My 99 xj has no muffler. Just straight piped. How much hp does this add?

40 on a 4.0L and 25 on a 2.5L

About 125+ decibels at WOT.

Thanks!

Would a better cat and headers improve the hp

A high-flow cat will, headers effect the KW and TQ the engine makes

So like high flow cat will prove hp and headers will increase torque

Kw?

What's with this guy and horsepower ? Does he realize he bought an XJ not a WRX ?

Straight piping doesn't really do anything for performance. These motors are tuned almost to their limit as is. But you can do small things like relocate AIT, different injectors, good high flow exhaust (Header, high flow cat and cat back exhaust) and high flow filter to help increase your throttle response. These motors are nothing more than "tractor motors" designed for torque more than horsepower. If HP is what your goal is, I suggest a stroker kit or engine swap. Either one will cost you a good penny.

How would I increase torque

Nice. Ranked in the pricey category.

If that's too expensive, email Dave at DT Machine: dtmach40@hotmail.com

He sells M90 supercharger kits for 4.0Ls at $1000. Great foot in the door kit.

Video of the kit installed: