Vacuum controlled ignition on B20E

[Stig Hornang]

On our Volvo 140 '72 (B20E) there is a distributor which has centrifugal
weights and a vaucuum system (as usual) to control the ignition timing
under the different speed and load conditions of the engine. How and why
the centrifugal works is pretty straightforward. When the engine speed is
higher the air/fuel mixture must be ignited earlier because the piston is
moving faster.

But I've never quite understood why the vacuum system works as it does on
our B20E. The theory says that when there is a lean mixture (and also low
pressure) in the sylinder, the burning prosess takes longer time. This is
the case when the throttle is (almost) closed. Vacuum builds up in the
intake manifoil and also in the sylinder. With the throttle closed there
is a lean mixture of gasoline in the sylinder.

The reason why the vacuum capsule was invented is (from what I've read)
that for best possible burning, the ignition timing should consider the
amount of fuel (and pressure) in the sylinder to "calculate" when the spark
on the plug should occur. If the mixture burns slow, the spark should
occur earlier and vice versa. The point is always to get maximum pressure
when the piston is at top. Since the pressure in the intake manifoil
represents the pressure and fuel amount in the sylinder, the ignition is
partly controlled by this vacuum.

Shouldn't it be so that a high vaccum in the intake manifoil (means low
sylinder pressure and low amount of fuel) should give an advanced spark?
This is not the case on our volvo. The hose for the vacuum capsule is
conected between the throttle and the engine so it "feels" the intake
pressure. Higher vacuum retards the ignition in our case. This is wrong?
If the throttle then opens (lets say at a low engine speed) then suddenly
the ignition would advance (go earlier) which is not right. It should
retard (be later) since a richer fuel mixture need less time to burn. Is
it suppose to be like this?

The Haynes manual also says that "an increase in engine vaccum results in
a retarded igntion" (not directly from the book) for the B20E and B20F
engines. It also says that "in this case the vacuum capsule is connected
to a point between the throttle and the engine". Could it be that the
distributor for the car is wrong? Since the car is fuel injected (which is
only on the 'E' and 'F' engines, i think), the distributor has an extra
connector so the control unit can measure engine speed. If it's the wrong
distributor it probably can only come from an 'F' engine.

 

[normalini]

It is a vacuum "retard" system, to retard the ignition at idle
only.......helps minimize exhaust emissions. Timing should be set to 10
degrees BTDC with the vacuum hose off. This was a common bandaid solution to
vehicle emissions in the early '70s.

 

[Jim Carriere]

It is a vacuum "retard" system, to retard the ignition at idle
only.......helps minimize exhaust emissions. Timing should be set to 10
degrees BTDC with the vacuum hose off. This was a common bandaid solution to
vehicle emissions in the early '70s.

I've seen this setup before too, but I thought the B20Es all had advance
(pretty sure the B20F had the retard system you describe, I know the B21F
had it over several years).

My B20B ('74) had ported advance. The vacuum line from the distributor went
to a nipple on one of the carburettors, right behind the throttle.

 

[TWE]

TWC:
Either Advance or Retard the engine timing had nothing related
to "exhause emission".
To eliminate the "engine knock" is the purpose.

Stig Hornang ¼g¤J¡G

 

[Stig Hornang]

I've seen this setup before too, but I thought the B20Es all had advance
(pretty sure the B20F had the retard system you describe, I know the
B21F had it over several years).

My B20B ('74) had ported advance. The vacuum line from the distributor
went to a nipple on one of the carburettors, right behind the throttle.

This cleared some of my confusion. Seems that vacuum retard has nothing
todo with fuel economy. But how can a retarded ignition on idle minimize
vehicle emmisions?

The distributor also need new centrifugal springs because they're not
strong enough anymore. So the basic timing isn't as should (must
compromise to not get too early ignition on high RPM), so right now I
can't set it to 10 degrees BTDC for normal running.

One other thing that is "strange" with the vacuum retard is that when you
start the engine the vacuum is obviously zero and that yields for an
earlier ignition. This makes the starter go very heavy in the beginning
because the engine is pushed backwards by to early ignition. You can also
hear a diesel like knocking when starting. Sounds something like when the
big american V8 engine stops. This also happens if you force the engine to
a low RPM at same time you throttle it, (e.g. starting in 2nd gear).

Wish I had the opportunity to go back in 1972 and try a brand new 140 with
B20E Strange if it should be like i described above originally. But I
as I mentioned, I haven't checked that the distributor is correct for the
car. The one that is on is called something like "Bosch J-FURX".

 

[Stig Hornang]

TWC:
Either Advance or Retard the engine timing had nothing related to
"exhause emission".
To eliminate the "engine knock" is the purpose.

Well, the following pages says it has todo with fuel economy (most
effective combustion, as described in my first posting).

http://www.2quicknovas.com/happytiming.html "Its function is to provide
extra advance at high vacuum to increase fuel economy."

http://www.tpub.com/content/construction/14273/css/14273_67.htm "The
vacuum advance increases FUEL ECONOMY"

http://www.leeric.lsu.edu/bgbb/7/ecep/auto/g/g.htm "...To understand the
relationship between vacuum and ignition timing and reduced fuel economy
in computer assisted engines."

Most pages I've seen says that the vacuum mechanism exists because it
improoves fuel economy. You may be right with your theory, but can you
explain how it could avoid knocking?

The pages I've referenced only talks about vacuum advance not retard. You
say it's not exhaust mission. I still wonder what vacuum retard is good
for since it actually does the opposite thing (makes fuel ecenonmy
worse?).

 

[Mike F]

On our Volvo 140 '72 (B20E) there is a distributor which has centrifugal
weights and a vaucuum system (as usual) to control the ignition timing
under the different speed and load conditions of the engine. How and why
the centrifugal works is pretty straightforward. When the engine speed is
higher the air/fuel mixture must be ignited earlier because the piston is
moving faster.

But I've never quite understood why the vacuum system works as it does on
our B20E. The theory says that when there is a lean mixture (and also low
pressure) in the sylinder, the burning prosess takes longer time. This is
the case when the throttle is (almost) closed. Vacuum builds up in the
intake manifoil and also in the sylinder. With the throttle closed there
is a lean mixture of gasoline in the sylinder.

The reason why the vacuum capsule was invented is (from what I've read)
that for best possible burning, the ignition timing should consider the
amount of fuel (and pressure) in the sylinder to "calculate" when the spark
on the plug should occur. If the mixture burns slow, the spark should
occur earlier and vice versa. The point is always to get maximum pressure
when the piston is at top. Since the pressure in the intake manifoil
represents the pressure and fuel amount in the sylinder, the ignition is
partly controlled by this vacuum.

Shouldn't it be so that a high vaccum in the intake manifoil (means low
sylinder pressure and low amount of fuel) should give an advanced spark?
This is not the case on our volvo. The hose for the vacuum capsule is
conected between the throttle and the engine so it "feels" the intake
pressure. Higher vacuum retards the ignition in our case. This is wrong?
If the throttle then opens (lets say at a low engine speed) then suddenly
the ignition would advance (go earlier) which is not right. It should
retard (be later) since a richer fuel mixture need less time to burn. Is
it suppose to be like this?

The Haynes manual also says that "an increase in engine vaccum results in
a retarded igntion" (not directly from the book) for the B20E and B20F
engines. It also says that "in this case the vacuum capsule is connected
to a point between the throttle and the engine". Could it be that the
distributor for the car is wrong? Since the car is fuel injected (which is
only on the 'E' and 'F' engines, i think), the distributor has an extra
connector so the control unit can measure engine speed. If it's the wrong
distributor it probably can only come from an 'F' engine.

I had a '71 142E (B20E) and I noticed the same thing. When I had it,
Canada was going through the withdrawal of lead in fuel and a general
drop in octane ratings, (premium fuel became lead free first). I
noticed the same thing - higher engine load retarded timing. The way
the original vacuum control was set up, more vacuum retarded timing. I
noticed a dual action vacuum control on a 164 in a junkyard (two vacuum
connections, one on each side of the diaphragm). This vacuum control
bolted right on to my distributor, and with only the "back" side
connected to a different spot on the intake manifold (I believe I "t"ed
into the line for the manifold pressure sensor), my fuel economy
improved and the engine didn't ping anymore.

--
Mike F.
Thornhill (near Toronto), Ont.

NOTE: new address!!
Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

[Stig Hornang]

I had a '71 142E (B20E) and I noticed the same thing. When I had it,
Canada was going through the withdrawal of lead in fuel and a general
drop in octane ratings, (premium fuel became lead free first). I
noticed the same thing - higher engine load retarded timing.

Well, the case is that higher engine load advances timing not retards in
my case! That means that low engine load retards timing. Remember that
high load creates low vacuum, and low loads create high vacuum.

The way the original vacuum control was set up, more vacuum retarded
timing.

Yes, same is in my case.

I noticed a dual action vacuum control on a 164 in a junkyard (two
vacuum connections, one on each side of the diaphragm). This vacuum
control bolted right on to my distributor, and with only the "back" side
connected to a different spot on the intake manifold (I believe I "t"ed
into the line for the manifold pressure sensor), my fuel economy
improved and the engine didn't ping anymore.

Which side of the diaphragm is the hose connected to? Does high intake
manifoil vacuum result in retarded or advanced ignition relative to the
ignition point when the hose is disconnected?

 

[Mike F]

Well, the case is that higher engine load advances timing not retards in
my case! That means that low engine load retards timing. Remember that
high load creates low vacuum, and low loads create high vacuum.

Oops, there's a good reason to proof read a little better. Higher
engine load indeed advanced timing - which made the pinging problem I
had much worse.

Which side of the diaphragm is the hose connected to? Does high intake
manifoil vacuum result in retarded or advanced ignition relative to the
ignition point when the hose is disconnected?

On the vacuum control from the 164, there are 2 hose connections. The
one on the outside, that is the same side that your existing advance is
on, I left disconnected. The one on the inside I connected to a "T" in
the hose to the manifold pressure sensor. Then I set timing by trial
and error - retarded until there was no more pinging. At part load and
idle this resulted in more advanced timing. This modification had the
side benefit of greatly increasing fuel economy.

--
Mike F.
Thornhill (near Toronto), Ont.

NOTE: new address!!
Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

[Stig Hornang]

On the vacuum control from the 164, there are 2 hose connections. The
one on the outside, that is the same side that your existing advance is
on, I left disconnected. The one on the inside I connected to a "T" in
the hose to the manifold pressure sensor. Then I set timing by trial
and error - retarded until there was no more pinging. At part load and
idle this resulted in more advanced timing. This modification had the
side benefit of greatly increasing fuel economy.

Thanks! That makes sense and works like a vacuum advance system as on many
other cars where the goal is better fuel economy. Did you try running
without either hose connected? It could be an idea for me to just
disconnect the vacuum capsule. One other volvo owner with a website did
that. The question is why volvo made the vacuum system the way it is
originally. Haven't found someone who can explain that.

 

[normalini]

"The question is why volvo made the vacuum system the way it is
originally. Haven't found someone who can explain that."

read original answer!

 

[ironwing]

Set the engine at the optimum BTDC timing ,means just
BEFORE the engine knock take place.
This Optimun timing vary depends on different engine
speed,engine load,and engine temperature.
If the optimum timing can be achieved,the car
will run fast and smooth.
That's will improve fuel economy..
I've to put an extra ignition advanced on my B200E,
this make it runs like rocket..


Stig Hornang ¼g¤J¡G

 

[Mike F]

Well, the following pages says it has todo with fuel economy (most
effective combustion, as described in my first posting).

http://www.2quicknovas.com/happytiming.html "Its function is to provide
extra advance at high vacuum to increase fuel economy."

http://www.tpub.com/content/construction/14273/css/14273_67.htm "The
vacuum advance increases FUEL ECONOMY"

http://www.leeric.lsu.edu/bgbb/7/ecep/auto/g/g.htm "...To understand the
relationship between vacuum and ignition timing and reduced fuel economy
in computer assisted engines."

Most pages I've seen says that the vacuum mechanism exists because it
improoves fuel economy. You may be right with your theory, but can you
explain how it could avoid knocking?

The pages I've referenced only talks about vacuum advance not retard. You
say it's not exhaust mission. I still wonder what vacuum retard is good
for since it actually does the opposite thing (makes fuel ecenonmy
worse?).

Retarding the timing any time decreases fuel economy, assuming that the
timing is set at the optimum place. What normal vacuum advances is set
the timing to the optimum for light loads, to maximize fuel economy.
Then the compression ratio is set a little too high (to maximize economy
and power) for this timing at full load causing ping. So a normal
vacuum advance retards the timing at high load for this reason. A
better designed the combustion chamber will promote a faster burn and
will require less ignition advance, and will produce more power since
there is less pressure inside the combustion chamber approaching the top
of the compression stroke. Anything that speeds up combustion will have
the same effect, such as installing multiple spark plugs.

I did a little reading last night on emission theory. Setting the
timing later will lower the peak cylinder pressure and temperature which
will reduce the formation of nitrogen oxides (NOx).
And quoting directly from my 1971 140 series workshop manual:
"The vacuum regulator on the B20A engine raises the firing when the load
on the engine reduces. On the B20B and B20E engines the vacuum
regulator lowers the firing below the basic setting during idling and
engine braking. Reducing the firing is part of exhaust emission control
and prevents the engine from emitting excessive, noxious exhaust gases
at idling and engine braking."

--
Mike F.
Thornhill (near Toronto), Ont.

NOTE: new address!!
Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

[Stig Hornang]

Retarding the timing any time decreases fuel economy, assuming that the
timing is set at the optimum place. What normal vacuum advances is set
the timing to the optimum for light loads, to maximize fuel economy.
Then the compression ratio is set a little too high (to maximize economy
and power) for this timing at full load causing ping. So a normal
vacuum advance retards the timing at high load for this reason. A
better designed the combustion chamber will promote a faster burn and
will require less ignition advance, and will produce more power since
there is less pressure inside the combustion chamber approaching the top
of the compression stroke. Anything that speeds up combustion will have
the same effect, such as installing multiple spark plugs.

I did a little reading last night on emission theory. Setting the
timing later will lower the peak cylinder pressure and temperature which
will reduce the formation of nitrogen oxides (NOx). And quoting directly
from my 1971 140 series workshop manual: "The vacuum regulator on the
B20A engine raises the firing when the load on the engine reduces. On
the B20B and B20E engines the vacuum regulator lowers the firing below
the basic setting during idling and engine braking. Reducing the firing
is part of exhaust emission control and prevents the engine from
emitting excessive, noxious exhaust gases at idling and engine braking."

Thanks a lot! The last part is very interesting and acutally explains why
the B20E among others got a vacuum retard system which was my main
question from the begining. The first reply to my posting (from normalini)
actually was correct. (Thanks normalini, you're right). Engine braking is
actually also an important part of the electronic fuel injection system.

It makes sense that retarding ignition makes the engine more unefficient.
That's what I've always thought, but since the reason has nothing todo
with the engine performance, but with exhaust emission preventing, it all
makes sense!

 

[Stig Hornang]

"The question is why volvo made the vacuum system the way it is
originally. Haven't found someone who can explain that."

read original answer!

Yes, you've right! Just one other guy said it had nothing todo with
reducing exhaust emission.

"TWC:
Either Advance or Retard the engine timing had nothing related to
"exhause emission".
To eliminate the "engine knock" is the purpose."

 

[Andy]

Thanks a lot! The last part is very interesting and acutally explains why
the B20E among others got a vacuum retard system which was my main
question from the begining. The first reply to my posting (from normalini)
actually was correct. (Thanks normalini, you're right). Engine braking is
actually also an important part of the electronic fuel injection system.

It makes sense that retarding ignition makes the engine more unefficient.
That's what I've always thought, but since the reason has nothing todo
with the engine performance, but with exhaust emission preventing, it all
makes sense!

Yes, to sum it all up, in my old 240 with a B21A, there are two vacuum
mechanisms: Retard AND Advance
The retard hose goes to manifold vacuum and the advance hose goes to ported
vacuum. This means that at idle (when manifold has the highest vacuum)
ignition is retarded by some 10 degrees keeping the absolute timing at about
0 deg BTDC. At part throttle, the ported vacuum overrides manifold and
ignition timing is advanced.

One personal experience of my car is from the delay valve on the advance
hose being clogged. That meant that the absolute timing never exceeded 15-20
deg BTDC! After removing that little gizmo, ignition advance started working
again, and both the fuel consumption dropped and the car responded a lot
quicker from idle. That made me a happy(ier) Volvo owner!