I have a C-75 with a 1B90/CM 7351 prop and dutifully cruise at 2275. I consistently get around 93 mph speed. Flying the other day I noticed I was getting around 105 mph then looked at the tach and I was running around 2,500 rpm. Now the original idea from ERCO was that you could get more performance by running the C-75 at a higher rpm without having to go to have a more powerful engine but the marketing department said people want horse power.
Then I ran into another forum that said the proper McCaully prop should not be run above 2275 and that McCauley props have a different vibration signature than the original wooden prop so I decided to do some sleuthing and found this:
TYPE CERTIFICATE DATA SHEET NO. P-842
rgl.faa.gov/Regulatory_and_Guidance_Libr...EBDF/$FILE/P-842.pdf
In effect, the C-75 and C-85 are mechanically identical with the difference being the Venturi and prop diameter. According to the data sheet the C-85 can be run at 2,600 with prop size between 70 to 78 diameter. I would expect the C-75 could be too as envisioned by ERCO to get more performance at cruise.
