HEI Module - Vacuum Retard?

A

alan bredbury

New member
Aug 5, 1999
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Sorry about causing confusion, I have been in and around
this stuff for so long I forgot how confusing the term
Vacuum advance can be. You know, Advances the timing with
vacuum applied and so on. Simply. With a stock set up if
you put the accelerator to the floor and it pings 5 times
if you put a weaker spring in the vacuum advance pot it
will ping less times, all other things kept the same. The
vacuum unit I was refering to has a Allen set screw that is
accessed through the hole that the hose plugs into and it
allows the user to adjust the spring tension. The original
post mentioned having an experienced individual or
organization "set up" the distributor. This is a good idea
if you know some one who can do it and can afford it.

=====
Alan Bredbury
Clinton, Ct.
1974 GMC 26' Classic Motorhome
http://www.finesttool.com/index.html/special.htm

__________________________________________________
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I'd like to hear from Emery, Tom and Jim Bounds on this subject. My vacuum
advance is ported and therefore the further I open the throttle the more
vacuum advance I get(that is, within the mechanical limits of the vacuum
advance).
Bob McLaughlin

- -----Original Message-----
From: owner-gmcmotorhome
[mailto:owner-gmcmotorhome]On Behalf Of
Patrick.Flowers
Sent: Wednesday, June 07, 2000 9:13 AM
To: gmcmotorhome
Subject: RE: GMC: HEI Module - Vacuum Retard?

Nope, Alan got it right. The timing advances for economy and must be
retarded to avoid pinging under load. More advance does give more power,
but there's always a price to pay. In this case it's preignition.

Patrick

>

>
> "If you want less pings when the throttle is applied you want
> the vacuum control to react more quickly (the vacuum control
> retards the timing as you apply throttle)"
>
> Alan,
>
> I'm pretty sure my vacuum control ADVANCES the timing.
> Retarding the timing under accelaration would be
> counter-productive, IMHO
 
>I'd like to hear from Emery, Tom and Jim Bounds on this subject. My vacuum
>advance is ported and therefore the further I open the throttle the more
>vacuum advance I get(that is, within the mechanical limits of the vacuum
>advance).
>Bob McLaughlin

Sure Bob - draw me into this controversy - thanks a whole lot! :) :)

OK, here's my two pennies. This is one time when both sides are
right. With regard to the vacuum advance mechanism, there are times
when you push down on the throttle and the vacuum increases therefore
advancing the timing. There are other times when you might floor it
and the vacuum decreases therefore retarding the timing. If you drop
the throttle to the floor it initially can loose vacuum until the
engine starts to rev up. So, for a short time the timing retards and
then begins to catch up as the vacuum increases due to the rpm
increase. If you were under heavy load, such as going up a steep
hill, you can floor it and find that the vacuum steadily decreases as
you start to loose speed. In both these examples, the vacuum advance
will retard the timing.

In general, though, when driving on a flat road, the more gas you
give it, the more RPM you'll get, the higher the vacuum goes, and the
more vacuum advance you'll get until the vacuum advance reaches its
limit. Don't forget, however, that your distributor also has a
mechanical advance that consists of weights that cause the
distributor to advance as the rpms increase. The spinning weights
pull to the outside (centrifugal force) and mechanically cause the
timing to advance at higher rpms. Your total advance is the sum of
the initial idle advance, the mechanical advance and the vacuum
advance.

Pinging (pre-detonation) can be caused by several things. One is too
much advance on your timing. Others are lower octane fuel and
improper air/fuel ratio. It can also be caused by a change in the
temperature of the incoming air into the carb (which can cause a
change in fuel ratio due to the air density change caused by the
temp. changes) or a change in altitude (which again changes the
density of the air). Lower density air has less oxygen.

With your timing set at a given position the engine might run fine
with no pinging but then you can get a bad batch of fuel (or lower
octane) and the engine will ping. The same thing can happen on a
hotter or colder day. The engine might begin to ping. You can
manually retard the initial timing to take care of this problem but
then you might have to manually advance it back later to get more
power.

I've found that with the Caspro EKE (electronic knock eliminator)
system, I can have my idle timing set to 12 or 13 deg. before top
dead center (instead of the factory recommended 8 deg.BTDC and when
I change fuels or altitude or go from a hot day driving to a cold
night driving, the EKE will automatically retard my timing as needed
to stop pinging (knocking). I get a couple of pings and then when
the EKE cuts in the pings go away. It'll automatically try to
advance the timing back later but will again retard it if the pinging
is still present.

This allows me to use regular gasoline (usually 86 or 87 octane) but
still get decent power and fuel economy.

As with any subject, the conditions have got to be defined before the
right answer can be given.

Emery Stora
77 Kingsley
Santa Fe, NM
 
Emery,
Thanks. You always say it so well, especially pointing out how the OEM
system can't automatically compensate for differing quality fuel and changes
in temperature and altitude. The EKE for the most part does compensate.
One thing I was hoping you might cover was the location of the port in
comparison to the venturi. I believe the physics are such that the air flow
through the venturi determines the vacuum in the port, not the manifold
pressure. Comments anyone?
Bob McLaughlin

- -----Original Message-----
From: owner-gmcmotorhome
[mailto:owner-gmcmotorhome]On Behalf Of Emery L. Stora
Sent: Wednesday, June 07, 2000 8:55 PM
To: gmcmotorhome
Subject: RE: GMC: HEI Module - Vacuum Retard?

>I'd like to hear from Emery, Tom and Jim Bounds on this subject. My vacuum
>advance is ported and therefore the further I open the throttle the more
>vacuum advance I get(that is, within the mechanical limits of the vacuum
>advance).
>Bob McLaughlin

Sure Bob - draw me into this controversy - thanks a whole lot! :) :)

OK, here's my two pennies. This is one time when both sides are
right. With regard to the vacuum advance mechanism, there are times
when you push down on the throttle and the vacuum increases therefore
advancing the timing. There are other times when you might floor it
and the vacuum decreases therefore retarding the timing. If you drop
the throttle to the floor it initially can loose vacuum until the
engine starts to rev up. So, for a short time the timing retards and
then begins to catch up as the vacuum increases due to the rpm
increase. If you were under heavy load, such as going up a steep
hill, you can floor it and find that the vacuum steadily decreases as
you start to loose speed. In both these examples, the vacuum advance
will retard the timing.

In general, though, when driving on a flat road, the more gas you
give it, the more RPM you'll get, the higher the vacuum goes, and the
more vacuum advance you'll get until the vacuum advance reaches its
limit. Don't forget, however, that your distributor also has a
mechanical advance that consists of weights that cause the
distributor to advance as the rpms increase. The spinning weights
pull to the outside (centrifugal force) and mechanically cause the
timing to advance at higher rpms. Your total advance is the sum of
the initial idle advance, the mechanical advance and the vacuum
advance.

Pinging (pre-detonation) can be caused by several things. One is too
much advance on your timing. Others are lower octane fuel and
improper air/fuel ratio. It can also be caused by a change in the
temperature of the incoming air into the carb (which can cause a
change in fuel ratio due to the air density change caused by the
temp. changes) or a change in altitude (which again changes the
density of the air). Lower density air has less oxygen.

With your timing set at a given position the engine might run fine
with no pinging but then you can get a bad batch of fuel (or lower
octane) and the engine will ping. The same thing can happen on a
hotter or colder day. The engine might begin to ping. You can
manually retard the initial timing to take care of this problem but
then you might have to manually advance it back later to get more
power.

I've found that with the Caspro EKE (electronic knock eliminator)
system, I can have my idle timing set to 12 or 13 deg. before top
dead center (instead of the factory recommended 8 deg.BTDC and when
I change fuels or altitude or go from a hot day driving to a cold
night driving, the EKE will automatically retard my timing as needed
to stop pinging (knocking). I get a couple of pings and then when
the EKE cuts in the pings go away. It'll automatically try to
advance the timing back later but will again retard it if the pinging
is still present.

This allows me to use regular gasoline (usually 86 or 87 octane) but
still get decent power and fuel economy.

As with any subject, the conditions have got to be defined before the
right answer can be given.

Emery Stora
77 Kingsley
Santa Fe, NM
 
The Venturi has some effect but most of the Ported effect is due to the
vac connection is above the butterfly.

you might want to read these references

http://www.california.com/~eagle/howell.html

gene

>Emery,
>Thanks. You always say it so well, especially pointing out how the OEM
>system can't automatically compensate for differing quality fuel and changes
>in temperature and altitude. The EKE for the most part does compensate.
>One thing I was hoping you might cover was the location of the port in
>comparison to the venturi. I believe the physics are such that the air flow
>through the venturi determines the vacuum in the port, not the manifold
>pressure. Comments anyone?
>Bob McLaughlin
>
>-----Original Message-----
>From: owner-gmcmotorhome
>[mailto:owner-gmcmotorhome]On Behalf Of Emery L. Stora
>Sent: Wednesday, June 07, 2000 8:55 PM
>To: gmcmotorhome
>Subject: RE: GMC: HEI Module - Vacuum Retard?
>
>
>>I'd like to hear from Emery, Tom and Jim Bounds on this subject. My vacuum
>>advance is ported and therefore the further I open the throttle the more
>>vacuum advance I get(that is, within the mechanical limits of the vacuum
>>advance).
>>Bob McLaughlin
>
>Sure Bob - draw me into this controversy - thanks a whole lot! :) :)
>
>OK, here's my two pennies. This is one time when both sides are
>right. With regard to the vacuum advance mechanism, there are times
>when you push down on the throttle and the vacuum increases therefore
>advancing the timing. There are other times when you might floor it
>and the vacuum decreases therefore retarding the timing. If you drop
>the throttle to the floor it initially can loose vacuum until the
>engine starts to rev up. So, for a short time the timing retards and
>then begins to catch up as the vacuum increases due to the rpm
>increase. If you were under heavy load, such as going up a steep
>hill, you can floor it and find that the vacuum steadily decreases as
>you start to loose speed. In both these examples, the vacuum advance
>will retard the timing.
>
>In general, though, when driving on a flat road, the more gas you
>give it, the more RPM you'll get, the higher the vacuum goes, and the
>more vacuum advance you'll get until the vacuum advance reaches its
>limit. Don't forget, however, that your distributor also has a
>mechanical advance that consists of weights that cause the
>distributor to advance as the rpms increase. The spinning weights
>pull to the outside (centrifugal force) and mechanically cause the
>timing to advance at higher rpms. Your total advance is the sum of
>the initial idle advance, the mechanical advance and the vacuum
>advance.
>
>Pinging (pre-detonation) can be caused by several things. One is too
>much advance on your timing. Others are lower octane fuel and
>improper air/fuel ratio. It can also be caused by a change in the
>temperature of the incoming air into the carb (which can cause a
>change in fuel ratio due to the air density change caused by the
>temp. changes) or a change in altitude (which again changes the
>density of the air). Lower density air has less oxygen.
>
>With your timing set at a given position the engine might run fine
>with no pinging but then you can get a bad batch of fuel (or lower
>octane) and the engine will ping. The same thing can happen on a
>hotter or colder day. The engine might begin to ping. You can
>manually retard the initial timing to take care of this problem but
>then you might have to manually advance it back later to get more
>power.
>
>I've found that with the Caspro EKE (electronic knock eliminator)
>system, I can have my idle timing set to 12 or 13 deg. before top
>dead center (instead of the factory recommended 8 deg.BTDC and when
>I change fuels or altitude or go from a hot day driving to a cold
>night driving, the EKE will automatically retard my timing as needed
>to stop pinging (knocking). I get a couple of pings and then when
>the EKE cuts in the pings go away. It'll automatically try to
>advance the timing back later but will again retard it if the pinging
>is still present.
>
>This allows me to use regular gasoline (usually 86 or 87 octane) but
>still get decent power and fuel economy.
>
>As with any subject, the conditions have got to be defined before the
>right answer can be given.
>
>Emery Stora
>77 Kingsley
>Santa Fe, NM
>
>
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/
 
GMCers,

Sorry if I caused (or increased) confusion with my vacuum advance/retard
comments this morning, I certainly didn't mean to. Thank you Emery for your
excellent post.

Peter
77 Eleganza II
Colorado
 
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