Marine Distributor

[TollyWally]

What makes a marine distibutor a marine Distibutor? What spark proof magic is needed to make one safe?

[CDK]

The marine one is painted in the engine color, the vacuum advance mechanism is removed and the price tag meets marine specifications.

[TollyWally]

LOL,
I can handle the paint. Is is as simple as removing the vacuum advance?

[Landlubber]

TollyWally,
The vacuum hose that connects to the distributor can be removed, and the distributor will not , obviously, then be affected by the vacuum advance. But be careful of the other end of the hose, it connects to the carby, and air will be sucked in there if you do not seal the nipple that the hose goes onto (making the engine run lean). A lead round sinker gently tapped in there is an easy way to do it.

[CDK]

Marine engines usually have no vacuum hose (except to the dashboard sometimes), so there's no problem.
Be sure to use a distributor that is designed for a car engine with approx. the same displacement. For smaller engines the centrifugal advance angle sometimes is larger.

[TollyWally]

Thanks all,
Looking at some other threads here I've come across the notion of venting.

http://www.boatdesign.net/forums/showthread.php?t=14723
http://www.boatdesign.net/forums/showthread.php?t=15777

One guy quite reasonably pointed out that he was perfectly capable of sealing holes and duplicating the screened venting needed.

I notice that the one thing in common that “spark proof” marine accessories seem to have is the use of fine meshed screening in the openings. What is it that makes screen contain spark or impede explosive vapor? When I look at examples it just doesn’t seem like it would be very effective. So obviously I don’t get it. Perhaps someone wiser than my self can explain.

I now realize that I probably should have posted this under DIY marinizing. I apologize, at the time I was lusting over one of those new electronic programmable distributors so I had electronics on the brain.

[CDK]

The wire mesh is supposed to absorb the heat in case of an explosion. The combustible mixture can get inside the distributor or alternator and may ignite if the air/fuel ratio allows so and a spark is present, but the wire mesh keeps the fire inside while the expanded gases pass through.

This flame arrester principle really works, but the material used must be copper or alu in several layers so the flame has to pass enough heat absorbing surface to extinguish, while no other escape is present.

In real life, a simple, but clearly visible screen is added to a standard automotive product while no further precautions are taken. Mercruiser uses marine alternators that are open at the belt side but have a screen in the opposite cover. Of course that doesn't work, but in an alternator sparks are extremely rare and the air/fuel mixture in the engine compartment seldom is suitable for an explosion.....

[Ike]

CDK has it right. Flame screens are used in many applications as a flame arrestor and are very effective. Take a look at the vent fitting for you fuel tank. Just inside is a flame screen. No that screen is not there to keep the bugs out! LOL Anyway, what happens is, if you have explosive vapors in the engine compartment they will enter the distributor. If a spark occurs that is hot enough, they ignite. The screen slows down the flame front, and cools it enough that it doesn't go out of the distributor to the engine compartment and make your boat go boom ruining your whole day!

USCG regs and regulations in Canada, Europe and almost everywhere that regulates boat safety require marine distributors to be ignition protected. If the distributor is sealed ( as some are) nothing else is required. If the distributor is vented then it needs to be ignition protected. This of course assumes it is a distributor that uses points. If it is solid state, magnetic induction or other sparkless distributor, no problem. In fact amy electrical or electronic equipment that is in a space where explosive fumes can accumulate is required to be ignition protected. That includes alternators starters, battery chargers, switches, circuit breakers and on and on.

Also. The advance curve for marine distributors is different than for automobiles because marine engines run under heavier loads and are almost always run at about 3/4 throttle which is the high point of the torque curve somewhere around 4000-4500 rpm on a typical v-8 ( depends on the prop and reduction gear) . On cars the engine runs at a variety of rpms because of the transmission and gearing and typically auto engines run at about 20-25% of max hp which is usually (depends on the engine, tranny and differential) 2000-2500 rpm

Other than that a distributor is a distributor. Marine distributors, as with all things "marine" are produced in much lower volume than automotive parts. With marine you are talking a few hundred thousand distributors produced a year for the entire marine industry. With autos you are talking millions and millions for just one manufacturer, a factor of 100 to 1 so simply economics makes the large volume item cheaper than the low volume item.

[TollyWally]

Well this has been enlightening. So if am understanding you guys correctly, the fine screen uses thermal mass to absorb the heat from a mini explosion in the electrical device, fuel vent etc. and physical mass to keep the mini flash contained within. All the while the permeable structure allows the cooled vapors out.

Further more in a distributor, if it is a pointless solid-state model, the risk of the spark isn’t there because without points arcing there isn’t a spark.

A solid state mechanical advance distributor ought to be ok for a marine application then, especially if any venting or penetrations are sealed or screened with the appropiate materials.

Now we’ve all seen old spark plug wires arc. Are there special marine plug wires or is it enough to use high quality automotive wires.

[Ike]

yes there is a specific SAE standard for marine ignition wiring. Here's what the fed reg says

Sec. 183.440 Secondary circuits of ignition systems.

(a) Each conductor in a secondary circuit of an ignition system must meet SAE Standard J557.

(b) The connection of each ignition conductor to a spark plug, coil, or distributor must have a tight fitting cap, boots, or nipple

The problem here is that SAE changed the standard. It's now SAE J2031.

Mallory, Magstar, MSD, and others make this stuff. Just google marine ignition wire. It's designed to work in a much damper and hotter environment than regular auto ignition wire.

[MattZ]

This of course assumes it is a distributor that uses points. If it is solid state, magnetic induction or other sparkless distributor, no problem. In fact amy electrical or electronic equipment that is in a space where explosive fumes can accumulate is required to be ignition protected. That includes alternators starters, battery chargers, switches, circuit breakers and on and on.


Even with solid state distributors there's a chance of spark from the rotor contact. Proper marinization is still required. The MSD units are very affordable.