I think my old 1994 CDS ran at around 2100 rpm. I'm not positive, you'd better call Hydramaster and ask.
Not to hijack the thread, but can someone tell me if the 4.2 CDS is a dual operation unit? I have looked at the manual on the Hydramaster website which mentions dual wanding but it is a combo manual for the 4.2 and 4.7 CDS models so I am not sure if it applies to both. Thanks! -Robert