[daustin]

The difference in the F and S is the EGR and a slight tuning difference. The 1/2 ton 1500's usually got the S staring in the 1994 model year. The 6.5 TD's are rated anywhere from 210HP ('92-93) down to 180HP (S later models). The F is usually in the 2500/3500's and some Suburbans, lots of people trash the S upper and lower intakes and replaced them with the F intakes. The F intakes don't have the EGR ports and that's blocked off in the lower intake. The F upper intakes aren't tapped for the EGR as well. Hope that helps.
Don