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That's sort of a complicated question. First, when you say 3 inch "intake piping," I assume you mean the piping from the intake air filter to the compressor inlet. Not the generic term "intake piping" to mean all of the intercooler/inlet piping. Is there any data to support the 3" intake piping? Not exactly. To get a real-world comparison, you would need to get back-to-back dynos using the exact same turbos and setup, but only changing the inlet piping from 2.5" to 3". Even then, it wouldn't be an exact comparison because the ambient temperature and humidity are changing while you're swapping the piping. Plus the heat in the motor/power system is changing. But you could compare the horsepower and torque difference and get a pretty good estimation of the benefit, if any. To answer your second question, you need to understand two things. First, that the inlet side of the turbo, and the compressor side of the turbo are completely different animals. One is under pressure, the other is not. And second, that you need to adjust for both independently. Intercooler piping (pressure side) is a balance between maximum flow and pressure drop (ignoring piping length for the moment). A 6” diameter intercooler pipe flows a ton, but the pressure drop would be huge because the turbos have to fill up all that volume of pipe. Likewise a 1” pipe would not drop much pressure, but flow would suck. The general consensus is that 2.5” diameter piping is the ideal balance for almost all VG30DETT turbos. The inlet piping on the other hand is not under pressure. Therefore, there is no such thing as pressure drop, only the atmospheric pressure of the environment (are you at sea level, or at 10,000ft?). Since there is no such thing as pressure drop, we want to make the inlet piping as large as possible to increase the flow. How do we do that? Well, if you read Corky Bell’s Maximum Boost, he says that the ideal way to increase flow to the inlet, is just to have the turbo inlet open to atmosphere. You see that on drag cars occasionally. Obviously that presents problems for a street car. So the second best way is to mount a filter directly on the turbo. The largest filter you can. That way the flow is the highest. Obviously we can’t do that on our cars. So then you need piping from the front of the car to the turbo inlet. Ideally you want the largest piping you can fit, with the shortest routing you can muster, and the largest filter you can accommodate at the end. So the ideal (currently available) bolt-on setup is Ash’s 3” inlets, with 2.5” charge pipes and 60mm TBs. Will the 54mm throttle bodies “negate” the 3” inlet piping? Probably not. But we also know based on thermodynamics that the more times the air has to change directions or diameters, the worse it is for flow. So ideally the piping would not change diameter once it’s under compression, there would be very few changes in direction (bends in the piping), the piping would be as short as possible, and there would be very few joints/couplers to disrupt air flow. So we’re limited in that department by feasibility and layout of the engine, turbos, intercoolers, and throttle bodies. I hope this answers your question. You should check out the Bell Intercooler website FAQ and read Maximum Boost if you’re super-interested.
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