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Distributor Timing Advance

Posted: Mon Jan 16, 2017 11:35 pm
by Rick McC.
If I'm understanding what I've read here correctly; why does everyone say it's best to disconnect the vacuum advance from the distributor, and run a static distributor timing when an automotive engine is used to power an airboat?

I'm pretty much lost on this...

Re: Distributor Timing Asvance

Posted: Tue Jan 17, 2017 3:05 am
by bkmail
A search will result in lots of threads cover this topic.
Bk

Re: Distributor Timing Asvance

Posted: Tue Jan 17, 2017 6:34 am
by SWAMPHUNTER45
Personally I think it is a bad idea to lock at full advance but the topic of much debate. As stated there are several threads to search.

The vacuum advance was designed for fuel economy in an automobile and is useless in a severe duty application.

What I find more appropriate is a properly curved mechanical advance. This allows for easy starts, no special switches or buttons and when done correctly a proper performance timing specification to match engine rpm.

Now this requires effort and resources (weights & springs) to get it right, could be why this best practice is avoided by some. Once set however it is a beautiful and simple thing.

Re: Distributor Timing Asvance

Posted: Tue Jan 17, 2017 9:47 am
by Deano
Rick McC. wrote:If I'm understanding what I've read here correctly; why does everyone say it's best to disconnect the vacuum advance from the distributor, and run a static distributor timing when an automotive engine is used to power an airboat? I'm pretty much lost on this...
You are trying to further your understanding of how the timing works, and there are those here that are willing to help toward that end. It may well only be my interpretation, but the way your question is worded makes it somewhat ambiguous. This subject can seem complicated to anyone on the front side, so don't worry about that. It is difficult in any case to pose the best question when your not familiar with what you are asking about. :wink:

There are basically three ways to set-up/alter/adjust the timing:

First . . . You could 'lock it out'. In other words, set your initial/base timing to a number that will NOT change, regardless of the RPMs. It is not variable, it is permanently fixed in one position. Consequently, it will only be optimal running in a SMALL rpm range, rather than across the board. This isn't generally recommended for that and other reasons.

Second . . . You could run 'mechanical advance' where the initial timing is set at idle, and the springs and weights in the distributor will add timing as the engine RPMs increase. This is overwhelmingly the recommendation by most people for most airboat applications running an automotive engine.

Third . . . You could run 'vacume advance' in addition to the mechanical previously mentioned. To oversimplify the purpose of vacume advance would be to say that it is to gain fuel efficiency when the engine is loafing around with only a partial load at lower RPMs. This generally doesn't happen on an airboat unless it is a very big engine on a very light hull, so generally it isn't used. It is worth noting that it can be beneficial in such a circumstance, but is generally not done due to the nearly non-existent difference it would make in most cases.

What wasn't really clear (to me anyway) about your question is exactly what you mean when you say 'run static distributor timing'. Depending on the context, "static" could be interpreted as fixed timing, or it would refer to the base/initial timing setting. Are you asking about running vacuum vs OPTION 1 or are you asking about running vacuum vs OPTION 2, or are you asking about OPTION 1 vs OPTION 2.

The first two responses figured you were asking about locked out (option 1) VS mechanical (option 2); but if that was your intended question, I'm not sure why you mentioned vacuum advance. If that was the case though, Swamphunter offers good advice and hits on the highpoints of the debate. Here is just one such recent thread: http://southernairboat.com/phpBB3/viewt ... 3&p=675097

If this and/or the previous posts answered your question, then that's great.
If not, re-wording your question will allow us to get more specifically to what is it you are wanting to know.

If this was just my confusion, then just say so, excuse me, and I will quit trying to help. 8)

Re: Distributor Timing Asvance

Posted: Tue Jan 17, 2017 2:29 pm
by glades cat
....the reason airboats don't benefit from the vacuum advance module is, they function during cruising RPMs and light throttle application. A vehicle can cruise down the road at highway speed and barely open the throttles...a light power demand. As a prop accelerates, it places increasing power demands on the engine. The engine therefore always sees high loads associated with high RPMS. The vacuum advance will stay retarded...except during idle and off-idle operation...
Which means...You do get a smoother idle with the additional timing.
A typical GM HEI distributor will give you about 20 deg of mechanical advance, plus 8-14 deg initial timing= your typical SBC 34deg total timing.
That is the problem with locking down a distributor on some engines. They don't like starting with too much base timing and can kick back or crank slowly, especially when hot. Some will diesel when shut down.

Re: Distributor Timing Asvance

Posted: Tue Jan 17, 2017 11:06 pm
by Rick McC.
Deano wrote:
Rick McC. wrote:If I'm understanding what I've read here correctly; why does everyone say it's best to disconnect the vacuum advance from the distributor, and run a static distributor timing when an automotive engine is used to power an airboat? I'm pretty much lost on this...
You are trying to further your understanding of how the timing works, and there are those here that are willing to help toward that end. It may well only be my interpretation, but the way your question is worded makes it somewhat ambiguous. This subject can seem complicated to anyone on the front side, so don't worry about that. It is difficult in any case to pose the best question when your not familiar with what you are asking about. :wink:

There are basically three ways to set-up/alter/adjust the timing:

First . . . You could 'lock it out'. In other words, set your initial/base timing to a number that will NOT change, regardless of the RPMs. It is not variable, it is permanently fixed in one position. Consequently, it will only be optimal running in a SMALL rpm range, rather than across the board. This isn't generally recommended for that and other reasons.

Second . . . You could run 'mechanical advance' where the initial timing is set at idle, and the springs and weights in the distributor will add timing as the engine RPMs increase. This is overwhelmingly the recommendation by most people for most airboat applications running an automotive engine.

Third . . . You could run 'vacume advance' in addition to the mechanical previously mentioned. To oversimplify the purpose of vacume advance would be to say that it is to gain fuel efficiency when the engine is loafing around with only a partial load at lower RPMs. This generally doesn't happen on an airboat unless it is a very big engine on a very light hull, so generally it isn't used. It is worth noting that it can be beneficial in such a circumstance, but is generally not done due to the nearly non-existent difference it would make in most cases.

What wasn't really clear (to me anyway) about your question is exactly what you mean when you say 'run static distributor timing'. Depending on the context, "static" could be interpreted as fixed timing, or it would refer to the base/initial timing setting. Are you asking about running vacuum vs OPTION 1 or are you asking about running vacuum vs OPTION 2, or are you asking about OPTION 1 vs OPTION 2.

The first two responses figured you were asking about locked out (option 1) VS mechanical (option 2); but if that was your intended question, I'm not sure why you mentioned vacuum advance. If that was the case though, Swamphunter offers good advice and hits on the highpoints of the debate. Here is just one such recent thread: http://southernairboat.com/phpBB3/viewt ... 3&p=675097

If this and/or the previous posts answered your question, then that's great.
If not, re-wording your question will allow us to get more specifically to what is it you are wanting to know.

If this was just my confusion, then just say so, excuse me, and I will quit trying to help. 8)
Thank you Sir.

I used the term "static" to imply a set timing advance that wouldn't change.

Re: Distributor Timing Asvance

Posted: Wed Jan 18, 2017 6:39 am
by jeepinocala1111
I run a locked timing distributer and it works great.

Re: Distributor Timing Asvance

Posted: Wed Jan 18, 2017 10:30 pm
by Rick McC.
jeepinocala1111 wrote:I run a locked timing distributer and it works great.

Thanks!