exhausts generate back pressure, keeping the gasses in the cylinders for longer. This is
sometimes necessary.
If your engine is not capable of correcting the mixture on-the-fly, then by removing the back pressure (by straightening the exhaust, removing the centre box, removing it entirely etc...) you might end up running too lean. (Too much air, not enough fuel). This will cause your combustion to burn hotter.
If your vehicle is capable of dealing with the increase airflow through the cylinders, then I guess it shouldn't be an issue, and more fuel should be shoved in there with the air to keep the mixture at the correct ratio.
Therefore... if you remove the center box of the exhaust, you will reduce back pressure. you will therefore burn more fuel because the MAF meter will detect more air passing through and will therefore put more fuel in.
If you still have a centre box and a cat, however, but your cat is collapsed and/or blocked, then you will could be running rich (not enough airflow). In reality, though, the MAF meter will detect lower airflow and put in less fuel, causing you to have to rev harder to achieve the same goal.
So... with a collapsed/blocked cat, you are having to push the accelerator harder to achieve the same goal (burning more fuel), and with no cat and no centre box the airflow is increased and you are using more fuel, but not necessarily with any power gain.
With the cat removed, but the center box still in place, you are allowing more air to pass through, but not
too much, so making the balance a little "nicer".
I think this makes sense, anyway... from my basic understanding of the four stroke cycle!!
Oh, and i'm no expert on landy's in particular, nor engines - this is just from my understanding, so please correct me if i'm wrong!! I'm keen to learn here too
