7.4L Mercruiser Starter Current Draw - Battery Switch

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What does the starter on my 7.4L Mercruiser draw for current? I'm planning on installing a battery switch in my 1993 Cruisers 2670 Rogue. There's a medium duty switch that is rated at 250 amps continuous, 360 amps intermittent capacity and then there's the heavy duty rated at 380 amps continous, 850 intermittent. The minimum rated battery recommended for the 7.4L is 650cca/825mca. I don't want to overkill it but I do want to be sure it's not going to melt the contacts.

Any thoughts on brands? Perko vs. Blue Sea

Went to start it this past weekend out on the water after an hour run and my 3 yo starting battery was getting weak. Almost had to get out the jumper cables but it finally started.

Thanks
Dave

 

[04fxdwgi]

Re: 7.4L Mercruiser Starter Current Draw - Battery Switch

What does the starter on my 7.4L Mercruiser draw for current? I'm planning on installing a battery switch in my 1993 Cruisers 2670 Rogue. There's a medium duty switch that is rated at 250 amps continuous, 360 amps intermittent capacity and then there's the heavy duty rated at 380 amps continous, 850 intermittent. The minimum rated battery recommended for the 7.4L is 650cca/825mca. I don't want to overkill it but I do want to be sure it's not going to melt the contacts.

Any thoughts on brands? Perko vs. Blue Sea

Went to start it this past weekend out on the water after an hour run and my 3 yo starting battery was getting weak. Almost had to get out the jumper cables but it finally started.

Thanks
Dave

Current draw on a 7.4 L starter is kind of a loaded question, as there are so many variables. May train of thought is:
1. It's a big block and it's spinning lots of big parts.
2. If it doesn't start right away, it's really giving the starter / battery a workout.
3. If it's older, (as yours and mine are) it should, thoretically, draw less due to lower compression and being loose. But, see item 1.
4. Hot engines can be harder to spin because of increased compression than cold ones or visa versa if oil is cold.


So, that being said, I would opt for the HD switch if I were to change the one on my 7.4L. The Perco and Blue Seas are very comperable, as far as I can see. Any other's, I wouldn't bother with.

 

[Bondo]

Re: 7.4L Mercruiser Starter Current Draw - Battery Switch

What does the starter on my 7.4L Mercruiser draw for current? I'm planning on installing a battery switch in my 1993 Cruisers 2670 Rogue. There's a medium duty switch that is rated at 250 amps continuous, 360 amps intermittent capacity and then there's the heavy duty rated at 380 amps continous, 850 intermittent. The minimum rated battery recommended for the 7.4L is 650cca/825mca. I don't want to overkill it but I do want to be sure it's not going to melt the contacts.

Any thoughts on brands? Perko vs. Blue Sea

Went to start it this past weekend out on the water after an hour run and my 3 yo starting battery was getting weak. Almost had to get out the jumper cables but it finally started.

Thanks
Dave

Ayuh,.... Either of 'em will work just Fine,... Normally,...

In the event of a hydro-lock, or other dead load, the bigger 1 might be better off...
The battery will be unloadin' Everything it's got...

 

[Mischief Managed]

Re: 7.4L Mercruiser Starter Current Draw - Battery Switch

My 7.4 MPI has been started through a Perko 250 amp rated switch for 11 years now. No problems at all.

 

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Re: 7.4L Mercruiser Starter Current Draw - Battery Switch

Thanks!

Here are the responses from Perko and Blue Seas.

Perko
You can easily go with my 8603DP Medium Duty battery switch for your application and you do not need to use the Alternator Field Disconnect feature.

Blue Seas
Thank you for your question. The answer depends on your starter amperage draw and alternator output, but just on the face of it, I'd be inclined to steer you toward the HD switch.

Alternator Field Disconnect is something few people need, and in fact it isn't even relevant unless you have an external voltage regulator. This is generally found with alternators that are either very old or very large. Here's an article that explains AFD in some detail:

http://bluesea.com/viewresource/91

The article is old enough that it only references the medium-duty part numbers, but Heavy Duty switches with AFD are PNs 3001, 3003, and 11003.

There is one other point about AFD: if you aren't using it to protect the alternator, you can connect an LED to the AFD terminals and it will indicate when the battery switch is in any position except OFF. This is potentially useful if you wish to monitor your battery switch position from a remote location.