Ported or manifold for vacuum advance?

[edwardgonzalez]

I know this is probably going to get a few people going because I know it's a heated subject but I'm going to ask anyway because I'm having trouble adjusting my carburetor. At the moment I have the vacuum advance connected to the port on the driver side which I believe is ported vacuum. It's been like that always but it also has run rich always. I set the idle screws with a vacuum gauge and seem to be getting around 22hg at idle. Should I be running the vacuum advance on the other port. I do not have anything with emission. No egr or smog pump. I do have the pcv connected. Any help would be greatly appreciated. Thank you

[Axle]

Try both and use whatever works best. I've run both myself, depending on engine setup, one sometimes outshines the other.

Alex.

[BMERDOC]

Ported was used in emissions vehicles. Unless you modify the distributor by changing the advance canister you need to make where you hook it up with where it was designed to be plugged in. Aftermarkets use manifold.

[rich weyand]

If you don't have a smog pump, use manifold vacuum.

Edelbrock carb from the sound of it?

[nonstop]

You will get a load of different opinions. Ported was used long before emissions was a concern. Manifold is supposed to give better performance. I have tried both on my truck. Manifold seems a little more responsive, but not what I was expecting based on what I've read. As stated, try both and see what you like.

[rich weyand]

You can read a bunch of stuff on ported versus vacuum advance, usually written by people who have never worked on a stock older vehicle. Also, these posts seldom discuss combustion chemistry. If they don't discuss combustion chemistry and cylinder pressures, they have no clue what they are talking about.

The advance used to be on the steering column, a manual lever like a turn signal. In 1938, Studebaker introduced vacuum advance, run on manifold vacuum. In 1968, the smog pump was introduced to cars, and ignition was moved to ported vacuum. The idea was to reduce emissions on cars sitting idling on big-city freeways during rush hour.

No car I had before 1968 had ported vacuum. 1961 Chrysler Newport with the 361 engine, 1962 Plymouth with a 225 slant six, 1962 Buick LeSabre with a 401 Nailhead, and a 1967-built 1968 Ford Country Squire station wagon with the Z-code 390.

So, what pre-1968 vehicle ran on ported vacuum? Not a big-three car, I don't think.

The issue is combustion chemistry. Let's start with mechanical advance.

When the throttle is wide open, and the engine is accelerating, the manifold vacuum is at zero, so the manifold is at atmospheric. If you have an 8:1 compression ratio, you are pulling in atmospheric-pressure mixture and compressing it to one-eighth the size. Your cylinder pressures at TDC are eight atmospheres. (I will leave out the complications of dynamic compression ratios and intake runner length and such here; assume a volumetric efficiency of 1.0 for this discussion.)

There is an optimum time in milliseconds of advance for that cylinder pressure. That is, there is a certain amount of time it takes that mixture to burn down to the piston and start pushing. At that point the piston should be starting down. So, the reason you need base timing to be before TDC is because it takes some time for the fire to get there.

The problem is that that time is in milliseconds, not degrees of rotation. As the engine turns faster and faster, it turns more degrees during those milliseconds of time. The mechanical advance keeps adding degrees of advance to the timing to try to get to that same value in milliseconds for optimal timing.

OK, so what's vacuum advance? When the throttle is shut, you might be pulling 15" of vacuum, say. One atmosphere is 30" of mercury, so the manifold has one-half atmosphere of mixture in it. The cylinders pull in half as many molecules of air and fuel. You now compress that at your 8:1 compression ratio and you have four atmospheres of cylinder pressure, not eight as before with WOT.

The issue is that that thinner mixture will burn slower. The molecules are farther apart, and the combustion process is slower. That means that, in order for the front of the combustion pressure wave to hit the piston at the right time, you have to ignite the charge sooner -- i.e., you need more advance.

Now to ported vs. manifold vacuum. Manifold vacuum is the signal from the manifold that tells how thin the mixture in the cylinder will be at TDC, and how much in advance of TDC the charge needs to be ignited to reach the piston at the optimum time. There is no difference in the combustion chemistry between idle and just-off-idle, and no reason for the timing to be different.

Ported vacuum purposefully ignites the charge late at idle. The idea is to dump the burning charge into the manifold so that the added air from the smog pump together with the still-burning charge will burn up some of the pollution before it exhausts the vehicle. This only happens at idle, so it reduces pollution from idling cars, such as those stuck on LA freeways at rush hour. That's the purpose.

Running on ported vacuum will require a higher idle throttle setting, because not as much of the energy of the charge is applied to the piston. Another compensation is to run a richer mixture, because a richer mixture will burn faster than a lean one, reducing the effect of igniting the charge late. This is why when someone moves the vacuum advance from ported to manifold without adjusting the idle mixture and idle throttle setting, the car might run worse. But lean out the idle mixture and reduce the idle throttle setting, and the engine will idle better, have no off-idle hesitation, and will run cooler, than on ported vacuum.

The one place ported vacuum will help is on starting. At the very low rpm of cranking the engine to start it, manifold vacuum will have the ignition timing more advanced than the best position for the 100-200 rpm of cranking the engine over. This makes it crank harder and start harder than it would on ported vacuum. The simple solution to that is to put a valve on the vacuum advance line to the distributor, either with a cable into the cab or a solenoid valve run off the oil pressure switch that runs the electric choke. I haven't needed it on mine -- it starts all the time with the gear-reduction starter -- but I've thought about it.

Anyway, do what you want. It's your truck after all. But us old geezers remember the emissions-controls-free cars we drove back in the day, and we remember how to tune them.

[Fullpower]

For best idle quality, best off idle throttle response, and fuel efficiency use FULL manifold vaccuum to the distributor.
If the vaccuum can on the distributor is a late model, 1968 or newer, it will have a bit TOO much advance, and you will likely have to fashion a limiter strap, so that vaccuum advance only makes 15 or so degrees.

Accel sells an adjustable vaccuum can, so you can dial in the point where the advance cuts out.

[rich weyand]

Originally Posted by Fullpower

For best idle quality, best off idle throttle response, and fuel efficiency use FULL manifold vaccuum to the distributor.
If the vaccuum can on the distributor is a late model, 1968 or newer, it will have a bit TOO much advance, and you will likely have to fashion a limiter strap, so that vaccuum advance only makes 15 or so degrees.

Accel sells an adjustable vaccuum can, so you can dial in the point where the advance cuts out.

Most of the pollution-era HEIs give 9-10 camshaft degrees (18-20 crankshaft degrees), while the pre-pollution and early-pollution points distributors give 8 camshaft degrees (16 crankshaft degrees). Not much difference there.

But, if you are worried about it, you can switch whatever is on your HEI to the VC1853/AR23 can, which is the 1975 350 4bbl can, which comes in at 5"-7" and tops out at 7.5 (camshaft) degrees at 11"-12.5". Compare this to the VC1807/B25 can that was the pre-HEI/pre-pollution can for 350s on the CKs: comes in at 5"-7" and tops out at 8 (camshaft) degrees at 13"-15".

Adjustable cans can be a PITA to get adjusted, based on people I have seen on the forums having problems getting them set up.

[rich weyand]

One addendum to my earlier comment, re: starting. Both ported and manifold vacuum will start with no vacuum advance, because there is no vacuum when the engine is not running. (duh) But as the engine cranks, it will start to pull vacuum, and if the throttle is closed manifold vacuum will start to pull in advance where the ported won't. My engine just about always catches on the first two revolutions, so no difference there.

[BMERDOC]

Ask yourself what is being powered by the vacuum signal...the advance canister. Port vacuum was for smog engines. These answers, however, are not answering your question. 22 in. hg. is plenty. What kind of carb troubles are you having?

[nonstop]

My DD, a 54 Dodge came from the factory using ported vacuum. None of my Motors manuals from the 50's refer to using manifold vacuum, and neither do my large truck manuals. On the contrary, I have not come across an older car that ran manifold vacuum from the factory. With that said, Rich is spot on with his posts. This is going to come down to personal trial and error, and whichever you like best. I too did the search for a long time comparing the two, and in the end deemed it to best go with trial and error. This is similar to asking an oil related question - you will get a boat load of answers that will range across the board.

[rich weyand]

Originally Posted by nonstop

My DD, a 54 Dodge came from the factory using ported vacuum. None of my Motors manuals from the 50's refer to using manifold vacuum, and neither do my large truck manuals. On the contrary, I have not come across an older car that ran manifold vacuum from the factory. With that said, Rich is spot on with his posts. This is going to come down to personal trial and error, and whichever you like best. I too did the search for a long time comparing the two, and in the end deemed it to best go with trial and error. This is similar to asking an oil related question - you will get a boat load of answers that will range across the board.

Does your '54 have the "Load-O-Matic" distributor?

[rich weyand]

Originally Posted by nonstop

My DD, a 54 Dodge came from the factory using ported vacuum. None of my Motors manuals from the 50's refer to using manifold vacuum, and neither do my large truck manuals. On the contrary, I have not come across an older car that ran manifold vacuum from the factory. With that said, Rich is spot on with his posts. This is going to come down to personal trial and error, and whichever you like best. I too did the search for a long time comparing the two, and in the end deemed it to best go with trial and error. This is similar to asking an oil related question - you will get a boat load of answers that will range across the board.

Well, let me make a couple comments here.

The Load-O-Matic distributor, which was used on a bunch of 1950's stuff into the 1960s, was a vacuum-vacuum distributor. No mechanical advance. Instead, the advance line off the carburetor had a spark control valve (SCV) that selected the higher vacuum of manifold vacuum and venturi vacuum. Venturi vacuum is from the narrowest part of the carb and will show vacuum at high flow rates -- i.e. high rpm. This advanced the spark instead of mechanical advance. So the vacuum signal would start at high vacuum at idle, go down to near-zero with the throttle open, and increase again as the rpms picked up. That is not ported vacuum, even though it is not (just) manifold vacuum and comes from a special vacuum port on the carb.

I have never seen an older car that ran on "manifold vacuum" either. We didn't call it "manifold" vacuum until the mfrs introduced ported vacuum under government mandate on January 1, 1968. We just called it "vacuum", and yes it came from the manifold. One popular mod on GM cars was to run it from the back of the manifold instead of the front of the carb so we could hide the vacuum line for a cleaner look. On the Fords, you left it, because the distributor is in the front anyway.

That change on 1/1/68 is why my dad ordered the 1968 Country Squire with the Z-code 390 early enough to be built before the end of the year.

Remarking on an earlier post, I have a stock GM HEI with the Accel cap, rotor, and coil, and the AR-23 vacuum can, which runs the lesser advance (7.5 camshaft degrees) from the pre-pollution period, rather than the stock vacuum can. 17*BTDC base + 20 mechnical + 15 vacuum, for 37* total timing and 52* at cruise.

[BMERDOC]

It all comes down to how engineers intended the vacuum advance and distributor to perform. If there are some pre-emissions examples out there then that's great but in the end it comes down to whether or not the vacuum advance was intend to have vacuum at idle or not. Running a distributor built to run manifold hooked to ported and vice versa would have negative consequences on performance. They must be matched.

[SkinnyG]

I've tried both manifold and ported, but I have a lot of cam and run 20° base timing. Adding manifold vacuum was a bit too much at idle.

Rich: got any tips or a link to adjusting a vacuum can properly?

[rich weyand]

Some questions:

- How much mechanical advance do you have? Alternatively, what is your total timing?

- How much vacuum advance are you running, in degrees, and what vacuum does it come in at?

- What is your engine idle vacuum?

- What do you mean, it was a bit too much at idle? What did it do?

[Firebirdjones]

Rich is asking all the right questions Skinny. Everyone's idea of a lot of cam is a bit different.

What I usually find with my own or a customer car that has a cam with some overlap, is that manifold vacuum will tend to work better in that situation.

To start out, I like to set them up with 16-18 initial with about 18-20 degrees of mechanical. Depends on where I want the total to sit, because engines vary quite a bit in that regard depending on a lot of factors like compression, gas used, camshaft, and the design of the combustion chamber of the cylinder head. A really efficient engine might make best power at just 30-34 degrees (usually large cube stroker motors of 540+ like less total) where some other engines with inefficient combustion chambers might make best power at 38 or even 40 degrees.

Anyway, once initial and total are set I'll throw vacuum in the mix. I'll limit this however to about 10-12 degrees and ramp it in fast or slow depending on what the engine will tolerate. It's a little trial and error.

With everything described above, and using manifold vacuum, I'm usually idling with my 18 initial and another 8-10 degrees of vacuum thrown in with it so I'll see 26-28 degrees idling with the timing light on the balancer and the vacuum plugged in. This really helps idle with those aggressive cams and also helps the engine make a little more vacuum, then dialing in the carb from there is a tad easier, I don't have to get as aggressive with things like drilling idle feed restrictors, or really low hg power valves etc...
With that said, throwing in initial, mechanical, and vacuum, I'm cruising lightloads at around 50 degrees give or take.

[rich weyand]

Originally Posted by Firebirdjones

Rich is asking all the right questions Skinny. Everyone's idea of a lot of cam is a bit different.

What I usually find with my own or a customer car that has a cam with some overlap, is that manifold vacuum will tend to work better in that situation.

To start out, I like to set them up with 16-18 initial with about 18-20 degrees of mechanical. Depends on where I want the total to sit, because engines vary quite a bit in that regard depending on a lot of factors like compression, gas used, camshaft, and the design of the combustion chamber of the cylinder head. A really efficient engine might make best power at just 30-34 degrees (usually large cube stroker motors of 540+ like less total) where some other engines with inefficient combustion chambers might make best power at 38 or even 40 degrees.

Anyway, once initial and total are set I'll throw vacuum in the mix. I'll limit this however to about 10-12 degrees and ramp it in fast or slow depending on what the engine will tolerate. It's a little trial and error.

With everything described above, and using manifold vacuum, I'm usually idling with my 18 initial and another 8-10 degrees of vacuum thrown in with it so I'll see 26-28 degrees idling with the timing light on the balancer and the vacuum plugged in. This really helps idle with those aggressive cams and also helps the engine make a little more vacuum, then dialing in the carb from there is a tad easier, I don't have to get as aggressive with things like drilling idle feed restrictors, or really low hg power valves etc...
With that said, throwing in initial, mechanical, and vacuum, I'm cruising lightloads at around 50 degrees give or take.

This. Plus setting the vac can to be all in at your idle vacuum so it doesn't hunt, which will make it idle like crap.

[SkinnyG]

Base timing: 20°
Total mech: 16°
Vacuum: 12° (limited to)
Dunno what vacuum it comes in at. The can is adjustable, and I have it set fairly low.
Vac @ idle: 8" in gear (when ported)

Too much at idle: really really stinky exhaust. Could be a tuning issue, and I may not have refined the tune enough.

I appreciate the input - I'm learning lots!

[Firebirdjones]

Switching back and forth from ported to manifold is definitely going to make a difference in your idle mixture requirements. I'll see pretty drastic changes on the wideband when doing that. Hooking to the manifold brings in more timing at idle and creates a higher vacuum situation and can pull more fuel from the idle feed restrictors and/or boosters.
Would be best to pick one and stick with it a while and dial in the carb to accommodate it, then see which one you like better.

[rich weyand]

Originally Posted by SkinnyG

Base timing: 20°
Total mech: 16°
Vacuum: 12° (limited to)
Dunno what vacuum it comes in at. The can is adjustable, and I have it set fairly low.
Vac @ idle: 8" in gear (when ported)

Too much at idle: really really stinky exhaust. Could be a tuning issue, and I may not have refined the tune enough.

I appreciate the input - I'm learning lots!

OK, what you've done is keep the ends of the timing curve (timing plotted on Y, rpm and idle plotted on x) at the right spots, and raised the curve in the middle with that high base timing. This will not give you the best performance at WOT in the lower rpms.

It would be interesting to try 16 base timing, 20 mechanical, which gets you to the same WOT/high rpm timing as you have now, but has less advance in the lower rpms when the throttle is open. This should be better timed for those rpms and will also reduce the chance for knocking.

Then on the vac advance, get a can with about 12 (crankshaft) degrees of advance, that starts at 3-5" and is all in by 7-8", so your timing won't hunt at idle. Hook that to manifold vacuum, and lean out the idle. You probably have stinky exhaust on manifold vacuum because you haven't leaned out the idle enough.

What you should get is better gas mileage and better pull in the lower rpm range, plus about 10 degree cooler engine temps at idle and cruise. You'll probably also get a little higher idle vacuum, which will help idle and off-idle carburetion.

Which is basically exactly what Firebirdjones is saying as well, and is close to what you have, *except* 1) swap the base timing and mechanical advance numbers to still get 36 total, but 16 base + 20 mechanical, 2) change to manifold vacuum, and 3) lean out the idle, a lot -- use a vacuum gauge (OK) or a broadband AF/R meter (better).

The B28/DV1810 can has the start vacuum/all-in vacuum you want. It is specifically for highly cammed engines. You may need to limit the advance it pulls in though, as it will add up to 16 crankshaft degrees. 32 degrees of advance at idle (on manifold vacuum) may be too much. But the curve is low enough for your weak idle vacuum signal. But it sounds like your vac advance is dialed in OK right now to work with a lower base timing on manifold vacuum.

[SkinnyG]

I had run manifold when I first built the truck, and tuned as lean as I felt I could get away with. It ran perfectly fine. Ran that for close to a year (daily driver).

Switched to ported almost a year ago, tweaked the idle mix and it runs absolutely fine like this too. I may go back, tune, and try again to compare.

To be honest, I haven't really noticed a significant difference between the two.

[nonstop]

Just for clarification - you say vacuum is 8" in gear, ported. Are you getting vacuum in the ported line? Or was it just a general statement?

[SkinnyG]

Originally Posted by nonstop

Just for clarification - you say vacuum is 8" in gear, ported. Are you getting vacuum in the ported line? Or was it just a general statement?

When I run ported vacuum, the engine idles at 8" in gear, if I remember correctly. Or it idled at 8" with manifold vacuum. I can't really remember. It was low, at any rate.

[rich weyand]

Thinking some more about it, the smelly exhaust might be that 20 base + 12 vac adv at idle on manifold vacuum was just too much advance at idle and it was missing and blowing raw fuel out the back from the misses. I think this might be it because you said you had leaned it out a lot on manifold vacuum before.

Another way to check that is back the base timing down to 16, switch to manifold vacuum, and lean out the idle mixture for best vacuum, then see how it idles and performs in the lower rpm range. You can do that really simply, then if you like that you can mess with the mechanical to get you back up to 36 total timing (that is, open the mechanical up to 20*) for the upper rpm range.

BTW, are you running regular or premium gas right now?