[4vxc]

just wondering where people hook vacum advance up to, its a LPG 351

 

[soverign]

just wondering where people hook vacum advance up to, its a LPG 351

is it dual (petrol/lpg),or is it straight lpg...

 

[4vxc]

straight gas, bosch electronic dizzy

 

[soverign]

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?

 

[4vxc]

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

 

[soverign]

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...

 

[73falcongt]

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.

 

[soverign]

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...

 

[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.

 

[soverign]

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...

 

[73falcongt]

Okay, so basically there can be two different type of vac advance setups, one where the vacuum to the vac advance unit comes from manifold vacuum, and the other from venturi vacuum. So these two setups do completely the opposite thing yeah? Venturi vacuum setup will increase timing as throttle opens off idle and manifold vacuum will retard it. So these are completely different things that happen, I'm wondering how come the rules don't stay the same for engine timing. Or is it that only the vacuum levels are reversed but the vac advance unit on different distributors read them differently, for example on some distributors maximum vacuum advance is with no vacuum and with others it's with full vacuum coming from the hose? No? Or are they simply two different methods of timing for different engines, one where an engine receives more advance as the throttle is opened, and another where the timing is retarded slightly when the throttle is opened. Thanks.

 

[soverign]

The vacuum advance mechanism,is basically an economy device...
an engine may require 35 degree's of total mechanical advance to allow the engine to produce peak power under load,however it can,and in most cases will demand more timing when under partial load (cruising conditions),and this extra advance will give far better economy due to conditions that i have mentioned previous...

for a quick test,go and advance the timing when the car is idling...what happens?the rpm picks up,without increasing the throttle angle...so,its burning more of the mix that you are putting into the chamber by giving it more time to burn it...and when you are cruising along at 60-80 km/hr,your engine is very low in the rpm range,and you will find that it is pulling around 15-20" of vacuum...so your vacuum advance will be in operation,just where the engine needs the extra burn time...soooo you can have less throttle angle for the same ground speed...

yes manifold and venturi vacuum are two different ways to bring this extra advance in,and depending on what YOUR dissy pod is set up to operate on,you could bring in to much if you had it hooked up to the wrong pick up point...and vice versa...if you have a dissy set up to run off of venturi vacuum,then that distributor is designed to read only a partial amount of the full manifold vacuum (2-5 degrees extra),whereas manifold vacuum setups will bring in up to 12-15 degree's extra...it just depends on what your particular combination requires...you can customise your timing setup to what ever you want,going on what your engine and car as a whole likes according to what it's APPLICATION is...

 

[73falcongt]

Yep, I get it, theres so many extra questions, but for the most part they're very specific and I've asked enough for now. Thanks for the info soverign, you know your stuff.