• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Ported vacuum advance or manifold. Let's debate!

Kern Dog

Life is full of turns. Build your car to handle.
FBBO Gold Member
Local time
9:44 AM
Joined
Apr 13, 2012
Messages
37,091
Reaction score
132,896
Location
Granite Bay CA
There are strong arguments on both sides. I'd like to hear yours.
I have always used ported vacuum. To me, it made sense to let the engine operate with simple initial/base timing at idle, let the distributor operate on it's own in regards to mechanical advance and then let the vacuum can add timing at cruise to burn the fuel more completely at cruise.
I base my habits here on what the factory did.
Am I wrong?
I could be. Nothing is original in the engine anymore except that the block is still cast iron. Everything else has been changed so maybe I need to change my thoughts on this subject. I have more compression, a bigger cam, a bigger carburetor and a bigger exhaust system.
I used to scoff at guys like "Don" at FBO ignition for his preference to run manifold vacuum. It seemed odd to me that you'd load the engine with initial and vacuum timing to some huge number around 30 degrees, then once you crack the throttle, the vacuum drops like a rock and you lose half your timing.
I'd love some explanations as to which you prefer and why.

IMG_6795.JPG
 
Think of MVA as PVA with benefits. MVA does everything that PVA does with these additional benefits: because it adds timing at idle, it improves idle quality; less fuel is used at idle; tip in response is improved; engine runs cooler at idle.

There is absolutely no reason not to use MVA over PVA. Chrysler missed the boat on MVA.
I did a search on this forum on this topic.
It was a piece here and a piece there but I was hoping to see more debate and info on the pros and cons on both.
I read that Ported was chosen partly to reduce NOX at idle.
If ported vacuum is mainly an emissions tactic, your words make sense.
 
Folks get so worked up over this. Same old internet story and lack of hearing another opinion.

My silly opinion goes like this: It Depends. The 383 4bbl engine in my 2 owner 69 I rebuilt bone stock. It lights up the tires, shifts hard and quick with the shifter in D and runs wonderfully with 11 initial and 34 total, ported vac advance on the factory Holley. If I had some highly modified 509 cam engine or a .610 roller cam race engine, I suspect manifold may be the way to go.
 
I too believe its engine dependent. If you are looking to get a high initial timing on a hot street engine without having a hot start issue with too much initial binding on the starter, then manifold vacuum makes the most sense. You can use the manifold vacuum at idle to add sometimes 10 degrees of intial and still have a good start without kickback on the starter. For your everyday run of the mill engine with 14-18ish even initial timing, then I think you are fine with ported.
 
I’m a fan of manifold vacuum advance because it helps keep the engine temps down, it smooths out the idle with a lumpy cam and initial throttle response in normal driving. I think with a real lumpy cam it kind of smooths out low gear/low speed driving where the engine wants to buck and trailer hitch with the mechanical advance swinging in and out. But you have to have a vacuum can that works with low vacuum and someway to limit the total vacuum advance amount in some cases.

I started to hook it up in my 66 Hemi to manifold vacuum but it added 20 - 21 degrees to my 15 degrees base timing and I was afraid that might be a bit too much at lower cruise speeds. So it’s still on ported vacuum but I run my GTX on manifold vacuum advance due to air condition loads at idle. I need to brush up on my Mopar advance cans and ways I can limit the amount of advance.

It‘s kind of funny, I’ve owned an old BB Corvette for decades and the Chevy/ Corvette guys are overwhelmingly for manifold vacuum advance. In the Mopar world it seems to be more of a 50-50 split, maybe a majority for ported advance.
 
 
Unfortunately I'm just a backyard hack. I know just enough to get me in trouble. I'm lost with this thread.
 
There are strong arguments on both sides. I'd like to hear yours.
I have always used ported vacuum. To me, it made sense to let the engine operate with simple initial/base timing at idle, let the distributor operate on it's own in regards to mechanical advance and then let the vacuum can add timing at cruise to burn the fuel more completely at cruise.
I base my habits here on what the factory did.
Am I wrong?
I could be. Nothing is original in the engine anymore except that the block is still cast iron. Everything else has been changed so maybe I need to change my thoughts on this subject. I have more compression, a bigger cam, a bigger carburetor and a bigger exhaust system.
I used to scoff at guys like "Don" at FBO ignition for his preference to run manifold vacuum. It seemed odd to me that you'd load the engine with initial and vacuum timing to some huge number around 30 degrees, then once you crack the throttle, the vacuum drops like a rock and you lose half your timing.
I'd love some explanations as to which you prefer and why.

View attachment 1415699
read this discussion Ported vs manifold vacuum for ignition advance? - Don Terrill’s Speed-Talk
The short answer is it depends on your engine and combo.
 
I’m a fan of manifold vacuum advance because it helps keep the engine temps down, it smooths out the idle with a lumpy cam and initial throttle response in normal driving. I think with a real lumpy cam it kind of smooths out low gear/low speed driving where the engine wants to buck and trailer hitch with the mechanical advance swinging in and out. But you have to have a vacuum can that works with low vacuum and someway to limit the total vacuum advance amount in some cases.

I started to hook it up in my 66 Hemi to manifold vacuum but it added 20 - 21 degrees to my 15 degrees base timing and I was afraid that might be a bit too much at lower cruise speeds. So it’s still on ported vacuum but I run my GTX on manifold vacuum advance due to air condition loads at idle. I need to brush up on my Mopar advance cans and ways I can limit the amount of advance.

It‘s kind of funny, I’ve owned an old BB Corvette for decades and the Chevy/ Corvette guys are overwhelmingly for manifold vacuum advance. In the Mopar world it seems to be more of a 50-50 split, maybe a majority for ported advance.


Same deal for me. I bought a vacuum advance canister that added less timing and that helped but I ended up back to ported as my combo preffered it.
 
I started a similar thread a little over a yr ago. I still believe it's personal preference, but I believe manifold has a place absolutely.
 
No vacuum advance is not the best for a street driven car.
 
Last edited:
Most engines I have are modified, including the distributor, I find best results with manifold vacuum.
 
[1] Cannot believe the ignorance, post #15.
[2] GM used MVA up to the late 60s, when tighter emissions forced the use of the USELESS PVA. MVA produced more HC than PVA, hence it's demise.
[3] Rarely would a street driven car/engine not benefit from MVA. One case would be locked timing, but even some of these would benefit from a small amount of timing added by MVA.
[4] If the engine was a candidate for MVA [ & most are ], then there is no such thing as it didn't work for me'. What happened was it wasn't dialled in properly. Unfortunately, I have never seen a method quoted, so I developed my own 25 yrs ago. Some of it is in the thread 'Carburetor Tuning' in this section. I suggest you read it & also see what D. Vizard said.
[5] GM cars. As an example. My GTO was driven off the showroom floor, idling at 26*. 6* initial + 20* added from MVA. 10.75:1 comp ratio, mild cam 200* @ 050.
[6] Chry missed the boat on MVA, that simple.
 
Post #15 could have made more sense if it was written properly.
I think he was trying to state was that a street engine needs some manner of vacuum advance.
 
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.
 
Auto Transport Service
Back
Top