Vac advance, ported or manifold?

WORTH said:

Not true, with high rpm and low throttle, they could be the same.

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How can one acheive a high rpm and not apply much throttle at the same time? I guess if he were running 9.90 gears.

Ozsum2 said:

How can one acheive a high rpm and not apply much throttle at the same time? I guess if he were running 9.90 gears.

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Simple, when your car's in park or neutral, or when going down a large hill in low gears.

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.

Anyone ever have a dual diaphragm distributor on their engine? How is that hooked up to vacuum?

ok, I read all this, and no ones addressed my questions, so I'll continue to wait

*poke poke*

bull999999 said:

Simple, when your car's in park or neutral, or when going down a large hill in low gears.

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What are you smokin? If the car is in neutral or park, are the rpms high?

351LX said:

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.

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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

Cdaniel said:

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.

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Good idea.

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.

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.

Cdaniel 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.

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Hang in there. If I can make mine purr at 2 miles high, you can do it.

Cdaniel said:

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.

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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