[Cousin_Joe]

I was wondering if there is any difference (or preference) in running vacuum advance through manifold or ported in the two ways below. The various items I've read on manifold -vs- ported aren't perfectly clear to me, I have a 383 (vortec heads, 9.7:1, 268xe cam, 2000 stall OD) which likes more initial so I have a vested interest. Perhaps it's related to my advance can, but the manifold seems to fluctuate the advance* for me.

A) Running 10* initial w/ ~10* from manifold vac advance, and using 'full' mechanical adv. or
B) Running 20* initial w/ 0* from ported, and limiting the to mechanical advance.

Thanks,
Joe

[techron]

i ALWAYS run manifold vacuum, set your total timing somewhere around 34-38* with the vac advance disconnected then hook it up. the ported vacuum was a stop-gap method to reduce idle emissions.

[curtis73]

I really get frustrated when people say that one is "always" better than another. The truth is, you use it to tune your curve like any other thing on the engine.

For instance, I needed a buttload of initial advance on my inefficient 454 with 8.5:1 compression. In fact, I locked the advance at 36*. Using manifold vacuum would have put me at 36 degrees initial and retard it all the way up to WOT. That is exactly the opposite of what it needs to do. Instead I set it up with ported vacuum, so that it was 36* base, then the vacuum advance added more during low-load cruise. Respectfully disagreeing with techron, ported vacuum was NOT a stop-gap for emissions. It was a drivability choice enacted by the manufacturers given the lower compression of the early smog-era vehicles.

On the other hand, if you have a high compression engine, you can use manifold vacuum to get extra initial timing that backs off a bit under load, then the mechanical puts it back in as you increase RPMs.

The bottom line is that the vacuum signal you use is simply another way of tailoring the ignition curve to match the engine's needs. One isn't "right" and the other "wrong." This debate is much like the vacuum secondary carb vs. the mechanical secondary carb. There is no "right" answer, only what is best for your application.

With your setup (good match of cam, heads, and compression by the way) either one could be tuned for proper operation, but I would lean toward the manifold source. Set it up for 8* initial (off the can) and then most vac cans give an additional 12* or so under vacuum. That will give you 20* at idle, but still allow easy cranking. Curve to give yourself 32-34* total. Play around with total, but the Vortec heads are very efficient and don't need the 36-38* like old school heads do.

In your two scenarios (and I'll add a third), let's look at how that would play out. Both scenarios start with 20* at idle

1) 10 initial + 10 from manifold vacuum. When you hit the accelerator, vacuum drops to nearly zero, meaning you instantly drop 6-9 degrees from your timing. Depending on your stall, you might need to curve the dizzy to start adding timing earlier to compensate.

2) 20 initial and no vac advance. You would be missing out on throttle response and acceleration by not having that extra, throttle-based timing adjustment.

3) 20 initial and ported vacuum. 20 at idle, then when you crack the throttle a little, the vac can responds with additional lead which tapers off as you add more throttle.

All three scenarios start at 20 degrees. The first example reduces lead when you add throttle. The second example doesn't do anything until you reach the speed when the mechanical kicks in. The third example adds lead when you add throttle. Then, once you pass about 1/4 throttle, all vacuum sources will have the same general reading. So the fine-tuning we're describing here only has to do with cruise, part throttle, and throttle response. Idle advance is very inconsequential. Engines will operate on a huge range of advances at idle. At WOT, all vacuum signals are basically zero, so the only part of the tune you have to really worry about are light throttle inputs. The manifold source scenario will make an aggressive idle advance that backs off for part throttle. The ported source makes the same idle advance, but attacks with a more aggressive off-idle lead.

I suggest you get (if you don't already have one) an adjustable vac canister. It can be set up for both sources. Try it on both. If you hook it up to ported (starts at 20 and increases) and find that its pinging under load/lugging, try backing off the vac advance. If you find that its still pinging, try manifold vacuum and keep going with the adjustment.

While on ported vacuum, the adjustment limits how much it adds when you hit the gas. On manifold vacuum, you're adjusting how much it takes off when you hit the gas.

[techron]

curtis, with all due respect, i stand by my statement. i have been messing with engines since the late 60s and was a professional mechanic for 30 years, so have probably built, tuned and modified more engines than most here with the exception of a few older guys who have been doing this for a living all their lives as i have. the OP asked a simple question and i gave a simple answer. while i agree with some of your statements, like each engine has to be tuned for what it likes-initial timing, total cintrifigal, total vacuum. i assume the OP knows this, otherwise i'm wasting my typing finger .

every car from any factory has an RPM controlled advance (centrifigal or computer controlled) advance curve to make the engine performs best at full throttle. the vacuum advance is to get the engine to perform best at cruise/high vacuum conditions, idle is a high vacuum condition with a stock or mild cam. the combo of centrifigal and vacuum works very well, just tweak them to suit your engine. don't try to outguess the factory engineers who designed these advance systems that are on millions of cars.

it sounds to me from your post like you are trying to use the vacuum advance for a substitute for centrifigal advance and locking the dist. at 36*. how does that engine crank with 36* advance without kicking back??? how is it at 1000/1500/2000 RPMs at full throttle with 36* advance???

i used to work on many 60s high compression, high performance corvettes in the 70s and they all used manifold vacuum. when the ported vacuum cars started showing up out here in cal. to meet smog regs. i would ALWAYS switch them to manifold vacuum-every customer would come back and say "HOLY CRAP, what did you do to my car, it runs tons better". out here in the early 70s the first smog regs. required all cars to have a kit installed on the upper rad. hose to lock out the vacuum advance until the engine reached hi temps. customers were pissed at the crappy performance. i would just tell them "come back after you pass the smog test and i'll hook you back up to full manifold vacuum"

SO, i stand by my earlier post:

ALWAYS use manifold vacuum...

PS: i still work on early vettes, i'm doing a body off resto-mod on my 64 coupe. i built a 383 stroker with 10.7-1CR (but i know how to make it live on pump gas) i'm also helping a friend/noob 65 vette owner build a late engine 383" stroker because he wants to be faster at the strip...

to the OP----> go with MANIFOLD VACUUM-->ALWAYS...

[curtis73]

Originally Posted by techron

it sounds to me from your post like you are trying to use the vacuum advance for a substitute for centrifigal advance and locking the dist. at 36*. how does that engine crank with 36* advance without kicking back??? how is it at 1000/1500/2000 RPMs at full throttle with 36* advance???

Not at all. 8.5:1, plus inefficient 049 stocker heads, plus a 218/224 cam meant needing a ton of initial advance. Having an automatic tranny (even with a stock stall) means you will never have 1000-2000 RPMs at WOT. My converter flashed to 1800, so a split second and you were past 2000.

The more important measurement (when using ported vac) was how it did just off idle at 2000 rpms with the vac can at max advance on a hot so-cal, sea-level day... and it was fine. I measured as high as 52* total advance at simulated cruise on the dyno with no detonation. If I tried really hard I could get a tiny touch of audible detonation on the hottest days, but a swap from 50/50 coolant mix to 70/30 water took care of that. Hot cranking never gave kickback, but I could tell it was working a little harder when hot.

SO, i stand by my earlier post:

ALWAYS use manifold vacuum...

And I'll stand by mine: Using absolutes to apply one "best" solution to the billions of possible combinations that exist in the new-school as well as the old-school is a little narrow-minded.

PS: i still work on early vettes, i'm doing a body off resto-mod on my 64 coupe. i built a 383 stroker with 10.7-1CR (but i know how to make it live on pump gas) i'm also helping a friend/noob 65 vette owner build a late engine 383" stroker because he wants to be faster at the strip...

Sounds sweet in a light Y-body like that. I'm currently building a 383 LT1, 11.03:1 that should make somewhere north of 445 hp, pass CA OBD2 smog, and it will actually burn 87 octane with the obvious use of the factory knock sensors and some very advanced chamber designs. I'll tune the PCM to run on 91 where its designed to run, but so far I've done a few that passed OBD2 with flying colors at 425-435 hp and 11:1, including a couple 396s. I'm working on some new head port and chamber designs with AI to see what else we can squeeze out of the old LT1.

to the OP----> go with MANIFOLD VACUUM-->ALWAYS...

And I'll stick by mine... Proper vacuum source --> always. There are no absolutes. techron, I totally respect your experience, but there are a lot of things that worked well with leaded gas in the 60's that just don't fly today, especially when considering the modern combustion chambers, quench theory, more modern valve event controls, etc... you have to admit that comparing a Vortec head and modern gas to a camel hump with leaded gas are two vastly different things... as evidenced by your 38* total timing recommendation. There is just so much that has changed that its not really logical to make absolute statements like that. I agree that for the most part your statement would be true up until about 1972, but in the 37 years since then, the widely varying range of engine combinations renders that kind of absolute advice obsolete.

Again, I'm coming from a totally respectful perspective. I don't mean to neglect your experience in the industry, but as a former engineer/consultant for Rochester Delphi, I was constantly required to know the latest and most advanced combustion physics on the planet. Of course, my greatest contribution was whittled down into the TBI/swirl port engines, so maybe I should keep my mouth shut

[Cousin_Joe]

I appreciate the quick feedback guys, and I do understand that each engine will have its own setup. I also understand that I won't quite need 38* with the vortec heads.

I kinda figured this would come to a try and error thing. I guess my goal was to clarify that I wasn't thinking/doing something that those with more experience would consider incorrect.

So let me pose this one... no matter what I go with for the best idle quality (manifold or ported), if I'm in @ 32* by say 2500-3000 rpms, what happens when I'm only cruising in OD @ 1800 rpm? Am I missing out on mechanical advance and is this hurting mileage? The other piece I'm missing is this 50-52* total idea. I'm not trying to rehash this to death, but just wanted to ask a couple things I hadn't been able to clear up thru searching. Thanks.

[oldbogie]

Originally Posted by Cousin_Joe

I was wondering if there is any difference (or preference) in running vacuum advance through manifold or ported in the two ways below. The various items I've read on manifold -vs- ported aren't perfectly clear to me, I have a 383 (vortec heads, 9.7:1, 268xe cam, 2000 stall OD) which likes more initial so I have a vested interest. Perhaps it's related to my advance can, but the manifold seems to fluctuate the advance* for me.

A) Running 10* initial w/ ~10* from manifold vac advance, and using 'full' mechanical adv. or
B) Running 20* initial w/ 0* from ported, and limiting the to mechanical advance.

Thanks,
Joe

Ported vacuum with-holds vacuum advance at idle, no vacuum will be applied till the throttle blade moves past the port thus exposing it to the pressure inside the manifold. As the throttle moves toward WOT at some point the manifold pressure and atmospheric pressure become nearly the same, thus no vacuum is present with a normally aspirated engine. So the vacuum advance amount becomes less and less as the throttle is opened.

Un-ported vacuum shows the advance mechanism manifold pressure from the moment the engine fires. To that end vacuum advance is present at idle but otherwise functions as outlined in the first paragraph. This can help a low compression engine or a long cammed engine to idle and transition to mid throttle more smoothly.

There really isn't a right or wrong here, it's more studying what the engine as it's installed in the vehicle likes best.

Bogie

[Rick WI]

Originally Posted by Cousin_Joe

manifold seems to fluctuate the advance* for me.

That comment is the problem. With that cam, cubic inches and compression the idle should be rock solid. The problem is not ported versus manifold, of which techron is correct in his comments, it's likely in either the tuning of fuel ratios or an issue with the distributor.

[joe_padavano]

Originally Posted by oldbogie

Ported vacuum with-holds vacuum advance at idle, no vacuum will be applied till the throttle blade moves past the port thus exposing it to the pressure inside the manifold. As the throttle moves toward WOT at some point the manifold pressure and atmospheric pressure become nearly the same, thus no vacuum is present with a normally aspirated engine. So the vacuum advance amount becomes less and less as the throttle is opened.

Un-ported vacuum shows the advance mechanism manifold pressure from the moment the engine fires. To that end vacuum advance is present at idle but otherwise functions as outlined in the first paragraph. This can help a low compression engine or a long cammed engine to idle and transition to mid throttle more smoothly.

There really isn't a right or wrong here, it's more studying what the engine as it's installed in the vehicle likes best.

Bogie

This is the best explanation I've seen here. The only difference between mainfold and ported vacuum is that the former is taken below the throttle blades and the latter is taken from a port (hence the term) just above the throttle blades. As soon as the throttle is opened enough to uncover the ports, manifold and ported vacuum are exactly the same. The only difference is when the throttle is closed. As pointed out, depending on how the engine and the distributor are set up, you want one or the other.

What hasn't been pointed out is that ported vac advance really came into favor in the late 1960s as an easy way to control NOx emissions at idle. By preventing the vac advance from working, the combustion was less efficient and NOx was reduced. This same concept is why cars of the late 60s and early 70s had Transmission Controlled Spark. The vac advance was disabled in all gears except top gear to further reduce NOx. Since this tended to make the car run hotter, these systems also had a temperature controlled valve that would bypass the TCS and run full manifold vacuum to the distributor in the event that the car was about to overheat. Note also that this disabling of the vac advance was the concept behind the "NOx device" that was required to be retrofitted to 66-70 cars in the state of California.

[red65mustang]

step #1: Read the dist install instructions!!!
(many dist are designed/engineered for and specify timed or manifold vac adv)

step#2: look up the calibration curve and min-max Hg versus degrees operating range of your specific vac model unit...
(there are dozens and dozens of different calibrations-curves models for different car/motor/gears combo's and it can add way to much (24*'s?) to be able to use manifold or not enough (6*'s?) for the base....

268 cam should be making 15Hg in park with 14-16* base which is plenty for fuel atomization at idle....
14* base plus 20* (normal) cent puts you right on the Vortec WOT mark...

best hwy mpg timing total=highest Hg at cruise=anywhere from 42 to 54* depend on the car...
rig your vac guage with a long hose into the car and test drive adding more vac advance timing till the guage Hg peaks holding the rpms steady at hwy speed
(if 46*=18Hg at 70mph and 48*-50*=18Hg at 70mph, set it to 46)
now test drive on a step grade with just heavy throttle to be sure there is no mid-throttle ping from to much added to soon....
it's not max degrees=best, it's max Hg=least pedal=best

my $.02
a 268 cam motor can better like using either port depending on the car....
there are definite good and bad points for using either!!!
as long as the dist instructions say you can use either, try both....
timed vacuum comes in at roughly 1200rpms, as soon as you step on the gas a fraction of a inch and open the butterflies, so does your car like vac adv added to the cent or does it like vac adv taken away from the cent with manifold from a stop sign

any bigger cam than 268 needs some manifold timing added, to raise the idle Hg and not have to use a 1000rpm idle to raise the Hg

[KULTULZ]

Originally Posted by oldbogie

Ported vacuum with-holds vacuum advance at idle, no vacuum will be applied till the throttle blade moves past the port thus exposing it to the pressure inside the manifold. As the throttle moves toward WOT at some point the manifold pressure and atmospheric pressure become nearly the same, thus no vacuum is present with a normally aspirated engine. So the vacuum advance amount becomes less and less as the throttle is opened.

I was going to try and stay out of this misinformed topic, but if what you say is correct, why does one have to remove the vacuum source to set base timing at recommended curb idle?

Researching the Venturi Effect will also answer a few misconceptions regarding vacuum signals.

[curtis73]

Originally Posted by KULTULZ

I was going to try and stay out of this misinformed topic, but if what you say is correct, why does one have to remove the vacuum source to set base timing at recommended curb idle?.

Because depending on the tuner and the idle requirements of the car, sometimes the throttles are open enough to send a tiny signal to the ported hole. I'm not saying its right, but to properly set base timing, the vacuum should be disconnected regardless of the vacuum source.

But on anything I do (street torque stuff mostly) the ported source is safely covered and showing 0-.5"

[woodz428]

Originally Posted by KULTULZ

I was going to try and stay out of this misinformed topic, but if what you say is correct, why does one have to remove the vacuum source to set base timing at recommended curb idle?

Researching the Venturi Effect will also answer a few misconceptions regarding vacuum signals.

Hallelujah, I second that. I will add to the confusion though . Anymore, when I am doing a performance build that isn't a restoration one, I plug the vacuum can on the dist and curve it for full mechanical advance, adding the amount of initial that works. That helps keep from having to tune around different cams,etc. Just a little more to confuse...some Ford advance cans can be adjusted for both the advance amount and the amount of "signal" it responds to.
That would have to be some engine to achieve atmospheric pressure within the intake....it is negative pressure( vacuum), by the way.....and the reason manifold vacuum drops IS the fact that it is below the throttle plates. It is the spring rate within the can that over powers the signal as it drops and returns it to it's rest position. If it became the same as atmospheric...how would the fuel enter the venturis??
Now you can research the Venturi Effect as Gary suggested....

[Rick WI]

Joe, your distributor should be setup to give you 20 degrees advance with 32/33 total mechical degrees in the 2500-3000 RPM range, assuming 12 degrees advance dialed in at idle. Your advance should start to come in at about 1000 RPM, as the engine will idle lower than this. That will be very close when setting up the mechanical curve.

The vacuum can should have around 8-10 degrees of additional advance and be plugged into the manifold port. This should give you a smooth idle assuming the carb is adjusted correctly and good part throttle performance.

You can not crutch one type of advance curve with the other, vacuum versus mechanical, they perform totally different functions. Vacuum advance is load based and mechanical is RPM based.

As to your question about cruise RPM, the only way you can have the best of all worlds with optimum timing at 1800 RPM is via a totally programmable ignition system. Not worth the expense on an application of your type.

[Cousin_Joe]

Thanks for all the feedback. I wasn't really trying to get into a discussion my specific setup, but just trying to understand more about timing.