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Vacuum Advance, port vs. manifold

Posted By Jack Groat 2 Weeks Ago
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Jack Groat
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I have a Holley 3x2 set-up and I have a port vacuum outlet on the main Holley.  I can provide either port or pure manifold vacuum to the distributor.

The difference is port vacuum disappears at idle.  You only have vacuum advance at cruise.  I have researched this and cannot find any recommendations.  As an afterthought, I don't see many port vacuum outlets on more modern carburetors.   I am comfortable with manifold vacuum sine that is what I have used on previous engines.

Any opinions??
55blacktie
Posted 2 Weeks Ago
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Holley's website recommends port vacuum, at least for single-carb applications. I don't know that multiple carbs makes a difference.
DryLakesRacer
Posted 2 Weeks Ago
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I have always used port vacuum for the low idle I like. I bought a HEI ignition for Tom Langdon especially made for a different brand in-line engine and he recommended intake vacuum which we have been using. That engine has 3 Holley 2 barrels and it takes a lot of tuning to get it to idle under 850rpm.

56 Vic, B'Ville 200 MPH Club Member, So Cal.
Ted
Posted 2 Weeks Ago
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All the new 4V carbs I purchase come equipped with both direct and ported vacuum outlets.  If you have an older L-O-M distributor, then a carburetor with a spark advance valve is required.  This would include the older three bolt 2V carbs and Holley 4V Teapot carbs unless modified appropriately for the later model distributors.  If you have upgraded to a distributor that uses both mechanical and vacuum advance, then a later model carburetor with a specific ported vacuum signal works with a typical mechanical advance curve.  If going to the trouble of shortening up the amount of advance supplied by the mechanical portion of the distributor, then you may find that using a direct vacuum advance signal works for you.
 
Here’s a past thread on the subject of ported versus direct vacuum to the distributor.
 
http://forums.y-blocksforever.com/Topic44648.aspx


Lorena, Texas (South of Waco)


Rusty_S85
Posted 2 Weeks Ago
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Honestly, If you have a regular mechanical advance distributor with vacuum advance I recommend trying ported and manifold and see what your engine likes.  I have seen some engines prefer manifold over ported and some prefer ported over manifold.  In searching you might come across a questionable kid that I dont agree with on youtube that claims ported vacuum will give you better fuel economy over manifold vacuum but cant explain how when vacuum is vacuum and it only depends on when it comes in.

Ford have used venturi vacuum, ported vacuum and manifold vacuum for vacuum advance.  I prefer using manifold vacuum personally as it provides a smoother idle by giving you more advance at idle.  But the engine I am building for my truck and going with the Holley Sniper Stealth 4150 I will be trying ported vacuum first since its an entirely new engine from my old engine.

You do how ever want to make sure you dont have too much timing, Fords dont typically like more than 35-36 total degrees of advance or they will tend to run hot or knock.  I have my Y block how ever setup at 12* initial but I have the LOM distributor and not sure exactly how much advance it provides but I do know it works up to 25* advance in the quick test I did.  On my truck it is not a Y block so I wont go into too much detail on it outside of the general aspects, I am going to have my distributor recurved by a guy I found in Washington state that has a sunmachine that recurves ford distributors for under $150.  He was in agreeance with me that he will be setting my distributor up to be around 35* total advance vacuum/mechanical based off my engine specs I provided.  Based off how much mechanical advance he uses will determine how much initial I can put in, dont have to worry too much about vacuum as vacuum advance is only advancing under low load conditions which isnt as prone to detonation and goes away with load which is when detonation has a higher chance of happening.  Im hopeful he can set it where I can retain my 12* initial as that seems to be a sweet spot I found out with my Ford V8s.

1956 Ford Fairlane Town Sedan - 292 Y8 - Ford-O-Matic - 155,000 mi

Tedster
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The very early repair manuals discussed "ported" vacuum source - that is, above the throttle plates, as a method to ensure a smooth steady idle. This predates any sort of smog considerations. Sometimes you'll hear people claim this port business was an EPA requirement foisted on everyone. That isn't really the case.

It is true that after the federal clean air mandates were imposed on the automakers especially by the early 1970s, everyone needed to use a ported connection to have any hope of getting their engines to pass. One concession they made, was a solenoid that would revert to a manifold vacuum connection of the distributor in the event of engine overheating. Very thoughtful of them, since lots of engines were now running on the ragged edge.

A number of strategies were employed, including a reduction in compression ratios, cylinder combustion chamber redesign, EGR, retarding of ignition curves, and camshaft modifications to include "late" valve timing, and very lean fuel mixtures, as well as excessively rich fuel mixtures depending.

What all this tended to do was create a very poorly running, inefficient engine that wasted prodigious amounts of fuel (during both a currency devaluation and an Oil Embargo ..)

You can try to experiment a bit and see what you think. The general rule that seems to hold is factory stock engines are best setup with a "ported" or timed vacuum source. Performance engines, particularly those with radical camshaft (and consequently very low manifold vacuum) may indeed benefit from a fulltime manifold vacuum connection to ensure a streetable engine. Remember, a true drag engine doesn't include vacuum advance - because "part throttle" just isn't part of their vocabulary. Thus, the vacuum advance mechanism itself is just something else to get in the way - and possibly fail - at an inopportune time.

Keep in mind "Total Timing" is also a drag racing term. Again, because vacuum advance doesn't exist in their world.

The 36° or 38° "total timing" we talk about on Y-Blocks isn't counting vacuum advance. This is strictly referring to the initial timing, + the internal distributor mechanical weights and springs. A lot of people get confused on this subject. The vacuum advance is always tuned separately, after the mechanical curve is satisfactory.

When cruising on flat ground 50° BTDC or more is typical with OHV engines. The engine will run cooler and smoother with appropriate ignition timing advance at idle and cruise and everywhere in between.

Another common error is errors when setting the ignition timing. There won't be any vacuum advance at idle in a factory stock engine, provided the RPM is set to factory specification.

Most people like to bump it up a little. For example if I recall Y blocks are factory 450-550 RPM. About 600 to 650 seems to run smoother for most - and if you check with a timing light, it's easy to see why. The vacuum advance, even connected to a "ported" connection, will start to pull in a fair amount of timing just past factory RPM specs. My experience with walking people through this stuff is they would be far better served with learning what is an appropriate timing curve, than worrying about what port to connect to, because without understanding, it doesn't matter what port is used, it won't run right anyway.
Rusty_S85
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Tedster (11/12/2020)
The very early repair manuals discussed "ported" vacuum source - that is, above the throttle plates, as a method to ensure a smooth steady idle. This predates any sort of smog considerations. Sometimes you'll hear people claim this port business was an EPA requirement foisted on everyone. That isn't really the case.

It is true that after the federal clean air mandates were imposed on the automakers especially by the early 1970s, everyone needed to use a ported connection to have any hope of getting their engines to pass. One concession they made, was a solenoid that would revert to a manifold vacuum connection of the distributor in the event of engine overheating. Very thoughtful of them, since lots of engines were now running on the ragged edge.

A number of strategies were employed, including a reduction in compression ratios, cylinder combustion chamber redesign, EGR, retarding of ignition curves, and camshaft modifications to include "late" valve timing, and very lean fuel mixtures, as well as excessively rich fuel mixtures depending.

What all this tended to do was create a very poorly running, inefficient engine that wasted prodigious amounts of fuel (during both a currency devaluation and an Oil Embargo ..)

You can try to experiment a bit and see what you think. The general rule that seems to hold is factory stock engines are best setup with a "ported" or timed vacuum source. Performance engines, particularly those with radical camshaft (and consequently very low manifold vacuum) may indeed benefit from a fulltime manifold vacuum connection to ensure a streetable engine. Remember, a true drag engine doesn't include vacuum advance - because "part throttle" just isn't part of their vocabulary. Thus, the vacuum advance mechanism itself is just something else to get in the way - and possibly fail - at an inopportune time.

Keep in mind "Total Timing" is also a drag racing term. Again, because vacuum advance doesn't exist in their world.

The 36° or 38° "total timing" we talk about on Y-Blocks isn't counting vacuum advance. This is strictly referring to the initial timing, + the internal distributor mechanical weights and springs. A lot of people get confused on this subject. The vacuum advance is always tuned separately, after the mechanical curve is satisfactory.

When cruising on flat ground 50° BTDC or more is typical with OHV engines. The engine will run cooler and smoother with appropriate ignition timing advance at idle and cruise and everywhere in between.

Another common error is errors when setting the ignition timing. There won't be any vacuum advance at idle in a factory stock engine, provided the RPM is set to factory specification.

Most people like to bump it up a little. For example if I recall Y blocks are factory 450-550 RPM. About 600 to 650 seems to run smoother for most - and if you check with a timing light, it's easy to see why. The vacuum advance, even connected to a "ported" connection, will start to pull in a fair amount of timing just past factory RPM specs. My experience with walking people through this stuff is they would be far better served with learning what is an appropriate timing curve, than worrying about what port to connect to, because without understanding, it doesn't matter what port is used, it won't run right anyway.


Correct, my truck is post emission as well as my Mercury both of which are manifold vacuum but they run through a vacuum switch that is plumbed into the coolant port to remove vacuum advance from the equation when below normal operating temperature.  My Mercury goes one step further in having the hard to find yellow strain relief that has another manifold vacuum line that attaches to what we would call today a map sensor.  This actually when working correctly puts my engine at 14* initial timing at idle to a total of nearly 45* of total timing at idle as the yellow strain relief DSII module is advancing the timing more than the mechanical/vacuum advance would be capable of in low load conditions including idle.  My truck how ever is supposed to be 8* initial mainly to help reduce detonation under load since you will be working a truck even a 1/2 ton truck harder than your standard production car.  Before the emission era most of it was ported vacuum to prevent transition issues when driving from closing the throttle plates too much to try and idle the engine down.  I didnt have this issue with my truck as I had to maintain a slightly higher idle than spec as I have dealer AC installed and there was no stepper solenoid so the extra vacuum advance at idle ontop of the 12* initial timing wasnt too much of a problem.  Im waiting to see just what I will have to do when it comes to my new engine build I am doing since I am turning the engine on its head in a sense from removing emission systems by back dating but then going more advance with better modern heads and fuel injection.

For me when I say total timing it is what the timing under load which would remove vacuum advance as vacuum advance is eliminated during load conditions so you want total timing which is your mechanical advance and your initial advance to add up to with in the 30* range for ford engines Ive seen 34* to 38* listed but I am going with aluminum AFR heads on my truck engine build which has the fast burn heart shaped chambers which I need to back my timing down on the lower side of around 34-35*.  I just hope the guy I will be using doesnt set my dist up to be 35* on mechanical and dont take initial into account cause from the research Ive done on my specific build I want to shoot for around 35* advance under load.  Ideally I would like to keep 12* initial which would limit mechanical advance to be no more than 23* itself.  If he sets it up around 20-21* mechanical that would be better as I know my mercury with its 351W loves 14* initial but I question a higher compression 302 with a OE starter at 14* initial.  I know DSII has the ignition retard on start function which will help.

For my 292 how ever, I have the petronix points delete installed which made my engine run so much better since the distributor probably is in need of a rebuild.  I also have the initial set at 12* initial which seem to be pretty nice, only issue I am having to deal with is the Holley 4000 choke doesnt seem to want to work right in the sense that to set it tight enough to close when cold with one press of the throttle to the floor it would never fully open up.  Got a new NOS choke thermostat and it worked considerably better but car has sat for a bit and now its doing that again.  Going to try and use a brush and carb clean on the linkage and see if I can try and free it up a bit with sticky ethanol blended fuels we have now.  But aside from that and my transmission issue I got my 292 running at almost OE spec, in gear I am at 550 rpm in gear and OE spec in my shop manual is 450 - 500 rpm.  After I do a band adjustment on my transmission if that corrects my issue I want to try and idle my 292 down to OE spec a bit more if possible.

I agree, its better to have a proper timing curve than worrying about ported vs manifold.  I have always told people to see what their engine likes best and throw that "ported is proper" nonsense out the window.  That may work with a 100% bone stock all original brand new engine but once you wear the engine in it doesnt always work that way.  Like my truck I tried switching to ported vacuum and I could not get the truck to idle and ive had this one kid try to tell me I need to add more timing to offset the reduction in total timing at idle with manifold vacuum or some nonsense like that.  I tried telling him that I have my timing set at 12* initial and 14* initial has starter drag on hot restarts I cant advance it anymore.  Then he tried to tell me the carb isnt set right but yet my truck with manifold vacuum idled dead smooth with AC on at 600 rpm in gear.  if the carb wasnt set right I dont believe you would have that low of a idle speed with the load of an AC compressor such as a York in gear and be dead smooth.  Only carb issue I had with my truck which will be gone now with my new planned engine build was the accelerator pump, I had to bump it up to the largest squirt size on the 2150 autolite carb to remove a stumble on tip in of the throttle.  It actually corrected it 99% of the way there was just a slight tip in problem.  I could probably work on it and get it better but its not worth it when I parked the truck 3 years ago and started planning and working on a new engine build from the ground up after the OE balancer broke apart and egg shaped the crank snout.  With the Sniper Fuel Injection they have a ignition port for vacuum but it is ported vacuum and I plan on using that as a start and see how my new package works and see what the dyno guy I will be using for tuning the FI and see what he thinks.

On that note I really wish someone like holley would make more retro carb like FI systems I would seriously consider a FI setup for my 292 if it looked just like a Holley 4000 carb.

1956 Ford Fairlane Town Sedan - 292 Y8 - Ford-O-Matic - 155,000 mi

Florida_Phil
Posted 2 Weeks Ago
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There is a lot to learn and digest on this subject. Ted's previous explanations of the vacuum advance system helped me.  Like many, I didn't understand it's purpose.  The key is experimentation.  Not all engines are the same.  Not all driver's are the same either.  I tried everything on my engine until I found what worked best for me and my car.  My TBird engine has a stock 1957 Ford distributor with a Pertronix unit and lighter advance springs.  My vacuum advance is connected to the primary side of my single Holley carb. I run 36 degrees advance with the vacuum line disconnected.  It starts, idles and runs great.




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Tedster
Posted 2 Weeks Ago
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It really is an ingenious system, I'm not sure who actually invented it. Kettering, I guess. Utilizing engine vacuum as a signal is used for many different automotive functions.

In the case of ignition advance, they had to figure something out because centrifugal weights and springs (RPM based) alone can not provide anywhere close to enough timing advance under low load, part throttle conditions.

In experimenting a little bit with a direct connection to manifold vacuum it ran fine, but I had trouble with getting a perfectly steady idle, something mentioned in the early manuals as the reason for the distributor port in the first place.

This is getting off into the weeds here, but what I don't understand is why the engineers were forced to basically cripple 70s era engines with all kinds of performance and economy robbing measures simply in order to reduce NOX emissions, but modern engines can run the typical timing advance curves. Better catalysts in the exhaust than back then? I don't know.
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This is getting off into the weeds here, but what I don't understand is why the engineers were forced to basically cripple 70s era engines with all kinds of performance and economy robbing measures simply in order to reduce NOX emissions, but modern engines can run the typical timing advance curves. Better catalysts in the exhaust than back then? I don't know.


The reason(s) being FEDERAL and STATE(S) EMISSION  LAWS.
Emmission System(s) then were mechanical. Modern day is computer controlled.


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