The whole point of funk music is finding a groove and working it, but too many really funky songs quit just as they're getting started. Stevie Wonder is especially guilty of this in the recorded versions of his songs, and many other songs are only super funky in longer live versions, as opposed to their somewhat stiff studio recordings.
So I thought I'd put together a list of a couple of really long (over ten minutes) funk songs that find a groove and then work out every possible variation without ever getting boring. Feel free to add in your own suggestions.
First is, naturally, the Godfather. James Brown's "Papa Don't Take No Mess" is 14 minutes of the best laid-back funk groove ever recorded. Though the song was originally released on Hell, that album's a bit uneven as James can't decide between trying to make his own soundtrack to Shaft and trying to update a few of his then-recent hits as salsified workouts. If you don't already own it, then I'd recommend picking up Star Time, which is still the definitive James Brown box set. The broad range of material in the box set helps make it even clearer what an accomplishment "Papa" is, not just for the formidable bass line and the signature guitar part, but for Jabo Starks's elegant and subtle drumming.
Next up is Funkadelic, with perhaps their best-known single, and one of the few moments where they unequivocally outshined sister group Parliament, with the track "(Not Just) Knee Deep" off of Uncle Jam Wants You. Though the album was ostensibly Funkadelic's attempt to be political and relevant, the song (fortunately!) isn't weighted down by any politics and is content to just be stupid fun for about a quarter of an hour. The album version of the song is sometimes misattributed as an extended version on various compliations, but you don't really need to bother with such things as the original Uncle Jam album has now been competently remastered on CD and has the whole song in its uncut version.
And I can't close a piece on funky tracks without mentioning Prince. During the nadir of his troubles with Warner Brothers over his recording contract, the little man was getting fairly creative, even by Prince standards, with the attribution and credit on his albums. So he released an album under the name of his New Power Generation band, entitled Exodus, which closes with the epic "The Exodus Has Begun". This 10-minute-plus jam has Prince and his band channeling P-Funk more explicitly than ever before, and has perhaps the densest mix of any track he's ever released. Unfortunately, the album was never formally released in the U.S. but if you're outside of the States it should be fairly easy to find, and Americans can dig around at Best Buy or Amazon, both of which regularly feature cutouts of the disc for around ten bucks.