vacuum advance on marine motor

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vxtech

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just curious question- how does a marine motor operate differently than auto concerning vacuum advance why is vacuum advance not needed on marine engine, thanks chris
 
Bt Doctur

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Re: vacuum advance on marine motor

Because most motors run at or near wide open throttle and no vac is present
 
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T-Bone Coug

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Re: vacuum advance on marine motor

Marine distributors use a mechanical advance driven by centrifugal force on the distributor shaft. The faster the shaft turns, the farther the weights move, increasing the advance.

As far as bt's description...I think he's mistaken. Vacuum leaks are always more evident at wide open throttle, leading me to believe that vacuum is always present. Ever hear the SUCKING of the carb at wide open throttle vs idle? By definition that is vacuum. :)
 
Bondo

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Re: vacuum advance on marine motor

As far as bt's description...I think he's mistaken. Vacuum leaks are always more evident at wide open throttle, leading me to believe that vacuum is always present. Ever hear the SUCKING of the carb at wide open throttle vs idle? By definition that is vacuum.
Click to expand...

Ayuh,... And I think you're mistaken, about BD being mistaken...

By your description,... Diesels have Vacuum,... Which they Don't...

The vacuum registered on a gasoline motor is the motor sucking against the throttle plate....
Diesels have No throttle plate, hence, No vacuum...
 
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T-Bone Coug

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Re: vacuum advance on marine motor

Bond-o said:
Ayuh,... And I think you're mistaken, about BD being mistaken...

By your description,... Diesels have Vacuum,... Which they Don't...

The vacuum registered on a gasoline motor is the motor sucking against the throttle plate....
Diesels have No throttle plate, hence, No vacuum...
Click to expand...

I stand corrected regarding vacuum, and WHY marine motors use mechanical or electronic advance. I learned something new. Thank you, Bond-o. :)

My description of how mechanical advance works is correct though.
 
Bondo

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Re: vacuum advance on marine motor

My description of how mechanical advance works is correct though.
Click to expand...

Ayuh,.... Absolutely,....;)
 
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mla2ofus

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Re: vacuum advance on marine motor

I believe most vacuum advance systems worked off of venturi vacuum, which is just the opposite of manifold vacuum. The further the throttle plate(s) are open the higher the vacuum.
Mike
 
Bt Doctur

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Re: vacuum advance on marine motor

Don`t think so. My 64 bonny had a manifold vac gauge in the dash. Under normal operation it shows 12-16 inches of vac. Kick the quads open and manifold vac went to zero.
 
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dan t.

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Re: vacuum advance on marine motor

yes MANIFOLD vacume will go to 0 at wot, VENTURI vacume increases with throttle opening and increase in air flow. vacume advance can be run off either depending on the advance curve. its purpose is mainly for added economy under cruise or light throttle conditions in automotive aplications. it would serve no purpose in a boat
 
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mla2ofus

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Re: vacuum advance on marine motor

So my question is: why can't marine distributors have vacuum advance running off venturi vacuum since they run at or near full throttle? Wouldn't that help increase power and mileage?
Mike
 
zbnutcase

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Re: vacuum advance on marine motor

No. At that engine speed you get the same results from the centrifugal advance; engines can take only a certain amount of advance before all kinds of bad (read expensive) things happen. :)
'nutcase
 
mkast

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Re: vacuum advance on marine motor

mla2ofus said:
why can't marine distributors have vacuum advance
Click to expand...

When points type distributors were used a centrifugal advance was used.
Corrosion of the movable point plate, springs and weights was a pain.
When electronic distributors became the norm, electronic spark advances are used.
Are you under the impression that because you don't see a vacuum diaphragm on the distributor that it doesn't have a spark advance?
 
bruceb58

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Re: vacuum advance on marine motor

mla2ofus said:
I believe most vacuum advance systems worked off of venturi vacuum, which is just the opposite of manifold vacuum. The further the throttle plate(s) are open the higher the vacuum.
Mike
Click to expand...
No, they work off of manifold vacuum. The theory for an auto is that you want less vacuum when you are under load and that happens when the vacuum drops.
 
Bondo

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Re: vacuum advance on marine motor

Ayuh,... I think it's been about a hundred years since I last played with a vacuum distributer,...
But,...
Isn't vacuum Advance abit of a misnomer,..??
If I remember right, the vacuum actually retards the flyweights advancement don't it,..??
Sorta primitive spark-knock control,..??

Somebody refresh this ole fart's memory, will ya,..??
 
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system-f

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Re: vacuum advance on marine motor

If I remember right, the vacuum actually retards the flyweights advancement don't it,..??
Sorta primitive spark-knock control,..??
Click to expand...

This is what I remember. That if the distributor had NO vacuum it would be at max advance and once it got manifold vacuum..say at idle..it would retard it back to 0 or whatever then as manifold vacuum decreased it would allow the springs to pull it into advance.



If I take a 350 long block out of a truck (lets say it already has an RV cam in it), marinize it except leave the vacuum advance distributor on it and throw it in a boat, why wouldn't it work? It worked just fine in the car at X rpm...and we haven't changed anything except for the freeze plugs, carb (which is assumed to have the right vacuum port), water pump, fuel pump, starter, alt, and anything else I forgot that would go boom in a boat.
 
slasmith1

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Re: vacuum advance on marine motor

system-f said:
If I take a 350 long block out of a truck (lets say it already has an RV cam in it), marinize it except leave the vacuum advance distributor on it and throw it in a boat, why wouldn't it work? It worked just fine in the car at X rpm...and we haven't changed anything except for the freeze plugs, carb (which is assumed to have the right vacuum port), water pump, fuel pump, starter, alt, and anything else I forgot that would go boom in a boat.
Click to expand...

you forgot that an auto distributor is not ignition protected or coast guard certified for marine use. So if you don't blow yourself up first, you will definitely get a ticket and be removed from use until you replace it the first time you are given a "courtesy inspection". the same goes for all fuel components, electrical components (starter, alternator, etc.), spark arrestor not air filter the list goes on auto parts are not designed for marine use. the only thing that is the exact same between auto and marine use is the block and internal parts except the cam and core plugs.
 
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ChrisCraftJohnny

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Re: vacuum advance on marine motor

Guys,

All large gas powered internal combustion engines need spark advance for maximum power. What you need to understand is that the rate of expansion of the exploding gases is the same if your at idle or if you are at WOT.

Therefore, as the RPM's increase, the spark has to fire a bit sooner during the ignition stroke to make up for the slower expansion of the exploding gases relative to the faster mechanical movement of the pistons at high RPM. In other words, at a certain RPM the pistons are moving towards TDC faster than the speed of the explosion.
By advancing the spark you allow the air/fuel mixture to ignite a bit sooner to allow the expanding gases to get a head start on fully exploding when it should. Most engines at idle in the 7-10 BTDC range. You see, even at idle, ignition is still advanced to make up for the expansion rate of the explosion.
On a electronic ignition, the advance is handled by a small microchip known as the ignition module.

Johnny
 
bruceb58

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Re: vacuum advance on marine motor

system-f said:
This is what I remember. That if the distributor had NO vacuum it would be at max advance and once it got manifold vacuum..say at idle..it would retard it back to 0 or whatever then as manifold vacuum decreased it would allow the springs to pull it into advance.
Click to expand...
That's not how it works. The lack of vacuum retards the spark. For example, let's say you are climbing a hill and want to pass someone. If you accelerate and add more load to the engine, you may start having a ping if the ignition stays in its current advanced state. When you punch the accelerator you will immediately get a vacuum drop which will in turn retard your spark because less vacuum will be going to the distributor.
 
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mla2ofus

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Re: vacuum advance on marine motor

mkast said:
When points type distributors were used a centrifugal advance was used.
Corrosion of the movable point plate, springs and weights was a pain.
When electronic distributors became the norm, electronic spark advances are used.
Are you under the impression that because you don't see a vacuum diaphragm on the distributor that it doesn't have a spark advance?
Click to expand...


I seem to have trouble making myself clear. I'm well aware electronic distributors don't have vac advance. I was referring to the point type w/ centrifugal advance.
My bad,
Mike
 
cr2k

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Re: vacuum advance on marine motor

Vac advances ADVANCE timing with application of throttle. If you dont believe me and have a car with vac adv. pull the hose and put it to manifold vac and tell me how well it runs.

However this is a completely different elephant on an ECU controlled car. The Vac timing unit there is 2 way adv/retard and driven by the O2 sensor.

Vac advances are for 2 things; 1. drive-ability. If the engine bogs the adv will retard and not backfire.

and B. The later models are tailored to reduce smog at certain engine loads and rpms.

So to answer your question; Marine engines due to being non smog equipped and not running constantly changing throttle settings like a car does i.e. idle, wot or cruise, don't need them and there is one less opening to seal against sparks and vapor.
 
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