Originally Posted by 2011 Nismo#91

I know this can and has been an issue for pre-cats. How ever this is just impossible for a normal Cat. Yes there is some gas sucked back during overlap but you have 5 other cylinders one of which will be exhausting completely. So at most the exhaust pressure would be reduced but as for an over all back flow of the exhaust is just BS.

Your blown engine was due to other issues, the destroyed cat is unrelated. And more then likely all the oil burning clogged the cat up and exhaust pressure just ripped through the clog. But all this is difficult to prove hence why they are just replacing the engine.