Not even sure of the number. We've used them many ways, many times. If you fix the last four inches of the ports, and work a little in the plenum, they can get very good. Big engines respond better to them. In my Super Stock stuff, before we raised the ports too high to use a Portosonic, an 8300 rpm 428 was just about
the same on the track with a mildly worked Victor vs. a severely worked Portosonic. They were only hundredths apart, if that.