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Old 05-09-2006, 03:31   #1 (permalink)
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Vac advance

just wondering where people hook vacum advance up to, its a LPG 351
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Old 05-09-2006, 05:03   #2 (permalink)
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Re: Vac advance

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Originally Posted by 4vxc
just wondering where people hook vacum advance up to, its a LPG 351
is it dual (petrol/lpg),or is it straight lpg...
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Old 05-09-2006, 05:12   #3 (permalink)
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Re: Vac advance

straight gas, bosch electronic dizzy
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Old 05-09-2006, 05:32   #4 (permalink)
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Re: Vac advance

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straight gas, bosch electronic dizzy
i know that both the bosh and motorcraft POINTS dissy's were intended to be run off the manifold vacuum...however unsure of the electronic dizzy's...

is it a fresh engine?what sought of combo?have you built it to use the LPG purposely?
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Old 05-09-2006, 06:06   #5 (permalink)
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Re: Vac advance

10 years fresh, built for gas 351 2v, impco 425, small cam for towing, been on gas from the start. I have the vac advance hooked up to manifold vac but am not sure if thats correct
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Old 05-09-2006, 06:39   #6 (permalink)
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Re: Vac advance

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10 years fresh, built for gas 351 2v, impco 425, small cam for towing, been on gas from the start. I have the vac advance hooked up to manifold vac but am not sure if thats correct
if it hasn't been giving you any problems,and hasn't been pinging or detonating,i wouldn't worry about it..

with a gas engine one of the most important things to have(not unlike any engine really),is a strong ignition system..and ,everything being equal,your amount of advance timing and advance RATE..

LPG will generally have a higher octane rating and a slower burn RATE...
so,this real gaseous mixture entering the chamber will have a higher electrical resistance...the ignition side of a LPG engine is VERY important...
due to the slower burn rate in the cylinder,generally to get the most out of this combo you need to have the ignition timing advanced(start the burn earlier)...with this type of set up,cylinder temp's will be higher,so a colder plug would generally be required,and you would also tend to keep the plug gap on the smaller side to help overcome this higher electrical resistance of the mixture...

this extra strain on the ignition system demands regular maintanince(especially if it is getting on in age)...generally you should have the ignition timing advanced by up to 4 degrees over a pertol engine (give or take),and have the ignition in very good condition...

vacuum advance should be alright taken from the manifold,the engine should like the extra advance down low in the rpm and with light engine loads..


just remember,with a petrol engine you CAN have a slightly weaker spark and it will run okay,however,with LPG it can cause great power loss and misfiring especially under load...
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Old 05-17-2006, 10:58   #7 (permalink)
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Re: Vac advance

Manifold Vacuum actually drops as throttle is opened. As far as I knew the hose connecting to the vac. advance nipple of the dizzy should increase with vacuum (that is suck more) as the throttle is opened. I don't think thats what you get with manifold vacuum it's the opposite. Someone correct me please if I'm wrong, I'm curious myself.
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Old 05-18-2006, 04:46   #8 (permalink)
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Re: Vac advance

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Manifold Vacuum actually drops as throttle is opened. As far as I knew the hose connecting to the vac. advance nipple of the dizzy should increase with vacuum (that is suck more) as the throttle is opened. I don't think thats what you get with manifold vacuum it's the opposite. Someone correct me please if I'm wrong, I'm curious myself.

Manifold vacuum is high when your engine is at idle...when your throttle blades are closed...as you open your throttle blades you are taking away the restriction and now your manifold vacuum drops...manifold pressure increases.

when you are using manifold vacuum your vacuum point is taken from below the throttle blades,so as you open the throttle blades you are taking away the pressure differential,which is what a vacuum is...

so,if you are running vacuum advance,and your pickup point is from below the throttle blades,then as you increase the throttle angle,your vacuum advance drops off,and that is relative to how much vacuum you had,and what rating is in your vacuum pod on the dissy...basically...
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Old 05-18-2006, 06:22   #9 (permalink)
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Re: Vac advance

Mmmmm, I thought that timing was advanced the more load the engine was under. Now I've found out the more load the engine is under, the less timing it receives. Why is this? I originally thought that the point of the vacuum advance was to add more timing as the throttle was OPENED, because more mixture requires longer to burn, why is it the opposite way?

Also, I thought that was one reason why old dizzy style cars would tend to ping under extreme load, like uphill or full throttle in high gear, because the vacuum advance mechanism would weaken and allow too much timing advance at full throttle under load. Mmmmmmmm, shows what I know.

My Suzuki I had, the dizzy vac. advance hose I'm pretty sure increased in vacuum, that is it would suck more the more load the engine was under, and the more vacuum the earlier the timing would be. So that hose would suck the most when opening the throttle.
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Old 05-18-2006, 07:13   #10 (permalink)
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Re: Vac advance

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Originally Posted by 73falcongt
Mmmmm, I thought that timing was advanced the more load the engine was under. Now I've found out the more load the engine is under, the less timing it receives. Why is this? I originally thought that the point of the vacuum advance was to add more timing as the throttle was OPENED, because more mixture requires longer to burn, why is it the opposite way?

Also, I thought that was one reason why old dizzy style cars would tend to ping under extreme load, like uphill or full throttle in high gear, because the vacuum advance mechanism would weaken and allow too much timing advance at full throttle under load. Mmmmmmmm, shows what I know.

My Suzuki I had, the dizzy vac. advance hose I'm pretty sure increased in vacuum, that is it would suck more the more load the engine was under, and the more vacuum the earlier the timing would be. So that hose would suck the most when opening the throttle.


say for instance...you will generally say,have 12 degrees of initial advance when you fire up the engine,that you have set with turning the distributor(setting it to fire the plugs 12 degrees before the piston is at TDC)...
now,as you raise the rpm of the engine,the distributor will add more advance timing via the weights,springs and plate,say for instance another 20 degrees...
this will come in,by a rate and total amount that you can set by making adjustments to weights,springs etc...this mechanical advance will end at a certain rpm and when this is reached,all of your timing(initial and mechanical together)will be in play...12+22=34...so you will have 34 degree's of total ignition timing...

vacuum advance will be a seperate amount that is brought in by means of vacuum,and depending on where you source the vacuum from will depend on WHEN it comes in,and WHEN it finishes...you have two types of vacuum generally,manifold,which is sourced from under the throttle blades and diminishes as you raise the rpm...and venturi vacuum which will increase to a certain degree as you raise the rpm...this is taken from ABOVE the throttle blades...

so,if you use manifold vacuum,when you start the engine up ,you will see the added vacuum advance straight away,typically up to 15 degrees of extra advance to help with the inefficiencies that the chamber see's at lower rpm...and as your rpm keeps raising ,your manifold vacuum will drop along with that vacuum advance...
if you use ported (venturi vacuum),when you start the engine up you see no extra vacuum advance,but just off of idle and into the rpm a certain amount(depending on a few things),the vacuum advance will add the extra degrees and then drop off as the load and rpm raise more and more...

so depending on when all your advances come in and end at, you could have quite a bit at some rpm(all of them together),you just have to work it all out,find out where your manifold vacuum starts to drop out in relationship with rpm and engine load and what your vacuum pod is set at...

but yes,it is used for lower rpm inefficiences,that the engine see's due to poor mixture quality,exhaust dilution,slow air veocity through the ports,things like this,that as you increase the throttle angle,all tend to get better,hence the engine dosen't need that extra vacuum advance any more...basically...
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