Alpha drive for a V8 whats the ratio?

[crazy charlie]

Have twin 5.0 1988 Alpha drives.Have an opportunity to buy a spare drive and it claims
to be a 1.47 ratio.I always thought the V8 was to be a 1.50 ratio?? Charlie

 

[dubs283]

Reason for difference is the ratio in the gear case.

1.47 will swap fine for a 1.50 and vice versa.

Is more important for twin setups to have exact ratio however with the difference being close enough it should work okay. Only item I'd verify with an increase of ratio is total advance of timing with the same prop.

 

[jimmbo]

I/O makers often offered an engine with more than one Ratio, to make it easier to Prop, over a larger variety of boats, mainly to reduce the number of Prop a dealer need to inventory, and to keep the Prop choices closer to where the Dia-Pitch relationship made for better Efficiency.
Both Drives need to be the same Ratio, as any mismatch will create a variety of problems/issues.
Close only works with Horseshoes, Grenades, and Nuclear Weapons

 

[Bondo]

Ayuh,.... I disagree Jim,.... The 1.47, 'n 1.50 are the same,.....

 

[dubs283]

The 1.47, 'n 1.50 are the same,.....

same...ish

Exact ratio is (example) 1.47 engine revolutions per 1 (one) revolution of propshaft.

Again, only caveat I have is if using the same prop it's best to determine the engine isn't consuming itself from too much timing advance

 

[crazy charlie]

Ayuh,.... I disagree Jim,.... The 1.47, 'n 1.50 are the same,.....

thanks Bondo ,thats essentially what the seller told me and I just
needed to confirm as I did not know that..Charlie

 

[Scott Danforth]

5.0's usually come with 1.62:1 drives

5.7's come with the 1.47:1

there actually isnt a 1.5:1 drive. it has always been 1.47:1 for them

Horsepower​	Engine​	Year​	Ratio​
120 thru 140	4 cyl. One drive	1971 thru 79	1.94 / 20-24
170 thru 190	4 cyl. One drive	1971 thru 79	1.81 / 17-19
165	6 cylinder	1971 thru 79	1.62 / 24-24
188 thru 260	V-8	1971 thru 79	1.47 / 20-22
120 thru 140	4 cyl. One drive	1980 thru 84	1.94 / 20-24
170 thru 190	4 cyl. One drive	1980 thru 84	1.81 / 17-19
165	6 cylinder	1980 thru 84	1.62 / 24-24
188 thru 260	V-8	1980 thru 84	1.47 / 20-22
120 & 140	4 cylinder	1985 thru 89	1.94 / 20-24
120 & 140	4 cylinder	1985 thru 89	1.94 / 20-24
170 thru 190	4 cylinder	1985 thru 89	1.81 / 17-19
170 thru 190	4 cylinder	1985 thru 89	1.81 / 17-19
198	V-8	1985 thru 89	1.62 / 24-24
198	V-8	1985 thru 89	1.62 / 24-24
198 thru 260	V-8	1985 thru 89	1.47 / 20-22
198 thru 260	V-8	1985 thru 89	1.47 / 20-22

 

[jimmbo]

Ayuh,.... I disagree Jim,.... The 1.47, 'n 1.50 are the same,.....

No , if one drive says 1.47 and another says 1.5, they are not the same

 

[Bondo]

No , if one drive says 1.47 and another says 1.5, they are not the same

Ok,.... They're Functionally, the same,.....

 

[jimmbo]

They are close, but keep in mind there is another drive, next to it, and that difference, however slight, may result in issues

 

[QBhoy]

I always thought that an older era alpha may have came in 1.5 ratio. Then later replaced with the 1.47 ratio, sometime in the mid 90’s or close.
Anyway, it’s unlikely that you would currently have either, mated to your 5.0…unless it’s the rarely seen combination of it and a 5.0 mpi.

 

[dubs283]

I always thought that an older era alpha may have came in 1.5 ratio. Then later replaced with the 1.47 ratio, sometime in the mid 90’s or close.

Yes

R/MR/Alpha One were only offered in 1.50, not 1.47

Gen 2 had both ratios available

I reread OP and see 1988 which would be in the vintage of Alpha One and if oem wouldn't be available in 1.47.

Guessing OP is looking at a gen 2 for replacement which wouldn't work on an Alpha One bellhousing

 

[QBhoy]

Yes

R/MR/Alpha One were only offered in 1.50, not 1.47

Gen 2 had both ratios available

I reread OP and see 1988 which would be in the vintage of Alpha One and if oem wouldn't be available in 1.47.

Guessing OP is looking at a gen 2 for replacement which wouldn't work on an Alpha One bellhousing

I’d say you’d be right about the above. But I still think it unlikely he would have or be looking for a ratio like that. Could be wrong. The exception might be twin engines. Not sure I’ve heard of 5.0 having as high a ratio, unless it’s an mpi and a performance hull.

 

[jimmbo]

It would have to be a very light hull to use a 1.5 on a 5 liter, otherwise props in the 14 - 17 inch pitch would be needed. The 1.5 ratio was more commonly used on the 5.7

 

[QBhoy]

It would have to be a very light hull to use a 1.5 on a 5 liter, otherwise props in the 14 - 17 inch pitch would be needed. The 1.5 ratio was more commonly used on the 5.7

100% spot on. I can’t think of one around here, apart from mine that has that ratio and a 5.0. Sometimes wonder if I’d rather a little less on the ratio. Just me idle wondering about the constant hunt for prop perfection though . Would love to be able to run a 23” on mine. But the only one I tried (old school square vent laser 2), just would hardly get her out the water. Not too bad once she was going though. I run one of three props on her. 21” laser 2, 20” enertia or a 19” tempest plus. Might have thrown on a 21” tempest too actually. She will limit easily with the 19” t and the 20” e…with the 21” laser 2 from memory. But don’t think she did with the 21” tempest. 20” enertia is the usual on her. Still goes well enough

 

[Scott Danforth]

Yes

R/MR/Alpha One were only offered in 1.50, not 1.47

Gen 2 had both ratios available

see post # 7 on R/MR/Alpha gen 1 ratios

see below for A1G2 ratios.

The OP has 1988 5.0 liters which should have came with 1.62:1 drives not the 1.47:1 drives

 

[dubs283]

The OP has 1988 5.0 liters which should have came with 1.62:1 drives not the 1.47:1 drives

True that.......

OP hasn't given us much feedback and I have read many threads about alpha drive swaps where information provided includes using a different ratio than was used oem. User is informed they should use a different prop to get the performance close enough to work

OP is a vice admiral which I'm guessing means they know a couple things about boats or at least have been provided a lot of info here. Maybe they tried a different ratio/prop combo at some point?

In any case this thread has some decent information for posterity

 

[Scott Danforth]

With twins, you need to keep them the same ratios and props

 

[dubs283]

With twins, you need to keep them the same ratios and props

Durr....post #2

At this point it's best to recommend OP upgrade to bravo drives/transom assys. After all they haven't said their NOT going to the moon

 

[Scott06]

With twins, you need to keep them the same ratios and props

Ye cant have one with a 1.62 and the other drive a 1.47 ...

If the engines are running 3000 rpm the prop shaft rpm diff between a 1.50 and 1.47 is 40 rpm. I doubt the throttle cables on a 1988 boat are synced that close.

At some point merc tweaked their upper and lower gear designs that the ratio changed from 1.47 to 1.50; 1.65 to 1.62; 1.84 to 1.81 etc. Merc lists them as interchangeable. We are overthinking this...

Gear ratio changes - SEI Forums

Here we will be posting Technical Articles covering common topics.

sterndrive.cc