Need Aftermaket Suggestions On 01 ISX

I suggest you talk to Mr. Haney . He's on here a bit. Does Cats , but I've seen him post for the older Cummins . Quickest way would be for you to PM him , but if you can't , find a recent post , and throw your question out there . May be awhile before he answers , he's not here everyday.
What , specifically , are you wanting ? Better mileage , more HP ? How much weight you pullin ? What part of the country you run in ? What rears you running?
You can check out Pittsburg Power , PDI , out of Utah , they have accredited shops all over , or a Bully Dog . How much cash you willin to spend ?
Most important , what is it you're tryin to accomplish , that a 525 isn't givin you ?

 

I have an 06 ISX and put a bully dog tuner on it and am very happy with it. The power difference was very noticeable and I gained a full 1 MPG.

 

Add a Filtration Solutions FS2500 bypass filter, use synthetic lube front to back, look into a Fleet Air washable air filter, low restriction mufflers, high flow exhaust manifold ( PDI, Pittsburgh Power, etc ) and find a tuner. The tuning is everything, there's so much left on the table when it comes to power and mpg it's silly.