The above sounds like "GMJim" might just be doing a bit of trolling.

I've covered the C4 ZR-1 and its LT5 engine for all kinds of magazines and web sites from mid-1987 until now. I remain in contact with people in the 2G ZR-1 community who worked at either Lotus, GM or MerCruiser.

The first inaccuracy of the above quoted post is that no 1993 ZR-1 had the "one-piece" or "header-cat" exhaust manifolds. They were only used on cat-equipped 89 prototype LT5s, or on production engines in 90, 91 and 92 ZR-1s. In fact, I re-confirmed that just two nights ago with the last GM LT5 Project Manager, who is now-retired.

All second-generation, 405-hp, 93, 94 and 95 LT5s had the later, revised exhaust manifolds and bolt on cats.

So, why did the exhaust manifolds and cats get redesigned?

During the first couple of years of C4 ZR-1 production, there were some unusual engine failures caused by ingestion of catalytic converter substrate fragments through open exhaust valves. These failures only occurred if two conditions were met: 1) a cat or cats with either fragmenting or fragmented substrate and 2) coast-down from very high rpm.

If those two situations occurred together, the low pressure between the exhaust valve and the cat, which exists during a period of high rpm and closed throttle, would suck cat substrate fragments through the open exhaust valve and into the engine. The next time the piston came up, engine damage or failure occurred.

Once GM understood that problem, it redesigned the exhaust manifold with some features that resisted flow reversion and moved the cats a little farther away from the exhaust valves which necessitated a bolt together connection between manifold and cat.

Around the same time, GM issued a Service Bulletin that instructed dealers that they could replace early header-cat assemblies with the late exhaust manifolds and bolt-on cats. At the same time, the header-cats were discontinued. Any 90-92 which came into a dealer needing a replacement exhaust manifold or replacement cat was upgraded with the new design parts.

While not widely known, the second design exhaust manifold design actually cost 5-hp compared to the early manifolds, so anyone who had the TSB performed on their early engine, unfortunately, ended up with a 370-hp engine.

How did the second design engines end up with 405-hp?

The changes made to get over 400-hp consisted of some cylinder head porting mods, injector housing mods, a change in cam timing and a new ECM calibration. That actually was worth 35-hp but, once the new style exhaust manifolds and cats went on, 5-hp of that increase was lost and the power rating was 405.

Ok, now...the rear brake rotors for C6es with J57:

First let me say this "Cultrag" likely does not say whether a part is available or not because they want you to make a purchase. After they have your money, you'll find out if the part is in-stock or on back-order. Trust me, Cultrag might have one or two of them, but they are not going to have stock of that part. No one has. Further, as "Konadog06" states, the link posted is to a front rotor not a rear rotor.

Next, the part has not been "discontinued" it is temporarily not available. What has been discontinued is the one iteration of the C6 rear J57 rotor which was PN 84023644. There were two other versions before that and, obviously, there will be a fourth version soon to come.

I have been researching this issue for almost a month. I had been emailing with two media contacts at GM's Customer Care and Aftersales division which includes the parts network. The first contact went on leave and referred me to a second contact who said she was looking into the issue for me, but then she stopped communicating. So far I've been unable to determine what happened to her or even if I can recontact any media rep for Customer Care and Aftersales.

I couldn't wait for GM to get its PR **** togther so, though other channels, I've developed some intel which indicates that a dealer out West discovered rear rotors in its stock did not fit properly. Once the rotor was in place and the wheel was bolted on, the wheel would not turn. In one emergency situation, this dealer modified one of these rotors so it would fit a customer's ZR-1. This same dealer notified GM of the problem. Further research I did found that at some point in the recent past, Brembo made a run of C6 J57 rear rotors but changed the part such that the inside edge of the rotor interfered with the parking brake backing plate. With that rotor, once you bolt on the wheel, you have a "permanent parking brake."

From there, I can only speculate at this point, but my guess is most of the rotors stocked at dealers around the country and at GM had were defective in the same manner, so they were all taken out of stock by GM and likely scrapped or maybe sent back to Brembo in Italy for them to "fix". That, of course, caused a back order and (I continue to speculate) Brembo is currently working to remedy the problem.

My guess is, at some time in the near-term, proper C6 J57 rear rotors will once again be available.

Ok, part numbers....
The discon. part, ie: the one that won't fit is PN 84023644 and that part should be avoided. Install that and you may end up with the permanent parking brake problem. The previous PN for rear C6 J57 discs was 22805790 and before that, the PN was 25843128. My guess is that if you have a new rotor which is either of those two PNs, then it will work ok.

Lastly here are a couple of images that show the problem.





If I get any more news about this, I'll post it.