6.4 with h&s tuner
 

I have a 2008 6.4l with a h&s minimax and 5" exhaust and egr and dpf delete kit it run good but fuel mileage really didn't change much but I put it on a dyno and had the tuner turned up to 300 hp gain and it still only put out 408hp I'm not real sure but I think it should have put out way more hp since it is 351hp stock any help would be appreciated.

Something sounds slightly odd, I would start with a reputable truck shop that does this stuff on a daily basis.

Where did you dyno it? Is the dyno set up for a turbo charged vehicle? The dyno should add resistance to the wheel to allow the turbo to spool properly. Inertia vs Load Cell

I had it dyno at a diesel truck show and pulls

Things I'd check immediately: DTC's, air filter and fuel filters.

I just pulled my S&B air filter out after 25,000 miles and it was so incredibly gunked up, I was astounded the filter minder hadn't tripped yet. What made me check it was a slight decrease in mileage and a slight decrease in the "get up and go" when the skinny pedal was pushed. It's minor and I doubt that's what is holding you back, but it's a few things to look at.

Keep in mind the 300HP Hott damn tune won't get you an additional 300 RWHP.

doesn't make any sense at all.... your saying the truck has been running good except it's missing about 150hp....lol.... are you positive the tuner wasn't set at a different level?? There is a lot of variables.. is the truck lifted...big rims?? I didn't see if you have an intake or not.. but definetly helps with the bigger hp tunes.... something seriously wrong with this thread!

I was leading there. Lol.

The newest H&S Flashes have changed quite a bit in how the tunes are uploaded. Maybe you accidentally loaded the 150 tune unknowingly?

Can you go into the settings in the tuner and confirm your version and current tune?

I don't normally run the truck on that setting so I'm not sure how it should run. I will try to reload the tuner again and see if there are any changes. Thx

Oh and the truck is not lifted stock rims.

5" exhaust and s&b intake.

I was like is RM gonna say it or me....lol!! but seriously with your mods...also depending on the dyno and climate wherever you are...you should be putting anywhere from 550-600RWHP to the ground... so if there we're no mistakes during the upload/dyno run then there's something seriously wrong.