The key difference, in my mind anyhow, is that when you have a mechanically supercharged engine you are forcing volumes of air under pressure into the engine, and if there is a blockage in the exhaust (i.e. cats) it's going to be a much bigger problem than a naturally aspirated engine with the same exhaust blockage.

The more air coming in, the more exhaust gases coming out. Naturally aspirated engines aren't pulling in anywhere near as much air as a mechanically supercharged engine of a similar capacity and rev range.

And if i'm not mistaken, with a naturally aspirated engine it's going to be inhaling less air if the exhaust is restricted. But the supercharged engine tries to pump in the same volume regardless...

No?

So the cats on your 30 year old 928 are probably choked up too. You just don't notice it because you're not trying to force-feed the engine more air than it can manage out the exhaust.

 

I think yes (for NA) and no (for SC) LOL

On either car (or any car), if the cats are (even part) blocked then the exhaust gas can't get out (statement of the obvious, but bear with me) so there is stuff still in the bores and so the engine is less of a vacuum pump than normal. It can only flow less air past the MAF (or whatever sensor it has, e.g. MAP will see less vacuum). So it doesn't matter whether it's supercharged or not - it'll be down on power because it's flowing less air/exhaust.

(The SC only compresses whatever made it past the MAF, which is reduced with blocked cats.)

You'll probably only see it at or near WOT.

Newer engines (OBD II etc) may show codes, sometimes weird ones.

A dyno would show the reduced power, though you have to figure the cause.

Also, MAF at WOT would be down - but you won't know unless you have values to compare against. (I suppose MAP would show it, too.)

On a 928 you may have less data / it's harder to come by.

 

Problem fixed. Or at least sort of...

Replaced cats, and also replaced Supercharger bearings.
Expensive, but problem is solved. Performance back to normal (that is, amazing!)

My new concern is now a subtle "flapping" noise, when at idle.
The noise is stronger when in reverse.
This car is going to drive me crazy...

 

Thanks for reporting back.

Another case of choked cats. I think as time goes by we will see more and more of these....

It's always the way though, you fix one then and then something else pops up.

Flapping noise... is it a metallic sound?

 

I am not sure whether I would describe it as metallic...
But for sure it happens at idle, especially when I put reverse...
When in Neutral (and idle), it is less intense.
I think basically the more "load" there is on the engine, it gets slightly louder.

When driving, I do not hear it though. And the more I drive, the more it fades (ie driving for 1-2 hours, and then reversing).

My first thought is the timing chain tensioners...
I don't know whether I should ignore it, or not...

Mechanic said it's "synchronizing with the new cats and produces this noise"... Sounds though like a bunch of bologni...