When my dad bought his 1688 4 years ago, it came with a 1680 manual, and although it is uselful for most of the combine, there are a few things that we would still like to have the proper information for. One of the biggest things is the factory engine speed. The guy that dad bought the combine from had it sped up to almost 2800 rpm and from what I can find after hours of google searches is that the 1688's were turned down to 2200 rpm. We did turn it down to 2300 but seems like we loose too much rotor and fan speed (rotor maxes out at 940 and fan at 950 when running 2300engine rpm). So we need to find out if something else has been changed (eg. slow down kit). If anyone has a manual they would like to sell or possibly give some elaboration on how these machines were set up as apposed to a 1680 would be tons of help. Thank you in advance!