I dont have a converter book handy now, but here is the breakdown of how the 258mm converters stall from lowest to highest and you can compare this with the K-factor ratings.
LOWEST stall rpm
"JSFM"
"JTFM"
"JZFM"
"JXFM"
HIGHEST stall rpm
JSFM converters where used with most NA Bonnies and larger cars as well as a lot of Buick and larger bodied cars with the L67. The JTFM is the converter that came with most W-body L67 cars from 97-02/03 . Around the 02/04 time frame most of the L67 and L32 cars started coming with the JZFM converters. The 3.5L dohc engine equipped cars such as the Oldsmobile lineup used the JXFM. Keep in mind the stall ratings again are different depending on which engine they are matched to, so a JSFM matched with an L67 may stall higher than than the JXFM with an NA engine. The K-factor info is not given on all converters either and I have even seen a few OE converters from GM that dont even have a code at all on the converter just a part number so that throws more of a mess into the pot!
