabotaging fuel delivery. The use of a return style regulator is necessary with this type of setup. The benefit of the return system is that volume demand is met before the fuel is bypassed back to the tank so the regulator can react instantly to the changing fuel demand of the engine. A pressure reference line is run from the carburetor inlet hat to the boost reference port of the regulator to maintain the required fuel pressure. The reference line should not be connected to intake manifold vacuum as that would lower fuel pressure and volume in the bowl during vacuum.