Hey, guys. I've only been on this forum for a short while, but it's been long enough that I've noticed that the distiction between ported and direct manifold vacuum when setting up distributor vacuum advance is something of a hot-button issue around here.

I've tried both ways, and to be perfectly honest, I can't notice any difference whatsoever. I've also tried disconnecting the unit entirely, which does lessen the tendency to pre-ignition, but that's about it.

What difference(s) am I supposed to notice between ported and direct manifold vacuum?

 

I believe it is whether it is there or not at idle.

 

Okay, but do I need it at idle? Do I want it? What difference is that supposed to make to the car's drivability?

 

Having vacuum advance at idle already (ie, using manifold vacuum) means the distributor doesn't have to transition as you roll into the throttle. This might give you better throttle response. It also might give you better idle quality. The more radical your cam, the more this might be noticeable. Or not. There's a lot of factors going into both throttle response and idle quality, and an adjustment here can make up for something else over there.

 

Lets back up a bit. With manifold vacuum your vacuum canister at idle will add the amount of advance that the canister is rated or set at. With ported is does not add any. So at idle there is a difference. Once you give the engine throttle the carb transitions out of the idle circuit and there is vacuum on the canister either way so there is no difference. At WOT vacuum drops to 0 therefor the vacuum canister provides no advance and the distributor only uses mechanical advance.

The main reason people have detonation when using their stock vacuum advance is when the distributor is tuned with way more advance than stock. Meaning you max'd your timing to 36-38 at a desired rpm around 2800-3000. Then you plug your vacuum advance in and your total timing with vacuum exceeds 50 degrees BTDC considerably at normal cruise and high vacuum.

So in review, if the distributor hooked to manifold vacuum the timing will advance and the rpms would go up considerably, you would have to readjust the idle speed down at the carb. If the distributor was hooked to ported there will be no noticeable difference at idle. Its not that the advance is connected to one or the other, its how the whole package works together.

 

All right, so I get it that with direct manifold vacuum there is advance right away at idle. But seeing as vacuum drops to zero as soon as the throttle is opened wide, allowing mechanical advance to take over, what advantage is actually gained by this? Or conversely, what disadvantage is there to using a ported source which will provide no vacuum advance at idle?

And what difference should I be able to notice in real-world driving? Is it better throttle response due to the fact that advance is already there as BlackGold suggests? Because as I say, I get no difference at all.

 

The advantage with hooking to manifold vacuum is the engine will run a bit cooler at idle. If you have a big cam it tends to smooth it out a bit. Once its off idle there is basically no difference, it works the same.

 

Okay, well that's something. I don't have a problem with overheating and my cam's only a little shaggier than factory ... so I guess I'll leave it running off ported. Thanks, Eric.

 

The better throttle response I was talking about (which is not guaranteed) is the "tip-in", as you just start pressing the pedal off of idle. This is a transition region -- for both the fuel and ignition -- and having the advance already engaged may help. If you see no difference, don't worry about it.

With an aggressive cam, added advance at idle allows you to adjust your throttle blades further closed. This in turn can ensure that your carb is really idling on its idle circuits and not draining the transition slot, thus preventing a lean condition as the throttle blades start to open and before the pump shot takes effect.

 

I was thinking about this again and I'm resurrecting this zombie thread for one more question before we bury it forever:

What did the factory do? When you bought an Oldsmobile fresh off the showroom floor back in the day, how was the vacuum advance set up, ported or direct manifold?

 

On the 69, it was ported.

 

X2, it was ported. Also the factory timing settings were very conservative leaving power on the table. Its not hard to experiment and fine tune the engine, just write your current settings down and note the changes you make. You can always go back.

 

Got it. Thanks!

 

The factory only went to ported vacuum in order to meet emission requirements. Having no vacuum advance at idle reduces certain emissions, however it raises engine temperature. There is a switch on the intake which allows for advance if temperatures get too high. By 71 advance only is allowed once in high gear or as stated if temperatures climb to a certain level. Prior to emission standards engines ran on manifold vacuum.

 

As rocketbrain posted, it depends upon what year you are talking about as both were used.