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Compound Boost - What you should know.
Compound Boost Facts PDF Print E-mail
Written by HellionCraig   
Wednesday, 01 April 2009 15:58

Because of the overwhelming popularity of the compound boost setup, and the many questions that surround it, we at Hellion Power Systems have decided to include a Compound Boost section to specifically answer the most common questions we receive and to also get some facts out in the open about the systems and their performance.


Q: Can I use the System WITHOUT the Supercharger?

A: Absolutely. This kit will attach to a stock 96-98 or 99 & 01 Cobra intake just like our other 96-04 Cobra single turbo kit, or will attach to a sheet-metal intake on GT500's. The Supercharger is simply an option for those who want to try the Compound Boost setup .But no, the supercharger is NOT a requirement.

Q: Will I Make More Power With a Smaller Pulley/More Boost?

A: The short answer is no, not necessarily. In the compound boost setup, the turbos are the more efficient power adder, and because of this, we want to try and utilize the power they can make as best we can. By turning the supercharger faster than it's factory setting of 8-10 psi, the compounding ratio is increased, and therefor total boost levels will increase at a more pronounced rate. This may limit the amount of boost that we can run from the turbochargers, which may limit top-end power. However, by turning the supercharger faster, it is possible to create more low-end power and torque, at the expense of top end power efficiency. 

Q: Can I use a Kenne Bell or Whipple Supercharger with the Turbos?

A: Yes. Keep in mind though, there are some precautions to take with the larger, twin screw superchargers. The amount of air that a twin screw supercharger would flow through it, as well as the manner in which the twin screw superchargers compress air will lead to much, much higher total boost levels versus a roots style supercharger. This is an important factor to keep in mind when tuning the car for maximum power. Turn the supercharger slower with a larger pulley, and any potential issues should be resolved. 

Q: Is the Supercharger a Restriction?

A: No, not at all. The supercharger is not a stagnant, solid piece that air has to travel around or through. Rather, the supercharger is simply doing what it does, and that's take air and essentially make it smaller. The supercharger will simply take the air that has been compressed by the turbochargers and further compress it as it travels into the engine. In fact, the turbochargers will basically help to improve the overall efficiency of the supercharger. Roots and screw-type superchargers experience a condition known as "back flow." Back flow refers to air that doesn't make it into the engine when it is compressed, and actually comes back up into the intake tract. This air is heated, and is the cause of the loud noise heard from positive displacement superchargers. When the turbos are added in conjunction with the supercharger, they help to keep the airflow directional, minimizing the amount and effect of the back flow. Also, it's important to remember that boost itself is actually a measure of restriction, so to speak. The number that we read as "boost" is positive manifold pressure, or the byproduct of the engine only being able to flow a certain amount of air through it at any given time. The air that is being forced into the engine will eventually overcome the amount of air that the engine can ingest during every revolution, and the resulting "back up" of air is positive manifold pressure, or boost. Therefore, by using the supercharger to create a higher level of pressure, it cannot be viewed as a restriction. 

Q: How Come the Turbos Alone Will Make More Peak Power at the Same Boost Level Then?

A: As previously stated, the turbochargers are the more efficient power adder (supercharger vs. turbo). In the compound setups, the turbo boost level is limited because the air will be compressed again by the supercharger. Therefor, 20psi of compounded boost will only be about 8 psi from the turbos and 8-9 psi from the supercharger. This means that you will see peak power characteristics that reflect the amount of turbo boost present, with low-end power characteristics of a supercharger. The total boost number is somewhat irrelevant. If we use the turbos only to create 20psi, they are in a more efficient airflow range, but will lack the low-end power of the supercharger. Also, the turbo combinations with lack the mechanical losses, however minimal they may be, which will lead to slightly higher peak power numbers. 

Q: What are the Real Benefits of the Compound Boost Setup?

A: The real world benefit of a compound boost setup is average power and driveability. With the compound setup, you can still have the instant boost and low-end grunt of a supercharger, but the addition of the turbochargers will allow for a much more efficient and powerful top-end. This is referred to as "power under the curve." The average amount of power that we are able to produce with the compound systems will provide better driveability while still producing hair-raising amounts of power. Here's a quick example of how to think about it:

Let's say that we have a car that makes 1,000 hp peak, but only makes 500 - 1,000 hp for around 3,000 rpm. Now, we have another car that makes 900 hp peak, but makes 500-900 hp for 5,000 rpm. If we use a linear progression of power to calculate the average power through 5,000 RPM, the second car in question would have an average power output of about 700 hp through 5,000 rpm, whereas the first car mentioned would only have an average output of about 550 - 600 hp through 5,000 rpm. Using this equation, the first car in question has a higher peak power output, however the second car will most likely out-accelerate it due to the fact that it makes more average power over the same RPM range. 

Last Updated on Monday, 24 August 2009 19:04

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