reverse vacuum load phenomenon

david jenkins · Post by **david jenkins** » Mon Oct 29, 2007 11:46 am

brentp wrote:Right, the map sensor is measuring absolute pressure-. More vacuum- less 'pressure'. As you open the throttle, the vacuum reduces- WOT and you are near 100KPa, atmospheric pressure.

Don't worry- I have more of those moments than I care to admit.

Now you've gone and confused me again!

I've just done some testing for the problem I've discussed elsewhere, and my vacuum works the other way round! As I increase the engine speed the vacuum increases - the KPa value reduces. This can be seen on the attached log file.

It also seems the logical result to me - or am I also having a 'moment'!

David

BTW: It's a Weber DGV carb (twin venturi vertical carb), with the MAP connection on the manifold below the carb base.

dr.occa · Post by **dr.occa** » Mon Oct 29, 2007 1:54 pm

david jenkins wrote:
brentp wrote:Right, the map sensor is measuring absolute pressure-. More vacuum- less 'pressure'. As you open the throttle, the vacuum reduces- WOT and you are near 100KPa, atmospheric pressure.

Don't worry- I have more of those moments than I care to admit.
Now you've gone and confused me again!

I've just done some testing for the problem I've discussed elsewhere, and my vacuum works the other way round! As I increase the engine speed the vacuum increases - the KPa value reduces. This can be seen on the attached log file.

It also seems the logical result to me - or am I also having a 'moment'!

David

BTW: It's a Weber DGV carb (twin venturi vertical carb), with the MAP connection on the manifold below the carb base.

david, i'm sorry to say but i'm glad i'm not the only one: i'm not so crazy after all.

i've also attached a quick data log from this morning.

if it helps any, i am running a pair of aftermarket performance cams (PDM custom made by Colt Cams).

Post by **brentp** » Mon Oct 29, 2007 3:09 pm

David's log looked like he was driving the car and dr.occa's log appears to be blipping the throttle from idle. Is that correct?

Perhaps it would be better to discuss in terms of engine load rather than RPM. For example- if you rev the engine and then snap the throttle closed you will see less pressure (more vacuum) as the engine coasts down than you will at idle.

Understanding why you're reading 100KPa at idle would be helpful, since your engine must be generating enough vacuum to draw fuel out of the carburetors.

david jenkins · Post by **david jenkins** » Mon Oct 29, 2007 3:21 pm

brentp wrote:David's log looked like he was driving the car and dr.occa's log appears to be blipping the throttle from idle. Is that correct?

I was blipping the throttle from idle...

dr.occa · Post by **dr.occa** » Mon Oct 29, 2007 3:49 pm

brentp wrote:David's log looked like he was driving the car and dr.occa's log appears to be blipping the throttle from idle. Is that correct?

Perhaps it would be better to discuss in terms of engine load rather than RPM. For example- if you rev the engine and then snap the throttle closed you will see less pressure (more vacuum) as the engine coasts down than you will at idle.

Understanding why you're reading 100KPa at idle would be helpful, since your engine must be generating enough vacuum to draw fuel out of the carburetors.

strangely enough, david stated his was from blipping the throttle from idle.

my log is from driving into work this morning.

Post by **brentp** » Mon Oct 29, 2007 4:14 pm

Actually, I got the two logs mixed up...

I did interpret them correctly. dr.occa, what part of your log were you at or near WOT?

dr.occa · Post by **dr.occa** » Mon Oct 29, 2007 7:48 pm

brentp wrote:Actually, I got the two logs mixed up... I did interpret them correctly. dr.occa, what part of your log were you at or near WOT?

from around 03:31.3 to 03:41.4 is where i'm getting on it.

Nick · Post by **Nick** » Fri Nov 02, 2007 12:58 am

Hi, May I throw in my twopennyworth? I'm sorry if I'm teaching the sucking of eggs, but here goes...It occurs to me that the symptoms are those of a connection to a 'vacuum' pick up placed as a cruise advance system. There are many 'economy' carbs that have a take off just above the leading (rising) edge of the throttle plate that will have 0in/hg at idle and as the plate moves past the hole so the pressure drops. This is converted to an additional vacuum advance for light throttle cruise economy. I have often been caught out by this placing, as it inverts the expected results.

dr.occa · Post by **dr.occa** » Fri Nov 02, 2007 10:10 am

Nick wrote:Hi, May I throw in my twopennyworth? I'm sorry if I'm teaching the sucking of eggs, but here goes...It occurs to me that the symptoms are those of a connection to a 'vacuum' pick up placed as a cruise advance system. There are many 'economy' carbs that have a take off just above the leading (rising) edge of the throttle plate that will have 0in/hg at idle and as the plate moves past the hole so the pressure drops. This is converted to an additional vacuum advance for light throttle cruise economy. I have often been caught out by this placing, as it inverts the expected results.

Thanks Nick but I don't know what to do with this information being that my vacuum ports are located on each runner just like in the diagram I linked to on page 1 of this thread.

I'm waiting on a vacuum gauge...

david jenkins · Post by **david jenkins** » Fri Nov 02, 2007 10:25 am

I am still unconvinced!

First of all, my vacuum take-off is the one that used to be connected to the mechanical distributor, so I'm using the correct port.

This take-off is between a very powerful air pump (the engine) and a significant air restrictor (the carb's butterfly and venturi). When the engine's rpm is increased it tries to suck in more air; as there is a restrictor in the way the pressure should drop (more vacuum, lower KPa reading). In the mechanical dizzy, the increasing vacuum worked on a diaphragm that in turn pulled the internal mechanism to advance the timing.

Is this confusion due to the differences between throttle bodies and carburettors? The throttle bodies are free-flow tubes with an injector that squirts into the air stream. The carb has to have a venturi to lower the pressure next to the jets, to suck petrol up into the air stream, so they are not 'free flow' by design.

Or am I talking out of my lower orifice (again)?

David

MartinM · Post by **MartinM** » Fri Nov 02, 2007 11:56 am

david jenkins wrote:I am still unconvinced!

...When the engine's rpm is increased it tries to suck in more air; as there is a restrictor in the way the pressure should drop (more vacuum, lower KPa reading). ...

..Is this confusion due to the differences between throttle bodies and carburettors? ...

When the engine's rpm is increased by pressing the accelerator (which means the restrictor opens) the pressure increases (less vacuum, higher kPa reading) - it must increase since....

...at idle the throttle (ie the restrictor) is closed and there is a big vacuum (engine sucking, big restriction in place) - which is why you normally see 20-30kPa at idle.

...then imagine just dumping the accelerator into the carpet from idle - the restrictor opens immediately (it's a mechanical link), the engine doesn't immediately change speed and the first thing you see is an increase in manifold pressure as the external air pressure forces air into the manifold (more kPa, less vacuum)

(the age old question/conundrum is whether engines suck in air, or is air forced into them by atmospheric pressure, but let's not talk about that one)

I don't know much/anything about throttle bodies, I must admit...

david jenkins · Post by **david jenkins** » Fri Nov 02, 2007 12:01 pm

Whichever way it works... I have set up my map to work in a similar way to the mechanical dizzy - more vacuum, more advance.

Exactly WHEN the vacuum increases or decreases is open for debate, but at least I'm matching the engine's original equipment! And it seems to be working OK...

regards,
David

MartinM · Post by **MartinM** » Fri Nov 02, 2007 12:13 pm

david jenkins wrote:...more vacuum, more advance.

Yep, that's the one!

Martin

dr.occa · Post by **dr.occa** » Fri Nov 02, 2007 6:27 pm

maybe i'm just confusing myself and need to face the reality of a problem unique to my situation but here's my understanding: typical vacuum at idle on most cars is ~ 20-22 in. Hg (inches of Mercury or 77 kilopascals) as the car accelerates on the vacuum/boost gauge that measures inHg the indicator moves from the higher number to 0 and then psi increases on the other side of 0 in boosted applications.

from what i'm getting is that MJ does the opposite and kpa numbers increase on acceleration. there's my confusion or lack of understanding.

MartinM · Post by **MartinM** » Fri Nov 02, 2007 9:28 pm

dr.occa wrote:typical vacuum at idle on most cars is ~ 20-22 in. Hg (inches of Mercury or 77 kilopascals)

Yes, and this is the amount of the vacuum, not the amount of absolute pressure.

No vacuum == atmospheric pressure - where you and me are right now

Complete vacuum == no pressure at all - outer space

They're called vacuum gauges because they measure vacuum. When they are measuring no vacuum, the pressure is atmospheric.

So taking atmospheric as 102 kPa, 77 kPa of vacuum is (102-77) = 25kPa of absolute pressure...which is exactly what my MJLJ reads at idle

Double check...
What does a mechanical vacuum gauge read when its in the open air - zero
What does an electrical vacuum gauge when it's powered up and in the open air - zero

Go find some automotive vacuum gauges on Google with a description of the values that they show...

as the car accelerates on the vacuum/boost gauge that measures inHg the indicator moves from the higher number to 0 and then psi increases on the other side of 0 in boosted applications.

Yes, exactly.... "vacuum...gauge...moves...to 0" i.e. the vacuum goes to zero/becomes less. So the absolute pressure is rising back towards atmospheric.

from what i'm getting is that MJ does the opposite and kpa numbers increase on acceleration. there's my confusion or lack of understanding.

Yes, exactly right as well.

The MJLJ measures absolute pressure.

What does a MJLJ read when its powered up and in the open air - 102 kPa

What does a MJLJ read when its powered up and engine idling - 25kPa (= 77 kPa of vacuum)

What does a MJLJ read when its powered up and an engine is at full throttle (open butterfly) - pretty close to 102kPa (= 0 kPa of vacuum)

So everything you say is entirely consistent