Confirming the ported vs. manifold vacuum debate

[Hemirunner]

As rpm increases, the amount of ignition lead required drops so let that guide you through your vacuum choices, lol.

 

[Geoff 2]

MVA v PVA.

[1] GM cars used MVA until about 1968, when we got the useless PVA. Not sure if Ford used MVA.
[2] Contrary to the belief of many, Chrysler DID use MVA. In the 1970s. The engines of the era, saddled with all the emissions gear could run hot. A temp switch switched PVA to MVA to increase idle speed & cool the engine. Those contemplating using MVA should think about why the idle speed increased......
[3] The link below. scroll down to post $6. Last few lines are most important: www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
[4] Guru D. Vizard said this in PHR magazine, Nov 04 while testing the then new Crane electronically adjustable dist. It had a mech curve....& a vac adv curve. Hmm....

"At idle & low speed operation, the amount of advance reqd to most effectively utilise the air & fuel entering the engine can be as much as 50-55 degrees. This is handled by the vac adv: a function many hot rodders believe is not needed because their favorite drag racer does not use it. Now is the time to listen up & listen up good. A functional vac adv is the single most effective camshaft tamer you can get. By taking the time to hook up the vac adv to a manifold vacuum source you can get a big cam to idle as if it were 20* less than it really is. Conversely, if you are looking for a decent idle the use of vac adv will allow you to use a cam of, at the very least, 5 * more duration/overlap than otherwise would be the case."

 

[MoparLeo]

On any car, once you modify something from stock, stock info seldom applies anymore.
No debate really. If you modify it you are on your own as is your setup and parts you used.
One factory distributor and many, many aftermarket ones all that say theirs is best.
Factory only gives you the results of the testing done with their setup.
Whenever something goes south, their parts manager smiles.

 

[Paul_G]

Vacuum or ported? Use whichever you like. The only difference between them is at idle, above idle they are the same.

 

[68 Sport Satellite]

Vacuum or ported? Use whichever you like. The only difference between them is at idle, above idle they are the same.

but you'll need to adjust your final initial timing differently for each

 

[Paul_G]

Nope. They are the same. Once the throttle plate opens the vacuum signal is the same.

 

[68 Sport Satellite]

Nope. They are the same. Once the throttle plate opens the vacuum signal is the same.

Correct. I'm talking about at idle, depending on manifold or PV. If instead, after connecting MV, you just lower your idle speed, now you've possibly messed up your carb tune.

 

[dammstrate]

OK. Got the Edel AFB hooked up. 10deg initial, 50deg total, 36deg max mechanical means the vaccum is adding about 14deg max. Pretty run of the mill. Completely by the seat of the pants, the ported vacuum seemed snappier off the line, with the manifold vacuum a bit more sluggish, which is consistent with how the two impact the vacuum advance. Time now to play around with the timing and curve.

 

[Ceedawg]

So I think have read or watched 20+ articles on the ported vs. manifold distribution advance topic. In conclusion for a 67 R/T 440 4sp with a very mild
hop-up to the engine (aluminum intake, 1-up cam, Edel Performer, stock heads 10:1, stock manifolds, etc.)
1 - earlier AFBs including the original one for this car (which I have) had only a ported vacuum hookup, that must mean something as it was engineered that way
2 - Edelbrock says to hook non-emissions cars (like my 67 R/T) to the manifold vacuum for a 750cfm Performer like on the car currently. It has both outputs.

--> end result, play around with it and see which you like.

Taaa Daaa!

For anyone who cares, I will do some "seat of the pants" testing with different initial timings, and put observations in this thread. Once I solve the current
fuel leak.....

Ported advance is the same as advancing the timing to what a light would show on the balancer, as soon as you floor it it goes back to where the hose was off, till centrifugal took over. *** backward all around.
I agree somewhat with Vizard but how is a VA going to drop 40 degrees to not detonate on acceleration?

 

[dammstrate]

Found this vid that explains things pretty well.

 

[Geoff 2]

Post #29. The VA in this case is only dropping 14*, not 40*. I am unaware of any VA unit that adds 40*. 30* is the highest I have seen.

 

[Geoff 2]

Damm,
Your engine could use a lot more than 10* initial for better off idle performance. 16-20* would be noticeably better. Like seat of the pants better. The stock starter may struggle, but a 3 hp mini starter would be fine. The centri curve would need to be adjusted to provide 36* @ WOT. I would leave the 14* vac adv. If the engine surges at cruising speed, reduce VA amount in 2* increments.
There can be many reasons for the odd behaviour in post #28. such as using a non-adj VA unit. Once the engine is modified, an adj VA unit should be used, Allen Key turned fully CW as starting point & hooked to manifold vacuum. Like Edel said................& heaps of others...

Chrysler missed the boat on VA by using PVA instead of MVA.

 

[Ceedawg]

Post #29. The VA in this case is only dropping 14*, not 40*. I am unaware of any VA unit that adds 40*. 30* is the highest I have seen.

What I was saying, Visard said “55 degrees at idle”, under accelation no vacuum advance (that I know of) can drop to a degree (say 15 degrees) that won’t spark knock. I have to disagree on the last part of what you said. Why have high initial advance and then after pushing pedal to floor the vacuum drops to nothing and so does advance advantage? Better way, I’d to set initial at 36, port the vacuum to another 20, THEN when the vacuum drops you’re back to initial. Duel pickups in distributor would allow start-run. Done all the time on crank trigger type system.

 

[68 HEMI GTS]

I personally like vacuum advance on anything that see’s street duty as long as it’s adjustable so you can limit the total advance. I run it on my 10 sec street car, (manifold vacuum, that apparently is no good according to above… lmfao). I also run one of those stupid PCV’s that everyone throws out. If I didn’t run a tach drive on my hemi, it would have vacuum advance also.. pretty hard debate to solve as every engine combo is different and should be tuned as such. Anybody that is dead set one way or the other is wrong.

Dual plane six pack, big single with a dominator. Works good hooked to manifold for this particular combo. If it were a milder engine, ported may be the way to go. Gotta experiment…

 

[furyus]

Frankly, being a tach drive no vacuum distributor user, I don't understand the conundrum. Only two ways, ported or manifold vacuum. Try it both ways, stick with the one that works best for you. DONE! (What is so freakin hard?)



Now, if you're talkin pinion snubbers and pinion angle.......

I remember reading some expert, maybe Eberg, saying like you said. Try both, and see what it likes.

 

[Paul_G]

Engines with big cams that idle high can benefit most from manifold vacuum. It raises the idle speed without opening the throttle plates and exposing to much of the transition slot. This type of engine will also require a quick acting mechanical advance. So quick that as soon as rpms come up and vacuum goes away the mechanical advance us already stepping up and raising the timing.

 

[dammstrate]

I remember reading some expert, maybe Eberg, saying like you said. Try both, and see what it likes.

I will never understand how Ehernberg remembers or has access to so much. I hope he outlives me.

 

[4406bbl]

Engines with big cams that idle high can benefit most from manifold vacuum. It raises the idle speed without opening the throttle plates and exposing to much of the transition slot. This type of engine will also require a quick acting mechanical advance. So quick that as soon as rpms come up and vacuum goes away the mechanical advance us already stepping up and raising the timing.

This is really good advice if you want to run manifold vacuum with a big cam and a 4 speed, also works well with automatics with loose converters. With big cams and tight converters you may want to try ported and a lot of initial. If you can't get 10 -12" of vacuum at idle in gear then manifold vacuum will be difficult to make run well with an automatic, with the currently available vacuum cans anyway.

 

[BSB67]

So I think have read or watched 20+ articles on the ported vs. manifold distribution advance topic. In conclusion for a 67 R/T 440 4sp with a very mild
hop-up to the engine (aluminum intake, 1-up cam, Edel Performer, stock heads 10:1, stock manifolds, etc………..

I think you confirmed it