Calculate optimal downpipe diameter for turbocharged engines based on exhaust flow, boost level, and turbine outlet size.
Input Parameters
Results
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Exhaust Flow Rate
cfm
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Optimal Downpipe ID
in
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Gas Velocity
ft/s
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Est. HP Gain Over Stock
hp
Note: Downpipe is the most restrictive point post-turbo. A 3" downpipe supports ~450hp; 3.5" to ~600hp. Always match or exceed turbine outlet size.
Reference
Exhaust Flow = HP x 1.5 CFM
Required Area = Flow / (60 x Target Velocity)
Optimal Dia = sqrt(4 x Area x 144 / pi)
Min Dia >= Turbine Outlet Size
HP Gain = (Stock BP - New BP) x Flow x 0.004