crank mounted water pump vs leg pump

[tricone]

i just bought a 350 today to replace a dead 4.3, the 350 has a raw water pump on the crank, i,m wondering if i need this pump.
would the leg pump supply enough water for the heat exchanger
Thanks for any tips

 

[Bondo]

Re: crank mounted water pump vs leg pump

i just bought a 350 today to replace a dead 4.3, the 350 has a raw water pump on the crank, i,m wondering if i need this pump.
would the leg pump supply enough water for the heat exchanger
Thanks for any tips

Ayuh,.... What drive ya marryin' it too,..?? a '74 would be a MC-1, Not an Alpha 1...

The gear ratio is gonna be Wrong...

 

[achris]

Re: crank mounted water pump vs leg pump

It's not just about the volume of water, what will be the maximum water temperature you will be operating in? If it's more than 80* then you need the crank mounted pump. If less than 80* then you will get away with just the drive pump.... But as Bondo said, the ratio is not correct for the 350. If the current ratio in the drive is the earlier of the 2 used with the V6 (1.84:1), then you are going to need to change it. If it's the later (1.62:1) you may be ok with just a prop change. If you post the drive serial number we can have a look and see what the ratio was when it was supplied. Or if the ratio is visible on the drive, read that....

Chris.....

 

[tricone]

Re: crank mounted water pump vs leg pump

hi guys the 4.3 and alpha 1 are of the 1986 vintage, the ratio is 1.8 : 1 i'm pretty sure.
I read one of Don S. posts that the crank pump alone would have to much vacum leaks to draw water from the leg.
what would happen with the 1.8 ratio and the v8
thanks i never even though about the leg ratio

 

[Bondo]

Re: crank mounted water pump vs leg pump

what would happen with the 1.8 ratio and the v8
thanks i never even though about the leg ratio

Ayuh,.... Basically, the motor will be Screamin', 'n the boat goin' Nowhere, Slowly...

 

[Don S]

Re: crank mounted water pump vs leg pump

hi guys the 4.3 and alpha 1 are of the 1986 vintage, the ratio is 1.8 : 1 i'm pretty sure.
I read one of Don S. posts that the crank pump alone would have to much vacum leaks to draw water from the leg.

If using the engine mounted pump with the Alpha style drives, you would need a through hull pickup of some sort.
Merc has one that mounts on the transom.

 

[tricone]

Re: crank mounted water pump vs leg pump

the drive ser. number is 0a597710 could you verify for me and chris the average temp im guessin would be 60 to 70 *

 

[Don S]

Re: crank mounted water pump vs leg pump

V8's were used with MC-1 thru Alpha I drives all the time. Only time you needed the bigger engine mounted pump was when a full closed cooling system (manifolds and engine) was installed. If you have a raw water cooled engine you should be fine.

 

[achris]

Re: crank mounted water pump vs leg pump

Yes, that's a 1.84:1, 1986. Unless it's in a big heavy boat, you won't find a propeller with enough pitch to keep the engine from over-revving. 2 things you can do. 1. have the upper gearbox gears replaced with a set of 1.50:1, or 2, buy a new upper box with a 1.50:1 ratio. -> 1972-1991 Mercruiser Stern Drive Upper Drive Assembly - iboats.com

 

[Fun Times]

Re: crank mounted water pump vs leg pump

would the leg pump supply enough water for the heat exchanger

Only time you needed the bigger engine mounted pump was when a full closed cooling system (manifolds and engine) was installed. If you have a raw water cooled engine you should be fine.

Don, chris & other's, Maybe you guys could help clear a lingering question up a little more.

Since he said he has a heat exchanger, i'm thinking he has a fresh water cooling systems, If so, wondering if his closed cooling system is half or full?

Then the question becomes, if it is closed cooling, should he or could he get away with using the the old alpha 1 impleller or should he use the engine mounted impeller like recommended by mercruiser?

The reason i'm asking is, in the past, i've seen a few different opinions about this subject on this forum. Some say they have no problems using the alpha impeller on a closed cooling system even though it's recommended.

What are your thought about this, Thanks guys!

 

[Don S]

Re: crank mounted water pump vs leg pump

I totally missed the part in his original post about having the heat exchanger.

He should still be fine if he is only cooling the block (Half system), but if he is cooling the exhaust manifolds along with the engine (Full system) then he will need the engine driven pump.

It's the full system that Merc wants the bigger pumps on. He has the pump already on the engine, just take the belt off the pump and use the drive pump alone and see if it works. if not, then throw the belt back on, hook the water to the inboard pump from a thru-hull fitting and have at it. Will also need to cut the hose under the bellhousing as well.

 

[Bondo]

Re: crank mounted water pump vs leg pump

Don, chris & other's, Maybe you guys could help clear a lingering question up a little more.

Since he said he has a heat exchanger, i'm thinking he has a fresh water cooling systems, If so, wondering if his closed cooling system is half or full?

Then the question becomes, if it is closed cooling, should he or could he get away with using the the old alpha 1 impleller or should he use the engine mounted impeller like recommended by mercruiser?

The reason i'm asking is, in the past, i've seen a few different opinions about this subject on this forum. Some say they have no problems using the alpha impeller on a closed cooling system even though it's recommended.

What are your thought about this, Thanks guys!

Ayuh,.... See the 2nd sentence, post #8....

 

[tricone]

Re: crank mounted water pump vs leg pump

thank you all
the 4.3 had a completely closed system (with antifreeze in engine and manifold) and water only to risers after cooling engine coolant
the 350 seems to only have raw water cooling only
the color of the engine is silver/grey is this a chris craft color ?

 

[achris]

Re: crank mounted water pump vs leg pump

Don, chris & other's, Maybe you guys could help clear a lingering question up a little more.

Since he said he has a heat exchanger, i'm thinking he has a fresh water cooling systems, If so, wondering if his closed cooling system is half or full?

Then the question becomes, if it is closed cooling, should he or could he get away with using the the old alpha 1 impleller or should he use the engine mounted impeller like recommended by mercruiser?

The reason i'm asking is, in the past, i've seen a few different opinions about this subject on this forum. Some say they have no problems using the alpha impeller on a closed cooling system even though it's recommended.

What are your thought about this, Thanks guys!

I had a full closed system (engine, heads and exhaust manifolds) on my previous engine, 1994 4.3LX and used the drive pump. Didn't have any problems, ever. I do remember when installing it, the instruction were quite clear about the water temperature. Anything over 80* and they recommended an engine mounted pump, with thru-hull or transom mounted pick-up.

With my new engine, 2006 4.3MPI, When I inquired about a closed cooling system I was told, unequivocally, that I would need an engine mounted pump, in addition to $4,000 for the cooling system... Needless to say that I now have a raw-water cooled engine.... and they have a funny expression on their faces. :faint2:

Chris......

 

[tricone]

Re: crank mounted water pump vs leg pump

So resised some pics and this is the motor i want to use
we need to up the gear ratio and maybe a through hull pick up is that right

 

[Don S]

Re: crank mounted water pump vs leg pump

Volvo style pump, motor mounts show it was a straight inboard, not an IO. It is a half system with an automotive distributor. You will also need to remove the wedge under the carb as it won't be setting at the steep angle like the inboard was.

Your motor mounts, bellhousing and maybe the coupler from the V6 should fit it as well. Wiring is going to be another story.

 

[tricone]

Re: crank mounted water pump vs leg pump

so don with the half system what pump system do you recommend, i'm thinking a lot of the 4.3 stuff will work on this
this was purchased yesterday mostly for the exhaust manifolds but came with alot of extras ....price was 500.00

 

[Don S]

Re: crank mounted water pump vs leg pump

If it was my boat, I would go with the engine driven pump even if it was raw water cooled. All it takes is a plastic bag over the intake, and the impeller is gone rather it's the drive pump or the engine pump.
Ever try to replace an impeller on an Alpha style drive in the water? It's easy on that pump if you have an impeller as a spare.
I had it happen on the sail boat one time, about 2 miles from home, dropped anchor, cleared the thru-hull fitting, replaced the impeller and went home.