My dad has a 97 F150 w/FlowMaster muffler mated to the stock piping and, since last night, a True Flow intake. He wants more power and better fuel economy, will putting a hi flow cat and 2.25 ro 2.5" exhuast piping help with this?

 

Very little if any improvement in fuel milage.
If he is already light on the throttle, changing an exhaust system won't improve flow over the factory system.

 

Just replacing the muffler won't do any real noticeable mileage increase with and intake it should help out maybe noticeably, but perhaps if it was a complete exhaust headers back it would help relieve some backpressure and with the greater airflow the fuel would be used more efficiently. Not quite sure if he is looking just for gas mileage if it would be worth the money though.

 

The difference (if any) would be so minimal that it would take years before any saving would be reaped after paying for the parts. IF that is all that he is wanting it for is better fuel mileage. Like Bluegrass said, the only time the stock plumbing becomes slightly restrictive is at wide open throttle in the high RPM's. If he isnt flooring it from stops or down the highway then all he would be changing is the sounds.

 

so will adding hi flow cats and larger diameter exhaust piping add any power?

 

Yes some.
Don't expect to bolt on an exhaust and blow the doors off of everybody. But it is a good first step to performance.