I know it sounds silly, but i really did it on a 5 gas analyzer at my tech school. using an OTC genysis scan tool and an adapter, with the tail pip sniffer and 2500 rpms with out magnets hc PPM were at 1300ish and after putting on magnets they would drop to 375-400. take them back off and they would go back up to 1300. idle HC PPM dropped also but not as much as the did at 2500rpm

1986 xtra-cab 22re
stock everything
1991 bed