CPSI is the density of the cells per square inch.
Cells are important, but you should consider the quantity of noble metals on the matrix too! That's what makes the cat expensive or cheap as Rhodium, Palladium and Platinum are not cheap.
I think the top brand is the German HJS, in the Fia GT, WRC and other international race champioship where the cats are compulsory they exclusively use HJS.
If the quantity of noble metal is insufficient you get CEL and you don't pass emission tests.
The diameter and lenght is important too...as with the same cells density if you have a larger cat you got more cells, obviously.
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