OK, think of it like this. As the gases cool they slow down so the further down the exhaust the cooler they get and therefore the slower. By expanding the exhaust further down the exhaust as some people do they are slowing down the exhaust gases even further. In actual fact you could reduce the tailpipe dia. slightly compared to the rest of the system to keep the gas velocity up. Think of a hose with water coming out. If the size of the hose is increased the same amount of water will still come out but it now comes out with less pressure. By wrapping the exhaust he has kept the heat into it and that keeps the velocity up. This helps with cylinder scavenging and helps to draw through the fresh charge to help fill the cylinder for the next burn. If you can get a better fill in the cylinder you get more power which is what a blower does on a bigger scale. The downside to wrapping is that it can cause corrosion to steel pipes.

hey! what happend to BIG muffs at the end of the exhaust system? i think they are cool

i think i will get some wrapping for my exhaust when i change it, i guess the manifold won't hurt by rust?

i know that a dyno test gives you a clue of what you are doing, but dyno testing is expensive.
the most importaint is that it responses when i put the lead shoe on

Actually I think the most important thing when you put your foot down is that it doesn't go BANG 

true, but i've adjusted the chip 2 times after i installed it.

you get a lot of air in when it's -20celcius

at +20c the GV made too much black smoke

cj has it correct.  Back pressure is not needed; velocity is.  Expanding the pipe reduces velocity, which affects scavenging (sucking burnt gas out of the exhausting cylinder before the valve closes).  The size of the exhaust pipe has been selected by the designer as a compromise; he can either optimise at low, medium or high revs.  People who fit big bore systems are sacrificing performance at low revs in favour of performance at high revs.

You can read this in automotive design literature, and an honest and knowledgable exhaust mechanic will tell you the same.  Anyone who tells you he has a big bore system which was an improvement low down either had a badly designed or partially blocked system to start with, or is using the butt dyno to back up his choice.

i spoke to a mecanic the other day, he told me it was safe to remove the cat.

Diesel engines has had the same "smog"? level reqirements the latest 20 years.

then i won't have problems with authoreties