3.0 cobra drive to diesel conversion

[jake-x]

Hi all

I am in the process of restoring a fjord consul, I also have 2 omc cobra 3.0 litre petrol engines and drives, I am considering using the drives and binning the motors for diesels, here is the problem I have been told that because the cobra drives have got dog clutch’s and not cone clutch’s, engaging the gears will be a problem??? Any thoughts on this would be appreciated? Thanks for the help.

[CDK]

Let the person who told you that story explain why the gears would not engage or release if you change the power source. I cannot think up a valid reason.
The only restriction is, that HP and torque must be in the range a Cobra drive can safely handle.

[jake-x]

The motors I’m looking at using are Toyota 3.0ltr turbo diesels, the very same engine the nanni diesel use for there 4.380 TDI, if you look at the standard engine’s specs
 Base: nanni / Toyota
 Max power: 130 hp (97 kw)
 RPM at max output: 3600 rpm
 29.3 kgf•m (287 N•m) @ 2000 rpm. reline is 4400 rpm
 Displacement: 2982 cc ( 181,95 ci)
 Number of cylinders: 4 in line
 Bore/stroke: 96 x 103mm
 Compression ratio: 19,7/1


The only information I have found for the 3.0ltr omc is
Horsepower: 130.0
Cylinders: 4
Type: 4 stroke
Gear Ratio: 1.86:1
Min WOT RPM: 4200
Max WOT RPM: 4600
so far this is the only information I have found?

[CDK]

I see no problem whatsoever with these Toyota engines. Except financial ones: you need the water cooled exhaust manifolds Nanni uses, which may be expensive. An alternative would be to use the OMC parts if they are still in good condition, but you'd have to make adapter flanges because the holes don't match.
You can use the OMC bell housings with an adapter plate and maybe the engine couplers too.
For a job like this you need welding equipment and a lathe or a good friend who has access to a machine shop.

Your old engines are known in the industry as Vortex3000 from GM, now obsolete. They were widely used in marine applications with performance figures up to 140 HP, but GM always rated them below 100 HP.
The Toyota's will do much better, especially if you use props 1-2" steeper than the ones you have now. Getting into plane is much easier: you'll have almost twice the torque around 3000 rpm. And they are quite happy with heating oil.

The OMC drives can easily handle the extra power, they were also used with 6 inline and small block V8 engines.

Good luck!

[mongo75]

The only difference internally in a 4 cyl Cobra drive vs. a 6 or 8 cyl drive is the gear ratio. I had one on my last boat and loved the simplicity of it.

[gonzo]

The drive will not get out of gear because the clutch dog needs a shift interrupt switch to release the torque. The switch momentarily cuts off the ignition to allow the clutch to disengage. I can't see how you could interrupt a diesel. When the switches malfunction it is necessary to turn off the engine to get in neutral.

[CDK]

The ignition cut-off switch is not really necessary, just an extra precaution in case the clutch doesn't disengage when the engine is throttled back. A new sterndrive switches in and out of gear without activating the cut-off switch; but when the mechanism ages more force is needed and the ignition must be momentarily shorted.

The switch is a normally open (NO) type for a gasoline engine. For a diesel it must be replaced by a NC type, wired in series with the fuel cut-off solenoid on the injection pump. This device immediately interrupts the fuel flow to the high pressure stage in the fuel pump.

I see no real problem there, any sturdy Honeywell or similar microswitch can do the job. The mechanical arrangement that is on top of the old GM engine can be used for the diesel as well.

[mongo75]

Also, ya gotta think that most diesels idle lower, or can be made to idle lower than a gas engine. Again the shift interrupt is to drop rpms from 850 to about 500 for a second to make shifting easier, both on the gears and the operator. As long as it works I think it's a great innovation OMC came up with. The biggest problem is the damn switch spring (looks like salad tongs with a coil at the joint) gets worn over time and doesn't open the ground up fast enough after shifting and causes the engine to shut off. That usually happens at the most inopertune time, such as approaching the bait dock, and causes you to look like a dumbass LOL

[gonzo]

Gas engines idle at between 650 and 750 RPM. 850 is too high. Sterndrives when new won't get out of gear without the interrupt switch. If anything, it gets easier when they are worn out. Eventually, the dogs wear out so much they won't stay in gear. Using an electric fuel cut-off won't work well. There is a delay between the solenoid activating and the engine stopping.