Normal red filters with SFX give results similar to HP5 with a red filter, since SFX is HP5 with extra sensitising dyes.

If you want the infrared effect you must use a Hoya R72 or Heliopan 715. You can’t see through them as they don’t allow light through that the eye can see. That’s how dark red the filter needs to be for SFX in order to produce the IR effect.

The B+W 092 filter is a very dark red filter which gives a small IR effect with SFX, you can just see through this. Ilford do a gel SFX filter too, which you can cut to size.