radek, I have a 2000 5.0 and I just had my exhaust put on a few months ago. The guy at the muffler shop I was at actually offered to let me try out both a 40 and 50 series muffler, but I wasn't able to since the muffler I needed (a single 3" center in/out) wasn't in stock.
So, since my Explorer is a Limited, I knew I didn't want it to be too loud. However, it is rather sporty for an SUV so I definitely wanted it to sound powerful. After a few days of thinking about it, I decided on the Flowmaster Deltaflow 50 Series. (My full system includes a Flowmaster 3" Y-pipe from the catalytic converters, to a single 3" Flowmaster 50 Series muffler, a single 3" pipe and 3.5" tip) The first time I started it up I was very happy with it. In my opinion its perfect, that is perfect for the way I wanted it to sound.
The 40 Series is more aggressive, whereas the 50 Series is more of a deep powerful sound, with less droning at highway speeds. My Explorer sounds awesome when I'm accelerating, but at 70mph its almost unnoticable.
Then again, if you have an Explorer Sport, than you obviously have a v6. My old '91 Explorer with the OHV 4.0L had just your average Turbo Muffler with dual outlets and dual 2 1/4" pipes with Borla intercooled tips, and it sounded good I'd say. Not very loud at all, not really THAT much louder than stock (although definitely noticeable when accelerating).
Personally, I don't care for the sound of Flowmasters on a v6 (I'm not saying they sound bad, but I think other things can sound better). For example, Flowmasters are generally going to be fairly loud on a v6, and while a v6 doesn't sound like a weed whacking 4 banger, if your exhaust is loud enough it might sound similar...
I don't mean to be rambling on and on, but I'm just trying to help you out as best I can, because I've been in your situation with both of my Explorers. I also work at a Ford Dealership and I've seen A LOT of trucks come in with custom exhaust systems. Some sound good... and some sound like echoing farts... (strange analogy isn't it?)
Anyway, if I were to put another exhaust on a 4.0L Explorer, I might try a Flowmaster Deltaflow 50 Series, but definitely not a 40. But don't be afraid to get a lesser known brand or even an off brand. Your best bet is to find a good honest (tough combo to find these days) muffler shop and talk to someone there. Tell them what you want your truck to sound like, and if they're good, they'll make it sound just like you want it.
Just don't buy a 50 Series thinking its going to be barely louder than stock, because you will be surprised and may end up being unhappy with your purchase.
/\
longest reply ever.
Sorry for taking up the space folks, just trying to do what I can. Hope that helps!
