how do you find out if youre cat is bad or robbing you power and does a highflow cat flow much more than the stock one .will i feel a difference on a L36 the cat is original 98 almost 10 year old whit 75,000 miles on it im asking this because i see lots of heat coming from it

If it rattles the honeycomb catalyst is broken on the inside, you will notice a severe loss of power. If your car is running rich or burning oil for whatever reason it may clog up.

Check out this article:
http://www.aa1car.com/library/converter.htm

Keep in mind, depending on your ebay skills, cats are nearly free to replace. OEM H-Body cats are worth around 70-80 dollars on ebay. Your local recycling center may give you 20-30 bucks for it... A high flow cat is around 50 bucks or so + install. I actually made money replacing my cat with a high flow version.

or get the 2.5 dp from zzp and send them your old cat (they give you 50 dollars for it). If you are going to go rockers don, you might as well getthe Downpipe now.

so in other words replace the cat with a straight pipe?i think thats a bad idea.you should go with any aftermarket parts that sale cats and buy one,the reason for that is that u burn ur gas more lest just say better,cause with the cat,the chamber inside heats up at 2500 degress and burns all the unburnt gasses,plus if you get caught by the bacon patrol,you wont get introuble if they look under your car.its your choice guys,sound and gass hogger,or sound and gass efficient?well at least here in california it has to be likke that i dont know about other states,but thats only my oppinion.

Then there'* always a Magnaflow hi-flow cat.

Had one of those on my old Beretta... very nice.

Woah! Where'd you get that from? Nobody said that. You need to get your terminology straight here bud. DOWNPIPE is not a STRAIGHTPIPE.

so a down pipe basically is from the exhaust manifold down :?:

Exactly