More C-85 questions

[flybuoy]

Just wondering …
Assuming a well-maintained C-85 with good compression …
What static rpm with a 7148 prop are you getting?
What static rpm with a 7146 prop are you getting?

Also, what rpm do you usually use at cruise?

[6597]

Here's some useful info off the Type Certificate, but I don't have a C85. These are your limits.

For C-85 Engine Installation
1. Propeller - Sensenich 74FC-47, 74FC-49, 74FK-47 or 74FK-49 or any other approved fixed pitch wood propeller which is eligible for the engine power speed and which meets the following limits:
Static rpm at max. permissible throttle setting:
Not over 2160, not under 1885.
No additional tolerance permitted.
Diameter: Not over 74 in., not under 72 in.
Not eligible on seaplane models.
2. Propeller - fixed pitch metal - McCauley 1A90-CF, 1A90-CH or 1B90-CM
Static rpm at max. permissible throttle setting:
(a) Landplane: Not over 2300 not under 2100.
(b) Seaplane : Not over 2225 not under 2200.
No additional tolerance permitted.
Diameter: Not over 71 in., not under 69.5 in.
7. Propeller - fixed pitch metal - Sensenich M74CK-2
(Eligible on landplane only.)
Static rpm at max. permissible throttle setting: Not over 2250, not
under 2150.
No additional tolerance permitted.
Diameter: Not over 72 in., not under 70.5 in.

[6643]

Unfortunately, I suspect the difference between props will be less than the difference between tachs.

I would say that if the engine has good compression and runs smoothly at all RPM and meets the static limits, it's probably a good engine. If it has good compression and runs smoothly at all RPM and doesn't meet the static limits the first thing I would check is the tach. Second would be the ignition timing.

Static RPM is most useful comparing your engine to itself, i.e. if it suddenly has lower static than it has had historically, you might want to find out why.

One more thing: make sure you know what prop you are really dealing with. A CM7148 may be only 69.5" long and that will make a difference, too.