For the most part all we have did so far with the CDF is remove reverse bars to install forward and remove all knifes. In an effort to get one to work as well as a sweeped up eight bar in edible bean I installed all forward bars, installed all knifes then took them all out. Didn't work so I had to custom build some sweeps to fit. Before even putting one of these in the field I install a minimum of an economy version of steep pitch thresher helicals (do that with any design of rotor). This has worked just fine for us so far but will be adapting more tricks as I hear of them.
Today I get a call from down south and it promted me to go ahead and answer your question I have been pondering for a couple days. He recently added a R75 to his allready hyped up R72s that had eight bar rotors with sweeps. 75 was all stock with a CDF in corn. Complaint was machine would be going along and then just pull down. He flipped the door open to peak at rotor and found left side of concave and right side of seperator grate packed with junk. This is exactly why I suggest steep thresher helicals and get rid of most if not all reverse bars.
Now I will share what I have learned from guys that have did more tweaking with CDF.
First I hear of some guys shimming out thresher bars by 1/2". This got the thresher side back out to original 25" diameter of the eight bar rotor. I believe this was mostly done for small grains for it would give a better fitup of whole concave to cylinder instead of just front half.
Second I hear of leaving thresher cylinder bars alone but installing steep thresher helicals shimmed out from 1/4 to 3/8". This has worked well in all crops but can likely be improved on.
Third is same as above and also shimmed the thresher cylinder bars by 1/4". This guy ran nice variety of crops including corn and didn't see any drawbacks.
Fourth is the steep thresher helicals as above shimmed out by 3/8" and no shimming of all forward thresher cylinder bars, removed every other bar in row B which is just to left of concave, removed every other bar in row A which is just next to discharge paddles, cut and made three two bolt reverse bars and installed one right next to discharge paddle, one in middle location of row A, and the third at right location of row A. These two bolt reverse bars were shimmed out by 3/8". This setup worked extremely good for the guy but he expanded on this by going with all shimmed out two bolt bars on seperator side. This gave him a setup of all straight bars over concave and maximum room by leaving at 24" diameter. Flow still controlled very well with the shimmed out steep helicals. The missing sections of bars on seperator side gained him room there while still providing positive flow and some mixing plus slightly higher tip speed of rasp bar. Short rasp bars overlap some from position to position so material that happens to slip around the edge will still get hit with next bar. A couple of the short rasps that end up overlaping concave a bit would have to be trimed to prevent concave contact at zero clearance. This setup has given the guy tremendous performance in barley, wheat, lentils, and peas. This guy had studied the good bad and otherwise of the Gleaner eight bar, Gleaner CDF, Sunnybrook gen 2, St. John, PFP, and Bison to help him deside what he wanted to give a try. He had been away from Gleaner for nearly twenty years and now is extremely happy to be back.
Now if I were to put my own twist on the whole deal I would set up the same no matter which crop including corn. First I would install steep thresher helicals shimmed out 1/4" and spaced apart an extra whole as I moved toward discharge. First helical would start against the gearbox wall and line up with helical on triangle across feeder opening, next helical would start one hole to left of original second helical and run paralell to first helical (this will preserve the distance between helicals that original ones had) and so on with two more long helicals and one medium length. Don't forget to trim triangle over feeder to prevent blocking flow from feeder and reposition that helical accordingly. Then I would make up 1/4" shim plate for the whole length of cylinder bars. Install all the forward thresher bars (maybe everyother or all narrow rasp if in tough to thresh small grains). Then use all forward two bolt bars on seperator side in a steggered setup. I would also want the high wide wire seperator grate to get wires within 1/4" of top of crossbar. I noticed a couple R66s had line bored and very sharp leading edges on seperator grate cross bars and I would prefer to grind leading edge on them to prevent posibly busting corn cobs or hanging material excessively. I believe this would make a very nice all around rotor and would have to give a try sometime. A guy could still easily install a couple or more reverse bars if ever needed and there is a guy creating some sweeps that will fit between the bar mounts in area of knife mounts so more options or attachments coming.