After writing about the 2000cc thumper of Franz Langer, I thought that was about as big as you go with singles and we could move on, but when records are involved, it's never quite that simple. A comment on the first post directs us to Claus Mees who has that little 2 liter thumper beat with this 3034cc monster which is called the HG3000.
There's BMW suspension bits, some Volkswagen and Porsche parts, plus contributions from a tractor as well. The whole page describing this build is in German so I'm a bit limited on what I can tell you but check it out for yourself. According to the writeup, it actually runs but I couldn't say how well. The whole project seems to have been the result of a bet, the wager, a bottle of beer. I wonder what Claus would build for a 6 pack?