I have an 02 Z06 and it something has gone wrong. Here is a step by step summary on the progress of the car.
1) 376/367 Vortex airbox, borla stingers,30 svo's, maft 11/09/02 11.656@117.75
2) 423/400 added Kooks 1.75 longtubes 3" x-pipe, comp cams 228/230 .571/.573 112 Xe grind 7/30/04
3) 447/408 added AS stage 2.5 5.3 heads 8/09/04-tune was not finished as the speed sensor on dyno died
4) 432/390 added dynatech 3" metal matrix cats(same as random tech's), then every pull after kept losing power until final pull was 420/380( car has always been dyno'd at the same dyno).

Now I made the mistake of not looking at the dyno sheets properly-all I did was look at peak #'s and A/F-I always kept A/F at 12.9-12.8.
I watched the pulls and scanned for knock-at 28 degrees I got only 1 degree around 4000-4800, at 30 I got 2 around the same rpm and then dropped timing to 24 and still got 2. With edit I dropped timing around 3 degrees around peak torque so if my timing was 30 total it was 27 at peak torque etc. Now I noticed that the car did not seem to pull like it normally does on the dyno-kind of seemed like it was labouring. However it did still pull to 6900 rpm redline. The dyno operator seemd to think my cats got plugged up. so over the weekend i pulled the cats and sure enough they look fine-u can see right through them-they're not plugged. Driving the car around normaly it seems fine-although I have noticed it holds the idle longer at stops-at wot it doesn't feel like it has the same *****. Now my AS heads have the crane duals as I suppled them to Jay. When I got the heads i pulled a valve to see the port work and did not notice any shims. I talked to Jay about my concern with these springs not having enough seat pressure and he told me he would set them up around 1.75 not 1.8. I never checked the installed height.
This weekend I'm going to do a leakdown test to see what's going on. Then I'm going to pull each spring and check the installed height. While the springs are off i'm going to check the pressure of the springs. I have ordered a set of "new" Cranes with the stripes and a set of PRC dual springs. I also ordered shims too. As you guys can see i'm thinking something has gone wrong with the springs. I've heard alot of times that Jay uses longer valves and without shims I think my installed height may not have enough seat pressure for the valves. My first dyno with the new heads was the day after the install-the next dyno was almost 2 months after-so I'm thinking the springs have prematurely fatigued.
I'm just wondering if I'm on the right track guys-I tried getting a hold of Jay but no luck-any input or suggestions would be appreciated-thanks!

 

So if I understand your timeline...you made more power with the heads, but the cats cost you power. Step one, remove items in reverse order. Start with the cats. and see if you can make ~447hp like before.

 

The cats have already been removed-BUT-I highly doubt it is the cats that can be attributed to the lose in power. I also did pulls with and without cats and this is the difference. 420/394 cats on, 423/399 cats off. This is before heads.

 

Check the valves/springs like you said. See if that's the problem.

 

sounds like they might have just worn out if they're not shimmed correctly. be glad they didn't snap on ya