Hi fellas I know the gear ratio in the alpha for the 5.7 is 1.5, However a good a shot @ getting a brand new complete alpha drive for peanuts but it has the 1.82 or 84 ratio know I know its worth it and just swap the gears but what I was thinking was you know how heavy these 268,s are and under powerd with the single SB what do you guys think about leaving the higher gears and making it up in the prop has anyone done this. I currently use a 3 blade 16x16 if any one has please let me know what prop you used and how did it afffect performance..
85 26 DA Alpha Ratio What do you guys think
- Thread starter Jsnap
- Start date
lime4x4
Member
When we first got are 86 268 weekend. It had a 17 pitch prop. The first time taking it out at full throttle it would go 5 mph and at full throttle i was hitting almost 6000 rpm,s. Came to find out owner had installed 1.84 ratio upper unit. It took me 2 weeks of trying different props to no avail.
BonBini
New Member
Based on this quick math: 260hp @5800rpm’s driving a 1.5:1 ratio turning a 15x15 @ 3866 rpm’s should push a 268 35mph.
With a 1.82:1 ratio I think you’d have to use something like a 15X26 prop to choke the rpm’s back to 5800. A crazy pitch prop like that just wouldn’t have enough low end bit to get a 268 moving.
My .02 is I think your on track with swapping out the upper gear set.
With a 1.82:1 ratio I think you’d have to use something like a 15X26 prop to choke the rpm’s back to 5800. A crazy pitch prop like that just wouldn’t have enough low end bit to get a 268 moving.
My .02 is I think your on track with swapping out the upper gear set.
sprink56
New Member
- Oct 28, 2008
- 1,288
- Boat Info
- 1979 255 Liberator
- Engines
- twin Mercruiser 450 HP I/O on Bravo I Drives
I have done it on lighter boats (23') with success but be prepared to mess around with props.
Going from 1.5 to 1.82 is a percentage swing of 21% so theoretically a 16" pitch would need to be replaced with a 19.5" pitch prop to make things even speed and RPM wise.
From my experience you will not be able to get an aluminum prop to work properly due to it's flexibility. A stainless is an absolute must.
In your case you might find a Stainless MerCruiser Mirage in 19" pitch to be the right wheel. This is a guess but a pretty good starting point. You might also find advantage in a 4 blade stainless in the same pitch. Forget about a "cleaver" design...they don't work well on a heavy boat and tend to cavitate.
Keep in mind also that if the new drive is a Series II Alpha, you will need to modify a few things to get it to work with a Series I Gimbal Housing. The Series II is a bit wider than the Series I and you have to grind about 1/16" of case in a small area on each side to create clearance (don't freak out...there is plenty of thickness in the area that you need to trim). You also need to replace the top shift lever in the Series I Gimbal Housing to one from a Series II in order for it to couple to the Series II intermediate shift shaft. The main shaft splines and shaft are the same so the two get married easily. MerCruiser says that you can't make this work but I have done it and can show you a running boat that it works on...I was just on it this past weekend.:thumbsup:
Going from 1.5 to 1.82 is a percentage swing of 21% so theoretically a 16" pitch would need to be replaced with a 19.5" pitch prop to make things even speed and RPM wise.
From my experience you will not be able to get an aluminum prop to work properly due to it's flexibility. A stainless is an absolute must.
In your case you might find a Stainless MerCruiser Mirage in 19" pitch to be the right wheel. This is a guess but a pretty good starting point. You might also find advantage in a 4 blade stainless in the same pitch. Forget about a "cleaver" design...they don't work well on a heavy boat and tend to cavitate.
Keep in mind also that if the new drive is a Series II Alpha, you will need to modify a few things to get it to work with a Series I Gimbal Housing. The Series II is a bit wider than the Series I and you have to grind about 1/16" of case in a small area on each side to create clearance (don't freak out...there is plenty of thickness in the area that you need to trim). You also need to replace the top shift lever in the Series I Gimbal Housing to one from a Series II in order for it to couple to the Series II intermediate shift shaft. The main shaft splines and shaft are the same so the two get married easily. MerCruiser says that you can't make this work but I have done it and can show you a running boat that it works on...I was just on it this past weekend.:thumbsup:
Alibuma
New Member
I have a 1985 DA with a 1.5:1 ratio and a 15x17 prop. WOT is 27mph at 4000 RPM. The boat is a pig with the prop and can barely get up on plane. It came on the boat but I am replacing it this summer with the factory standard 15.25x15.
Have you played around with other props to arrive at the 16x16 you have on there now?
Have you played around with other props to arrive at the 16x16 you have on there now?
