X pipes work by using the crossover, or X, to draw gasses from one bank using the venturi effect of the other side. The effect is to help the engine ‘breathe’ more easily and is very noticeable above 4000rpm when the balancing butterfly in the manifold opens. There is an increase in power and another benefit is a subtle and very pleasant change in engine note.
'X’ over pipes were discovered during the Winston cup race series. One of the cars seemed to be consistently faster than anything else out there, and it wasn’t until the car in question had a racing accident and turned over that the reason became obvious. ‘X‘ pipes!
We have developed our own version of X pipes involving many iterations and hours on a dyno rig resulting on the design you see.
Part of the problem is in the existing CATs. On later 928s (from 1990 on) CATs are now at least 25 years old, and sometimes over 30 and may have done over 200,000 miles.
The ceramic core of the cat breaks up, and, along with exhaust by-products the cat becomes blinded and effectively chokes the engine - we saw a power drop of 60bhp on seriously blinded cats. Thats more than an old Mini developed in the first place!