I am about to add a k & n air filter on my cadillac eldorado.I heard they inrease hp and help gas mileage.But does anybody know if they help give it a better sound.

IMO it will not have a better sound. Need to modify the exhaust for that. As far as HP I don't think you will notice the difference, however it is an excellant filter.

I have the whole K&N Cold Air setup on my 94 STS and love it. I got an extra 3-4 MPG and I can really feel the diference in power. It take air in from right in front of the drivers side tire.