My lower is a 2:1 ratio. So would the 2.0 ratio upper work fine with no change in the prop? Boat was running around 6000 rpm with the 1.65, so I assume that'll drop with the correct unit.
So, if your lower is a 14/28 box, that doesn't work, because with that lower you CAN NOT have a 1.65:1 overall. None of the upper boxes will give 1.65 with a 14/28 lower. BTW, a 14/28 lower is quite rare. Most shops prefer not to sell them as they are a fully hunting gear set. Setting them up, then repairing them is a pain. They also generate harmonics, which are very undesirable.
The 'correct' ratio for a 3Litre engine is 1.94:1, not 2:1. If you currently have a 1.65 and are pulling 6,000rpm, then putting a 2:1 with the same prop would (in theory) allow the engine to go to 7,200rpm. You'd need to increase the prop pitch to 23" just to hold the 6,000rpm, which is still WAY too much. To get down to the correct 4,400 to 4,800, increase the current pitch to about 25", or you'll need a roughly 30" prop with the 2:1....
Now, my spider sense is telling me something is WAY off with all these numbers, because unless your boat is a 6' flyer, it's not making any sense. First thing you need to do is find out exactly what gearsets you have. The next is to get a shop tacho to check your dash tacho against.
Checking gearsets. I assume you have the top box apart. Easy, just count the teeth. The gear that is on the end of the yoke is the 'drive' gear, the one in the bottom of the housing is the 'driven'.
For the lower, because there are just 2 possible combinations, it's also dead easy. Mark the prop shaft (easy to count the turns). Put it in gear and turn the drive shaft exactly 2 turns. Look at the propshaft. Exactly one turn, it's a 14/28. if the propshaft turned a little over 1 (1.2 to be exact), then it's a 13/21. If you're having trouble seeing the difference between 1 and 1.2 turns, then turn the drive shaft 10 times. Prop shaft should turn 5 times (14/28) or 6 times (13/21)....
Hope this helps...
Chris.......
