Ported vs manifold vacuum for ignition advance?

[Geoff2]

HQ,
Couple of things that do not make sense....& some that do.
If I read it correctly, your idle rpm increased 150 rpm with the extra 16* at idle provided by MVA. About what I would expect for your engine specs. So your engine is making more hp. If it made less hp, rpm would have dropped. The engine liked MVA.

What I find odd & hard to understand is cruise. At 2100 rpm, the PVA port should have been well & truly exposed to manifold vacuum. The same vacuum as MVA & under those conditions, there should be no difference between MVA & PVA. That assumes the same VA unit was used & adjusted the same for both tests. MVA [ compared to PVA ] only adds a benefit at idle & off idle.

[HQM383]

HQ,
Couple of things that do not make sense....& some that do.
If I read it correctly, your idle rpm increased 150 rpm with the extra 16* at idle provided by MVA. About what I would expect for your engine specs. So your engine is making more hp. If it made less hp, rpm would have dropped. The engine liked MVA.

What I find odd & hard to understand is cruise. At 2100 rpm, the PVA port should have been well & truly exposed to manifold vacuum. The same vacuum as MVA & under those conditions, there should be no difference between MVA & PVA. That assumes the same VA unit was used & adjusted the same for both tests. MVA [ compared to PVA ] only adds a benefit at idle & off idle.

Re-read my post. Idle increased by around 150 rpm with an extra 8*. Any more additional timing did not gain any more rpm, so why advance more than the 28* that equates to? If I were interested in more hp at idle I would get the guys that recurved my distributor to reduce mechanical advance to 6* making 28* initial and 34* total. Easier than adding another system to apply 8* for little effect. Oh yeah, it was just more timing the engine liked not MVA in particular. Same result is achieved by tuning the distributor. But it’s not just that:

1) the idle is more than acceptable for a weekend street strip vehicle, temp in traffic included with 20* initial. Carb transition isn’t an issue either.
2) with 4K converter and 3.89:1 diff and 26” time spent 1000 to 2000rpm is next to nothing. I don’t see any point looking for extra power there. With mech in at 3k it’s likely awfully close to 28* by then anyway.
3) vacuum at idle in gear is around 9” so MVA would need to be giving me that extra 8* for more idle power with 7”- 8” of vac to avoid an annoying varying idle.
4) all driving conditions on the street is way more than 7-8” of vac so that 8* will always be applied when not giving it burst of pri/sec driving or about 99.9% of cruising.
5) I would have to make a new stopper plate to limit to 8* added MVA. The case for it is just not strong enough to bother. As I said, leaving it at 16* applied MVA is too much with a noticeable effect on part throttle performance.
6) ported I can keep the total of 16* but applied incrementally as throttle opens. Net effect may well be more applied vac advance cruising at highway speeds ported than if I had MVA and having to tame it with reduced total applied.

The difference between MVA and PVA is there and not in a neat theoretical box it’s thought to be. At 2100 rpm maintaining steady speed my 1.75” throttle blades are barely cracked. They start with trans slot square. If I had a Carter AFB or Thermoquad with tiny primaries then throttle rotation would be more, exposing more to the ported vac hole in the venturi. Mate, I saw it with my own eyes on the same vacuum gauge hooked up the same way bar swapping carb ports between test and on my engine the amount of vacuum applied at each rpm point was chalk and cheese between MVA and PVA. It’s not up for discussion of what it should or shouldn’t otherwise be, that’s what it is. To blow your hair back even more it’s all with a single plane manifold. Remember, if the test doesn’t fit your theory then get a new theory and that’s what I have done. Your hero in MVA isn’t required to save everyone.

MVA has its place, it did on several of my old daily driver 308s so I am not averse to it and have so used it where it is effective. Believing it should be applied to all and sundry and being a mistake not to is misguided.

[Nut124]

If applying this to a Fiat with full IR intake set-up some care will be needed in creating a steady source of vacuum. The solution needs to be one which steadies the pulsing at idle, this done by way of a small reserve canister. To take from all 4 cylinders it is advised to choke each vacuum line near its runner with a small idle jet, this to minimize cyl-cyl interference, then combine all at the canister. If this engine is also that which has been cammed somewhat then idle vac will be limited, and will fall off quite rapidly when the IR throttles are opening in any case, correct?

The FI set-up will be plenum type if stock, quite straightforward for establishing manifold vacuum. No way either will facilitate ported vacuum as I see it.

Cheers
Steve

In my IR manifold Fiat I take the manifold vac from a single runner and use a dampening system similar to a can, consisting of volume and a small orifice. This setup draws 10" of vac at 1000rpm with 10* static and 14-15* vac for a total of about 25* advance. If I disconnect and plug the vac hose, the rpm drops by almost 200rpm to about 820.

With idle AFR at an indicated 12.5, header primary temps are 225C with 25* of advance, rising to about 290C at lower rpm and 10* advance. AFR shows the same.

The vacuum diphragm in my Fiat gives no advance until full 5" of vac so I think vac advance drops off pretty quick as plates open.

Thank you for all the interesting responses. I can appreciate now why opinions may differ on this.

I still cannot quite understand how any port above the throttle plates could produce more than an inch or two of vacuum, partcicularly al lower speeds. It would seem to me that this kind of vac should be the highest at top rpm where flow is the highest.

Could the vac at the ported source ever be higher than the manifold vacuum below?

[Geoff2]

HQ,
Thank you for your 'advice'.
I am not misguided, nor is MVA my hero. It was somebody else's hero loooong before I came along & I just tagged along......
And I don't need a new theory, because the old one was perfectly sound & tested. You proved it....
You said MVA worked on your old 308s; it also worked on your 383. Because your engine rpm increased 150 rpm when added 8* of timing. Pretty good win rate....
What you are not taking into account is that you have 20* initial static timing. Many tuners will be starting with less, closer to the factory 6-12*. Some will be of necessity because their starters, particularly if they are stock, will not crank with a lot of initial timing. Also, for some engines, a lot of init timing might cause detonation, so another reason to cut back on initial & add timing with MVA, which drops away with load. So in these cases, & there are lots of them, they add quite a bit of MVA, more than your 8*.

[Geoff2]

Nut,
Very clever what you have done with the pulsating vacuum. I did a similar thing with Webers so that I could run MVA.

If the PVA port is below the t/blades at cruise, then the same vacuum will be on the MVA port below.

[HQM383]

HQ,
Thank you for your 'advice'.
I am not misguided, nor is MVA my hero. It was somebody else's hero loooong before I came along & I just tagged along......

On a multitude of forums I have never once seen you say “use ported because it would work better for you than MVA”. You push MVA at every opportunity. Many have said it’s not for them or didn’t work or their ignition is dialed in fine thanks (much like these last few post) and that’s where you begin to tell them where they are wrong and don’t know what they are doing along with David Vizards CV. Adjectives used are apt.

And I don't need a new theory, because the old one was perfectly sound & tested. You proved it....
You said MVA worked on your old 308s;

Yes it did because those engines configurations responded well to MVA. I understand it’s horses for courses.
MVA is sound when integrated into an engine that benefits from it, I’m all for that. But I’ve displayed it’s not always the best or one and only solution.

it also worked on your 383. Because your engine rpm increased 150 rpm when added 8* of timing. Pretty good win rate....

Adding 8* of timing is a win for added timing not MVA. But let’s continue to play this out, I add 8* of timing with MVA, rpm goes up 150rpm in gear, I then back the idle speed screw out to get idle rpm back where it should be and I’ve lost a square transition slot. Advantage nil. Transition slot stays square - still minimal impact not worth the effort as explained in previous post.
No it didn’t work on the 383. Performance suffered, I thought I was clear on that point. To maintain performance and add 8 almost useless degrees of timing to idle I’d just get the distributor modified.

What you are not taking into account is that you have 20* initial static timing. Many tuners will be starting with less, closer to the factory 6-12*. Some will be of necessity because their starters, particularly if they are stock, will not crank with a lot of initial timing. Also, for some engines, a lot of init timing might cause detonation, so another reason to cut back on initial & add timing with MVA, which drops away with load. So in these cases, & there are lots of them, they add quite a bit of MVA, more than your 8*.

I do take that into account as I have mentioned in another post on this very thread. Not just that, I wouldn’t have used it on other engines if I had not taken those factors into account. It was learned back in trade school and I have taken into account ever since.

I like these debates Geoff, it’s fun

See you at 7 o’clock tomorrow!

[steve cowan]

I done a vortec headed 350 for my uncle 5 years ago approx 400hp.
Rpm air gap and QFT vac sec carbs around 600cfm.
Mallory uni light with adjustable vac advance.
Engine has around 18" vac at idle.
After a lot of testing and changing springs in distributor,adjusting vac advance
.if initial timing was low with alot of vac advance the engine would fall on its face if you put your foot in to it from idle.more initial timing with limited mechanical advance and vac cannister set to minimum.
Actually believe it or not but this mild 350 ran the best ever with locked 32 degrees but did not like the hard hot start,but idled well and good power through the range.this is with stock converter and 3.08:1 rear gear.
Every application is different.

[novadude]

Debate ? for sure.

I only feel it is proper to add to the others that that said the use of ported vacuum existed, and was even used by choice, by some manufacturers way before even the toothless first Clean Air Act was the LAW in 1963.
Despite what GM (manufacturing) Engineer, auto enthusiast and 2009 Corvette Hall of Fame Inductee John Hinckley said in "that" article.

THANK YOU for posting this. I am so sick of seeing people spread the LIE that ported was engineered in the emissions era, and that manifold is the only way to go.

I am running ported, and I will continue to run ported (yes - I HAVE tried both ways).

[Geoff2]

Yes, Dude, ported VA [ PVA ] was used for emissions reasons. Sounds like you are unaware that MVA was used alongside PVA through the 1970s. MVA makes the engine more efficient at idle.

Here is some info:
- GM Combined Combustion System: 'Because of the increased possibility of engine overheating at idle, the thermo switch is added to some engines...if temp reaches 225 *, the switch moves to allow manifold vacuum to reach the dist to advance the timing & allow the engine to run cooler'

Another:
- Chrysler: ' When engine coolant at idle reaches 225*, the valve opens & applies manifold vac to the dist.....This increases the engine idle speed & provides additional engine cooling'

Another:
- Ford: ' When the coolant temp reaches 23, manifold vac is applied...advancing the timing & thereby lowering operating temp'.

Finally:
- Ford: ' If the engine overheats at idle, increased vacuum will flow to the dist to increase engine speed.'

[novadude]

Yes, Dude, ported VA [ PVA ] was used for emissions reasons. Sounds like you are unaware that MVA was used alongside PVA through the 1970s. MVA makes the engine more efficient at idle.

Not sure how that proves that PVA was used for emissions reasons? PVA was used in the 40s and 50s when performance and efficiency were a concern and emissions weren't given a second thought. This is well documented in automotive engineering texts and service manuals. I guess we'll have to agree to disagree on this.

[Geoff2]

At least two GM divisions used MVA during the 60s. About 1968, the cars did not meet tightening emission targets & had to switch to PVA to meet the new target.

[FishFry]

Well if you can pull the vacuum hose from your carb and nothing happens, you probably have ported vacuum. But most of the time your manual says pull the hose and plug the carb for initial timing. If it would be true PVA, that would make absolute no sense.

Many of the stock vacuum ports that the factory used for the vacuum advance were on the side of the carb. With out tracing the port, one would think that it is above the butterflies and therefore ported vacuum. It is not. The vacuum port on the Rochester 2 bbl and many 4 bbls goes straight down through the carb base below the butterflies which is manifold vacuum. Just because it goes into the side of the carb it does not mean that it is ported. Hook up a vacuum gauge to it if you don't believe me.

Like the 97, there is no port on the Rochester carb to get ported vacuum. The hot rod replacement carbs are made to adapt to many different applications and supply both types.

I also always hear "use whatever your engine likes the most".
What would be the difference in engines, that would not like advance at idle or cruising speed (and the benefits of it)?
Are there really such massive differences in engines that would do that, or is something else wrong, when it doesn't like advance?
Also why do all modern (computer controlled) motors love advance (look at your ignition tables) - why are there no differences ?

Frank

[Geoff2]

Manifold Vac Adv always works when set up properly.

It gets a bad rap because people use a factory non-adj VA unit with a low vac cam & find the timing varies & is erratic. Then they get on a forum like this & say 'MVA didn't work for me'...... [ Just one example of many where MVA would have worked if set up properly].

Modified engines MUST use an adj vac unit. Also, idle vac lower than about 6" is unlikely to operate the VA unit reliably. It might, but it would need to be tested.

[Tom68]

Not sure how that proves that PVA was used for emissions reasons? PVA was used in the 40s and 50s when performance and efficiency were a concern and emissions weren't given a second thought. This is well documented in automotive engineering texts and service manuals. I guess we'll have to agree to disagree on this.

What examples have you got ?

[Tom68]

Well if you can pull the vacuum hose from your carb and nothing happens, you probably have ported vacuum. But most of the time your manual says pull the hose and plug the carb for initial timing. If it would be true PVA, that would make absolute no sense.

Many of the stock vacuum ports that the factory used for the vacuum advance were on the side of the carb. With out tracing the port, one would think that it is above the butterflies and therefore ported vacuum. It is not. The vacuum port on the Rochester 2 bbl and many 4 bbls goes straight down through the carb base below the butterflies which is manifold vacuum. Just because it goes into the side of the carb it does not mean that it is ported. Hook up a vacuum gauge to it if you don't believe me.

Like the 97, there is no port on the Rochester carb to get ported vacuum. The hot rod replacement carbs are made to adapt to many different applications and supply both types.

I also always hear "use whatever your engine likes the most".
What would be the difference in engines, that would not like advance at idle or cruising speed (and the benefits of it)?
Are there really such massive differences in engines that would do that, or is something else wrong, when it doesn't like advance?
Also why do all modern (computer controlled) motors love advance (look at your ignition tables) - why are there no differences ?

Frank

The sidevalve 2 barrels had vacuum brake distributors, high air speed through the venturi ported distributor port braked the advance in the dizzy.