Not a good idea to use a 2-1/2" crossover pipe on your build?
For quicker spooling you need velocity from the driver side header, and the larger diameter pipe will slow down the exhaust gas flow. Maybe if you had Stage II head ports with almost double the exhaust flow of production heads a larger crossover would be in order?
You only have exhaust flow from 3 cylinders that need to get to the turbo with as much velocity as possible when it meets up with the passenger side especially at low RPM.
The larger 2-1/2" diameter pipe as compared to a 2" pipe will have more surface area which will also cool the hot gas as it passes through the larger pipe.
When we have engine output at the 1000 and up HP level we not only increase the crossover pipe diameter, but the primary tubes are increased from 1-1/2" to 1-5/8" for better flow and less restriction, but we still can maintain the needed flow velocity at the turbo.
When designing and testing of the TA headers we did not use a computer for the final design, but used common sense and lots of field testing to arrive at the best configuration in obtaining optimum performance which would produce an affordable product.
