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shadow3621

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Vacuums advance vs mechanical for my distributor
« on: April 24, 2016, 07:16:35 PM »
I have a friend that has recently gotten into jet boats, and has started building a bbf for his. He is sure that a mechanical advance is best for his setup, I am of the vacuum advance persuasion. Any thoughts/arguments for or against either of these setups?


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GT Jets

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Re: Vacuums advance vs mechanical for my distributor
« Reply #1 on: April 24, 2016, 08:20:38 PM »
I have a friend that has recently gotten into jet boats, and has started building a bbf for his. He is sure that a mechanical advance is best for his setup, I am of the vacuum advance persuasion. Any thoughts/arguments for or against either of these setups?


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A vacuum advance should really be called a vacuum retard.

Tell me why you think a vacuum advance is the way to go on an engine that,  if tuned correctly,  will create virtually no vacuum at wide open throttle.

The literal only way you can make a vacuum advance work is if you were able to determine how much advance the engine can remove from itself (again,  works like a timing retard) and still sustain acceleration without detonating itself to pieces at intermediate throttle settings. It would be very difficult to tune without the aid of an extremely adaptive ecu.

As the advance starts to drop when the engine comes up to wide open throttle (speaking strictly jet boat) the vacuum numbers waver as the timing goes advance and retard,  this vacuum has a direct effect on timing and vice versa.

The vacuum advance is actually designed to optimize engine efficiency and performance at a cruise,  in a car. The torque curve on a car, versus the curve on the jet pump is opposite. The jet is almost perfectly linear,  increasing constantly with RPM. The car on the other hand,  the torque completely falls off from extreme under acceleration,  reducing the torque in the upper rpm ranges.  This is called "pulling a gear". A jet has virtually no load until about 3,800 rpm. It's a constant torque vs variable torque scenerio.

I'll be up for a bit. Be more Than happy to debate this further and looking forward to being converted to vacuum advance systems.

Whatcha got?

GT

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« Last Edit: April 25, 2016, 08:08:55 AM by GT Jets »
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

shadow3621

  • Karma: +1/-0
Re: Vacuums advance vs mechanical for my distributor
« Reply #2 on: April 24, 2016, 10:58:03 PM »
Actually, GT, I have read a lot about it tonight, and after reading what you wrote, it all makes sense to me now. Since I used to be a car guy and now have a jet boat, lol, I seem to have had my brain stuck on gears. As in transmission. I keep forgetting that things are "simpler" with a jet drive, no transmission. I will start the hunt for a mechanical advance distributor. Thank you very much for the clarification.


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propless

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Re: Vacuums advance vs mechanical for my distributor
« Reply #3 on: April 25, 2016, 04:09:56 PM »

A vacuum advance should really be called a vacuum retard.

Tell me why you think a vacuum advance is the way to go on an engine that,  if tuned correctly,  will create virtually no vacuum at wide open throttle.

The literal only way you can make a vacuum advance work is if you were able to determine how much advance the engine can remove from itself (again,  works like a timing retard) and still sustain acceleration without detonating itself to pieces at intermediate throttle settings. It would be very difficult to tune without the aid of an extremely adaptive ecu.

As the advance starts to drop when the engine comes up to wide open throttle (speaking strictly jet boat) the vacuum numbers waver as the timing goes advance and retard,  this vacuum has a direct effect on timing and vice versa.

The vacuum advance is actually designed to optimize engine efficiency and performance at a cruise,  in a car. The torque curve on a car, versus the curve on the jet pump is opposite. The jet is almost perfectly linear,  increasing constantly with RPM. The car on the other hand,  the torque completely falls off from extreme under acceleration,  reducing the torque in the upper rpm ranges.  This is called "pulling a gear". A jet has virtually no load until about 3,800 rpm. It's a constant torque vs variable torque scenerio.

I'll be up for a bit. Be more Than happy to debate this further and looking forward to being converted to vacuum advance systems.

 Whatcha got?

GT

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 Hi GT , yea I'm still here.  And I got something for you. :P   You still don't understand vacuum ADVANCE systems.

 They don't RETARD timing, they ADVANCE timing over and above the mechanical advance curve of the distributor at times of high vacuum. At times of low vacuum the timing will fall back to the same mechanical advance curve you would have with a non vacuum advance distributor.   Less vacuum = less timing ADDED above the base mechanical timing curve.   
 You set the mechanical advance curve to give the timing the engine needs/wants at WOT, just like you would if there was no vacuum advance because the vacuum system will not be active at WOT.  Then you hook up the vacuum advance, It will add timing at times of increased vacuum/reduced load such as idle and cruise.  With less load (idle/cruise) the engine can handle way more timing than it can get from the mechanical advance system that is set for proper timing at WOT.
 A properly set up vacuum advance system will increase efficiency at idle/low load/part throttle situations on any engine no matter if its hooked to a transmission, a V-drive, a jet, or whatever as long as the engine sees a varying load.  And even a jet has this. In your own words "A jet has virtually no load until about 3,800 rpm".  At 3000 RPM cruise, you don't have the throttle wide open. The engine has vacuum, and the system will ADD timing over and above the base timing curve because this is a low load situation. If it wasn't a low load situation, you would need full throttle to maintain that RPM.  The engine may only be able to handle 36 deg advance at WOT. But at cruise it may be able to handle 46 deg because of the reduced load.  No mechanical advance system can change timing based on engine load, only a vacuum advance or computer controlled system can do that. 
 Don't believe me ? Go out and advance you initial (idle) timing 10 deg and see what it does to idle RPM.  It will increase RPM, you will have to turn down the idle speed at the carb. So now the carb is supplying less air/fuel to hold the same RPM.  Same thing will happed at cruise, it will need slightly less throttle to hold the same RPM, as long as there is enough vacuum to activate the vacuum advance system.

 All of this is doubly true if you carb has power valves. At light load the power valves will be closed providing a lean mixture (cruise) vs the proper (WOT) mixture.  This lean mixture will not bun properly, it actually burns slower increasing exhaust gas temps. The lean mixture at light load/cruise needs more ignition advance.

 What are the down sides to vacuum advance ?

 1. They are ugly.

 2. They have no advantage on a race engine that spends its life at WOT. The system would only be active at idle, and nobody really cares how well their race engine idles.

 3. Things can get complicated and most people don't take the time to learn how they work or take the time to find the right vacuum canister to work with their engines vacuum levels.  A vacuum canister for a stock 454 truck engine will be useless on a performance boat engine, you wont have enough vacuum to fully advance the system. You need a vacuum canister that goes to full advance with slightly less vacuum than your engine makes at idle, and the vacuum advance needs to be hooked to manifold vacuum. If the canister doesn't have enough vacuum to hold it at full advance your idle timing will fluctuate and idle speed will be erratic. It also needs to be the right canister to give the right amount of added timing at cruise (based on load). If you get it wrong you can end up with to much advance for the load (detonation). Or not enough advance for the load (not taking full advantage of the system).  If you take the time to get it right, advantage always goes to the vacuum advance system.



 At idle = advantage vacuum advance.  Timing will be advanced beyond base timing curve.  Better idle quality, less throttle opening, same RPM. Increased efficiency

 At cruise = advantage vacuum advance. Timing will be advanced beyond base timing curve. Less throttle opening to maintain the same RPM. Increased efficiency 

 At WOT = No advantage to either.  Both will be following the same base timing curve. 
 
 
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GT Jets

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Re: Vacuums advance vs mechanical for my distributor
« Reply #4 on: April 25, 2016, 05:52:30 PM »
Hi GT , yea I'm still here.  And I got something for you. :P   You still don't understand vacuum ADVANCE systems.


Holy fukknuckles.......    8) 8) 8) :thumbup: :thumbup: :thumbup: :thumbup:

You want to know something funny.... The whole time I wrote my response I was thinking of you..... HAHAHAHHA!

 
They don't RETARD timing, they ADVANCE timing over and above the mechanical advance curve of the distributor at times of high vacuum. At times of low vacuum the timing will fall back to the same mechanical advance curve you would have with a non vacuum advance distributor.   Less vacuum = less timing ADDED above the base mechanical timing curve.

In our application, for all intents and purposes, they actually retard the timing "to" the mechanical advance settings. Literally the only time it will have any vacuum advance brought in is either going to be under deceleration or just above an idle when the boat just starts to move....  By the time the secondaries start to open, the point would be moot, because as the mechanical is all in, all that can happen is the vacuum pot will start to return to normal because the vacuum numbers are falling like a rock, which is what it's intended to do in an automotive application, drop the timing out to "pull a gear". A jet pump never ever ever sees this happen.... It's virtually working backward of what you want to see in a jet boat. So in essence, you would need to set the mechanical advance up to obtain (for sake of argument) 36BTDC at the higher RPM range to be able to obtain optimum WOT ignition timing. Mainly (but not only) because the engine cannot create enough vacuum to pull the pot in. Making the terminology flip to vacuum retard.

   
 
You set the mechanical advance curve to give the timing the engine needs/wants at WOT, just like you would if there was no vacuum advance because the vacuum system will not be active at WOT.  Then you hook up the vacuum advance, It will add timing at times of increased vacuum/reduced load such as idle and cruise.  With less load (idle/cruise) the engine can handle way more timing than it can get from the mechanical advance system that is set for proper timing at WOT.
 A properly set up vacuum advance system will increase efficiency at idle/low load/part throttle situations on any engine no matter if its hooked to a transmission, a V-drive, a jet, or whatever as long as the engine sees a varying load.  And even a jet has this. In your own words "A jet has virtually no load until about 3,800 rpm".  At 3000 RPM cruise, you don't have the throttle wide open. The engine has vacuum, and the system will ADD timing over and above the base timing curve because this is a low load situation. If it wasn't a low load situation, you would need full throttle to maintain that RPM.  The engine may only be able to handle 36 deg advance at WOT. But at cruise it may be able to handle 46 deg because of the reduced load.  No mechanical advance system can change timing based on engine load, only a vacuum advance or computer controlled system can do that. 

This is the entire idea of having timing advance on the mechanical "all in by a low number, even locked out work "OK" because the load simply is not there. I don't feel any real gains in fuel mileage could be realized, because to take advantage, the engine is only turning 2,500 RPM, must hulls wont even plane at those crank speeds.

 
 
Don't believe me ? Go out and advance you initial (idle) timing 10 deg and see what it does to idle RPM.  It will increase RPM, you will have to turn down the idle speed at the carb. So now the carb is supplying less air/fuel to hold the same RPM.  Same thing will happed at cruise, it will need slightly less throttle to hold the same RPM, as long as there is enough vacuum to activate the vacuum advance system.

What do you think would happen if you snapped the throttle at that particular ignition timing? Where is the fuel efficiency coming from? you are not making any more HP buthte CFM into the carburetor will increase with engine RPM and the mixture isn't changing, so RPM for RPM, the fuel usage is the same... I could see if you only pulling 50 HP like what you would see in a car going down the freeway, but it''s not at all like that........ You let off the loud pedal, thing thing stops like it's tied to a tree.....

 
All of this is doubly true if you carb has power valves. At light load the power valves will be closed providing a lean mixture (cruise) vs the proper (WOT) mixture.  This lean mixture will not bun properly, it actually burns slower increasing exhaust gas temps. The lean mixture at light load/cruise needs more ignition advance.


Up to a point, engine load has a direct counter response.

 
What are the down sides to vacuum advance ?


They are extremely difficult to get to do what you want, when you want... I would rather see an ECM controlled advance, then you would have me....

1. They are ugly.
indeed.

 
2. They have no advantage on a race engine that spends its life at WOT. The system would only be active at idle, and nobody really cares how well their race engine idles.
True that.

 
3. Things can get complicated and most people don't take the time to learn how they work or take the time to find the right vacuum canister to work with their engines vacuum levels.  A vacuum canister for a stock 454 truck engine will be useless on a performance boat engine, you wont have enough vacuum to fully advance the system. You need a vacuum canister that goes to full advance with slightly less vacuum than your engine makes at idle, and the vacuum advance needs to be hooked to manifold vacuum. If the canister doesn't have enough vacuum to hold it at full advance your idle timing will fluctuate and idle speed will be erratic. It also needs to be the right canister to give the right amount of added timing at cruise (based on load). If you get it wrong you can end up with to much advance for the load (detonation). Or not enough advance for the load (not taking full advantage of the system).  If you take the time to get it right, advantage always goes to the vacuum advance system.


So are you running a vacuum advance then?



 
At idle = advantage vacuum advance.  Timing will be advanced beyond base timing curve.  Better idle quality, less throttle opening, same RPM. Increased efficiency
By how much? 2%, for an engine that idles about 10% of its life? 

 
At cruise = advantage vacuum advance. Timing will be advanced beyond base timing curve. Less throttle opening to maintain the same RPM. Increased efficiency   

Not above about 3,000 RPM because of the linear torque curve...... My 18' v-bottom does about 20 MPH at 3K and is barely on a plane.

 
At WOT = No advantage to either.  Both will be following the same base timing curve. 
 
Most jet boats I have owned run anywhere between 3,800 and 5,500 RPM, I really don't see it affecting my fuel budget.


What the hell have you been up to? Tell me you bought the Daytona back from John.......... Please tell me that......

$10k was criminal cheap.....

GT
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

GT Jets

  • Karma: +192/-0
Re: Vacuums advance vs mechanical for my distributor
« Reply #5 on: April 25, 2016, 10:46:33 PM »
So, I think I read the article you are referring to. Good read. And quite accurate for an automotive application. There is something that gets lost in the write up for our application. Throttle position. The throttle position at say 65 MPH on your vato will be ridiculously low, probably in the 10-15% range. This puts the power valve totally in play on your car, this leans the burn a bit. For the sake of keeping the discussion moving seamlessly, take the same exact car and run it up a steep hill. That is exactly what the jet boat engine is doing, all the time, the power valve is now open, the timing falls off (retards  ;) ) and it all goes out the window... Here's why.


The throttle blades will be much further open (closer to 50-60%) and the vacuum numbers will be lower (probably about 4"h.g.) most out of the box carburetors come from the factory with a 6.5" PV. This is why I always recommend dropping the power valve to a 4.5" It keeps the fuel inrichment "off" longer. This lower point will make a dramatic improvement on part throttle running, as will making sure the timing advance is "all in" at 2000-2500RPM, this also increases the part throttle running. The great part is it has zero chances of detonation. I have a really difficult time thinking any more timing would make any difference at all.

Another thing to consider is the the manifold vaccum will have a flase "low" when an aggressive cam grind is used. The large overlaps and long durations will reduce the effectiveness of any vaccum advance set up. In a normal set up, the vacuum advance will be good for 12-15 degrees advance, 15 at 20"hg and about 2 at 5"hg. the advance is not going to have that large of an effect.

And BTW, I know how they work..... I wrote ECM code for a while when I was at Chrysler. The OEM made a ginormous mistake when they programmed some of the first California EFI systems and when they came off the truck, they would hardley move............... They did not take into account the aggressive EGR operations and were turds....



GT
« Last Edit: April 25, 2016, 10:49:07 PM by GT Jets »
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

propless

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Re: Vacuums advance vs mechanical for my distributor
« Reply #6 on: April 26, 2016, 02:05:13 AM »

Another thing to consider is the the manifold vaccum will have a flase "low" when an aggressive cam grind is used. The large overlaps and long durations will reduce the effectiveness of any vaccum advance set up. In a normal set up, the vacuum advance will be good for 12-15 degrees advance, 15 at 20"hg and about 2 at 5"hg. the advance is not going to have that large of an effect.


 This why you are having trouble with it. You cant use a "normal" vacuum advance canister with a performance engine. There are canisters out the that will give you 20 deg advance with 10" of vacuum. Its all abut using the right canister. It has to be right for the vacuum levels of the engine, and give the right amount of advance.  There are so many available finding the right one can be worse than trying to find unicorn feathers.  That's why they make adjustable ones.  You will never get a canister like you described to work. It needs to be maxed out at idle, and nobody with a respectable boat can pull 20" of vacuum at idle, heck I cant even pull that much vacuum with the El Camino.  If you have 12" of vacuum at idle, you need a canister that will go full advance at about 10" of vacuum.  A canister like that would still be giving you about 10 deg advance at cruise. 10 deg will make a difference in efficiency at cruise.  If you have 4" of vacuum at cruise now with your mechanical advance only. And you add vacuum advance. It will add timing, adding timing will increase vacuum. You will probably end up with closer to 5 or 5.5" at the same RPM.  It will also lower the high EGTs you have because the power valves are closed and the engine is in "lean mode".

 Side note. If your running an HEI, forget about it. There are NO canisters for the HEI that will work with a healthy performance cam/low vacuum. Trust me, I searched.  :banghead: The "good" canisters only fit the early (pre HEI) GM distributors or aftermarket "small body" distributors. By the time GM went to the HEI they didn't have any engines that had anything close to a performance cam, and the adjustable cans for the HEI don't adjust low enough to go full advance with a healthy cam.  Vacuum advance HEIs are only useful on MILD engines or when hooked to ported vacuum, but then you loose all the advantages of the vacuum advance at idle. 



   Most jet boats I have owned run anywhere between 3,800 and 5,500 RPM, I really don't see it affecting my fuel budget.


What the hell have you been up to? Tell me you bought the Daytona back from John.......... Please tell me that......

$10k was criminal cheap.....

GT

 Nothing will effect YOUR fuel budget, if you don't try it. The idea is correct, and it will work, just needs the right parts.  And no, my mag doesn't have vacuum advance.  But if my boat was a lake boat that had to go out all day, pull kids around on floaties, cruise around as much as possible, and if it was a fuel sucking jet, I would defiantly use any possible thing out there to help with efficiency.

 John sold the Daytona ? I didn't know that.  10K was way to cheap for that boat !!!  My Daytona now is a Daytona "sprint", V-drive flatbottom.  >:D
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73sidewinderguy

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Re: Vacuums advance vs mechanical for my distributor
« Reply #7 on: April 26, 2016, 10:17:54 PM »
Thank you both for the knowledge . It definitely made a good read


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Re: Vacuums advance vs mechanical for my distributor
« Reply #8 on: April 26, 2016, 10:39:24 PM »
I guess where you're losing me is the fact that you think it will provide a noticeable increase in fuel economy and performance. I don't think it would be measurable in s jet boat. I played with it on an I/O (not "vacuum" but a programmable ECU in a configurable laptop) and the window of  improvement was so miniscule that I found the tuning to be more trouble than it was worth.

How can we prove it? I have three potential Guinea pigs.
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
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Ray

propless

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Re: Vacuums advance vs mechanical for my distributor
« Reply #9 on: April 27, 2016, 01:25:19 PM »
Thank you both for the knowledge . It definitely made a good read


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 LOL, you should see some our older "discussions". GT is a smart guy and full of great knowledge, but sometimes he gets stuck in his old "just lock the timing and run it" caveman style ideas.  ::) 

I guess where you're losing me is the fact that you think it will provide a noticeable increase in fuel economy and performance. I don't think it would be measurable in s jet boat. I played with it on an I/O (not "vacuum" but a programmable ECU in a configurable laptop) and the window of  improvement was so miniscule that I found the tuning to be more trouble than it was worth.

How can we prove it? I have three potential Guinea pigs.

 Wont be any "performance" (terminology ?) increase, just a potential increase in efficiency at lighter load situations.   Just think, if a typical jet boat with a 20gallon fuel load can increase fuel economy just 1 mile per gallon (proper carp tuning and vacuum advance, its all a combo). That can add quite a bit to the days fun time till the tanks are empty. Come on GT, how many hours have you spent trying to find ways to get that last little bit of efficiency out of the pumps, the hulls, and finding ways to save weight. You know that all helps with efficiency at cruise too. So as long as this idea doesn't hurt your WOT performance (and it wont) why not give it a chance and try it ?  Remember, not all jet boats are race boats.  Some people use them the same way you use that half wacker boat of yours.  It could mean the difference between having the fuel to give the kids that one last tow around the lake on the tube vs just trying to get back to the ramp without a tow rope or paddle.

 To be fair. Even your usual answer of "lock the timing at 36 deg" is better than a typical mechanical advance only distributor on a boat.  How many time have you heard and seen people with a typical "billet" mech advance only distributors they ordered from summit because that's what the "racers" use, they set the total timing, and whatever they get at idle is whatever they get.  They don't have enough timing at low RPM, there is no vacuum, power valves are open at idle,  they idle like crap because fuel is dribbling out of the boosters, and they blame the carb.  ???  We've all seen them, blubbering out through the no wake zone barely able to keep the things running, ands usually getting towed back in.
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mash on it

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Re: Vacuums advance vs mechanical for my distributor
« Reply #10 on: April 27, 2016, 01:42:36 PM »
Proppy-

At 'just 1 mile per gallon' is nearly a 50% increase (on my junk). Not gonna happen. On my sbc jet, 1 mpg would be 20% increase...still ain't gonna happen.

And use the gas dock, if ya need more fuel. Get a sailboat if ya want efficiency.

What about spark chatter, directly resulting from a vacuum advance can?

Float test ALL vacuum advance canisters, first.

Dan'l
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GT Jets

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Re: Vacuums advance vs mechanical for my distributor
« Reply #11 on: April 27, 2016, 01:50:41 PM »
I've never once insisted locking it down was the answer. I am an "all in" by 2500 rpm guy though.

And it's smiles power gallon. Not miles. It's not about how far you go,  it's how much fun you had getting there.

GT

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  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

FordLover

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Re: Vacuums advance vs mechanical for my distributor
« Reply #12 on: April 27, 2016, 01:59:38 PM »

It's not about how far you go,  it's how much fun you had getting there.

GT

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Preach it brother. Might be the best thing you ever said.
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propless

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Re: Vacuums advance vs mechanical for my distributor
« Reply #13 on: April 27, 2016, 06:05:10 PM »
Proppy-

At 'just 1 mile per gallon' is nearly a 50% increase (on my junk). Not gonna happen. On my sbc jet, 1 mpg would be 20% increase...still ain't gonna happen.

And use the gas dock, if ya need more fuel. Get a sailboat if ya want efficiency.

What about spark chatter, directly resulting from a vacuum advance can?

Float test ALL vacuum advance canisters, first.

Dan'l

 1 MPG, probably at least 30 gal on board (I assume its a 21 cruiser ?), even if it can increase to 1.2 MPG thats an extra 6 miles of play time.

 Please explain what you refer to as "spark chatter, directly resulting from a vacuum advance can".

 BTW, heres a quote that might make you think about your set up.
 
  I ran my boat yesterday - Tunnel ram, twin 450's, 8.5 PV's (looks like I need 7.5's) 3 adults,2 kids coolers with drinks, lunch etc. 50 gal of gas on board, crusing the channel, driving to steamboat, pulling the kids on the raft, idling for long periods of time. Averaged 5.5MPG (GPS based Miles).



I've never once insisted locking it down was the answer. I am an "all in" by 2500 rpm guy though.

And it's smiles power gallon. Not miles. It's not about how far you go,  it's how much fun you had getting there.

GT

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 Don't make me dig up the last time we "discussed" this and quote you, because I don't remember what forum it was.  ;) You basically stated that locking timing will give you the advance needed to give the engine a clean idle with a healthy cam and it wont hurt anything because jets have no/low load at low RPM. 

 Smiles per gallon, great plan.  And fits right into my plan.  The longer that gallon lasts, the more smiles can be had, as long as it doesn't cost you performance.  :thumbup:
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Re: Vacuums advance vs mechanical for my distributor
« Reply #14 on: April 28, 2016, 11:52:39 AM »
1 MPG, probably at least 30 gal on board (I assume its a 21 cruiser ?), even if it can increase to 1.2 MPG thats an extra 6 miles of play time.

 Please explain what you refer to as "spark chatter, directly resulting from a vacuum advance can".

 BTW, here's a quote that might make you think about your set up.


My day cruiser is at 2-2.5mpg, holds 46-48 gals. I use 'river miles' for calcs, 26 mi, 11-13 gals.

Lil 18' bowrider, SBC powered, ~5 mpg, holds 11/side, 70 mi, 13-14 gals. Both were on the river, not a lake.

Because the 'vacuum can(ister)' moves the pick-up coil, the timing (spark) is all over the place, about a 10-15* area under high(er) rpm, 4500+. This was without vacuum to the can. With the vacuum can removed, and the pick-up coil mounted solid, this went away. Timing stayed where it should, ~36*. Motor seemed happiest there.

Dan'l

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Re: Vacuums advance vs mechanical for my distributor
« Reply #15 on: April 28, 2016, 01:14:44 PM »
My day cruiser is at 2-2.5mpg, holds 46-48 gals. I use 'river miles' for calcs, 26 mi, 11-13 gals.

Lil 18' bowrider, SBC powered, ~5 mpg, holds 11/side, 70 mi, 13-14 gals. Both were on the river, not a lake.

Because the 'vacuum can(ister)' moves the pick-up coil, the timing (spark) is all over the place, about a 10-15* area under high(er) rpm, 4500+. This was without vacuum to the can. With the vacuum can removed, and the pick-up coil mounted solid, this went away. Timing stayed where it should, ~36*. Motor seemed happiest there.

Dan'l

 Interesting, not sure what would make the timing change like that at high RPM if the canister was in good shape.  The spring in the canister should hold timing steady at your base mechanical curve if there is no vacuum pulling on the diaphragm.  My guess without seeing checking the canister would be a damaged/broken/overly weak spring in the canister letting the diaphragm flop around. There should be no "slop" on the diaphragms rod going to the pick up. Spring pressure holds the diaphragm / rod at base timing curve until vacuum on the diaphragm is great enough to compress the spring and advance the timing. 
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