Do the 100 cell cats effectively remove the smell and drone? Or will I need 200 cell?

i don't think you can remove the drone, the C63 mufflers are too small to eliminate any drone...but the more cell's the more the smell is reduced.

From what I heard, the higher the cells, the more filtering (and less sound)

I think most of us running HFC's have 200

Generally, the higher the CPI count, the more effective the chemical conversion of exhaust gasses. Although, some performance cat manufacturers use some more exotic materials and/or manufacturing processes that can give higher conversion with lower cell count, or improved flow with higher cell counts.

I've only used one 100 cell cat worth a damn, but we stopped using them because they were ungodly expensive, lead-times were sometimes horrendous, and in some applications they could still smell some.

We've always used HJS 200 cell units for Porsche stuff, but are going to be trying a different brand, 400 cell count that claims to support some crazy power level. We shall see.

My mileage is getting high so I think I’m past the point of making crazy power, would the HJS unit be fine for a stock M156?

don't ever doubt the ability of the M156 engine. it is well engineered and my own at 155k is making more power than it ever has reliably and without burning any oil. I drive this thing hard every single day.

Remember, cats aren´t silencers.
They do very minor thing to the sound.

My dB meter would argue this after I removed my primaries...

Ya totally agree with this, stock primaries kill like 50% of the noise, everything else no change went from secondary cat delete to removing stock primaries and replacing with 200 cell keep the rest the same and waaaaay louder.

Stock Cats 100% decrease noise level there are some race cats which don’t reduce sound levels too much

I remember i tested my old c36 with cat and no cat, almost no difference in sound, also performance, no difference.
And many friends cars showed the same result.

But, there are always exceptions.,

I would imagine that with modern era cats the power difference with or without is negligible. I would also imagine without cats it would be a bit louder

The only time I ever burn oil is during the winter, and it’s only about 0.25Qt to 0.5Qt.


Maybe I will get a leak down test and see how things are going.

Yes the exception could be the 6.2L
From my experience the drone can be killed by the factory resonator if put back in.
And the volume can be reduced by 200cell cats.