Well That pull you see there was making 15 PSI and as you can see even at the graph I will show you, and at the limited RPM we did pull it was not slipping. The engine was tuned beyond that and made even more power. However over driving the S/C can lead to issues. And yes open A/C for drag racing use only or larger filter with custom built parts will yield more. And that cam is larger has way more duration than the 640 .. Look up the specs on the RX267 SE cam
Yes they have had to make some changes however it can be adjusted and not slip and not have a bearing failure on the pulley as well.
But again when you install them, build them and tune them you get to know what to look for and a few simple things that can be done to improve the performance of the kit .
The S/C is not very picky when it comes to lift more so duration. The 640 Pops the ex valve open very soon.. But again I have run bigger cams and made great low end power. However a MVA head stock will outflow the S&S head , chamber config etc all play a apart in that.
But try the 585 should be helpful .. BTW what EX??
Here is some boost info as well
Here is some basic info on a crate engine 124 HC with 13 psi and 1500 feet above sea level
640 cam
Your dynamic compression ratio is 8.86 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 181.36 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 13 PSI is 16.70 :1.
So if you run a cam with a early close point like the 635
19.5° 43.5° 58.5° 37.5° 243° 276° .635" .635" 102° 100.5° .115" .175"
Your dynamic compression ratio is 9.66 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 202.56 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 13 PSI is 18.20 :1.
all based on 13 PSI of boost..
Good luck.