According to Corky Bell, Maximum Boost pg 61, 304 MPH or 0.4 mach is the point at which airflow meets increased resistance (drag) and flow losses are experienced.
Using the calculator http://www.thermalloy.com/product-group/extrusions-na/flowcalc to calculate velocity in MPH
and
http://www.turbofast.com.au/tfcalc.html to estimate air flow in cfm
I modeled an engine with 2.5" IC piping, 6000rpm and 12 psi. This is roughly 670cfm, or 175mph.
Those calculators indicate that you would need to nearly double your airflow in CFM before a 2.5" tube would become a problem!
Using the calculator http://www.thermalloy.com/product-group/extrusions-na/flowcalc to calculate velocity in MPH
and
http://www.turbofast.com.au/tfcalc.html to estimate air flow in cfm
I modeled an engine with 2.5" IC piping, 6000rpm and 12 psi. This is roughly 670cfm, or 175mph.
Those calculators indicate that you would need to nearly double your airflow in CFM before a 2.5" tube would become a problem!