Ported vacuum advance or manifold. Let's debate!

[4406bbl]

Geoff, if you hit +Quote and then Insert Quote on the post you agree/disagree with, it includes it in your next post. That way you don't have to keep referring to post numbers and making members go back to pages to see what you're on about.
Post #15 couldn't be taken 2 ways.
You just read it wrong and shot from the hip.
The response of "No vacuum advance is not the best for a street driven car." was correct and in direct response to post #14 which said
"What about, no vacuum advance at all?"

Thank you, and yes it was in direct response to #14, I should have constructed it as a stand alone comment to avoid the drama.

 

[Photon440]

 

[Kern Dog]

I'll admit, I was put off by Don/FBO and his abrasive manner. He could have been 100% right but his pushy and arrogant ways just pissed me off.
Manifold vacuum seems like a good idea if one wants to increase idle speed without cracking open the throttle blades to the point of inducing dieseling/run-on when you shut it down. I still get the dieseling sometimes and it is annoying.
Trying all of this costs nothing except some time.

 

[threewood]

I run ported because...
1) I do not have to pull hoses, adjust carburetor every time I want to throw a timing light on it. No vacuum pulled at idle = my timing and carb setting never have to be altered.
2) I run 21 degrees initial, 37 total. I don't need the car pull more while at idle. It is dialed in with light mechanical springs and does not ping. Don't fix what isn't broke.

 

[AR67GTX]

Don't fix what isn't broke.

I think we can all agree with this.

 

[AR67GTX]

What Geoff said X 2. If you’re restoring one of these heaps, use PVA like the factory did so it’s “correct”. (Air quotes because I’m not a purist) For everything else use MVA.
I have 50 degrees of total timing in at idle.

In that case maybe I’ll give it a try on my Hemi as I’m much less at about 36 - 37 at idle with manifold vacuum advance.

 

[Cheapsunglasses]

I think we can all agree with this.

Nah, it’s the nature of the beast. This works, but can it be better? If we never try to fix what’s not broken, we’d be on horse back, prescribing meth to people with a simple headache, believing all planets revolve around us.

Hopefully I didn’t open a can of worms and derail the thread, I’ve still got my popcorn if I did

 

[Dragon Slayer]

Start at 50 and work down to 34 or 36 while vacuum is coming down and mechanical is coming up. Start at 12 and work up to 34 or 36 while vacuum may or may not be coming up and mechanical is coming up. It is a curve. Carb, cam, compression, manifold all matter. Chrysler did have a dyno and they did run motors 24 hr a day testing and jetting etc... It was PRE emissions for Chrysler using ported. So don't fall for that Red Herring argument. They chose ported even for HP cars pre emission. But do what works for you and your combo.

 

[Geoff 2]

I apologise to 440 6bbl.
I took his comment to mean that no vac adv was best .

 

[4406bbl]

I get it.

 

[Geoff 2]

Dragon Slayer, post #48.
You trot out this tired old argument that the switch from MVA to PVA was not emissions related & that it is a 'Red Herring'. You were wrong before & are still wrong.

Of course Chry didn't have to change anything because they never used MVA, only PVA. GM did use MVA & had to switch to PVA because it reduced HCs.

 

[Geoff 2]

Post #20 gets it! I am sure you have a sweet running engine.

 

[Geoff 2]

Kern,
Post #43. MVA does not cause dieseling, just the opposite!!
Whether the timing is locked or MVA is being used to add timing at idle, the result is the same: much improved idle quality.
The idle rpm can be lowered with more timing.

Example.
[1] Engine has sizeable cam 15* init timing, 1100 rpm idle. Engine runs too rough with lower idle speed.
[2] Timing at idle was found to be best with 40* of idle timing. Keeping the 40*, the idle speed will be able to be lowered to 900-1000 rpm & be as smooth as or better as example [1].

More idle timing results in a cooler engine, & high eng temps contribute to dieseling.

If lowering the idle rpm closes off too much t slot, reduce the size of the bypass air being supplied at idle.

 

[Geoff 2]

Thanks to those who told me about quotes & how to insert etc but I am a cranky old bastard & probably will forget....

 

[WileERobby]

The answer is very clear if one knows, or does the research, as to WHY ported vacuum advance was implemented.

 

[Kern Dog]

Kern,
Post #43. MVA does not cause dieseling, just the opposite!!

I'm on board with you. I think that you misinterpreted me....

Manifold vacuum seems like a good idea if one wants to increase idle speed without cracking open the throttle blades to the point of inducing dieseling/run-on when you shut it down. I still get the dieseling sometimes and it is annoying.

I am on ported vacuum on everything now. The 440-495 in my red car has the biggest cam, the highest compression and the highest idle speed.
I started this thread to get opinions on both sides.

 

[Wietse]

A simple attempt to find what the engine likes is disconnect the vac can, swing that distributor to advance and set it where it runs nicely, you likely will find the idle rpm starts to increase when going the right way.
If you run out of adjustment you can use the v-can also and pull in more timing, don't look at the actual number on the balancer but just listen to the engine. When too far it starts to run worse again. Let the advance number surprise you when you check it afterwards with a timing light.
Let's say you found it idling very nicely with 35* advance and idle 800-850 (or lower) rpm, now you decide how to get there with a good curve.
You will quickly realize you will not get it there with using ported vacuum.
Say 20* initial (or whatever you currently may have) and the rest you need to pull in with the v-can, some cans pull 10 or 15, some custom adjustable will pull more.
Once you hit the pedal all that advance is almost gone and you are on your normal initial advance.
You may find you have to dial it back as you could face some pinging in the light throttle cruising area's but you have to listen out for that with a test drive.

Also to remember, a lean mixture has a slow burn...advancing this far cannot be combined with a fat mixture and vice versa. An AFR gauge will be of good assistance with this.

 

[Kern Dog]

I was surprised when I started this engine in October. It idled better than I expected.
I ran this lumpy cam for awhile years ago. I pulled it because it idled rougher than I wanted and it made low idle vacuum. The power was great though.
This time, the compression is 3 tenths of a point lower but I have quench. Usually when you lower the compression, the cam starts to act bigger and less streetable. This went the opposite way.
Mine idles at 1000-1100. I could drop it some but it feels lazier starting from a lower idle speed. Maybe it does need more advance at idle. I'm at approximately 19 degrees BTDC now. I've always noticed how idle speed creeps up with more and more initial advance but that always resulted in detonation at WOT due to the sliding scale of the curve. This MP distributor had 24 degrees of curve in it at first. 16 degrees initial at idle meant 40 degrees when all in. WTF were they thinking? 24 degrees of curve in a performance distributor ??
It is adjustable by using small plastic pegs of different sizes. In theory I guess I could also just adjust it to have 10 degrees of curve and set the total to 35 with initial at 25. It would probably be hard to start at that point though.
If the manifold vacuum gives more advance at idle along with more advance at cruising speed, it seems like a good way to go.

 

[Wietse]

I guess the 24 degrees of mechanical advance was there so the initial can be kept low more easy, so 10* initial which would suffice with a stock or slightly modified engine and have 34* total. Then a v-can could be used to pull in more.
Like my MSD distributor gives a minimum of 18* mechanical advance, so i am stuck at maximum 18* initial to not exceed the 36* total timing....that is being stuck the other way. (i could make a bigger bushing which would lower the mech. advance though)
Mine still starts ok at 20* initial, but when swinging it to 25* or so it just doesn't wanna start.
For me to have 20* initial and ported vacuum pulling in more timing when driving off just makes it ping straight away, therefore i am on manifold so the vac advance drops off when driving off.

I believe that what i described is the most important part in choosing what vacuum source to use for your application, it does not matter it was Chrysler or Chevy who used this or what a person might "prefer", just what your engine needs depending on the type of build.

 

[Hemirunner]

In that case maybe I’ll give it a try on my Hemi as I’m much less at about 36 - 37 at idle with manifold vacuum advance.

Give it a try and report back. It’ll depend on the cam and compression as to where it’s happiest. Mine is a 9.5:1 turbo combo with 258/[email protected] stick. Mine likes timing with the low compression/cam size combo.