Re: [Chevelle-list] Fw: Vacuum advance

[SEVENTYCHEVY]

Not all manufacturer's distributors and vacuum advance work the same-  with 
most GM V8's the distributor is at the back of the block and the  distributor 
shaft rotates clockwise.  The vacuum advance is on the left  side of the 
distributor and when vacuum is applied, it pulls the pick up coil in  a 
counter-clockwise direction(against spring pressure) and advances the  timing.  
When 
vacuum is removed from the vacuum advance, spring  pressure returns the pick up 
coil to its original position and timing  is returned to initial plus 
mechanical 
advance.  
 
That is why on a non-computer controlled Chevy V8 you need to use a ported  
vacuum source- which will be above the throttle blades- for the vacuum  
advance.  Vacuum to operate the advance is created by airflow through the  
carb- as 
rpm's increase, ie: you accelerate quickly- the increased airflow  through the 
carb creates vacuum, pulls on the vacuum advance, rotates the pick  up coil, 
and advances the timing.  Under load, manifold vacuum  decrease.  So, if you 
were to hook up to a manifold vacuum port- a source  below the throttle blades- 
timing would be fully advanced at cruise rpm and  as soon as you mashed the 
throttle, as in to pass someone, timing would  retard.
 
Hook a vacuum gauge to different ports on the carb.  At idle, a ported  
vacuum source will have very little to no vacuum.  As you increase  engine 
speed, 
vacuum should increase.  A manifold vacuum  source at idle, will have full 
vacuum.  As engine speed  increases, vacuum will decrease.
 
I don't know which of the other manufacturers distributors work the same; I  
have heard that some F**ds work differently.  I don't own a F**d so I can't  
say from experience.  Hope this helps...
 
Eric



** See what's free at http://www.aol.com.

Re: [Chevelle-list] Fw: Vacuum advance

[Dale]

John,

 

Glad to see you did further research - even when two sites contradict each
other on the same setups.  I'd think if you have an adjustable vacuum
advance canister it might not make as much difference as it would with a
stock-type vacuum advance unit.

 

Keep us informed as to what you come with on your setup and what works best
for you.

 

Dale

 

  _  

From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of John
Sent: Tuesday, May 22, 2007 10:25 AM
To: The Chevelle Mailing List
Subject: Re: [Chevelle-list] Fw: Vacuum advance

 

Well...after reading about 10 different websites including Crane..MSD and
Chevy High Performance..(.By the way...On the Crane web page it says...When
using their adjustable vacuum advance cannister to hook it up to full
manifold vacuum...On the Chevy High performance page it says..When using the
Crane or MSD adjustable vacuum advances cannisters to hook them up to ported
vacuum...They just contradicted each other)...I think either way has it
pro's and con's...I will play around with both ways and see which is the
best for me...Its cold and rainy here anyways so may as well rip something
apart..!!!

John Palmer

66 Chevelle

327 4 speed



  _  

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Re: [Chevelle-list] Fw: Vacuum advance

[John]

Well...after reading about 10 different websites including Crane..MSD and Chevy 
High Performance..(.By the way...On the Crane web page it says...When using 
their adjustable vacuum advance cannister to hook it up to full manifold 
vacuum...On the Chevy High performance page it says..When using the Crane or 
MSD adjustable vacuum advances cannisters to hook them up to ported 
vacuum...They just contradicted each other)...I think either way has it pro's 
and con's...I will play around with both ways and see which is the best for 
me...Its cold and rainy here anyways so may as well rip something apart..!!!
John Palmer
66 Chevelle
327 4 speed

Re: [Chevelle-list] Fw: Vacuum advance

[Dave Corgill]

At 04:57 AM 5/22/2007, you wrote:

No problem Keith, but like you said, hopefully the additional info 
doesn't add to the confusion!


Herb


I have always had better performance with full vacuum
on all my cars. Running my 383 with 10 deg can on full
vacuum & my ZZ3 with 15 deg can the same.

The 383 with Vortec heads needs the 10 deg can.
   

Re: [Chevelle-list] Fw: Vacuum advance

[Tom Tomlinson]

If it helps at all, here is the timing curve for my '71 Malibu, 350 
engine, with Edelbrock 1406 carb connected to the timed port (port to 
the left side of carb, facing the engine), stock distributor with vacuum 
advance.


RPM  Mech only  Mech+Vacuum
500  15 deg 15 deg
100015 deg15 deg
1500 15 deg15 deg
2000 20 deg25 deg
2500 24 deg38 deg
3000 25 deg38 deg

The vacuum adds about 13 deg of advance, and I set the total advance to 
38 deg, which is the most it will take without pinging. The vacuum 
doesn't kick in until about 1500 rpm, so using the steady vacuum may 
advance the timing sooner, which may give slightly better performance.


Tom Tomlinson


Johnny Palmer wrote:

 
- Original Message -

*From:* John & Chris 
*To:* The Chevelle Mailing List 
*Sent:* Saturday, May 19, 2007 3:54 PM
*Subject:* Vacuum advance

Is it better to hook the vacuum line from the distributor advance to 
the timed port or the steady vacuum port on the carb..???...My friend 
and I are having a discussion on that and either one of us are 
completely sure..Just wanting some other input..

Thanx..
John Palmer
66 Malibu
327 4 speed

Re: [Chevelle-list] Fw: Vacuum advance

[Herb Lumpp]

No problem Keith, but like you said, hopefully the additional info doesn't
add to the confusion!
 
Herb

  _  

From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of KW
Sent: Monday, May 21, 2007 10:53 PM
To: The Chevelle Mailing List
Subject: Re: [Chevelle-list] Fw: Vacuum advance


Did a little searching myself. A good read can be found here
http://www.highperformancepontiac.com/tech/vacuum_advance_tech/
and here
http://www.leeric.lsu.edu/bgbb/7/ecep/auto/h/h.htm
Read and make your own conclusions. (in other words, it may confuse you
more!)
>From the Pontiac site, it looks like Ported may be the easiest, but manifold
may give some slight performance gains IF you have the ability to properly
tune it!
 
Thanks Herb, I learned something new.
 
Keith
 
 
 
 Original Message - 

From: Herb  <mailto:[EMAIL PROTECTED]> Lumpp 
To: 'The Chevelle Mailing List' <mailto:chevelle-list@chevelles.net>  
Sent: Monday, May 21, 2007 10:31 PM
Subject: Re: [Chevelle-list] Fw: Vacuum advance

It's been a while Dale, but I think I found it on Pro-touring.com in the
archives, but I'm not 100% sure.
 
Herb

  _  

From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Dale
Sent: Monday, May 21, 2007 10:20 PM
To: 'The Chevelle Mailing List'
Subject: Re: [Chevelle-list] Fw: Vacuum advance



That's 180-degrees out from what I've always heard and read on various
forums like http://www.moparchat.com/forums/archive/index.php?t-64032.html  

 

Which forum did yours come from Herb?

 

Dale 

 


  _  


From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Herb Lumpp
Sent: Monday, May 21, 2007 9:08 PM
To: 'The Chevelle Mailing List'
Subject: Re: [Chevelle-list] Fw: Vacuum advance

 

I snagged this from a forum, it's a bit long, but it should answer most of
your questions...

 

 

As many of you are aware, timing and vacuum advance is one of my favorite
subjects, as I was involved in the development of some of those systems in
my GM days and I understand it. Many people don't, as there has been very
little written about it anywhere that makes sense, and as a result, a lot of
folks are under the misunderstanding that vacuum advance somehow compromises
performance. Nothing could be further from the truth. I finally sat down the
other day and wrote up a primer on the subject, with the objective of
helping more folks to understand vacuum advance and how it works together
with initial timing and centrifugal advance to optimize all-around operation
and performance. I have this as a Word document if anyone wants it sent to
them - I've cut-and-pasted it here; it's long, but hopefully it's also
informative.

 

TIMING AND VACUUM ADVANCE 101

 

The most important concept to understand is that lean mixtures, such as at
idle and steady highway cruise, take longer to burn than rich mixtures; idle
in particular, as idle mixture is affected by exhaust gas dilution. This
requires that lean mixtures have "the fire lit" earlier in the compression
cycle (spark timing advanced), allowing more burn time so that peak cylinder
pressure is reached just after TDC for peak efficiency and reduced exhaust
gas temperature (wasted combustion energy). Rich mixtures, on the other
hand, burn faster than lean mixtures, so they need to have "the fire lit"
later in the compression cycle (spark timing retarded slightly) so maximum
cylinder pressure is still achieved at the same point after TDC as with the
lean mixture, for maximum efficiency.

 

The centrifugal advance system in a distributor advances spark timing purely
as a function of engine rpm (irrespective of engine load or operating
conditions), with the amount of advance and the rate at which it comes in
determined by the weights and springs on top of the autocam mechanism. The
amount of advance added by the distributor, combined with initial static
timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most
SBC's like). Vacuum advance has absolutely nothing to do with total timing
or performance, as when the throttle is opened, manifold vacuum drops
essentially to zero, and the vacuum advance drops out entirely; it has no
part in the "total timing" equation.

 

At idle, the engine needs additional spark advance in order to fire that
lean, diluted mixture earlier in order to develop maximum cylinder pressure
at the proper point, so the vacuum advance can (connected to manifold
vacuum, not "ported" vacuum - more on that aberration later) is activated by
the high manifold vacuum, and adds about 15 degrees of spark advance, on top
of the initial static timing setting (i.e., if your static timing is at 10
degrees, at idle it's actually around 25 degrees with the vacuum advance
connected). The same thing occurs at steady-state highway cruise; the
mixture is lean, takes longer to burn, the load on the

Re: [Chevelle-list] Fw: Vacuum advance

[richard sierra]

Good Evening - 
I'm looking at a 70 Chevelle for a friend and the Cowl Tag has the following information on it:
Second Line:  TR  796  B80  B90  48G  PNT
Third Line: 09D   002465  060146
I assume the 09D is the production date September 4th week
Any help on the remaining numbers would be greatly appreciated - Thanks



  Like the way Microsoft Office Outlook works? You’ll love Windows Live Hotmail. 

Re: [Chevelle-list] Fw: Vacuum advance

[KW]

Did a little searching myself. A good read can be found here
http://www.highperformancepontiac.com/tech/vacuum_advance_tech/
and here
http://www.leeric.lsu.edu/bgbb/7/ecep/auto/h/h.htm
Read and make your own conclusions. (in other words, it may confuse you more!)
>From the Pontiac site, it looks like Ported may be the easiest, but manifold 
>may give some slight performance gains IF you have the ability to properly 
>tune it!

Thanks Herb, I learned something new.

Keith



 Original Message - 
  From: Herb Lumpp 
  To: 'The Chevelle Mailing List' 
  Sent: Monday, May 21, 2007 10:31 PM
  Subject: Re: [Chevelle-list] Fw: Vacuum advance


  It's been a while Dale, but I think I found it on Pro-touring.com in the 
archives, but I'm not 100% sure.

  Herb



--
  From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Dale
  Sent: Monday, May 21, 2007 10:20 PM
  To: 'The Chevelle Mailing List'
  Subject: Re: [Chevelle-list] Fw: Vacuum advance


  That's 180-degrees out from what I've always heard and read on various forums 
like http://www.moparchat.com/forums/archive/index.php?t-64032.html  

   

  Which forum did yours come from Herb?

   

  Dale 

   


--

  From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Herb Lumpp
  Sent: Monday, May 21, 2007 9:08 PM
  To: 'The Chevelle Mailing List'
  Subject: Re: [Chevelle-list] Fw: Vacuum advance

   

  I snagged this from a forum, it's a bit long, but it should answer most of 
your questions...

   

   

  As many of you are aware, timing and vacuum advance is one of my favorite 
subjects, as I was involved in the development of some of those systems in my 
GM days and I understand it. Many people don't, as there has been very little 
written about it anywhere that makes sense, and as a result, a lot of folks are 
under the misunderstanding that vacuum advance somehow compromises performance. 
Nothing could be further from the truth. I finally sat down the other day and 
wrote up a primer on the subject, with the objective of helping more folks to 
understand vacuum advance and how it works together with initial timing and 
centrifugal advance to optimize all-around operation and performance. I have 
this as a Word document if anyone wants it sent to them - I've cut-and-pasted 
it here; it's long, but hopefully it's also informative.

   

  TIMING AND VACUUM ADVANCE 101

   

  The most important concept to understand is that lean mixtures, such as at 
idle and steady highway cruise, take longer to burn than rich mixtures; idle in 
particular, as idle mixture is affected by exhaust gas dilution. This requires 
that lean mixtures have "the fire lit" earlier in the compression cycle (spark 
timing advanced), allowing more burn time so that peak cylinder pressure is 
reached just after TDC for peak efficiency and reduced exhaust gas temperature 
(wasted combustion energy). Rich mixtures, on the other hand, burn faster than 
lean mixtures, so they need to have "the fire lit" later in the compression 
cycle (spark timing retarded slightly) so maximum cylinder pressure is still 
achieved at the same point after TDC as with the lean mixture, for maximum 
efficiency.

   

  The centrifugal advance system in a distributor advances spark timing purely 
as a function of engine rpm (irrespective of engine load or operating 
conditions), with the amount of advance and the rate at which it comes in 
determined by the weights and springs on top of the autocam mechanism. The 
amount of advance added by the distributor, combined with initial static 
timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's 
like). Vacuum advance has absolutely nothing to do with total timing or 
performance, as when the throttle is opened, manifold vacuum drops essentially 
to zero, and the vacuum advance drops out entirely; it has no part in the 
"total timing" equation.

   

  At idle, the engine needs additional spark advance in order to fire that 
lean, diluted mixture earlier in order to develop maximum cylinder pressure at 
the proper point, so the vacuum advance can (connected to manifold vacuum, not 
"ported" vacuum - more on that aberration later) is activated by the high 
manifold vacuum, and adds about 15 degrees of spark advance, on top of the 
initial static timing setting (i.e., if your static timing is at 10 degrees, at 
idle it's actually around 25 degrees with the vacuum advance connected). The 
same thing occurs at steady-state highway cruise; the mixture is lean, takes 
longer to burn, the load on the engine is low, the manifold vacuum is high, so 
the vacuum advance is again deployed, and if you had a timing light set up so

Re: [Chevelle-list] Fw: Vacuum advance

[Herb Lumpp]

It's been a while Dale, but I think I found it on Pro-touring.com in the
archives, but I'm not 100% sure.
 
Herb

  _  

From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Dale
Sent: Monday, May 21, 2007 10:20 PM
To: 'The Chevelle Mailing List'
Subject: Re: [Chevelle-list] Fw: Vacuum advance



That's 180-degrees out from what I've always heard and read on various
forums like http://www.moparchat.com/forums/archive/index.php?t-64032.html  

 

Which forum did yours come from Herb?

 

Dale 

 

  _  

From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Herb Lumpp
Sent: Monday, May 21, 2007 9:08 PM
To: 'The Chevelle Mailing List'
Subject: Re: [Chevelle-list] Fw: Vacuum advance

 

I snagged this from a forum, it's a bit long, but it should answer most of
your questions...

 

 

As many of you are aware, timing and vacuum advance is one of my favorite
subjects, as I was involved in the development of some of those systems in
my GM days and I understand it. Many people don't, as there has been very
little written about it anywhere that makes sense, and as a result, a lot of
folks are under the misunderstanding that vacuum advance somehow compromises
performance. Nothing could be further from the truth. I finally sat down the
other day and wrote up a primer on the subject, with the objective of
helping more folks to understand vacuum advance and how it works together
with initial timing and centrifugal advance to optimize all-around operation
and performance. I have this as a Word document if anyone wants it sent to
them - I've cut-and-pasted it here; it's long, but hopefully it's also
informative.

 

TIMING AND VACUUM ADVANCE 101

 

The most important concept to understand is that lean mixtures, such as at
idle and steady highway cruise, take longer to burn than rich mixtures; idle
in particular, as idle mixture is affected by exhaust gas dilution. This
requires that lean mixtures have "the fire lit" earlier in the compression
cycle (spark timing advanced), allowing more burn time so that peak cylinder
pressure is reached just after TDC for peak efficiency and reduced exhaust
gas temperature (wasted combustion energy). Rich mixtures, on the other
hand, burn faster than lean mixtures, so they need to have "the fire lit"
later in the compression cycle (spark timing retarded slightly) so maximum
cylinder pressure is still achieved at the same point after TDC as with the
lean mixture, for maximum efficiency.

 

The centrifugal advance system in a distributor advances spark timing purely
as a function of engine rpm (irrespective of engine load or operating
conditions), with the amount of advance and the rate at which it comes in
determined by the weights and springs on top of the autocam mechanism. The
amount of advance added by the distributor, combined with initial static
timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most
SBC's like). Vacuum advance has absolutely nothing to do with total timing
or performance, as when the throttle is opened, manifold vacuum drops
essentially to zero, and the vacuum advance drops out entirely; it has no
part in the "total timing" equation.

 

At idle, the engine needs additional spark advance in order to fire that
lean, diluted mixture earlier in order to develop maximum cylinder pressure
at the proper point, so the vacuum advance can (connected to manifold
vacuum, not "ported" vacuum - more on that aberration later) is activated by
the high manifold vacuum, and adds about 15 degrees of spark advance, on top
of the initial static timing setting (i.e., if your static timing is at 10
degrees, at idle it's actually around 25 degrees with the vacuum advance
connected). The same thing occurs at steady-state highway cruise; the
mixture is lean, takes longer to burn, the load on the engine is low, the
manifold vacuum is high, so the vacuum advance is again deployed, and if you
had a timing light set up so you could see the balancer as you were going
down the highway, you'd see about 50 degrees advance (10 degrees initial,
20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum
advance) at steady-state cruise (it only takes about 40 horsepower to cruise
at 50mph).

 

When you accelerate, the mixture is instantly enriched (by the accelerator
pump, power valve, etc.), burns faster, doesn't need the additional spark
advance, and when the throttle plates open, manifold vacuum drops, and the
vacuum advance can returns to zero, retarding the spark timing back to what
is provided by the initial static timing plus the centrifugal advance
provided by the distributor at that engine rpm; the vacuum advance doesn't
come back into play until you back off the gas and manifold vacuum increases
again as you return to steady-state cruise, when the mixture again becomes
lean.

 

The

Re: [Chevelle-list] Fw: Vacuum advance

[Dale]

That's 180-degrees out from what I've always heard and read on various
forums like http://www.moparchat.com/forums/archive/index.php?t-64032.html  

 

Which forum did yours come from Herb?

 

Dale 

 

  _  

From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Herb Lumpp
Sent: Monday, May 21, 2007 9:08 PM
To: 'The Chevelle Mailing List'
Subject: Re: [Chevelle-list] Fw: Vacuum advance

 

I snagged this from a forum, it's a bit long, but it should answer most of
your questions...

 

 

As many of you are aware, timing and vacuum advance is one of my favorite
subjects, as I was involved in the development of some of those systems in
my GM days and I understand it. Many people don't, as there has been very
little written about it anywhere that makes sense, and as a result, a lot of
folks are under the misunderstanding that vacuum advance somehow compromises
performance. Nothing could be further from the truth. I finally sat down the
other day and wrote up a primer on the subject, with the objective of
helping more folks to understand vacuum advance and how it works together
with initial timing and centrifugal advance to optimize all-around operation
and performance. I have this as a Word document if anyone wants it sent to
them - I've cut-and-pasted it here; it's long, but hopefully it's also
informative.

 

TIMING AND VACUUM ADVANCE 101

 

The most important concept to understand is that lean mixtures, such as at
idle and steady highway cruise, take longer to burn than rich mixtures; idle
in particular, as idle mixture is affected by exhaust gas dilution. This
requires that lean mixtures have "the fire lit" earlier in the compression
cycle (spark timing advanced), allowing more burn time so that peak cylinder
pressure is reached just after TDC for peak efficiency and reduced exhaust
gas temperature (wasted combustion energy). Rich mixtures, on the other
hand, burn faster than lean mixtures, so they need to have "the fire lit"
later in the compression cycle (spark timing retarded slightly) so maximum
cylinder pressure is still achieved at the same point after TDC as with the
lean mixture, for maximum efficiency.

 

The centrifugal advance system in a distributor advances spark timing purely
as a function of engine rpm (irrespective of engine load or operating
conditions), with the amount of advance and the rate at which it comes in
determined by the weights and springs on top of the autocam mechanism. The
amount of advance added by the distributor, combined with initial static
timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most
SBC's like). Vacuum advance has absolutely nothing to do with total timing
or performance, as when the throttle is opened, manifold vacuum drops
essentially to zero, and the vacuum advance drops out entirely; it has no
part in the "total timing" equation.

 

At idle, the engine needs additional spark advance in order to fire that
lean, diluted mixture earlier in order to develop maximum cylinder pressure
at the proper point, so the vacuum advance can (connected to manifold
vacuum, not "ported" vacuum - more on that aberration later) is activated by
the high manifold vacuum, and adds about 15 degrees of spark advance, on top
of the initial static timing setting (i.e., if your static timing is at 10
degrees, at idle it's actually around 25 degrees with the vacuum advance
connected). The same thing occurs at steady-state highway cruise; the
mixture is lean, takes longer to burn, the load on the engine is low, the
manifold vacuum is high, so the vacuum advance is again deployed, and if you
had a timing light set up so you could see the balancer as you were going
down the highway, you'd see about 50 degrees advance (10 degrees initial,
20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum
advance) at steady-state cruise (it only takes about 40 horsepower to cruise
at 50mph).

 

When you accelerate, the mixture is instantly enriched (by the accelerator
pump, power valve, etc.), burns faster, doesn't need the additional spark
advance, and when the throttle plates open, manifold vacuum drops, and the
vacuum advance can returns to zero, retarding the spark timing back to what
is provided by the initial static timing plus the centrifugal advance
provided by the distributor at that engine rpm; the vacuum advance doesn't
come back into play until you back off the gas and manifold vacuum increases
again as you return to steady-state cruise, when the mixture again becomes
lean.

 

The key difference is that centrifugal advance (in the distributor autocam
via weights and springs) is purely rpm-sensitive; nothing changes it except
changes in rpm. Vacuum advance, on the other hand, responds to engine load
and rapidly-changing operating conditions, providing the correct degree of
spark advance at any point in time

Re: [Chevelle-list] Fw: Vacuum advance

[Herb Lumpp]

I snagged this from a forum, it's a bit long, but it should answer most of
your questions...
 
 
As many of you are aware, timing and vacuum advance is one of my favorite
subjects, as I was involved in the development of some of those systems in
my GM days and I understand it. Many people don't, as there has been very
little written about it anywhere that makes sense, and as a result, a lot of
folks are under the misunderstanding that vacuum advance somehow compromises
performance. Nothing could be further from the truth. I finally sat down the
other day and wrote up a primer on the subject, with the objective of
helping more folks to understand vacuum advance and how it works together
with initial timing and centrifugal advance to optimize all-around operation
and performance. I have this as a Word document if anyone wants it sent to
them - I've cut-and-pasted it here; it's long, but hopefully it's also
informative.

 

TIMING AND VACUUM ADVANCE 101

 

The most important concept to understand is that lean mixtures, such as at
idle and steady highway cruise, take longer to burn than rich mixtures; idle
in particular, as idle mixture is affected by exhaust gas dilution. This
requires that lean mixtures have "the fire lit" earlier in the compression
cycle (spark timing advanced), allowing more burn time so that peak cylinder
pressure is reached just after TDC for peak efficiency and reduced exhaust
gas temperature (wasted combustion energy). Rich mixtures, on the other
hand, burn faster than lean mixtures, so they need to have "the fire lit"
later in the compression cycle (spark timing retarded slightly) so maximum
cylinder pressure is still achieved at the same point after TDC as with the
lean mixture, for maximum efficiency.

 

The centrifugal advance system in a distributor advances spark timing purely
as a function of engine rpm (irrespective of engine load or operating
conditions), with the amount of advance and the rate at which it comes in
determined by the weights and springs on top of the autocam mechanism. The
amount of advance added by the distributor, combined with initial static
timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most
SBC's like). Vacuum advance has absolutely nothing to do with total timing
or performance, as when the throttle is opened, manifold vacuum drops
essentially to zero, and the vacuum advance drops out entirely; it has no
part in the "total timing" equation.

 

At idle, the engine needs additional spark advance in order to fire that
lean, diluted mixture earlier in order to develop maximum cylinder pressure
at the proper point, so the vacuum advance can (connected to manifold
vacuum, not "ported" vacuum - more on that aberration later) is activated by
the high manifold vacuum, and adds about 15 degrees of spark advance, on top
of the initial static timing setting (i.e., if your static timing is at 10
degrees, at idle it's actually around 25 degrees with the vacuum advance
connected). The same thing occurs at steady-state highway cruise; the
mixture is lean, takes longer to burn, the load on the engine is low, the
manifold vacuum is high, so the vacuum advance is again deployed, and if you
had a timing light set up so you could see the balancer as you were going
down the highway, you'd see about 50 degrees advance (10 degrees initial,
20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum
advance) at steady-state cruise (it only takes about 40 horsepower to cruise
at 50mph).

 

When you accelerate, the mixture is instantly enriched (by the accelerator
pump, power valve, etc.), burns faster, doesn't need the additional spark
advance, and when the throttle plates open, manifold vacuum drops, and the
vacuum advance can returns to zero, retarding the spark timing back to what
is provided by the initial static timing plus the centrifugal advance
provided by the distributor at that engine rpm; the vacuum advance doesn't
come back into play until you back off the gas and manifold vacuum increases
again as you return to steady-state cruise, when the mixture again becomes
lean.

 

The key difference is that centrifugal advance (in the distributor autocam
via weights and springs) is purely rpm-sensitive; nothing changes it except
changes in rpm. Vacuum advance, on the other hand, responds to engine load
and rapidly-changing operating conditions, providing the correct degree of
spark advance at any point in time based on engine load, to deal with both
lean and rich mixture conditions. By today's terms, this was a relatively
crude mechanical system, but it did a good job of optimizing engine
efficiency, throttle response, fuel economy, and idle cooling, with
absolutely ZERO effect on wide-open throttle performance, as vacuum advance
is inoperative under wide-open throttle conditions. In modern cars with
computerized engine controllers, all those sensors and the controller change
both mixture and spark timing 50 to 100 times per second, and 

Re: [Chevelle-list] Fw: Vacuum advance

[KW]

Lots of the emission controlled vehicles used a vacuum retard on the 
distributor, at idle it retards the vacuum with manifold vacuum.
Most vehicles use a vacuum advance distributor which uses ported vacuum (no 
vacuum at idle) to add additional advance to the distributor as the RPM's 
increase. If you are insure if you have a vacuum advance or a vacuum retard 
distributor you can hook up a timing light, draw a vacuum on the port on the 
distributor and watch the timing. If the timing retards with the addition of 
vacuum then you have a vacuum retard distributor. If the timing advances then 
you have a vacuum advance distributor (and need to use ported vacuum [no vacuum 
at idle] for your distributor)

Keith

Re: [Chevelle-list] Fw: Vacuum advance

[Dale]

The increase of timing at idle is due to a full manifold vacuum connection.

 

Dale  <http://www.chevellecd.com>  

 

  _  

From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of John
Sent: Monday, May 21, 2007 7:40 PM
To: The Chevelle Mailing List
Subject: Re: [Chevelle-list] Fw: Vacuum advance

 

OK...I will hook mine up to the timed port and see what happens...I know
that I set my initial timing on the dist. to 8 degrees BTDC and when I hook
up the vacuum line it shoots up to around 16 BTDC...Then I have to lower the
curb idle speed back down to 700rpm...I will try it out...

Thanx very much..

John Palmer



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Re: [Chevelle-list] Fw: Vacuum advance

[John]

OK...I will hook mine up to the timed port and see what happens...I know that I 
set my initial timing on the dist. to 8 degrees BTDC and when I hook up the 
vacuum line it shoots up to around 16 BTDC...Then I have to lower the curb idle 
speed back down to 700rpm...I will try it out...
Thanx very much..
John Palmer

Re: [Chevelle-list] Fw: Vacuum advance

[Dale]

Timed and manifold vacuum are determined by the port it comes from.  Another
use for manifold vacuum would be something like a vacuum gauge connection.

 

Dale 

 

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From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of John
Sent: Monday, May 21, 2007 7:18 PM
To: The Chevelle Mailing List
Subject: Re: [Chevelle-list] Fw: Vacuum advance

 

I know my distributor has an adjustable vacuum advance but how do you tell
if it has a timed vacuum advance..??..Or is that determined by the port you
attach it too..??

John



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Re: [Chevelle-list] Fw: Vacuum advance

[John]

I know my distributor has an adjustable vacuum advance but how do you tell if 
it has a timed vacuum advance..??..Or is that determined by the port you attach 
it too..??
John

Re: [Chevelle-list] Fw: Vacuum advance

[Dale]

Use the timed port.  My Edelbrock 1406 has two ports; on the left (facing
engine) is the timed port and the right is manifold vacuum.  Instructions
say the timed port is for '.emissions controlled engines' and vacuum port is
for '.non-emissions controlled engines' making one think the manifold vacuum
would be correct for (in my case) my 67 327 since it's a non-emissions
controlled engine.  Not so, with manifold vacuum you get full vacuum advance
at idle and with timed port there's no vacuum until you get in the 1200-1500
RPM range. Instructions go on to say, "If your distributor has timed vacuum
advance, you will hook the vacuum hose from the distributor to the passenger
side vacuum port on the new carburetor. If it has full vacuum advance, it
will be hook up to the driver side port."   Except for maybe street use with
a very large cam, everyone I've talked to says to use the timed port.so I
do.

 

Dale McIntosh

 <http://www.chevellecd.com> 1966/67 Chevelle Reference CDs

ChevelleStuff.com <http://www.chevellestuff.com> 

 <http://www.chevellecd.com>  

 

  _  

From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Johnny Palmer
Sent: Monday, May 21, 2007 5:30 PM
To: The Chevelle Mailing List
Subject: [Chevelle-list] Fw: Vacuum advance

 

 

- Original Message - 

From: John  <mailto:[EMAIL PROTECTED]> & Chris 

To: The <mailto:Chevelle-list@chevelles.net>  Chevelle Mailing List 

Sent: Saturday, May 19, 2007 3:54 PM

Subject: Vacuum advance

 

Is it better to hook the vacuum line from the distributor advance to the
timed port or the steady vacuum port on the carb..???...My friend and I are
having a discussion on that and either one of us are completely sure..Just
wanting some other input..

Thanx..

John Palmer

66 Malibu

327 4 speed



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Re: [Chevelle-list] Fw: Vacuum advance

[TinIndian47]

I have been wondering the same thing, my Holley directions tell me the  timed 
port, but on my Edelbrock directions they  say check your old carb  first, if 
it has a vacuum on idle then put on full vacuum, if it doesn't have a  vacuum 
at idle then put it on the timed port. I would like to hear some   others  on 
what to do. 



** See what's free at http://www.aol.com.

[Chevelle-list] Fw: Vacuum advance

[Johnny Palmer]

- Original Message - 
From: John & Chris 
To: The Chevelle Mailing List 
Sent: Saturday, May 19, 2007 3:54 PM
Subject: Vacuum advance


Is it better to hook the vacuum line from the distributor advance to the timed 
port or the steady vacuum port on the carb..???...My friend and I are having a 
discussion on that and either one of us are completely sure..Just wanting some 
other input..
Thanx..
John Palmer
66 Malibu
327 4 speed