PRP Velocity Lessons
The Speedmaster head that I am working with can get to 285 cfm on a 186cc port with a 4.937in centerline length. That is pretty much max velocity/flow when the port starts shutting down due to separation/turbulence.
So, (186cc)x(1ci/16.387cc)=11.35 cubic inches port volume
(11.35ci)/4.937in=2.3 square inch average cross sectional area
(285cfm)x(1m/60s)x(144si/1sf)x(1/2.3si)=297 fps average velocity
This port is having problems before the 297fps point, so 260-285 fps would probably be better for this port and my porting skill level. Good racing heads would certainly have higher average velocities at the target rpm. Really good street heads could as well, but it would probably be better for a street engine to shoot for the 285 fps velocity at a lower rpm, which makes the CSA smaller for the street than for race.
Look at this chart. This is not my information, this is used by guys like Darin Morgan and Larry Meaux in their seminars and calculations. For the street 260-300 seems to be a sweet spot.
The Speedmaster head that I am working with can get to 285 cfm on a 186cc port with a 4.937in centerline length. That is pretty much max velocity/flow when the port starts shutting down due to separation/turbulence.
So, (186cc)x(1ci/16.387cc)=11.35 cubic inches port volume
(11.35ci)/4.937in=2.3 square inch average cross sectional area
(285cfm)x(1m/60s)x(144si/1sf)x(1/2.3si)=297 fps average velocity
This port is having problems before the 297fps point, so 260-285 fps would probably be better for this port and my porting skill level. Good racing heads would certainly have higher average velocities at the target rpm. Really good street heads could as well, but it would probably be better for a street engine to shoot for the 285 fps velocity at a lower rpm, which makes the CSA smaller for the street than for race.
