Effects of altitude on vacuum control and timing

[furgo]

Recently someone at the German Bay forum pointed out a potential issue with an electronic, programmable ignition system (*). The issue being, that it senses the vacuum with a Manifold Absolute Pressure (MAP) sensor, as opposed to the stock distributors, which measure the vacuum relative to atmospheric pressure.

That got me thinking about how the stock vacuum can works, and the effects of altitude on its operation. Before delving into it, let me double-check if I got the facts straight:

Stock distributor, relative pressure

If I understand it correctly, the way original vacuum cans operate is by being controlled by a measurement relative to atmospheric pressure. On the diaphragm:

• The distributor side side is exposed to atmospheric pressure
• The engine side is exposed to manifold pressure by means of ported vacuum. With closed or wide open throttle, pressure on this side equals atmospheric pressure; at part throttle pressure on this side is lower than atmospheric pressure (i.e. vacuum)

So if the amount of pull the diaphragm gets is relative to atmospheric pressure, should this make its operation independent of altitude? That is, given the following example (vacuum can with 10° of max advance at 7.9 inHg):

• Pressure at sea level: 29.9 inHg absolute
• Max advance: 10° @ 7.9 inHg relative vacuum or 29.9 - 7.9 = 22 inHg absolute pressure

• Pressure at 8265 ft: 22 inHg absolute (**)
• Max advance: 10° @ 7.9 inHg relative vacuum or 22 - 7.9 = 16.1 inHg absolute pressure

This doesn't sound quite right, though. With increasing altitude and decreasing pressure (push) on the distributor side, the pressure on the engine side should decrease more to achieve the same travel (pull) of the vacuum can's arm to keep the advance constant on the breaker plate.

In other words, I somehow don't quite follow my own logic for altitude independence, so I'm thinking there might be a flaw in the thinking here...

Aftermarket distributor, absolute pressure

On the aftermarket distributor, there is only one MAP sensor that measures pressure at the same ported vacuum port –in kPa, but let's use inHg to keep the same units throughout. The max advance is programmed with the same values as before, but assuming sea level and an absolute pressure: 10° at 22 inHg.

• Pressure at sea level: 29.9 inHg absolute
• Max advance: 10° @ 22 inHg absolute pressure

• Pressure at 8265 ft: 22 inHg absolute (**)
• Max advance: 10° @ 22 inHg absolute pressure
• Issue: at wide open throttle, pressure will equal atmospheric pressure, which in turn will equal the setpoint for max advance. There will be no part throttle regulation and vacuum advance will be stuck at max (10°) across the whole load range.

Does this make sense?

(*) It's the 123tune distributor. The system looks great, but they're IMHO sorely lacking in the marketing department... of their own product. TLDR; quality-built original-looking distributor, electronic ignition, programmable centrifugal and vacuum curves, quite pricey.
(**) Calculated using the barometric formula. Can be done with an online form, e.g. https://www.mide.com/pages/air-pressure-at-altitude-calculator
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)

Decode your M-Plate

[Wildthings]

This is part of the reason it is common to advance your timing as you go up in elevation. Above 7000ft or so a SVDA dizzy doesn't offer much advantage over an 009 because you have little or no vacuum advance when cruising at highway speeds on level ground.

The rule of thumb is one degree of advance for every thousand feet above 4000', but if you are starting with your timing delayed to 28° BTDC at 3500rpms then you could start adding extra advance earlier, say at 2000'. This is very subjective, but the factory spec is basically 30+/-2° so you shouldn't be doing much of any harm by beginning to add advance early on if you are starting at 28°.

[SGKent]

As you climb in altitude three things happen that affect the internal combustion engine

(1) the amount of oxygen available drops
(2) the compression in the cylinder falls so less power is produced by compression heating
(3) the molecules are farther apart so it takes longer for the flame front to travel.

All these require advancing the timing.

Furgo these are good questions but you are in the wrong forum. You need to find an engineering forum. When it comes to engines you can put one on a dyno and set the timing at a given number known to work well for that engine. You can advance the timing one or two degrees and the needle will barely move. You can go one or two degrees the other way and the needle will barely move. Your HP will go from 131 to 131.000001 and you won't even be sure is isn't because someone farted in the room or had onions for lunch. Advance it 3 or 4 degrees and the engine will blow up. Retard it 3 or 4 degrees and the engine will slow down. Once you get close to the sweet spot the engine is really happy being around there. Playing with mind games is fun but from a practicality standpoint you won't see any improvement in your VW engine - it will still go slow up hills, and it is still a 40 year old car.
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[Wildthings]

[richparker]

I run my real 009 at 30* full advance at 8000’.
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[furgo]

Thanks everyone for the replies.

[Wildthings]

I am going to make a few simplifications here to make the subject easier to explain. Lets say that you have an engine that puts out 50 HP at wide open throttle (WOT) at 60 mph at seal level, but only requires 30 HP to maintain 60 mph and thus when cruising at 60 mph at sea level sees an engine vacuum of 8 inch of mercury and has 10° of vacuum advance.

Now lets take that same engine up to 9000'. In order to cruise at 60 mph it still takes 30 HP, but for the engine to make 30 HP at this elevation the engine has to run at WOT and has zero manifold vacuum and zero vacuum advance as a result. It is quite possible that at 9000' that because your engine is now lacking 10° of vacuum advance it can not make 30 HP so instead of being able to maintain 60 mph in fourth gear you end up slowing to 40 mph and may even have to down shift just to maintain that speed. The solution is to give the engine more initial advance. If you bump your initial mechanical timing up to 30° at 2000' in elevation and then add 1° of additional advance for every thousand feet above that then at 9000' you will have about 9° of additional advance compared to running 28° of mechanical advance at sea level, pretty much fully offsetting the loss of the vacuum advance.

The negative is that you now have a static timing of somewhere around 16 1/2° verses the book 7 1/2°, which may could play heck with the starter when trying to get a hot engine to restart. With old fashion American steel that had a 10:1 compression ratio this extra advance may well have led to the engine not cranking, but with an ACVW with only 7.5:1 compression it is probably not a big deal. YMMV

[furgo]

Thanks for the example, that's really helped me better understand the topic.

So to make sure I got it right:

On operation of the stock vacuum can at varying altitudes

• To keep the same cruising condition at sea level and at 9000 ft, at 9000 ft the throttle needs to open more to let more air volume in.
• This is due to air being thinner as altitude increases: as the oxygen/nitrogen per unit of volume decreases, more air volume (more throttle) is required to achieve the same AFR as at sea level.
• As altitude increases, a point is reached where WOT is required to keep the same cruising condition as sea level. That nullifies the operation of the vacuum can, as at WOT it will not see a vacuum and thus vacuum advance will be 0°. For this same cruising condition at sea level, only part throttle would be required, which would imply some degrees of vacuum advance.

So it might well be that the operation of the vacuum can itself is independent of altitude, but as the driver operates the throttle and effectively decreases vacuum at increasing altitudes, that ends up affecting the amount of vacuum it sees.

Again, in one sentence: less vacuum is measured at increasing altitudes.

On relative vs. absolute pressure measurement

To answer my other original question:

• Stock vacuum can (relative pressure): less vacuum (pressure differential) at more altitude means less vacuum advance and same centrifugal advance, which in turn means less total advance. The distributor itself cannot be modified, so to achieve the same advance as sea level, the only option is to compensate by adding more static advance.
• Aftermarket vacuum sensor (absolute pressure): less absolute pressure at more altitude means more vacuum advance and same centrifugal advance, which in turn means more total advance. The vacuum setpoint of the distributor can be modified, so to achieve the same advance as sea level, the setpoint needs to be changed to a lower absolute pressure value.

So if we compare the (for the want of a better word) failure mode of each device at say 9000 ft, the stock vacuum can fails in a safer mode (less advance), whereas the aftermarket distributor adds a constant additional advance, which is less safe, but as long it's not too much advance, it might end up being compensated by the altitude.

I think the discussion can be distilled into:

• In both cases, to achieve the best performance, a tuneup would be required before driving at 9000 ft. I.e. neither device is altitude independent.
• One needs to bear in mind how each device works to understand how to compensate it for altitude.
• With the stock distributor, one can choose to not do the altitude tuneup and live with less performance and a less advanced timing. With the aftermarket one, given the amount of advance added, it's probably advisable (i.e. safer) to do the tuneup.

Does this all now make sense?
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)

Decode your M-Plate

[raygreenwood]

Good information in this thread.

Its absolutely true that as the atmosphere becomes thinner the amount of air available to mix with fuel is reduced so HP qnd TQ are reduced.....and the maximum level of vacuum tjat can be produced in the manifold is reduced as well.

This is not a bad basic article that pretty much lays out what SGKent and others have noted.

https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge/

However...it is not an absolute certainty that your vacuum advance unit and power brake units will be reduced in function. Both of these run from ported vacuum and yes......as the available manifold vacuum drops....what these sub systems see at their port is reduced.

However...for instance with vacuum advance....at sea level it may see 20 inches of vacuum at its port. This does not mean that it requires 20 inches to complete its full stroke of advance.
Vacuum advance unit diaphragms are about 3" in diameter.....for a reason. The diameter is a multiplication factor to allow proper, full operation at lower vacuum levels. They are not a 1:1 ratio of operation. This allows them to work to a degree at part throttle operation. The macimum stroke length control is not the vacuum supply level. Its the notches on the advance arm and the corresponding stop notches in the advance can body.

For example....reading this thread...I just tested four seperate low mileage, dual vacuum cans...two from a middle years 411, one from a late 412 and one from an unknown year type 4 bus.....with my hand pump.
All of them began vacuum stroke at between 2.5 and 3 inches of vacuum. They were all at hald stroke or better at 4.5 to 5 inches of vacuum. All four reached maximum vacuum stroke....meaning the arm notch was hard up against the body of the can....by between 7 and 9 inches or vacuum.

Yes...they were not attached to a distributor....but unless you have a really cruddy advance plate...it should not add more than 1 inches of vacuum requirement.

You should still have slightly more vacuum than this in a healthy engine in the 9000 ft range. Ray

[TomWesty]

[Amskeptic]

[furgo]

[Amskeptic]

[raygreenwood]

[Wildthings]

With my old engine which had a moderately upgraded cam with close to 0.5" of lift, but not an excessive duration or overlap, I would get maybe 10-12 inches of vacuum at 60 mph on the level at sea level. With my present engine which has a high mileage stock cam, I am lucky to get 5 inches of vacuum in the same conditions. Neither engine would give more than 2 inches of vacuum at 9000 feet when trying to do 60 mph on level ground and thus there was no vacuum advance, zero, nil, nada.

At sea level, when I am starting out on level ground even with my present worn stock cam, I do maintain sufficient manifold vacuum as I pull away from a stop on level ground to give full vacuum advance unless I really punch it, it is not until I shift into second that my manifold vacuum is guarantied to drop to the point where I loose vacuum advance.

I think it is important to note that there are three main reason for not having much vacuum advance at idle. None have to do with performance.

1. Delayed timing will help the engine crank
2. Delayed timing will give more decel when you let off on the throttle
3. Delayed timing will allow you to run a larger throttle opening and thus make for a smoother less polluting idle.

Note that the last two reason are why VW added the dual vacuum can to many distributors, giving even that much more retard when compared to a SVDA distributor.

[raygreenwood]

[Wildthings]

[SGKent]

all over my head. I though a DVDA was about California smog rules to lower NOX and CO.

All I ever learned about this came from what affected lap times, kept engines from blowing up and let the cars I built out accelerate the competitors. I learned not to advance the timing too much on VW air cooled engines by melting a piston on my daily driver in the dead of winter when the denser colder air made the engine more prone to detonation. I also learned that while we did have to change timing and mixtures (that is what practice laps are for) depending on the elevation of the track etc., we were racing at, the difference of a couple degrees here or there on the dyno did not make significant differences unless the threshold for detonation was crossed, or the combustion heat was significantly wasted (Colins situation that he corrected). If you want to go faster at elevation either use a steam engine, or an electric/hybrid engine.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin