actual power gains

[delinquent543]

What types of gains are experienced with adding a full exhaust (1-3/4 LT's, ORY pipe etc...). I know what the estimated gains listed by the manufacturers are, but what kind of real world rwhp gains are to be expected? reduction in quarter mile times at all? better throttle response? and so forth. I guess what im asking is how noticiable are the gains?

(on a side note, are there any setups/methods that allow me to add/remove cats as i please? I live in MA and they are pricks aboue emissions, so is there a way to simply slap on the cats for that dreaded one hour a year, then take them off for the rest of teh time?) i searched, but didnt find the answer i was looking for

[rotwiler]

There are LT headers that have removable cats and a piece to take the place of the cat when you don't want it in. I have some older FLP headers that do just that. Dynatech makes some too, off the same design as the FLP's.
When I did the install the car felt great after the LT install, then after a day or so it started to use alot of gas and felt slower. I took the car to the dyno and it showed I gained right around 11 h.p, but my PCM changed my A/F and was at 10.9:1. brought it up on the dyno using the Predator handheld tuner to 12.9:1 and was sitting at right around 345 rwhp. Before the LT's on the same dyno my best was 307.1. I don't have my dyno sheet here but was around 35 rwhp gain on a A4 car. This is with cats.
I decided to take my cats off after a few weeks and the car was so loud with the LT's and Magnaflow I was a cop target, plus it was abnoxious. I took the car to the track 30+ times with the cats off and about 10 times with the cats back on, my best was with the cats on and my best 60 foot too, so I prefer them on for sound and in my situation, performance. I guess if I tuned it both ways and switched back and forth my results would be better.
I didn't see much of a gain with them though in the 1/4(1/10th to 2/10ths), but was hot out at those runs with DA's at 4000+ feet at times and day by day results vary. With the LT's, K&N Fipk, Magnaflow and Predator tune and 3400 stall plus nitto DR's I ran a best of 12.696. My car is heavy though and with me in it is about 3800-3900 lbs, plus it was a 2000 foot D/A that day.
The guy I bought them from used(he put in a turbo) seen a 4/10ths gain on a M6 car.
I had to make a plate with a smaller hole on the air tubes due to they flapped and sounded like a ticking sound(eventually will burn out if this mod isn't done). Also I recommend locking headers bolts. I have had to tighten mine 4 times in the last 4 months and might as well do it when you first do the install. GM header metal gaskets like stock are best and I reused mine and worked fine. Add new spark plugs too, is easier when the headers are out. Will need to tweak exhaust most likely too. People claim a 5 h.p gain with cats off, but the LT headers with the removable cats cost a few hundred more and isn't worth 5 h.p to me, plus I hate the sound, sounds like a machine gun with a roar.