Just found this SB from Continental and talked to them - they are now recomending not less than 2300. Interesting read - it is CSB09-11
SUBJECT: MINIMUM CRUISE RPM LIMITS
PURPOSE: To inform operators of the possible long term effects of low engine RPM in cruise conditions. To establish limitation of minimum engine RPM in cruise.
COMPLIANCE Upon issuance of this bulletin
MODELSAFFECTED: O-470-G; IO-470-N; IO-520-BB, CB, MB, P; IO-550-A, B, C, D, E, F, G, L, M, N, P, R; IOF-550-B, C, D, E, F, L, N, P, R;
TSIO-520-AE, BB, BE, CE, DB, EB, JB, KB, LB, NB, UB, VB, WB; LTSIO-520AE; TSIO-550-A, B, C, E, K; TSIOF-550- J;
TSIOL-550-A, B, C
Teledyne Continental Motors (TCM) has examined recent occurrences of crankshaft counterweight release and subsequent engine stoppage in two high time IO-520 and two high time TSIO-520 engine models. Investigation and reported service history lead us to believe that these occurrences are associated with engine operation at sustained cruise engine RPM of less than 2300 RPM. Power settings of less than 2300 RPM have been within the recommended cruise range allowed by TCMa