Vacuum advance.. timed/ported or manifold

[DJCarbine]

So I am doing my little bit of researching on the net and stumble across a webpage about windsor engines. Cool, I decide to read it.... and come upon this.

Vacuum advance...GM engines used manifold vacuum to provide full vacuum assisted advance at idle and part throttle cruise to promote smooth running and increased mileage. Some early Fords used ported vacuum to the distributor. Switching to a manifold vacuum source and making the appropriate timing adjustments will smooth out the small Ford's idle and make it more efficient around town and on the freeway. Be carefull to avoid spark knock, though. Use good fuel!

Taken from - http://www.jalopyjournal.com/forum/archive/index.php/t-1525.html


Is there any truth to his saying? I was always in the mindset that ford distributors needed ported vacuum, not full manifold vacuum

Any input on this?

 

[66Runt]

Yes it can, depending on the application

Ported vacuum is effective through a fairly short area of throttle valve opening. Where manifold vacuum is somewhat independent of throttle valve position and is mostly dependent on engine load, and the range is much broader.
So if you spend the time to tweak your static, mechanical and vacuum advance, you get a broader range of variable timing.
It is the broad range of variable timing based on engine load that is the key to performance and economy.

HTH,

Scott

 

[180 Out]

There is no good reason to run ported vacuum in preference to unported. For some reason the "fancy" way seems to have gained popularity, but the reason you run vacuum advance is for improved fuel economy at part throttle, and at part throttle ported vacuum and unported are identical.

 

[Max Power]

The only benefit I have heard, and this topic is hotly debated, is that sometimes ported vacuum cures a small stumble or hesitation at idle.

This is especially true on cars with vacuum challenges from things like large camshafts.

Consider this scenario. You have a big cam and moderate vacuum. You use a manifold source for your vacuum advance, which actually improves idle a little bit. The problem is, when you crack the throttle, or apply a slight load, like putting an automatic trans in gear, the rpms drop and the vacuum drops. This causes your vacuum advance to back off, and the timing actually retards at a time when you least need it to. This will cause a stumble just off idle, and/or a stumble when putting the car in gear.

Engineering your advance curve so that your car idles and runs well strictly on mechanical advance and using ported for additional advance is better than relying on manifold vacuum advance in the same scenario. It is more consistent, especially in performance applications.

That being said, many cars come from the factory using manifold vacuum for vacuum advance. This works OK on stock engines, but one should seriously consider re-engineering to ported for low vacuum performance cam applications.

Both can work, but I have come to believe that ported is more effective for performance applications. Each need their mechanical curves tailored appropriately, and most Fords from the factory are curved for ported.

 

[66Runt]

Idle set too low with agressive cams

I've seen this where the idle is set too low with an agressive cam. Usually compounded by a stock torque converter, and a rich idle mixture.
Timing is a poor band aid for that.

 

[180 Out]

I've seen this where the idle is set too low with an agressive cam. Usually compounded by a stock torque converter, and a rich idle mixture.
Timing is a poor band aid for that.

I see what Max Power is saying, and it makes sense: if you set your base timing with the vacuum canister attached to un-ported vacuum, you will lose that vacuum source on tip in, which will pull out timing and which will cause a stumble. Thing is, I thought you were always supposed to set base timing with the hose removed from the vacuum canister and plugged. I never thought about the "why," I just did it, but this must be the "why."