Mercruiser 470 Repower

[Katoh]

Good evening All
I am currently building a 7m Aluminium vessel and just recently purchased a second hand Mercruiser 470 stern drive complete unit. The motor was seized but does not bother me as I have a motor that I want to use for this vessel.
I have a friend that will strip the leg down and bring it back to new for me, so that's Ok. I have a 3ltre turbo diesel Toyota motor which is new I wish to use on the the boat, so here my questions go.
Firstly I think power wise the Toyota and the original 3.7 merc have roughly the same power, do you think the 470 leg will stand up to the diesel? Will I have to change ratios or prop size to achieve an acceptable performance say 25 Knots at 2500rpm or is this a suck it and see equation? Because the vessel is custom built or being built i can play around with mounts and adaptors to suite the conversion, one thing I'm not sure about is the marinise of the motor. I have the heat exchangers off the original motor and all the parts, but I don't see how I'm going to convert the exhaust manifold to have the water running through it? or should I just keep all that part dry and feed the water back into the exhaust after the turbo outlet?
would it be an idea to put a fan back on the so it constantly moves air over the unit?
Is there a someone out there that makes kits for them or will i be looking down the barrel of making it all to suite
Thank
Katoh

[powerabout]

I would not put a diesel in front of a Mercruiser 1 drive you need a Bravo

[CDK]

Some issues to consider(and there are more):

-An adapter plate to use the Merc flywheel cover/ bell housing.
-Modifications to the flywheel to adapt the Merc engine coupler.
-A water cooled exhaust/riser for the Toyota or adapter flanges to use another manifold.
-A suitable heat exchanger for both water and oil.
-Relocation of the turbo charger.
-A prop with more pitch, the Toyota has a lower rpm.

I see no problem using the Merc drive unit, it was also factory installed with 5.7 an 6.2 L. V-8 blocks, although in some cases the vertical shaft snapped after a few years. As Powerabout says, a Bravo would be a better choice, but you still have to solve all the issues above.

[Katoh]

Gentlemen
Thank you for the replies. An adaptor plate from motor to bell housing and fly wheel to motor, motor to drive are all not really an issue, I have access to machinery and people with expertise in this field.
Ok lets start, according to CDK the leg should be able to take the the TDI when you look at the specs there's not much between the 470 and the Toyota. With the exhaust if I make up a fitting to introduce the coolant water straight after the turbo and duct into the original piping from there would that be OK? So from the head to the turbo it would be a dry manifold and from the turbo onwards would be flushed with water.
I have two original heat exchangers from the original 470 motor, could I use one for coolant and the other for oil just simply run one into the other?
Is it better to change the prop pitch for the diesel or simply go to the next ratio down gear. At the moment a 1:8 say go to a 1:6, or will this put too much strain on the leg?.
Thanks Again
Katoh

[powerabout]

That will be the problem, if you tried to run the correct ratio, which will be lower the drive will not handle the torque.
470 used 1:84:1
When the 1 drive ran on its biggest horsepower engine (about 280ftlbs) they used the 1.32:1 ratio to make it live.
How much torque does a 3ltr turbo diesel make?
Of course if your boat is real light and this rig will do over 60mph it might live.

[Katoh]

Maybe This will help I got some figures on the motor, max hp 80kw or 120hp @ 3000rpm and max torque is 260NM or 200ftlbs @ 2400rpm, this surly cant be overkill for the leg? or can it? the hull weight should come in at 900kg or 2000lbs
dry I cant see it fully load going over 1800kg or 4000lb that's with say 5 blokes on board each on average 100kg or 220lb

[CDK]

I've used this leg with a 250 hp V8 and a 1450 kg boat (factory installed).
The engine died from a birth defect and was replaced with a 320 hp Merc. That was when the shaft snapped.

Your Toyota does only 3000 rpm instead of 4200, so you need another gear ratio or another prop. I thought the prop was easier and cheaper.

Using a dry manifold is not a good idea unless you're prepared to provide the same wind speed as in a car.

[Katoh]

The leg is going to get totally pulled down, new bearings seals ect I think a gear set is around $400 easy to do while the whole thing is pieces.
This exhaust might be a stumbling block, I am not that keen of pulling the turbo off and trying to manufacture something to suite, as I doubt there's one already made on the shelf. Maybe I can just simply refit the fan to the motor and put in a couple of vents front and back of the engine box? That will give it airflow and have the wet exhaust from the turbo back to run into the leg, probably loose a couple of horses from the fan, but might be an easy solution.

[Katoh]

Good Evening Again
If I was to change the gear ratios of the leg I have a Choice of the 1:64 or the 1:50 gears available for the leg. Considering the original leg 1:84 with the 3.7ltre motor What would would you recommend for the Toyota diesel, Say the the cruise rpm of 22 to 25hundred rpm?
Thanks
Katoh

[CDK]

I think 1.64 would be the best choice. If the Toyota doesn't reach max. rpm with that, the prop can be repitched downward, which is a simple operation I'm told.

The temperature issue should not be underestimated. If you insist on using the dry manifold, put an air guide over it with lots of air coming from the fan. You should also install a riser to keep water from sloshing into the turbo charger housing.

[Katoh]

CDK thanks for your input, Easy enough to put in the 1:64 gears. Just thinking of this exhaust cooling problem, I have come up with another solution, Why cant I leave the original manifold as is and run a series of of 3/4" either stainless or alloy pipe loops around the manifold with the return cooling water running through it, from there it can run into a vertical riser were the exhaust can be joined after the turbo. i still have all the parts from the original 470 wouldn't be that hard to pipe the exhaust from turbo to the original flanges. Is it just the exhaust that needs cooling or the inlet as well?

[CDK]

If you think you are able to wind loops around the manifold with a reasonable degree of thermal contact, do it that way.
For every KW the Toyota produces at the flywheel, it dumps the same amount of heat in the exhaust manifold. You have to remove that with air or coolant fluid.

I am not familiar with the Toyota engine. If it had an inter-cooler in the car and not in the boat, the performance will drop 10-15%. Should you want maximum power in the boat as well, include a car heater in the intake plumbing. Connect the water hose from the stern drive to the heater's coil and feed the output to the engine's heat exchanger. That's the cheapest way to make a marine inter-cooler.

[Katoh]

Actually I'm not sure if there is an inter-cooler on the motor I presume it must have but not 100%, The main aspect of my questions has been answered and that is if the leg could handle the motor which it clearly should. I have to go back now and start machining all the parts for the hull, I hope to have the hull built by new year and in the mean time I can strip and rebuild the leg.
Just another thought with the mercruiser legs are the models interchangeable? say leg to gimble to bell housing? The reason I ask I will use the 470 leg which I think is an R drive, as the cut out in the transom, you never know I might find a bravo leg before I do the rebuild will the two marry up?
Something off the track now is what would you recommend as a backup propulsion unit, many vessels I have seen in the same size with single inboard don't seem to have any backups.

[powerabout]

You can fit a bravo transom plate where a 1 drive came from with some extra drilling unless you have a late model 1 drive the Gen II then it has the same number of holes
The bell housings are different between all the drives

One question on a diesel on a 1 drive...how will you momenterily cut the engine to get it out of gear? ( set up the electronic cutout throught he shift switch?)
ALso when you convert to Bravo you will need a seawater pump mounted on the engine as the drive does not have one

[CDK]

Converting to Diesel, What about cut out switch?