Edit: Just to shorten it down a bit
http://forums.mg-rover.org/showthread.ph...ost1885537
"With respect to the F/TF, then the space available limits primary and secondary pipe length to being less than ideal; tertiary pipe length availability is even worse (especially on non 'purpose fabricated' systems). This causes a limitation in how efficient are the after-market systems (in terms of tuning). The VVC engines will suffer most in this respect because they have a hotter inlet cam timing than do the MPi's. What seems to be happening with the power loss on de-catted VVC motors is that the tertiary pipe length is far too short (hence the drop in power), whereas, on their catted (or sports catted) cousins, the actual distance the gas flow (and therefore the standing wave) 'sees' is longer (because of the nature of the flow through the cat), hence catted VVC motors do better than de-catted ones."
The non vvc apparently do better without a cat, it's just the 200 shell that does better with one
http://forums.mg-rover.org/showthread.ph...ost1885537
"With respect to the F/TF, then the space available limits primary and secondary pipe length to being less than ideal; tertiary pipe length availability is even worse (especially on non 'purpose fabricated' systems). This causes a limitation in how efficient are the after-market systems (in terms of tuning). The VVC engines will suffer most in this respect because they have a hotter inlet cam timing than do the MPi's. What seems to be happening with the power loss on de-catted VVC motors is that the tertiary pipe length is far too short (hence the drop in power), whereas, on their catted (or sports catted) cousins, the actual distance the gas flow (and therefore the standing wave) 'sees' is longer (because of the nature of the flow through the cat), hence catted VVC motors do better than de-catted ones."
The non vvc apparently do better without a cat, it's just the 200 shell that does better with one
![[Image: mcetg0.png]](http://i57.tinypic.com/mcetg0.png)