Now that you’ve taken care of the biggest restriction in the system with the Stealthback Exhaust, it’s time to tackle the intake and finish off the exhaust system. You say you want to keep your car looking and sounding like stock or that you don’t want to drop your sub-frame or cut off your stock catback? OK, skip the catback. You’ll give up a bit of power, but not too much. Plus your car will be much quieter. Don’t, however, skip the intake.
Time for a bit of a lesson in turbocharging. When the turbocharger pressurizes the intake air it also heats the air. That is why you have an intercooler after the turbocharger. How much the turbocharger heats up the air depends on many factors and is described as the turbochargers compressor efficiency. Efficiency varies with boost pressure and airflow. This efficiency can be “mapped” on a graph of Pressure Ratio vs. Airlow (usually in CFM) and is called a Compressor Map. Generally speaking, as Airflow increases, compressor efficiency drops off. And as Pressure Ratio Increases, compressor efficiency drops off. You still with me? If you are you are probably asking yourself “WTF does this have to do with anything?” I’m getting to it. Pressure Ratio is the total absolute pressure produced by the turbo divided by the pressure before the turbo. If you have a restrictive intake, you actually create negative pressure (vacuum) in your intake. This negative pressure has the effect of INCREASING the Pressure Ratio. For example say you want to make 15 psi in your intake manifold. Atmospheric Pressure is 14.7 psi. Intake A causes a 2 psi of negative pressure in your intake. So the Pressure Ratio is (15 + 14.7)/(14.7 - 2) = 2.34. Intake B causes 0.5 psi of negative pressure in your intake. So the Pressure Ratio is (15 + 14.7)/(14.7 – 0.5) = 2.09. This means that in order to make 15 psi in your manifold with a restrictive intake, your turbocharger has to operate at a higher pressure ratio, and is therefore less efficient. This causes the intake charge to be hotter (less dense) than it would be at a lower Pressure Ratio. This causes your intercooler to have to do more work, and will cause it to heat soak earlier. All this only gets worse if you start to increase your boost pressure.
Lesson over. Get a less restrictive intake next and give your turbocharger and intercooler a break.