Re: Horsepower difference ?
 

"Catalyst reversion"...LOL
More like "catalyst regurgitation."

I don't remember what Graham called it but he did explain that because of lower pressure in the cylinder, the engine sucked pieces of the cat substrate past the exhaust valve. I'd imagine, if that happens, the damage sometimes had to be more severe than just a broken oil ring. GM wouldn't have spent the money to redesign the exhaust manifold if it hadn't spent too mucy money on warranty for that.

Re: Horsepower difference ?
 

For sure! This ring isn't broken. The lands have been completely eroded away. The cylinder liner was toast & piston badly scored. I just don't have photos of the other components. The damage wasn't confined to just a single cylinder.

In chats with Chris Allen & Graham, I don't recall this problem being associated with the few warranty returns. Maybe so, I just can't say.

My perspective on the manifold re-design and fabrication experience. Separating the converter from the manifold is a win-win. Simpler to manufacture & a lot less costly to replace a bad catalytic converter, in terms of both labor and components.

Re: Horsepower difference ?
 

So... Is that still a potential issue to all 375HP LT-5 that run with OEM cat?

E.g. when I bought mine the left cat was crumbled and had to be replaced.

Just out of curiosity. I don't think it's a common issue as up to that day I never heard about it.

Re: Horsepower difference ?
 

Matt,
I don't think it's a common issue at all. As I understand catalyst reversion, it may occur when the engine is operating @ high rpm, throttle is closed and clutch is engaged. Think in terms of road racing and using the engine for braking, i.e. engine braking.
As Hib stated in his last post, "lower pressure in the cylinder." I think it's likely negative pressure, occurring on the downstroke. The intake air is shut off when the throttle closes, and exhaust reverses direction, flowing towards the negative pressure on the downstroke, picking up catalyst "dust" on the way.
I don't imagine it takes long, considering the high rpm. Likely not a good idea to be at very high rpm & just dump the accelerator. Engine braking during heel-toe is generally in the mid-range of rpm.

Not an expert by any stretch and just my 2 cents, so I will probably draw some flaming. ;)

Re: Horsepower difference ?
 

Very interesting consideration! Thanks Marc. Indeed your thoughts don't indicate a common issue. And it shows that cylinder head, camshaft and exhaust design is way more complex than an average brain would expect.