B18c7 -> 100/200 cell cat

[rhys.l]

I was told by a mechanic a 200 cell rather than 100 cell cat would be better suited to a b18c7 as it is a high compression engine and some back pressure is a good thing.

Good advice?

[TODA AU]

I was told by a mechanic a 200 cell rather than 100 cell cat would be better suited to a b18c7 as it is a high compression engine and some back pressure is a good thing.

Good advice?

Not bad, but not entirely correct either.
Best analogy is if there was a strong vacuum force applied to the exit end of the exhaust system, the engine would make more power. Other effects aside, the obvious one is a significant reduction in pressure. But the power gain isn’t from that, it’s from the increase in velocity.
& it’s the maintaining of velocity that is critical in any engine.
Too big an exhaust & you’ll lose velocity, this is where the “you need some back pressure” argument comes from. So if the cat is too big, you’ll lose velocity & thereby horsepower too.
So the argument is sound when understood, just the term “back pressure” & the need for it is not thought through thoroughly.
(Now say those last 3 words as fast as you can 10 times… LOL)

Regarding your cat dilemma; If the engine is a std B18C7, there will be little difference whether you use a 10cel or 200cel cat. (Vs. each other) Both would be an improvement from stock assuming the pipe size is 60mm~65mm & the cat body isn’t larger than say 4~4.5” in diameter.
& FWIW, we use 100cel cats where ever possible.
Hope that helps

[rhys.l]

Thanks mate, appreciated. It is a relatively standard b18 and the cat I'm interested in is a 2.5" in/out magnaflow with a 4" body so it's looking like a good choice