Vac advance, ported or manifold?
- Thread starter Cdaniel
- Start date
-
Sponsors (?)
WORTH
Dirt-Old 20+Year Member
Ozsum2 said:Now wait a sec, you are saying that there is the same vac on the intake manifold, say where the PCV hooks up as there is on the line that goes to the vac advance on the dist?No way!
There can be, but it's just a coincidence. Manifold vacuum is created by the carborator being a restriction and the pistons sucking out of the manifold.
Venturi vacuum is caused by air rushing past a small port in the carborator wall.
All manifold vacuum needs to get to max is a partially closed throttle, but Venturi vacuum needs air rushing by the port.
when you go to WOT the pistons can no longer suck a vacuum in the manifold because you have a GIANT vacuum leak threw the carborator, but as long as air is rushing threw the carborator you will have venturi vacuum.
WORTH
Dirt-Old 20+Year Member
351LX
Founding Member
Last summer I hooked one vacuum gauge to a manifold source and one to a ported source and drove around.Once I got past part throttle, the two gauges were the same.The ported source only shuts off vacuum when the throttle is almost closed, other than that they are the same thing.Don't believe me, then try it yourself.I always hook it to manifold as I get a better idle.
LMan
Founding Member
351LX said:
I don't believe you. Take a slow breath and reread your post. You say the ported shuts off at no throttle. It is supposed to. You say the manifold is more vac and that is why it idles better.
Somebody just put a gun to my head now please. I can't take it anymore. It idles better because you increase vacumm? Naw. It should idle better with no vacumm. Vacumm shouldn't change anything about the idle. You're not supposed to have vacumm at idle. Only at higher rpms. What do you need vac at idle for? The thing just sits there and idles! If you have idle problems, I'd suggest a carb adjustment or rebuild, or look at your points if you stilll have them, or faulty plug wires, etc. NOT low vacumm.
the main difference between hooking your vac advance between man and ported is if you have a ford or a chevy. the old bow ties idle like crap(huffing, dieing when put in gear, etc) at the 8-10 degree setting they like to idle best around 15-20(no joke) but setting the dist there with a small cam you just cant turn the damn things over when they get warm, so man vac for chebbys.
just for a little clarification on ported vacuum, ported vac source is a little slot starting just above the throttle plates when the plates are opened a little, the slot is now partially below the plates giving some vac, at about 1/6 throttle they are completely below the plates giving the same vac as would be on the manifold.
ported vac would give more part throttle torque at lower rpms. the reson it drops off at more throttle is that cylinder pressures increase and the mixture burns faster making ping at lower engine speeds so we need less timing at wot. idealy timeing would always be maxed out to the point before pre ignition at wot and tame at idle but that is very hard to do without the use an ecm and detonation sensor. so we back the mechanical curve down at lower speeds and throw in vac advance for part throttle
but as for us computer impared engine users, 3 reasons for not using ported vac.
1 you have a relatively stock chevy
2 you have a higher compression engine(9.5:1 or so and above) with a low stroke to rod ratio(302, 351c) alot of times this will create ping without retarding the hell out of the timing, or you can get an adjustable vac pod to limit the vac advance.
3you have a hefty cam(280 grind or above) this with me at cruising speed tends to hop skip misfire after total timing has been set to 38 degrees.
hope this answeres your question
just for a little clarification on ported vacuum, ported vac source is a little slot starting just above the throttle plates when the plates are opened a little, the slot is now partially below the plates giving some vac, at about 1/6 throttle they are completely below the plates giving the same vac as would be on the manifold.
ported vac would give more part throttle torque at lower rpms. the reson it drops off at more throttle is that cylinder pressures increase and the mixture burns faster making ping at lower engine speeds so we need less timing at wot. idealy timeing would always be maxed out to the point before pre ignition at wot and tame at idle but that is very hard to do without the use an ecm and detonation sensor. so we back the mechanical curve down at lower speeds and throw in vac advance for part throttle
but as for us computer impared engine users, 3 reasons for not using ported vac.
1 you have a relatively stock chevy
2 you have a higher compression engine(9.5:1 or so and above) with a low stroke to rod ratio(302, 351c) alot of times this will create ping without retarding the hell out of the timing, or you can get an adjustable vac pod to limit the vac advance.
3you have a hefty cam(280 grind or above) this with me at cruising speed tends to hop skip misfire after total timing has been set to 38 degrees.
hope this answeres your question
I don't know about Lman, but that helps/validates things for me a little. Getting back my original question after all this input, I'm thinking the manifold "vac retard" approach is probably flawed thinking. I'll try backing off the initial and adjust idle settings from there until I can find the sweet spot for no part throttle ping. Or, I may just disconnect the vac at the dist. It's adjusted to the minimum anyway at this point.
Cdaniel,
If you are having part throttle pinging under most circumstances it is highly possible that your pistons/combustion chambers are carboned up and the host spots are causing the problem. You might try an upper cylinder cleaner additive to your gasoline. All of the commercial fuel injector cleaners would probably help. I like to do a little steam-air decoking on my older engines but it can be dangerous if you are not careful. Basically you rev the engine at about 2000-2500 RPM while pouring water into the carb....it blows all the nasty stuff out of there but you have to be careful. You can follow up with pouring tranny fluid in there while running as it really cleans well too....plus it makes the biggest and densest white cloud of smoke you have ever seen. If you use tranny fluid I like to change the pugs afterwards. These are old school shade tree tricks that actually work if you are careful.
If you are having part throttle pinging under most circumstances it is highly possible that your pistons/combustion chambers are carboned up and the host spots are causing the problem. You might try an upper cylinder cleaner additive to your gasoline. All of the commercial fuel injector cleaners would probably help. I like to do a little steam-air decoking on my older engines but it can be dangerous if you are not careful. Basically you rev the engine at about 2000-2500 RPM while pouring water into the carb....it blows all the nasty stuff out of there but you have to be careful. You can follow up with pouring tranny fluid in there while running as it really cleans well too....plus it makes the biggest and densest white cloud of smoke you have ever seen. If you use tranny fluid I like to change the pugs afterwards. These are old school shade tree tricks that actually work if you are careful.
pabear89
Active Member
I may just disconnect the vac at the dist. It's adjusted to the minimum anyway at this point. 
Are you saying that it is the lowest point of vac drawing pressure setting now, to make advance move?
Best way I have found to set the vac adv from the PORTED supply.
Need, timing light,vac guage tee'd to ported supply to dist adv.
Check timing and Idle base settings.
Run RPM's above 2500 and record vac reading on port and check timing for amount of full advance.
With vac pump, draw on dist adv and watch the breaker plate movement.
Set the advance adjustment screw so the plate starts to move at 3-5 less than total ported vac reading.
Test and adjust accordingly to get total advance within spec's.
PB
P.s. Those with Duel Diaphrams are to the manifold vac source thru a temp control valve.
Are you saying that it is the lowest point of vac drawing pressure setting now, to make advance move?
Best way I have found to set the vac adv from the PORTED supply.
Need, timing light,vac guage tee'd to ported supply to dist adv.
Check timing and Idle base settings.
Run RPM's above 2500 and record vac reading on port and check timing for amount of full advance.
With vac pump, draw on dist adv and watch the breaker plate movement.
Set the advance adjustment screw so the plate starts to move at 3-5 less than total ported vac reading.
Test and adjust accordingly to get total advance within spec's.
PB
P.s. Those with Duel Diaphrams are to the manifold vac source thru a temp control valve.
Ronstang said:
Thanks, but the engine has about 500 miles on it at this point. Don't think it's had time to make a carbon problem. But thanks for the old school tricks reminders. I'm sure most of my issues are due to the "boutique blends" of crap gas we get forced down our throats by the EPA in Arizona. I'm just still trying chase the elusive perfect combo of adjustments. And it's usually a bunch of compromise to make it operate in the real world of summer desert driving.
pabear89 said:I may just disconnect the vac at the dist. It's adjusted to the minimum anyway at this point.
Are you saying that it is the lowest point of vac drawing pressure setting now, to make advance move?
.
The diaphragm is adjusted to the minimum amount of travel it can apply to the breaker plate. It's gotten to this point by trial and error. I need to get a vaccum pump some day so I can get a bit more scientific about this stuff.
pabear89
Active Member
Cdaniel said:
What you need is the spring tension set to control the timing advance.
not just the movement travel amount.
20 Bucks for something that is alomst a must have item.Might keep you from pulling your Hair out.
PB
Similar threads
