the exhaust is the first thing that gets dumped if tuning a VW aircooled engine...
As mentioned there really is so much science in exhaust design, volumes and the resonance within the system also affect the sound produced as the pulses from the exhaust port need be in harmony with the system and make so much difference, bends and elbows are killers to flow...
Whilst you can simply "knock up" and exhaust from off the shelf bends and boxes, it may mean the engine many not perform to it's optimum.
I wasn't suggesting anyone fit a VW exhaust to their 500, rather to have a look at what VW enthusiasts do to upgrade, given similar space restrictions.
There might be a silencer that could be used and fabricate the pipes oneself.
Ever read the book - "The Scientific Design of Exhaust and Intake Systems" by Philip H. Smith and John C. Morrison. Very interesting but a bit too much for my limited brain power.
I wonder how much testing is done on 'Sports' exhausts, especially before i.e. standard exhaust, and after, on
the same engine.. I mentioned comparing the 850 standard exhaust with the 850 coupe 'sports' exhaust.
There wasn't any real difference inside the silencers, just 4-1 pipes which were quite short and reduced down very abruptly to enter the silencer. So not a great design imho. Which is why I suggested fabricating one's own system. You're not likely to do worse and possibly will do better. How much better? As others have suggested, given the small power output, you'll probably only see maybe 1 or 2 hp. I suspect most people fitting a 'sports' exhaust just want a 'sporty' sound. This is fine when showing-off a little in the car park or pulling away from a gathering of fellow enthusiasts but a noisy/droney exhaust will likely become a right pain in the ... head, on a long run. As others have alluded to above.
I'd agree that elbows are not good but you have to have bends to fit on a road car, unless you want to emulate some of the early race/speed record cars which had straight pipes exiting the bonnet....Methinks the police etc. might have something to say if you did this...
I modified an exhaust box on a Fiat 600, back in the day. Tail pipe fell off due to rust. I opened out the hole and welded a larger tail-pipe on in it's place. Gave a deeper, slightly sporty sound but wasn't loud or annoying. I doubt there was any power increase as the rest of the system was the original but I was happy.
. Some people thought the car was tuned due to the sound and way it went, or maybe it was because of the aluminium Fiat 782 badge (in the same style as the Fiat 600 one) on the engine lid.....
(I must look to see if I still have the badge...)
AL.