Mopar Vacuum advance is RETARDING throttle...???

[jenkins71]

Hey Guys. So I've been tuning the 440 in my 71 charger lately. Trying to get the best fuel economy out of it. about 300 miles on the 440 rebuild. Mild hop-up.

So, I was checking the timing with a light yesterday, and I had the vacuum advance hooked up. At idle I was set at a steady 20*. Then I cracked the throttle, and to my surprise, the timing went DOWN probably about 10*! I unhooked and plugged the vacuum advance, and revved it up again and watched my normal mechanical advance work as intended, going up to about 38* total.

So, I kept the vacuum advance disconnected, and went for a drive, and OH MY GOD, so much better at part-throttle. No more surging, and fuel economy seems significantly increased.

Now, I really want to run my vacuum advance as I do a lot of highway driving and want the extra MPGs. Anyone have any idea why the vacuum advance was RETARDING the timing? Any idea for a fix?

It's a stock Mopar distributor, the old style.

 

[bigmanjbmopar]

Was it hooked up to ported on the carb or full manifold? Sounds like manifold which explains what your describing.

Read this.

http://www.mopar1.us/engine2.html

 

[jenkins71]

No, it's hooked up properly to ported.

 

[SlinktRR]

Hey Guys. So I've been tuning the 440 in my 71 charger lately. Trying to get the best fuel economy out of it. about 300 miles on the 440 rebuild. Mild hop-up.

So, I was checking the timing with a light yesterday, and I had the vacuum advance hooked up. At idle I was set at a steady 20*. Then I cracked the throttle, and to my surprise, the timing went DOWN probably about 10*! I unhooked and plugged the vacuum advance, and revved it up again and watched my normal mechanical advance work as intended, going up to about 38* total.

So, I kept the vacuum advance disconnected, and went for a drive, and OH MY GOD, so much better at part-throttle. No more surging, and fuel economy seems significantly increased.

Now, I really want to run my vacuum advance as I do a lot of highway driving and want the extra MPGs. Anyone have any idea why the vacuum advance was RETARDING the timing? Any idea for a fix?

It's a stock Mopar distributor, the old style.

Which ECU are you using. This is common with some aftermarket ECU's.

 

[440roadrunner]

You might be able to play with ..............can't recall what they are called.........Ford used them...........vacuum delay units. These were small devices went in series with the vacuum hose and delayed vacuum "come on."

AAAAHHHHH!!! "spark delay valve"

Also "different carbs" have different port part throttle characteristics. "Back" in the 70's I used to run QJ on my 340. Had trouble at part throttle.....pinging.......caused by early "come on" with vacuum advance. You'd "take off" and "light part throttle" it might ping, give it more---which killed the vacuum some.......and it would stop ping.

 

[440beep]

Is the vacuum advance adjusted properly? Improper vacuum advance will cause the surging while normal cruising.

 

[jenkins71]

Ok, so I took the cap off and sucked on the hose to the vacuum canister directly with my mouth. The little arm definitely moves properly. My concern is maybe it is pulling it the wrong way. This distributor was put together with parts from other distributors. I have another vacuum canister, which appears to be the mirror image of the one in it now, and it won't fit. I am on to something?

 

[Homstead]

Post a pic of your dist. Small blocks turn in the opposite direction. Maybe some small block parts in it? Just guessing.....

 

[A383Wing]

yea, I'm thinking it has a small block vac advance on it

 

[vitamin c]

I don't think a small block vacuum advance will fit on a B or RB distributor

 

[AMX364]

If it runs better on manifold vacuum isn't the engine telling you something? Ported helped on emissions, not on performance. The Ford delay valves slow down the changes and can SOMETIMES helps on tuning, try different things, keep a record and use what helps.

 

[A383Wing]

I don't think a small block vacuum advance will fit on a B or RB distributor

he said he used parts from other distributors....wonder if he used a small block plate inside with small block vac can?

 

[383man]

Many need to remember that ported vacum is still intake manifold vacum but its just above the throttle blade so it dont have any vacum at idle. But once the ported vacum port is opened its the same as manifold vacum and will be high at a steady part throttle cruise and will drop to 0 at full throttle. It can come on a bit slower then full manifold vacum because of where the port is located in the carb but it is basically manifold vacum. Ron

 

[vitamin c]

possible, but that's some pretty creative distributor work!

 

[pishta]

Many need to remember that ported vacum is still intake manifold vacum but its just above the throttle blade so it dont have any vacum at idle. But once the ported vacum port is opened its the same as manifold vacum and will be high at a steady part throttle cruise and will drop to 0 at full throttle. It can come on a bit slower then full manifold vacum because of where the port is located in the carb but it is basically manifold vacum. Ron

I believe you are incorrect... Ported vacuum increases with the velocity within the carb venturi, ie when the car is under WOT. The velocity through the venturi runs across the ported vacuum port on the side of the venturi (or a little under it) sucking vacuum out of the nipple, or creating vacuum. Its just like a header evac system... or a venturi vacuum pump that uses compressed air across a pipe end to create (a lot of) vacuum. The faster the velocity across the port, the more vacuum it creates, ie draft tubes in the early 60's cars for crankcase ventilation didnt work unless the car was moving. Manifold vacuum is almost the opposite: It is highest when the carb is restricted, ie at idle. When the carb cracks the throttle, vacuum (below the throttle plate, ie manifold) is bled off and manifold vacuum goes down. There is 3 times when both ported and manifold vacuum are the same, one is when the car is accelerating and the ported vacuum is rising due to the increase in the velocity of air through the venturi, and the manifold vacuum is falling as it approaches WOT, The vacuum values will cross at one point. 2nd is the opposite circumstance, they will cross again when decelerating. 3rd is when the car is not running. The vacuum delay valve (from what I have read in a Mazda FSM) allows vacuum (ported in this case) to pull the advance in a typical manner in real time to meet engine demands, but will HOLD that vacuum level between the valve and the distributor for 2-8 seconds to allow the distributor to NOT snap back to 0 vacuum advance when the throttle is closed abruptly, ie braking. This reduces NOx emissions, Thats all it does.

 

[Photon440]

I believe you are incorrect... Ported vacuum increases with the velocity within the carb venturi, ie when the car is under WOT. The velocity through the venturi runs across the ported vacuum port on the side of the venturi (or a little under it) sucking vacuum out of the nipple, or creating vacuum. Its just like a header evac system... or a venturi vacuum pump that uses compressed air across a pipe end to create (a lot of) vacuum. The faster the velocity across the port, the more vacuum it creates, ie draft tubes in the early 60's cars for crankcase ventilation didnt work unless the car was moving. Manifold vacuum is almost the opposite: It is highest when the carb is restricted, ie at idle. When the carb cracks the throttle, vacuum (below the throttle plate, ie manifold) is bled off and manifold vacuum goes down. There is 3 times when both ported and manifold vacuum are the same, one is when the car is accelerating and the ported vacuum is rising due to the increase in the velocity of air through the venturi, and the manifold vacuum is falling as it approaches WOT, The vacuum values will cross at one point. 2nd is the opposite circumstance, they will cross again when decelerating. 3rd is when the car is not running. The vacuum delay valve (from what I have read in a Mazda FSM) allows vacuum (ported in this case) to pull the advance in a typical manner in real time to meet engine demands, but will HOLD that vacuum level between the valve and the distributor for 2-8 seconds to allow the distributor to NOT snap back to 0 vacuum advance when the throttle is closed abruptly, ie braking. This reduces NOx emissions, Thats all it does.

That is not all correct. Ported vacuum is not connected to the center venturi itself and is not relying on venturi pressure drop. It operates just like manifold vacuum with the main difference that it only begins to function after part throttle tip-in. At wide open throttle, both ported and manifold vacuum will be the same; nearly zero. Ported vacuum, in this example, is 'C', waiting for the throttle plate to move to expose it to underlying manifold vacuum. Regular manifold vacuum would be routed through 'E'.

 

[pishta]

Thank you...explain what and how vacuum secondaries open on a 3310.

 

[Geoff 2]

Photon is correct.

Holley Vac secs get the signal for the vac pod from the primary venturi, not the pri t/bore. As airflow through the pri venturi increases with rpm, there is a low pressure [ suction ] created in the pri venturi & this 'sucks' on the sec diaphragm to open the secs.