I recently bought an '88 635 CSi, and would like to grab some M6 parts to improve performance. For example, replacing the SOHC with DOHC, installing an M6 throttle body assembly (individual throttle bodies per cylinder), etc. I know a lot of E30 guys like to grab M3 parts for their 318's and 325's, is this still feasible for the E24 6 series?
BMW E24 6 Series question?spyware remover
My thought is that it is not very feasible upgrade, and I do not believe I have seen anybody do it.. The SOHC and DOHC are different engines by a lot of internal pieces, including the engine management unit, exhaust, and lots of little things, as well as the M6 pieces tend to be significantly higher and grabbing them is not an easy or inexpensive proposition. If your pockets are deep, nothing is impossible, but you would really be better buying an M6 and getting all the parts and packaging as I would bet it is easier and cheaper in the longer run. The M6 was about a $57K car in 1989 and the 635csi was $47k, so they are relative bargains and still popular with enthusiasts, and they keep their values pretty well.
If you can not find a M6 as your add on indicates, use the 635csi as the learning tool as you gain experience in doing maintenance and fixing the common issues within the series. You really do not want to have more money in a 635 than an M6. Also, consider looking beyond Hawaii for prospects if you have been finding the market too limiting. Happens all the time, you just need to look beyond your existing box!! Good luck! The M6 is a classic and timeless design.