Hi- just installed a Pertronix distributor with vacuum
advance on my 1971 Series 2 E-type. Original distributor
was vacuum retard. Original ignition timing was 10
degrees BTDC, @750 RPM, with vacuum retard disconnected.
I’m looking for the proper ignition requirements with the
vacuum advance distributor. If anyone has gone through
this process, I’d be interested in what timing procedure
worked. Thanks.–
Mark Campbell
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Crookedmouth sent Mon 29 Aug 2016:
Morning Mark,
In the FWIW department here’s how I do it. The most important
aspect of your timing is what it’s set at all in, optimal is
32-34 degrees BTDC at 3500 RPM vacuum line plugged(this is
assuming that your timing marks are correct-if not then you
need to research how to find TDC). Many distributors for
emissions equipped cars had a 20 degree distributor (40 at the
crank) and a vacuum retard dizzy as you know, this would set
your at idle timing around 8 degrees ATDC. Frankly I don’t
care what mine is set at at idle since the engine is under
minimal load at idle and if it idles well then just leave it
there. What I do care about is where its at when it’s
operating under a load hence setting it with an ‘adjustable’
timing light for 32 degrees BTDC at 3500 RPM when the dizzy
should be ‘all in’.
Then the car needs to be road tested to make sure that it’s
running properly.
Cheers,
LLynn–
The original message included these comments:
I’m looking for the proper ignition requirements with the
vacuum advance distributor. If anyone has gone through
this process, I’d be interested in what timing procedure
worked. Thanks.
Mark Campbell
–
Lynn G.
68/85 ots, 73 2+2, Boise, Id., United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Crookedmouth sent Mon 29 Aug 2016:
I have the retard dizzy on my '70 dizzy completely
disconnected,Plugged at the manifold) and rely on the
centrifugal advance…I have found my optimal idle advance to
end up around 12 degrees…
I also removed my secondary throttle plates, and rod, and
plugged the holes…Car runs like a scalded cat, and idles
around 900 rpm when warm…(knock on wood , has run very well
all season).
Edward–
It’s not a car, it’s a career! '70 OTS 4.2L
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from L.Lynn sent Mon 29 Aug 2016:
Hi Lynn- thanks for that valuable info. My real problem
was finding the proper ported vacuum source for a vacuum
advance distributor. The existing one on the bottom of
the front carb cannot be used, as it produces max vacuum
at idle, and then tapers off as the throttle is oprned. I
will have to drill a ‘‘ported’’ vacuum source into the carb
to work with the vacuum advance. I knew none of this when
I posted the question. Once I do that, I can actually set
the timing correctly.
Pertronix conveniently forgot to mention that I would have
to mod the carb for the distributor.–
Mark Campbell
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Crookedmouth sent Fri 2 Sep 2016:
Not sure how you could possibly know the correct location for
the vacuum port. It has to be precisely located relative to
the throttle plate to give the correct response. Too close,
and you’ll get too much advance, and/or too soon. Too far
away, and you’ll get too little advance, and/or too late. The
odds of getting it anywhere near right without a factory carb
to duplicate is about zero.
Bottom line: If your carb it not setup for vacuum advance,
you should get a non-vacuum-advance distributor.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Crookedmouth sent Fri 2 Sep 2016:
Mark
Many years ago switched to Pertronix dist. on my 70 with vacuum
retard. Figured even with my limited knowledge there would be a
problem with vacuum source. Ordered the non-vacuum unit with
mechanical advance only. Have been very happy with it and
advance was very similar to stock unit.
One point to consider is if your car is a 1970 or 1971 the
IgniterII module (red) that is included in this dist. will not
work. Must switch out to original Igniter (black) module. Took
months to learn this on my own Pertronix had no clue as they
have no clue on where to pull vacuum from. My guess is Ray has
got it right!
Heads up- do not leave ignition on with original Igniter
(black) module, will cook module. IgniterII (red) is protected.
If I am doing work on car and ignition switch needs to be on I
always pull coil wire.
Glenn
70E–
The original message included these comments:
Hi Lynn- thanks for that valuable info. My real problem
was finding the proper ported vacuum source for a vacuum
advance distributor. The existing one on the bottom of
the front carb cannot be used, as it produces max vacuum
at idle, and then tapers off as the throttle is oprned. I
will have to drill a ‘‘ported’’ vacuum source into the carb
to work with the vacuum advance. I knew none of this when
I posted the question. Once I do that, I can actually set
the timing correctly.
Pertronix conveniently forgot to mention that I would have
to mod the carb for the distributor.
–
melloyello
Lake Elsinore California, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Crookedmouth sent Fri 2 Sep 2016:
why do you feel you want ported vacuum, and not manifold
vacuum? Ported vac is mainly an emissions method for later
model cars with computer and/or emissions equipment. Vac
advance is supposed to be reduced as throttle opens,then come
in at cruise. Vintage Race cars at WOT would use no vac
advance at all. The mechanical advance takes care of the
advance needed as rpm increases.
just…sayin…look into it.
Nick–
The original message included these comments:
was finding the proper ported vacuum source for a vacuum
advance distributor. The existing one on the bottom of
the front carb cannot be used, as it produces max vacuum
will have to drill a ‘‘ported’’ vacuum source into the carb
–
Nick53XK120S
Spokane WA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Nick53XK120S sent Sat 3 Sep 2016:
‘‘later model cars with computer and/or emissions equipment’’
like most '50-'60s Jags with SU carbs, including early E-
types, that used ported vacuum advance??
Manifold vacuum would give you full advance at idle, which
will not do you any good at all. Using ported vacuum provides
NO advance at idle, some advance at part-throttle to help
getting off the line, and lots of advance at higher RPMs and
part-throttle (i.e. - cruise) for improved gas mileage. It
has nothing to do with emissions, and never did. Emissions
concerns created the abomination of vacuum RETARD.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Ray Livingston sent Sat 3 Sep 2016:
with due respect to your experience, I invite you Ray…and
anyone to read up on ported and manifold vacuum, rather than
me argue the point here. But I can tell you that when you do,
you are likely to revise your opinion. Both work, but work
differently.
Nick–
Nick53XK120S
Spokane WA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Nick53XK120S sent Sat 3 Sep 2016:
Of course they work differently. But your comment that
‘‘Ported vac is mainly an emissions method for later model cars
with computer and/or emissions equipment.’’ is patently false.
As I pointed out, MANY Jags from the '50s and '60s (including
E-types up to about ‘68’), and thousands of other cars from
that era USED ported vacuum. NONE of them were ‘‘later model
cars’’, NONE had computers, ALL were built long before
emissions were a concern.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Ray Livingston sent Sat 3 Sep 2016:
I shall try to ignore the impolite and incorrect post…and
hope this post, author Dick Maury helps.
BACK
Vacuum Advance versus Vacuum Retard
By Richard Maury
There has been a lot of misinformation out there about
vacuum advance versus vacuum retard and their applications.
They each have their purpose and that is what I will try to
explain here. I think that a definition of each is in order
first. Vacuum advance does just what the name implies. When
vacuum is applied to the unit, the timing is advanced. The
amount depends on the calibration of the unit and the amount
of vacuum. Not all vacuum units are the same and care should
be taken before mixing them. The advance unit is usually
hooked up to a port that supplies vacuum when the throttle
is just opening or at light engine loads. This is because
the engine can tolerate more timing advance under light load
which improves efficiency and fuel economy. The use of
vacuum advance pre dates the Jaguar company.
Jaguar used the advance unit in the Series 1 XKE 3.8
and 4.2 models. Due to the differences in the two engines,
the advance rates of the distributors and vacuum units were
different. If you are not sure which you have, check the
parts manual for the part number of the distributor. As a
rough guide, the 3.8 distributor has a mechanical advance of
13 degrees which translates to 26 crankshaft degrees. Add
initial timing of 10 degrees and you get a total of 36. The
4.2 uses an 11 degree mechanical advance curve for a total
of 32 degrees using initial timing of 10. The difference is
because the 3.8 cylinders match the chambers in the head.
The 4.2 had to have the crank changed so the cylinders are
all spaced evenly. Jaguar did not redesign the head so only
cylinders 2 & 5 match the head with the others offset. This
required the timing to be changed to maximize power without
pinging.
Vacuum retard was not used by Jaguar until emission
controls mandated its use. Even then, Jaguar did not use the
retard unit until 1970. The 68 & 69 models used a
distributor without either unit. The mechanical advance on
this unit was 20 degrees which gave 40 at the crank. Add
initial of 5 and you get 45 degrees total advance. The extra
advance was necessary as the leaner mixture slowed down the
flame propagation so combustion took longer. This required
the spark to happen sooner. One has to be careful if
converting one of these cars to triple SU carburetors that
the proper distributor is also used. To much high speed
advance can melt the top out of the pistons with expensive
results.
Vacuum retard came on the XKE in 1970 when Jaguar
changed the air filter design to a flat style mounted
directly on the face of the carburetors. The retard was
necessary to control Nitrides of Oxygen or NOx. Peak
combustion chamber temperatures caused excessive NOx so
retarding the timing kept the combustion from completing and
getting to hot. Timing on these cars was usually set with
the vacuum line disconnected and plugged. You would notice
when you hooked it back up that the engine would slow down.
This is because the timing was less than optimal for good
running. This also caused the engine to put off more heat
and consume more fuel. Even though full throttle allowed the
retard unit to be inactive, most of the driving was part
throttle and idling. Some owners have put a vacuum advance
unit on their Series 2 cars and hooked it up to manifold
vacuum. The biggest problem with this is that the timing is
over advanced at certain ranges and can cause engine damage
unless the mechanical advance is also modified to suit.
Vacuum retard was used on the V12 XKE also. However,
it was a smarter system as it had a thermostat system on the
right rear of the engine. This system disabled the retard
system when the engine got to hot. This in effect advances
the timing allowing the engine to run cooler and at a higher
idle. Timing on these engines is 4 ATDC. This is with the
retard system hooked up. One of the big problems is not
knowing that the vacuum unit is working or not and setting
the timing. If the unit has developed a leak and is not
working, the timing could be 16 degrees off. Initial timing
on the V12 can easily be set by disconnecting the retard
unit and setting the timing at 12 BTDC. Hook back up the
retard unit if you wish or need to for emissions.
Putting in a Vacuum advance unit from a later car onto
a V12 car can be done. One of the carburetors should be
drilled so that the advance unit can have ported vacuum at
light throttle, not full time throttle. Jaguar did this on
the later XJ6 cars with carburetors if you need a pattern as
to where to drill. The hole should be small and not exposed
to manifold vacuum until the throttle just starts to open.
All of the timing curves assume that the distributors
are in good working order and the weights not rusted up.
This is very common on the V12 engines because of the heat
and location of the distributor.–
Nick53XK120S
Spokane WA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from mouton sent Sun 4 Sep 2016:
Yup. I agree with every word of what Dick wrote. Again,
ported vacuum advance was used on many Jaguars, along with
thousands of other cars, throughout the '50s and '60s, and it
was in common use before Jaguar even existed. Near as I can
tell, it was first used on a production car by Studebaker in
1930.–
The original message included these comments:
Nick,
Doesn’t Dick’s post fully back up what Ray is saying?
Jerry
–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Nick53XK120S sent Sat 3 Sep 2016:
Nick,
Having, at your behest, taken the time to read up on this
topic, I can see that it is a source of much confusion among
many self-styled ‘‘experts’’ on various web forums - particularly
those devoted to Corvettes and other ‘‘American Iron’’. The
confusion, however, also extends to followers of British makes
other than Jaguar. I read a particularly animated discussion on
an MG forum where Ported Vacuum was presented as an invention
of the ‘‘emissions control’’ era by several posters, and
contradicted by another who indicated that all MGs of the 40s,
50s and 60s used Ported Vacuum, and for reasons nothing to do
with emissions. As far as Jaguar is concerned, referring to the
Service Manuals it is clear that the XK120, XK140, XK150 and
Series 1 E-Type also all used Ported rather than Manifold
Vacuum for Vacuum advance functionality before emissions became
an issue. In the end what is important is to connect the
distributor vacuum port to the type of vacuum source it was
designed for. A distributor designed for a ported vacuum
source will likely have a very different mechanical advance
curve, and vacuum sensitivity than one designed for manifold
vacuum. I presume that the Pertronix distributor acquired by
the original poster was designed as a replacement for the Lucas
distributor used with the triple SU carbs, rather than the
model used with the twin Strombergs. As such, it will have been
designed for a ported vacuum source, and not manifold vacuum.
My recommendation, for what it’s worth, would be to either swap
out the purchased Pertronix for the correct model for the US
Series 2. This would likely be the model that compares with the
Lucas distributor used on the '69 cars which has no vacuum
unit, rather than the later vacuum retard unit fitted in '70-
'71 cars. Thanks to this lively discussion, I now know more
about Ported and Manifold vacuum than I did before (and maybe
more than I will ever need…)
-David–
The original message included these comments:
with due respect to your experience, I invite you Ray…and
anyone to read up on ported and manifold vacuum, rather than
me argue the point here. But I can tell you that when you do,
you are likely to revise your opinion. Both work, but work
differently.
In reply to a message from Nick53XK120S sent Sun 4 Sep 2016:
It is not often that someone confidently posts highly-
respected and reliable evidence that directly contradicts
their own point of view
Dick being right means your confusion has hopefully been
cleared up Nick. As long as you learned something and have
revised your opinion, all is good. ‘‘Just sayin…look into
it.’’
We’ve all learned new stuff here…
Pete–
The original message included these comments:
hope this post, author Dick Maury helps.
–
66 ‘UberLynx’ D, 70 FHC, 79 S2 XJ12L, 97 XJ6L
Gaithersburg, Maryland, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from PeterCrespin sent Mon 5 Sep 2016:
I have a question, which I’ll present as a hypothesis.
As mentioned, ported vacuum advance has been around since
well-before pollution control days.
It’s clear that the leaner conditions at part throttle call
for longer burn time, which is why vacuum advance provides
greater fuel economy. But these conditions also exist at
tickover/idle–so why not advance the spark when the
throttle is closed as well? Why was ported vacuum
implemented to replace vacuum advance straight from the
manifold?
Here’s what I’ll trot out to answer that question:
Idling is a relatively unstable, because energy stored in
the flywheel is just barely adequate at low speed. If the
idle speed happens to increase spontaneously, more vacuum
will be created, and (if the vacuum advance is operating)
the spark will advance, leading to sill more idle speed.
This enhances the instability. Same if idle speed
spontaneously decreases. So you get a stabler, less
‘‘surging’’ idle speed by disconnecting the positive feedback
created by vacuum advance. You of course have to set the
idle fuel/air flow higher to compensate for the decreased
spark advance.
Is this correct? I just Googled the subject and couldn’t
find anything that agrees. There is a lot of stuff about
ported vacuum being a pollution tool–no doubt true later
on, but what about in the 50-60’s?–
Bob Wilkinson, 73 XJ6
Saint Louis, MO, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from Robert Wilkinson sent Mon 5 Sep 2016:
The effect of ported vacuum is more complex… Manifold
vacuum basically gives you a fairly direct measure of engine
load - more vacuum = less load. At idle, you’ll see close
to maximum vacuum, and it will decrease as the throttle is
opened, and be minimum at WOT. Ported vacuum provides a
highly non-linear response to throttle and load. At idle,
there is basically no vacuum advance, because airflow is
very low, and the port is upstream of the throttle. As the
throttle is opened, the vacuum level increases rapidly, as
the throttle plate approaches the port. Once the throttle
plate passes the port, the response reverts to basically the
same as manifold vacuum. The result is an advance ‘‘kick’’
off-idle, which can slightly help acceleration off-the-kine
with light or moderate throttle. At high-speed cruise, the
throttle opening is still small enough that the port
placement provides a bit more vacuum, hence a bit more
advance, than you’d get just from manifold tap.
More advance at idle doesn’t really do anything useful, but
can make idle a bit more unstable, and make starting a bit
harder.
Again, the PRIMARY function of ported vacuum always was and
still is to provide more advance under cruise conditions -
mid-RPM, light load - for the purpose of slightly improved
fuel economy.–
Ray Livingston - '64 OTS Santa Cruz, CA
Santa Cruz, CA, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
Ok… and Ray, how does the vacuum retard work?
LLoyd----- Original Message -----
In reply to a message from Robert Wilkinson sent Mon 5 Sep 2016:
The effect of ported vacuum is more complex… Manifold
vacuum basically gives you a fairly direct measure of engine
load - more vacuum = less load. At idle, you’ll see close
to maximum vacuum, an…
