Vacuum power

[human-potato_hybrid]

I have a question on manifold vacuum power. Since using this power draws air into the manifold, wouldn't it throw off the manifold air-fuel mixture?

 

I know manifold vacuum power is ubiquitous in a solid half-century or so of cars, so I feel like I'm missing something about why this works without apparent side effects.

Edited April 15 by human-potato_hybrid (see edit history)

[playswithbrass]

No because there is not a leak letting air into the intake. It is a closed system ,think of a brake booster the engine vacuum causes a low pressure area(vacuum)on one side of the diaphragm and atmospheric (14.7)on the other side allows the booster to move and assist in braking. Same with vacuum advance the low pressure area caused by vacuum on one side allows atmospheric pressure to push the diaphram,which pulls the breaker plate. Remember vacuum only provides a low pressure area that allows atmospheric pressure to do the work.Hope this helps

[human-potato_hybrid]

Yeah that makes sense for like a vacuum advance. But for something like vacuum powered wipers, about twice per second you are pulling a vacuum from a chamber filled with air, so that's a bit of a "leak". Maybe not enough to matter.

[8E45E]

3 minutes ago, human-potato_hybrid said:

Yeah that makes sense for like a vacuum advance. But for something like vacuum powered wipers, about twice per second you are pulling a vacuum from a chamber filled with air, so that's a bit of a "leak". Maybe not enough to matter.

The carburator does the actual, precise air/fuel mixture ratio; not the intake manifold.

 

Craig

[human-potato_hybrid]

8 minutes ago, 8E45E said:

The carburator does the actual, precise air/fuel mixture ratio; not the intake manifold.

 

Craig

I know, but the vacuum port on the carb goes to below the throttle plates. So that's after the carb.

[joe_padavano]

52 minutes ago, human-potato_hybrid said:

I know, but the vacuum port on the carb goes to below the throttle plates. So that's after the carb.

You are correct. If there is additional air coming into the manifold through a vacuum operated accessory, that will cause a lean condition unless compensated for in the carb. Using the vacuum brake booster as an example, there are vacuum chambers on both sides of the diaphragm, and both are evacuated by manifold vacuum. When you apply the brakes, an air admittance valve closes the connection from the rear chamber to the intake and opens a valve to atmosphere. The 14.7 psi on the rear side of the diaphram provides the brake assist. When you release the pedal, the air admittance valve closes and that chamber is again connected to manifold vacuum. Yes, this small additional air volume enters the engine after the carb metering circuits, so yes it makes a tiny change to A/F ratio. The actual air volume is a small fraction of a cubic foot, so in terms of CFM into the engine from this "leak", the air volume is meaningless. Vacuum wipers use a fraction of the air volume that the power brake booster uses, though the do tend to operate for a much longer period of time. Again, the CFM number is negligible.

[joe_padavano]

I'll add that if you've ever pulled the check valve out of the brake booster with the engine running, the massive vacuum leak that results will usually kill the engine.

[human-potato_hybrid]

Thanks @joe_padavano. So my understanding about the device operation was correct, it's just that the air volume is small enough that the engine doesn't really notice/care about it.

 

The pressure difference at idle is only like 5 PSI as I understand (30 inHg atmosphere -> 20 inHg manifold absolute pressure) so it's understandable that 5PSI times the volume of a wiper motor or brake booster is really not that much air.

 

As a point of comparison, a 350 ci engine idling at 400 RPM will pull in its displacement once per 2 rotations, so it sucks in about 40 CFM, or 27 NCFM.

[ABear]

Generally, yes, running an accessory that is "open" to air which tends to cause a leaner fuel mixture over all like you would have with a vacuum hose leak. But the volume of a leak with say a wiper motor is much smaller than a open vacuum line.

 

However, generally your carbs mixture for best operation will tend to be a bit on the "rich" side rather than on the lean side. Being slightly to the rich side can cover for open vacuum accessories or even changes in elevation with out your engine starving for fuel with a lean condition.

[human-potato_hybrid]

Good point. A 1949 carb manual gives this blurb.

 

[ABear]

Should also point out, running slightly to the rich side to start with, results in less heat build up in the engine, running too lean causes engine to run hotter.

[human-potato_hybrid]

44 minutes ago, ABear said:

Should also point out, running slightly to the rich side to start with, results in less heat build up in the engine, running too lean causes engine to run hotter.

Now that you mention it, why does running lean make the engine run hotter?

[Bloo]

4 hours ago, human-potato_hybrid said:

I have a question on manifold vacuum power. Since using this power draws air into the manifold, wouldn't it throw off the manifold air-fuel mixture?

 

I know manifold vacuum power is ubiquitous in a solid half-century or so of cars, so I feel like I'm missing something about why this works without apparent side effects.

As others mentioned, it is a tiny gulp of air to re-evacuate the booster after use, and it happens on a transition so you don't notice it. If it were a constant leak like PCV, the air would have to be let in at a spot where it mixes evenly with the air/fuel coming from the carburetor, and the carburetor would have to supply any extra fuel needed. PCV can't be way out on one intake runner like a power brake port usually is. Vacuum wipers leak whenever they are on, and in the prewar era were often fed from a less-than-ideal spot on the manifold. It worked because the engine had a little too much fuel.

[ABear]

24 minutes ago, human-potato_hybrid said:

Now that you mention it, why does running lean make the engine run hotter?

Has to do with the speed of the flame front, technically speaking the fuel doesn't explode, it BURNS in a wave like fashion (it does with a high enough speed to us it looks like and explosion). Not enough fuel mixed with air results in uneven heating in the cylinder creating hot and cold spots along the way. The flame front creates pressure and it is the pressure that pushes the piston on the down stroke. As a side note, because the fuel is burning instead of exploding as you increase RPMs you must also increase the ignition timing (spark advance) so the spark happens faster BTDC (Before Top Dead Center).

 

Slightly rich air to fuel mixture results in a much more even burning of the flame front and much more evenly burning flame front.

[Bloo]

1 hour ago, human-potato_hybrid said:

Good point. A 1949 carb manual gives this blurb.

 

There are 3 points of maximum efficiency. Maximum power occurs around 10.5 to 1, although it varies by engine. It is more difficult to get more air into the cylinders, but easy to get more fuel. When you use every possible bit of oxygen available, you get the most power, but waste some fuel. The best burn efficiency is at the stoichiometric point, or 14.7 to 1. The best fuel economy occurs when you burn every possible bit of gasoline, but that leaves some oxygen left over. It varies by engine but might be about 16.5 to 1 or so.

 

The blurb in the carburetor manual is probably intentional oversimplification. A couple of automotive engineering texts I have show they knew all this in the 40s, they just did not have economical enough hardware to control the mixture that tightly in mass production.

 

In 1981 oxygen sensor fuel control became ubiquitous in the US. Overnight everything new ran at 14.7 to 1 under idle and cruise conditions due to limitations of the sensor, instead of chasing a point near the lean limit as had been common in the first part of the smog control era. Suddenly new cars ran terrific as long as the system was working. It turns out 14.7 to 1 under idle and cruise conditions is a very good place to be. Full power mixtures for hard acceleration were still set much richer as before.

 

[rocketraider]

Hoo boy. Now we're getting into heat transfer and stoichiometry, which as the 1949 excerpt shows you ideally need 15 volumes of air (14.7 actually) to completely burn 1 volume of fuel. And you're making me dig up old powerplant knowledge after 8 years away from it!🤪

 

The concept of "excess air" is used to lean out a fuel mixture for more complete combustion of your designated fuel. So you're creating more heat thru more complete combustion, and in most cases higher heat = greater fuel efficiency. 

 

Running an automotive combustion engine that lean isn't one of those cases as engine damage can result. But excess air causing more complete fuel burn is great for exhaust emissions at the expense of power output.

 

Running stoichometric A/F ratios or even a little fuel rich has the opposite effect. The extra fuel tends to cool both combustion and exhaust temperatures, as does exhaust gas recirculation. Keep reburning exhaust till it burns up any remaining hydrocarbons.

 

The advantage to EGR is that it cooled combustion temperatures enough to reduce detonation caused by lean mixtures with resulting high combustion temperatures, though its true purpose was to cool them enough to reduce nitrogen oxide emissions.

 

Have mercy.🤯

 

###

 

Some engineers figured out that, in the case of vacuum wipers and HVAC controls, vacuum could be sourced from a mechanical fuel pump's diaphragm. This had the advantage of a more constant vacuum supply and less chance of engine vacuum leaks. The greatest benefit was that vacuum wipers weren't as likely to slow down under heavy engine loads and then go to maximum speed when decelerating. It also kept vacuum controlled HVAC systems from switching between floor, dash and defrost vents depending on engine load. 

 

I've seen this setup on Fords and Oldsmobiles. Pretty sure others used it too. By the 60s they'd figured out electric wipers, and vacuum storage tanks and check valves.

[Bloo]

45 minutes ago, human-potato_hybrid said:

Now that you mention it, why does running lean make the engine run hotter?

In addition to what others said, maybe because you are burning as much of the fuel as is possible? The same thing occurs in gas torches. The flame gets hotter when there is excess oxygen. Why doesn't excess oxygen make more power in an engine then? Because to make maximum power you have to use all the oxygen available at the expense of everything else. Oxygen is the toughest thing to get in.

 

Edited April 15 by Bloo (see edit history)

[human-potato_hybrid]

15 minutes ago, rocketraider said:

Some engineers figured out that, in the case of vacuum wipers and HVAC controls, vacuum could be sourced from a mechanical fuel pump's diaphragm. This had the advantage of a more constant vacuum supply and less chance of engine vacuum leaks. The greatest benefit was that vacuum wipers weren't as likely to slow down under heavy engine loads and then go to maximum speed when decelerating. It also kept vacuum controlled HVAC systems from switching between floor, dash and defrost vents depending on engine load. 

 

I've seen this setup on Fords and Oldsmobiles. Pretty sure others used it too. By the 60s they'd figured out electric wipers, and vacuum storage tanks and check valves.

Yeah, the 55 Packards I have from the factory came with an auxiliary pump mounted under the oil pump. Other cars, and the same engines used on Hudson and Nash, used an auxiliary pump combined with the fuel pump.

 

I have heard that the engines (320/352) are big enough that wipers etc. will still work fine without the auxiliary pump, but I plan on adding mine back anyway. 

[Bloo]

26 minutes ago, human-potato_hybrid said:

I have heard that the engines (320/352) are big enough that wipers etc. will still work fine without the auxiliary pump,

 

26 minutes ago, human-potato_hybrid said:

but I plan on adding mine back anyway. 

Good plan.

 

27 minutes ago, human-potato_hybrid said:

the 55 Packards I have from the factory came with an auxiliary pump mounted under the oil pump. Other cars, and the same engines used on Hudson and Nash, used an auxiliary pump combined with the fuel pump.

I don't really have any familiarity with the oil pump driven pump, though Buick also did that at some point. My gut feeling is that I would prefer the fuel pump driven one because it would be easier to get to if it needs service. Vacuum wipers on their best day are not great. Without a booster I think you will hate them.

 

Edited April 15 by Bloo (see edit history)

[ABear]

33 minutes ago, rocketraider said:

By the 60s they'd figured out electric wipers, and vacuum storage tanks and check valves.

Vacuum cans which have check valves helped by "storing" a small amount of vacuum. The stored vacuum helped to reduce the chance of vacuum accessories affecting engine operation and also became a temporary source of vacuum for vacuum motors used to operate Heating and A/C doors under heavy engine acceleration (wide open throttle results in almost no vacuum depending on where the vacuum port is).

[human-potato_hybrid]

20 minutes ago, Bloo said:

I don't really have any familiarity with the oil pump driven pump, though Buick also did that at some point. My gut feeling is that I would prefer the fuel pump driven one because it would be easier to get to if it needs service.

The main problem was the oil pump being so high made air get sucked in from the driveshaft. There was also no weep hole to lubricate the driveshaft from the pressure feed.

 

A number of retrofits etc. have been made to these over the years to combat this issue, with varying degrees of success.

[kgreen]

I’ve recently been struggling with a vacuum leak on a pneumatic convertible top mechanism. The vacuum cylinder is about 4 in dia by 18-20 in in length. That’s a good amount of vacuum loss when operating the top. It is not a closed system. The vacuum is applied to one or the other side of the cylinders position. Air from the opposite side of the piston is relieved through the valve. 

[human-potato_hybrid]

@kgreen That's still only about 0.13 cu ft. so assuming it takes a typical amount of time to operate, I don't see it being any worse than vacuum wipers.

 

I would check that the piston seal itself is not leaking. If you have the piston out of the car, depress the piston nearly all the way and plug the depressed side. You should be able to pull the piston away some and it should spring back. If it loses "springiness" quickly then it is leaking.