I am rehabbing a 1980 25 ft boat that has a Volvo Penta 290 sp outdrive.
1980 is a bit too early for the 290 drive. If this boat does have the 290, it was very likely a retro-fit.
No identification of an a, b,c ect.
During this year model, the B, C or D is used as a reference to the final drive reduction.
In order to learn what the Duo Prop ratio is, you will need to remove the props, remove the ring anode and wire brush the AFT most area of the prop shaft bearing carrier and expose several numbers.
You will see one of the following set of numbers:
2.30
1.95
1.78
This is ONLY good in the event that no one has replaced a DP gear set and has failed to "re-stamp" the carrier!
I am looking at replacing it with a 1989 290dp. Would the transom shield cutout be the same?
If the boat had been retro-fit with a 290, your proposed 290 Duo Prop system will fit.
the only designation I have for the 89 is an aq271dp, and it is a 290dp
"AQ" generally refers to the engine model.... such as AQ225, AQ260, etc.
Have you seen a PZNR # on the transmission (upper gear unit) ?