Ported vacuum advance or manifold. Let's debate!

[Kern Dog]

Now THAT makes sense.

 

[RJRENTON]

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

Should have asked this question b4: WHY? This had been discussed 1000's of times. The FBBO server is on the verge self destruction being inundated with the same information. This argument is boundless/endless; its like debating the best: spark plug brands, oil, wax, beer, tires, gasoline......why doesn't the OP just use what ever method suits his car or provides the results anticipated....and suffer or rejoice in his/her own way. The self appointed experts never seem to offer results just continuous endless speculation as to why their way is best. Do any of these experts have any theories as to reasons or limitations of the spark advance purpose? Very likely not......just my opinion of course......
BOB RENTON

 

[Photon440]

I hadn't realized that FBBO was about to self destruct. Do you think it would last longer with ported vacuum?

 

[Geoff 2]

Bob Renton.

If you are not liking the topics being discussed here, there are plenty of other forums for you to go to......& complain about them....

 

[mopar jack]

From the 1971 factory service manual Chrysler did utilize manifold vacuum to help control temperature by advancing the distributor to raise rpm.

 

[Kern Dog]

Bob Renton.

If you are not liking the topics being discussed here, there are plenty of other forums for you to go to......& complain about them....

It must be human nature.
I fall victim to it sometimes too.
We all know the guy that sees another guy washing his car and he is compelled to comment:
* Hey, you missed a spot!
* Hey, can I pull in next?
* oh no, now you’re going to make it rain!

People sometimes feel the need to jump into a thread, provide no assistance and just complain.
Renton is usually a good contributor though. I’ll just shrug off this one.

 

[Geoff 2]

Kern,
You make good points. But what irks me is this forum is free. No $$ reqd to be a member. I presume somebody has paid for this internet service; also, moderators are on hand, giving up their time freely, to keep the forum in check.

 

[Kern Dog]

Kern,
You make good points. But what irks me is this forum is free. No $$ reqd to be a member.

It is free when you are not a GOLD member....

 

[Dragon Slayer]

An everyone missed that SAT's car is now pinging at partial throttle on MVA, even though it idles and transitions better. There is an ideal ignition curve for your combination of parts on your motor. You also need to know how your carburetor ported works. Plenty of carbs out there were ported gets exposed at idle speeds because of throttle blade position.
The combination Mopar used worked. Can you get better idle speed with more timing, sure, you also get it with mixture screws.

I have said this before, in one case your adding timing as you accelerate to meet the idle ignition curve (ported). In the other case you pulling timing out initially and then adding back at high rpm (MVA). Whatever works best for your combination and needs.

 

[66 Sat]

Well I've got the Manifold Vacuum to work and it is better than Ported from my experience.
My engine is susceptible to pinging due to the 10.8:1 compression and fairly small cam. Plus it's pretty hot here. With the MVA it was pinging but I pulled the vacuum advance can and JB Welded 2 small tabs of steel to limit the amount of vacuum advance it can add. It now runs great on MVA. There is far less smell of exhaust after I've backed the car into my shed compared to Ported. There also used to be quite a strong smell just after I was getting on it driving down the road, but that's gone too. Don't know why, shouldn't be related.
Now I'm going to experiment to see if I can put a thinner tab in there to get a bit more vacuum advance.
Or buy a distributor with an adjustable vac advance of course.

 

[RJRENTON]

Bob Renton.

If you are not liking the topics being discussed here, there are plenty of other forums for you to go to......& complain about them....

Then there are the proponents of NO VACUUM advance......just use the mechanical advance mechanism or fixed advance......how many variations of the ported vs manifold argument exist??? Just two.....its totally amazing just how many experts there are.
BOB RENTON

 

[33 IMP]

Then there are the proponents of NO VACUUM advance......just use the mechanical advance mechanism or fixed advance......how many variations of the ported vs manifold argument exist??? Just two.....its totally amazing just how many experts there are.
BOB RENTON

Agree. NINE pages?!?
It's the simplest thing in automotive-dom. Try both, see which you like! It's literally swap a plug for a hose or vice-versa. Take a drive, see what happens. Done!

 

[Geoff 2]

No, it is not as simple as swapping a plug for a hose.....
Both PVA & MVA have to be integrated into the ign curve. Many, if not most cases, will require the rate of VA application & the amount, to be changed to suit the particular engine.
Just plugging in VA & doing nothing else is what leads to dopes getting on forums like this one & saying 'VA didn't work on my engine' when it most likely would have if was dialled in.

Would you just bolt on new cyl heads & not check valve lash or pre-load?
Would you just bolt on a new carb & not adjust float level, idle mixture etc?

 

[Geoff 2]

MVA.
D. Vizard 'gets' it......

 

[Photon440]

Then there are the proponents of NO VACUUM advance......just use the mechanical advance mechanism or fixed advance......how many variations of the ported vs manifold argument exist??? Just two.....its totally amazing just how many experts there are.
BOB RENTON

The only proponents of NO VACUUM that I've spoken to are drag racers who have no vacuum to make use of during the cycle of the race and have no need of possible fuel mileage cruise benefits.

 

[hunt2elk]

The only proponents of NO VACUUM that I've spoken to are drag racers who have no vacuum to make use of during the cycle of the race and have no need of possible fuel mileage cruise benefits.

I actually know quite a few guys, some I even consider to be good friends, who refuse to use any form of vacuum advance in their street cars. Mostly because a buddy of theirs who does a bit of racing says that vacuum advance is no good on any car. Never really understood that thinking...

 

[68 HEMI GTS]

The only proponents of NO VACUUM that I've spoken to are drag racers who have no vacuum to make use of during the cycle of the race and have no need of possible fuel mileage cruise benefits.

I actually use it in my race car. But I’m sure someone will have something to say about it.. lol

Street/strip car as well, ignore those PCV valves on either car. Those have no business on 10sec cars

 

[33 IMP]

The only proponents of NO VACUUM that I've spoken to are drag racers who have no vacuum to make use of during the cycle of the race and have no need of possible fuel mileage cruise benefits.

That's ME!
(Iron tach-drive distributor, no vacuum can.)

 

[68 HEMI GTS]

That's ME!
(Iron tach-drive distributor, no vacuum can.)

That’s what I use on my HEMI (alum version). Gotta keep the moroso tach drive, it’s been in the car longer than I’ve been alive..

 

[Geoff 2]

If you run a locked dist, as some race cars do, then you more than likely do not need VA because the locked timing provides sufficient ign timing.
My point in post #174 was that IF you are going to use VA, it needs to be dialled in & made part of the whole ign timing curve.