Guys,

I am still tinkering on my Charger and I am curious if I should once again hook up the distributor to vacuum or not.

The backstory is that when I had my car dyno'ed a few years ago, we found that the engine pinged at WOT (still does on warm days) so the dyno guy said to just leave the distributor disconnected from vacuum to help fight that. It has been that way ever since.

Right now the engine is at 16* initial timing and it advances to 36* total by 3000RPM's or so. This is with the dizzy vacuum plugged.

Would I gain anything by connected the distributor back to manifold vacuum? I know that the ported vacuum is more of an emissions thing as it kills the high vacuum signal at idle. If I use the manifold vacuum, will that improve my idle by advancing my idle timing further or am I just wasting time here?

Thoughts?

First you need to find a new dyno guy. Vacuum at WOT is zero or very near zero so the vacuum advance has absolutely nothing to do with your detonation problem.

Hooking the dist up to manifold vacuum should clean up your idle IF you have enough idle vacuum to operate the advance.

A little more info on the combo will help to dial it in.

Kevin

Stock, there is no vacuum advance at idle, either. One time I accidentally hooked up vacuum advance at idle and I could not get my idle speed to settle down properly. Once I figured out what I had done and fixed it things were fine.

The motor specs are in my signature. Vacuum is about 14" idling in gear. I have noticed that the engine kind of "wants" more idle timing as the RPM's pick up if I turn the distributor to advance the timing. I just have not left it past 16* initial due to the pinging thing at WOT.

One thing I have always noticed is that it seems the idle drops more than it should when I put it in to gear (about 100 RPM drop) and then if I add A/C, that's another 50 RPMs.

Will it hurt me if the idle vacuum advances the distributor well beyond the 16* @ idle it has now?

There used to be a great write-up on the Davis Unified Ignition site about vacuum advance but it appears to be gone now.. The bottom line being there are more advantages than disadvantages.

Here is what it currently says:

Q: What’s the difference between mechanical and vacuum advance?
A: Mechanical advance is the centrifugal weight and spring assembly located underneath the rotor that provides the largest part of the
timing curve. The vacuum advance canister, located on the side of the distributor, provides additional timing only when the engine is pulling
vacuum. The mechanical advance along with the initial advance (base timing set at idle) is what gives you total timing. An example of total
timing would be 12° initial plus 24° of mechanical which equals to 36° total. Vacuum advance is not figured into total timing because it will
only function when the engine is not under a load. Example: If your vehicle is cruising on a flat stretch of road or going downhill, vacuum
advance will come in and add as much as 15° to the total timing for increased fuel mileage and cleaner spark plugs. It is not uncommon to
see as much as 50° – 52° of timing with vacuum advance. But don’t be alarmed by this being too much timing as the vacuum advance will
decrease once you accelerate and put the engine under a load. Under hard acceleration the timing will go back to the original total with no
vacuum advance.

16/36/all in at 3K ain't outrageous. I'd start by plugging the can in to ported & if it only pings at WOT I'd slow the curve with a slightly heavier spring on one side. Best would be a bit more octane. If you're right on the edge a next colder plug MAY help. Experiment with manifold too & you'll for sure need to shorten the slots. With manifold I'd set the initial with the vacuum gauge method then shorten the slots to 35/36 then a heavy enough spring combo to stay just under the pinging point at WOT up thru the gears on your hottest/driest day. Best is more octane but most individuals are set in stone where they are at with that choice. Bottom line NO pinging allowed at any time.

What I am trying to do is clean up my idle/part throttle some so wouldn't I want to skip the ported vacuum as it has no signal at idle? I know this won't fix the WOT pinging by itself as it can only advance the timing.

What I was envisioning is maybe I could dial back the distributor to maybe 10-12* base timing and use the vacuum advance to keep the advance up at idle-part throttle but drop off at WOT which would then top out at 30-32* which might help the pinging?

Does this make sense or am I missing something??

First, I'd get the pinging situation solved. Either limit the total, or reduce the initial. It should never ping, and adding the vac advance won't do anything to change it.

Two schools of thought on ported vs manifold for the distributor:

Ported:
No vacuum at idle. You can run a high(er) initial timing, but too high can cause kickback/starting issues. Easier to tune, since you know the timing won't change between neutral and drive (at least, it shouldn't).

Manifold:
Vacuum at idle adds timing, but only after the engine is running, so starting won't be fighting a high initial. Vacuum drops out at WOT, which is fine. You MUST make sure that your idle and drive vacuum levels are higher than the vacuum the can needs to provide the advance, otherwise your idle will bounce all over the place.

What you're envisioning should work just fine, presuming you have enough vacuum to make sure the can is always advanced at idle/in drive.

what I would do is set the initial with the vac gauge method then shorten the slots to get the std total (which is initial+slots) for your eng type (ie 35-SB) then springs then plugin/adj the can. this would be for ported. then I would hook the can to manifold and same vac gauge method for initial then shorten the slots for the same correct (for your eng) total then springs & see which one works the best for you. there may be a better approach to this as modding the slots ain't easy & I have no exp with manifold but what I've read up on it, it does sound appealling/worth doing. first, more info on whats going on with cleaning up the idle/part throttle response problem. #1 dont allow it to ping! you want close to it for max power and or economy but never over that line

Yeah I think if I have to shorten the slots, I'm just going to buy the FBO plate as I don't have the time/patience/welding skill to do that myself.

FWIW, this is the Mopar Performance distributor that I got from Summit maybe back circa 2006-ish. I don't think they sell it anymore. Any idea what the vacuum advance is on this?

I'm wondering if I role the timing back maybe 2* to 14* base (34 total), then that might cure the pinging issue. However, I'm wondering what the vacuum advance will do to the idle advance...if it will be too far advanced or not.

(1) that's an excellent plan/easy to do/easily reverseable, try 2 or 3 degrees (2) plugging the can in to manifold will definitely require resetting the initial with the dist housing as a can with 7 stamped on the arm will add 14 initial & (as said/VG point) in regards to the idle vac you'd want a can that maxes out at a in hg number lower than your idle in hg reading

If that dist is the Mallory built MP unit, you can adjust the advance. The procedure should be in the Tech Archive.

No welding required.

Kevin

I would say get the pinging fixed quickly and then go from there

I just read the tech archives and see this:

"Although not discussed in MP's instructions, the mechanical advance is a Mallory adjustable YH type (same as used in most Mallory distributors). It is fully adjustable from zero (locked out) to 28 degrees. We set them at 28 degrees when we ship them to MP. All you need to adjust (or lock out) the advance is a Torx driver to loosen two button head screws. "

How do I know if mine is adjustable? Also where do these torx screws reside?

**EDIT**
I think I got this figured out. If my distributor has this, I will need a set of the keys to set the gaps between those tabs.

So do most people with performance STREET motors run vacuum advance or leave it disconnected?

2 second google search

http://prestoliteperformance.com/media/i...e_kit_29014.pdf

If you're running on the street, there's no reason not to run vac advance. Better mileage and burn during cruise for one reason.

If the car is only used on the strip, I wouldn't use vac advance. Why? On the strip, the car only ever sees WOT, at which point the vac advance is inactive. So may as well not have it at all.

I am worried about having too much timing at idle if I use manifold vacuum. Any idea of a ballpark of how much is too much for an engine combo like mine? I can tell it wants more timing at idle as when I advance it the idle picks up and feel stronger but I have been restricted by the total timing.

It seems like you know what you're doing, or at least have an idea.

Your idea to use a limited mechanical advance with the vacuum advance providing extra at idle is interesting and it could work. Years ago I was convinced that manifold vacuum was the only way to go. I have moderated some, but your idea is sound, unless you develop a bog right off idle when you put your foot down, because advance will drop from 20 to 6 degrees. It's worth a try.

Mazda runs very high compression by limiting heat input into the intake charge. The cooler the intake manifold, the cooler the air sucked through the filter, the more the engine will like it. What I didn't expect is they found that the exhaust residual could also add heat, so on their Skyactiv engines, the exhaust manifold is like a header. Scavenging gets the hot air out.

Points to ponder,
R.

Im missing something here, Why is there no ported vacuum an WOT. I would think that is where there would be the highest ported vacuum as you have the most air flow through the Venturi where ported vacuum comes is created so to speak.

Because there is very little vacuum present ANYWHERE when your butterflies are wide open.

The "ported" vacuum exists off ide, and part throttle conditions. that way vacuum advance exists only at part throttle conditions, by design.