L28 NA.. Port or Manifold Vacuum

[Dkyle]

I am encountering contrasting opinions on where the vacuum advance should be connected to while wrestling with getting my 280Z NA tuned correctly.

http://datsunzgarage.com/engine/index.htm states.."For the vacuum advance dashpot to work properly it needs to be connected to a "manifold vacuum" inlet as opposed to a "port vacuum" type".

Others, including the shop that will put my car on the Dyno this Saturday says it should be ported vacuum.

I've been running it disconnected for now, set the ignition timing at 14deg with total timing around 30deg.  While it accelerates OK, I'm trying to fix the "stumbling problem that the engine has at rpms lower than 2500.

So.. that problem aside.. why is their a difference of opinion on where to get the vacuum advance?  What is the consensus here?

[NewZed]

As soon as you touch the throttle you go to manifold vacuum.   Idle settings and driving settings are two completely separate environments. So when you're driving, you're always on manifold.  Ported vacuum is used to give you a low advance and a steady idle.  Not jumping around due to vacuum pulses.  It's pretty simple if you think it through and the arguments about it are just nonsense.  Hit the gas and the port opens.

 

The datsunzgarage site is just one guy with a Z who's good at putting a web site together.  Not a team of experts.  Of course, I'm just one guy on Hybridz, so it's a break-even.

[AleksS30]

I have experimented with this aswell and I now have my vacuum advance going to manifold vacuum on my L28 with SU's. I like the part throttle behavior of the motor much better using manifold vacuum. I recently did some reading about this and came across an interesting read.

About half way down there is a long post which explain's it all

http://www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

[NewZed]

That guy wrote a lot of words but never addressed what he said he was talking about.

 

The one thing that could be a factor in ported versus straight-to-manifold is the Venturi effect and Bernoulli's principle.  The pressure difference at the throttle body port might be greater than at the fairly static wall of the intake manifold, because the air is moving faster through the small opening at the throttle blade.  That would lead to more advance at low throttle opening for ported than for direct.  You'd have to generate a timing versus throttle opening curve to know for sure, or pressure (vacuum) versus throttle opening, at the two vacuum sites.  Seems like there should be that effect so my comment about "once you open the throttle they're the same" is probably wrong.

 

Logically though, assuming the same pressure differential at both positions, the only reason you should see a change in performance is because you retarded your static timing to lower the idle speed.  Which everybody will do because idle timing will be more advanced with direct.  You'll see an effect but it's not caused directly by the vacuum source.  It's an indirect consequence.  The logic just doesn't work at larger throttle openings.  Timing will be more retarded everywhere with direct.  Maybe your timing was just too advanced, in general.

 

With a mild cam you could probably get both to work fine, but the distributor advance curve might need to be tuned for the vacuum source.

 

 

Just finished a pot of coffee....

[beermanpete]

All the cars I have ever worked on, including the Z, use ported vacuum for the spark advance servo. I have heard mention of the odd car here and there that use manifold vacuum but I have never seen one.