Hey,

truck:
'98 F150 5.4 4x4
Has the 99 model year 5.4
E4OD

when I push the throttle I hear a light pinging noise. Also I think I think I'm lagging some low end grunt. Last year I installed shorty headers because my manifold was broken and I got them really cheap (I know they dont improve performance).
Moreover at the end of last year I replaced the upper two o2 sensors. Except from the pinging and lacking torque the truck is running great, it's LPG converted and today I had the LPG system checked and they also said it doesn't run fat or lean but the sound and the missing low end torque might be a clogged up cat.
I had that idea before but didn't want to order new cats because I just suspected it. I got no CELs except one in the last months that said insufficient EGR ventilation, I cleared the code and since then it hasn't come back (ever and way more then 20 ignition cycles since then).
Because OEM replacement cats are expensive I thought about adding some Magnaflow HiFlows with the o2 sensor mount.
But as the '98 has 4 cats, do two of those hiflows suffice? I'm German and we have strict emission laws here so if those two hiflows wouldn't clean the exhaust gases enough I won't pass inspection. I'd also like to have more tone, I'm running a very long (I think about 26inch) CherryBomb Glasspack for now but it's far away from being too loud but I think the HiFlows would add some nice tone...

Thanks for any input guys! Greets from Germany, Johannes

 

I recommend calling or emailing magnaflow and asking them. You can replace all 4 with 2 high flows, but the emissions I would double check with them. I'm sure they replacements much cheaper than OEM replacements.