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I decided I wanted my GPZ to sound like a "real bike" so I have modified the stock exhaust to have a different note and be louder. It's nowhere near Harley loud and it only sounds a little bit dirty so I'm happy with it. It took quite a lot of tinkering to get it how I wanted as with the baffles completely removed it was too loud and poppy (not that i could completely remove one of the baffles anyway!
All of my inspiration and help came from here http://www.ex-500.com/index.php/topic,1721.0.html, here http://www.ex-500.com/index.php/topic,7122.0.html and someone here I can't remember who told me to buy a slide hammer (I can't remember who).
The first thread has loads of info but it also has gone off into loads of random side issues and is currently 13 pages long! I copied all the info and pictures from it that looked like they would be of any use and saved them so I didn't have to wade through loads of dross. It's attached to this post as a PDF file if anyone wants it.
Firstly I removed the cans from the bike and then drilled out the three rivets holding the end can off (this was pretty easy but you have to be careful the drill doesn't slip and mark the can.
Then I used an angle grinder to grind down quite a lot of metal until the outer end of the baffle was loose and could be wobbled around. I spent ages messing around with the first one trying to get it out and in the end it broke at either end when I pulled it out with pliers (after I had pulled out the fibreglass insulation separately. The proper way to get it out (which I did on the second) was to get a pair of locking pliers, then lock them to inner part of the baffle, then attach a slide hammer I bought for about £10 to the end of the locking pliers, then bang away on the slide hammer until the baffle just popped out in one piece. You can see the mangled baffle below:-
I was left with one baffle snapped off inside and one completely removed. Luckily I realised that it sounds waaay too loud with the baffle entirely removed so I chopped off the bottom end of the baffle with a hacksaw and shoved it back into the exhaust on the end of a long pole. This meant that exhausts were deeper and louder but not straight through as some people have.
This shows the remains of the broken baffle and the one that came out in one piece:-
This shows where I cut the good baffle to get the little piece to shove back into the can.
I then decided I wanted a bit more depth and volume so I drilled 4 10mm holes around the end can and then put the end cans on. You can see the holes here:-
You can see the right exhaust can here with the snapped off baffle end:-
You can see the left exhaust can here with the sawed off baffle end stuck back in:-
Here is a picture of the completed exhaust:-
I found that after doing this, my idle speed rose quite a bit - to around 1500 rpms at high engine temnperature after this mod. This is due to there being less resistance for the expelled gasses etc from the engine. This actually improved the reponse-time I got when twisting the throttle but it also meant that i had to buy petrol 30 miles sooner than I should have. It actually ran out completely at 210 miles when I normally fill up at 200 miles and hit the reserve level around 205 miles. I was lazy and just adjusted the single throttle stop knob under the carbs to bring it down again. I'm not sure yet if this has bring my miles per gallon down. It is probably running a bit lean and I will check the spark plugs in a few months. There are no flat spots or lowered performance however.
One of the things I did that I think will be of most use to other people was record the sound levels the different setups produced. I just used my phone placed just underneath the rear wheel with the microphone pointing up so it wouldn't be drowned out. I made it into two videos with text explaining each sound as it is played. They are linked to below:-
This video just shows the difference between my stock setup and the new setup:-
This is a longer video that shows all the different options I tried and just for a laugh - the bike with no headers at all:-
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