putting aftermarket radios in newer german cars is a pain in the ... not to mention the stock units are so nice you'd want to put in something equivalent. which would easily run north of $1k just for the headunit.
I'm not familiar with the cls models but for Mercedes the general rule of thumb if you want more power... is to sell and buy an AMG. There's an extremely limited amount of performance parts; and an even smaller group of mechanics who know what they're doing. Your cost/hp ratio will be insane, and you're not likely too see significant gains. It's possible, but unlikely to be forced induction kits, if there are "bolt on" FI kits that's the route to go. Either way you REALLY have to search and find a specialized and qualified MB mechanic, the dealership mechanics will likely be clueless.
There's likely to be a couple extremely expensive exhaust kits, you'll have to do the research. Having your resonators removed will make the exhaust more lively and aggressive, also likely to be a couple different mufflers that will improve the sound. don't expect and horsepower gains, even from a $7k remus exhaust.
there is a wide selection of rims, 5x112 fits all MB's, vw's and audi's. once again you'll have to do the research and figure out what sizes/offsets fit your cls. having your roof painted black, or buying amg bumpers/rockers for your car is a great way to change it up. just make sure they're QUALITY, 99% of all aftermarket bumpers fit horribly and are junk.
tried to keep it as short as possible. I'm clearly not familiar with the cls models but that's a good "rule of thumb" for all Mercedes. If you're absolutely set on changing your car up be prepared to get out your wallet. Mercedes are built extremely well, and don't use cheap parts. Treat your benz well and it'll return the favor
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