Ive had a look on some other land rover forums (yes they do exist, really they do 
)
There is talk on them about damage to the valves (I assume in the engine) if you reduce the back pressure in the exhaust by removing the mid section.
Now I know that 2 strokes require back pressure & a specific length to the pipe to give the right frequency to draw gasses out. So mucking about with the pipes can reduce the power in a huge way.
I also know that you may loose some performance on a 4 stroke if you reduce the back pressure too much. Something to do with the way the gasses flow into the cylinders or some such black magic involving complicated equations.
But Ive never heard of it doing any physical damage before, can anyone comment on this? It sounds to me like scare mongering.
Same sort of reasoning as damage on a two-smoke, where the expansion chamber resonanse 'scavenges' the cylinder of residual exhaust gasses.
On a 4-stroke, removing silencing, and reducing back pressure does a similar thing, but as the gasses are mostly pushed out by the piston, so not as critical as a highly tunes stink-wheel.
On a two stroke, the spannies are designed so that as the leading edge of the exhaust 'pulse' hits the change in section of the pipe, it creates a shock wave that 'echoes' back along the pipe, in order to create a vacuum over the exhaust port to help suck the last bits of gas out, then a pressure pulse to push charge being squashed into the cylinder from the crank case ,back in, and stop it escaping down the exhaust while the exhaust port is open.
On a 4-stroke, musch simpler, valve opens, piston pushes exhaust gas down the pipe; reduce the back-pressure, gasses can get out more easily, and travel faster, sucking more exhaust after them, as the piston gets towardss TDC and isn't pushing so hard.
This means tha the exhaust valves get hotter, as they have more hot gasses passing over them while they are sat in the gas-stream unable to 'dump' that heat into the valve seat.....
Main effect, though, is that extra 'scavenging' taking lingering exhaust fumes out of the cylinder, means that the engine effectively runs 'weak' as the mixture is tuned for a burn that's not as clean...
That means knock, it also means higher running temperatures, and associated maledies..... so usual consequence is the hotter running exhuast valves burn out....
BUT, depends on whether the ignition and carburation have been adjusted to suit.
Old 'problem' on Jap multi-cyl motorbikes was that a 'straight through' 4-1 Motad or whatever would be fitted when the Jap pipe rotted out; power would go up from free'r flowing exhaust, but the carburation & ignition wouldn't get set up, cos of the cost and availability of Mikuni main jets, and needing four of them, and eventually exhaust valves would start burning.......
On cars, same affliction, but not so frequent, old mini's had SU carbs that were a lot easier to 'adjust' to get the mixure right to suit the new pipe, as do old rangies.
And on more modern motors, with EFi, many of them will 'self tune' to compensate......
Can screw the emmissions though for the MOT test, though added noise may be more of an issue depending on tester....... My local freindly chappy applies the 'unduly loud' principle, and to him the sound of a V8 is aural bliss.... others may not be so apreciative though!
