[Mad Maggot]

I know that hooking up the vacuum advance to manifold vacuum will produce advance whenever there is a vacuum in the manifold, such as at idle, cruise, and light throttle, and will produce no vacuum at WOT. I also know that hooking up the vacuum advance to the port on the carburetor will only provide vacuum when the throttle blades are high enough to cover the vacuum port, such as at cruise and part throttle, and will produce little vacuum at idle and WOT. The question is, what is more desirable for a high performance motor?

[topfuel]

Which port you connect the vacuum advance to will ultimately depend on your combination. I can tell you the factory used a basic rule of thumb to determine which port the hose was connected to. If a vehicle had a manual transmission, it was connected to a ported vacuum source. The port is above the throttle blades.This produced no advance at idle as there was no vacuum at idle. Vacuum increases as the throttle is opened.
In a car with an automatic trans the hose is connected to a full time vacuum port. Full manifold vacuum is present at idle and decreases as the throttle is opened. The port is below the throttle blades. The reason for this is to increase low speed torque with the increased spark advance.

I hope this helps.

[Mad Maggot]

seeing as I have an automatic tranny, I will assume that I should connect it to the manifold vacuum. I always thought this defeats the purpose of setting initial timing at a set degree, because the vacuum advance will always be present, and to varying degrees from different camshafts causing different idle vacuum levels. If I run the vacuum advance from the manifold, should I set the initial advance to produce the most efficient idle, determined by the amount of vacuum present?

[topfuel]

You want to plug the line to the vacuum advance when setting the timing. After setting the timing, hook the vacuum line back up and adjust your idle speed if necessary. When checking you mechanical (centrifugal) advance, the vacuum line must be plugged as well.

[willys36@aol.com]

http://www.hotrodders.com/t55364.htm...VACUUM+ADVANCE

http://www.hotrodders.com/t55248.htm...VACUUM+ADVANCE

http://www.hotrodders.com/t40086.htm...VACUUM+ADVANCE

http://www.hotrodders.com/t38205.htm...VACUUM+ADVANCE

[454C10]

Man. You are openning a can of worms!

FYI, in the most recent Chevyhiperformance, in the Q&A section in the back, In the answer, the tech guy wrote the ported source was in most cases, the best one to use.

[camaroman7d]

454C10

Man. You are openning a can or worms!

I thought the exact same thing when I read the title. This is one topic that will never be agreed on. Use the search function and you will find more info (opinions) than you want.

I will go as far, as to say it depends on the engine/vehicle.

Royce

[Mad Maggot]

thanks for the wake up call, willys36. I should have checked this

[65elky]

myself i like ported most sbc with much cam likes at least 18* initial and 36* at 3000 unless vortecs and they still like at least 18* initial and 32-34* at 3000 set like that and then adjust pump cams and tune carb

[454C10]

Wow, this post is back from the dead. Where do you find these old posts?

In 2005 I preferred ported but now (7 years later) I found manifold works best for my setup.

Turns out ported vacuum advance is a type of emissions setup.

read this

http://www.corvette-restoration.com/.../Timing101.pdf

[timothale]

57 and later Fords used manifold vacuum. (prior to all the hoses and various devices for emission control) 56 and earlier were different dist and carb's. and needed to be matched, Old Y blocks that run a newer carb also need the newer distributor.