Originally Posted by Phimosis
(Post 679893)
About spreading false information: Headers don't cause supercharged cars to lose boost.
From a physics standpoint, increasing exhaust backpressure makes it harder to get exahust gas out, which makes it harder to get fresh air in. If you apply resistance to the outlet side of a compressor, the pressure goes up. Reducing the resistance increases volume of airflow and decreases pressure. Think of the tire compressor at a gas station. When the valve is shut, it is flowing zero cubic feet per minute and making 150 psi. When you open the valve and it's flowing air into your tire, it's flowing, say 10 cfm and making 35 psi because of the resistance in your tire. When you open the valve and let it blow into the atmosphere, it's flowing, say 20 cfm, but making 0 psi of pressure.
Now, cars make hp by burning gasoline with air at a fixed ratio. Having air under pressure doesn't let you burn more gas and make more power. Having more AIR let's you burn more gas and make more power. Boost helps you get more air in, but like in the analogy above, if there is less resistance, like blowing the tire filler into the atmoshpere, there will be less boost, but more total air moved, which will give more power.
So, long tube headers are NOT decreasing boost (with the implication being that if you have less boost, you are making less power). Headers simply decrease the outflow resistance so the boost level is whatever your compressor can make against the closed intake valves.
In the case of the GTM supercharger, they are using a smaller unit that reaches max boost and flow characteristics at a lower RPM, but then at higher rpm, the unit does not increase airflow in the linear fashion that engine consumes air. This leads to a drop off in boost. It gives a nice fat torque curve that falls off at high rpm, similar to what you see on most turbo cars. Then, when you couple that with a free flowing exhaust, the boost numbers are lower than predicted in the higher rpm range, but will likely be same as predicted in the lower and mid range.
GTM's higher stages of their supercharger kit use a larger compressor and you probably won't see the fall off in boost at high rpm (even with headers), but the torque curve will probably look more like the unit that Stillen has developed.
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