| Vacuum Advance [message #65853] |
Sat, 28 November 2009 12:46  |
jwillard
Messages: 118
Registered: May 2004
Location: Silver City, NM
Karma: 0
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Quick dummy question. I just discovered that the vacuum advance on my HEI distributor is connected to what appears to be a "ported vacuum" port on the carburator on the front, left side of the carb. As I recall from my old car days, it should be running off of manifold vacuum.
Comments?
Jeff Willard
Silver City, NM
1973 ex-Glacier
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| Re: Vacuum Advance [message #65858 is a reply to message #65853] |
Sat, 28 November 2009 14:02  |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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The vacuum advance goes to and through the TVS to the "Ported Vacuum" port on the carb. If you eliminate the TVS then it goes direct to the "Ported vacuum" port on the carb. The ported vacuum port is the higher port on the driver's side front of the carb.
There is a picture of this hose connection in the GMC maintenance manual.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Vacuum Advance [message #65873 is a reply to message #65866] |
Sat, 28 November 2009 17:10 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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| Steven Ferguson wrote on Sat, 28 November 2009 16:18 |
Roger,
What Ken said is correct.
On Sat, Nov 28, 2009 at 12:38 PM, Roger P. Gleason <rgleas@peoplepc.com> wrote:
>
>
> I would have to go measure it, but I would think that if you eliminate the TVS switch then the vacuum line would go to the manifold.
>
> According to the manual the carburator vacuum is lower to assist in cold engine conditions. Since most of the time we are driving these they are warm. I would think you would get poor gas mileage at the very least.
Steve Ferguson
'76 EII
Sierra Vista, AZ
Urethane bushing source
www.bdub.net/ferguson/
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ALl right. here we go.
THe TVS is a device that sits in the intake manifold and monitors hot water. At a certain temperature over 200 degrees (I believe is was 225) a little wax pellet inside the TVS melts and allows the vacuum advance to get it's source directly from the manifold. The change increases the idle speed to try and get more air flows to cool down the engine. That is it's whole purpose. It is a device designed to primarily work on an overheated engine at idle.
When using ported vacuum at an idle there is no vacuum applied to the advance. So switching sources causes the advance to go full tilt and increase idle engine speed. Once you step on the throttle the ported port is opened up and vacuum is applied to the spark advance. If you switch vacuum sources from ported to manifold then you will also have to adjust your idle speed and base timing accordingly.
Second point.
The vacuum advance can on the distributor used with ported vacuum runs in a different range than a vacuum can running on manifold vacuum. I had these numbers a few years back but do not have them now. At the time I did the research I did not think anyone cared so I did not bother to document them. It was something like full advance on a can designed for ported vacuum was at 11 inches and full advanced on a can for manifold vacuum was 15 inches.
If you change your source I would get a new vacuum advance can used on an older GM car that used manifold vacuum. Also make sure that you get one you get advances the correct degrees at full vacuum. Dick Paterson uses a vacuum advance that only goes to 10 degrees full advance. GMC OEM advances went higher depending on 455 /403 and federal vs. California distributors. Toronado ones were even higher yet.
I have tracked the vacuum on the manifold vs. the carb port. They do run a couple of inches different. They run up and down together (except at idle) but a couple of inches apart. I think this is because of their distance from the open air meaning the carb throat.
MY advice is to leave it where it is as GM designed it.
The purpose of all of the advances (static, centrifugal, and vacuum) is to get as much advance as possible with out causing detonation (pinging) under varying engine conditions. The optimum maximum advance is around 39 to 40 degrees. On my Dick Paterson distributor this is done buy 13 degrees of static advance, 10 degrees of vacuum advance, and 18 degrees of centrifical advance at 2800 rpm. (Yes I know that totals to 41.) The static will never change. The centrifical changes as the engine speed goes higher. The vacuum goes full tilt and retards as you put more load on the engine by pressing on the accelerator. Also at altitude the vacuum advance retards some. This is why the gas companies get by selling 85 octane fuels at altitudes of 4000 feet or more.
My point is if you change one of these parameters (like vacuum advance), you will have to change another one (like static advance) to make up for it.
Ken B.
"Waiting for Rick and Larry to pounce on me"
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Vacuum Advance [message #65879 is a reply to message #65873] |
Sat, 28 November 2009 17:51 |
jwillard
Messages: 118
Registered: May 2004
Location: Silver City, NM
Karma: 0
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Very good explanation. I no longer have the TVS switch on mine and didn't know if it had been connected wrong some time ago. There is no vacuum at idle on the carb port, which was why I thought it seemed wrong.
Thanks for all the info
Jeff Willard
Silver City, NM
1973 ex-Glacier
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| Re: [GMCnet] Vacuum Advance [message #65881 is a reply to message #65879] |
Sat, 28 November 2009 18:22 |
dmumert
Messages: 46
Registered: December 2006
Location: Canada
Karma: 0
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Hi Jeff
Here is quote from Bob de Kruyff on April 3 of this year.
"Start Quote"
This is a volatile subject, but here is the way an engineer of the times
sees it. The engines of this vintage used a retarded timing scheme at idle
in order to control NOx emissions. That was accomplished with a fancy name
called ported vacuum which meant very little if any vacuum to the
distributer advance at idle. This caused several problems such as poor cold
start performance and hot engine temps while idling. The tvs will sense hot
engine temperatures and allow full manifold vacuum to the distributer
advance allowing not only a higher idle speed, but more importantly greater
efficiency and much less heat rejection. Other than complying with
emissions, it is a very detrimental feature of the times. Get rid of it and
go to manifold vacuum to the distributer. This has become such a contentious
issue on this forum that even the engineer who developed the system and
promoted manifold vacuum was chased away and resigned from the net because
of the nay sayers.
-------------------------------------------------------------------------
Bob de Kruyff
"End Quote"
I suspect Bob knows more about this than anyone else on the GMCnet. I accept
his word as gospel. My GMC runs manifold vacuum just like all cars before
the pollution 'fixes' of the seventies. If your engine is properly tuned it
should catch on the first revolution, long before there is any vacuum built
up in the intake.
Dave
> Subject: Re: [GMCnet] Vacuum Advance
>
> Very good explanation. I no longer have the TVS switch on mine and
> didn't know if it had been connected wrong some time ago. There is no
> vacuum at idle on the carb port, which was why I thought it seemed
> wrong.
>
> Thanks for all the info
> --
> Jeff Willard
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Dave Mumert
Olds, AB
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| Re: Vacuum Advance [message #65884 is a reply to message #65853] |
Sat, 28 November 2009 18:57 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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IIRC in the pre HEI days it was ported vac to te dist not manifold vac. Also the TVS is not a 'one time' deal but a thermal switch. I've got mine setup as factory and it runs perfectly.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Vacuum Advance [message #65891 is a reply to message #65881] |
Sat, 28 November 2009 20:50 |
Gary Casey
Messages: 448
Registered: September 2009
Karma: 0
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Wow - I'm amazed that an issue like this would be so controversial. I think Bob has the grasp of the subject, but here are a few clarifications: The retarded spark at idle was primarily to reduce idle hydrocarbon emissions, not NOX, although it probably does that as well. There were no NOX standards at the time. There is one benefit of a retarded spark at idle - it is easier to get a stable idle at a lower rpm. It is probably better to get rid of it, but I guess I wouldn't call it a "very" detrimental feature, just a detrimental feature. Yes, because more idle fuel flow is required the engine will run hotter at idle, but I don't think a little extra heating at idle is a big deal - we have a big radiator and lots of underhood space for ventilation at idle. There is a possibility - a slight one - that the idle ports in the carburetor will be better matched to the "no-vacuum" condition, so it might be more difficult to get a really good idle
adjustment. Oh, and I think having full vacuum at idle will reduce the tendency for dieseling. But at cruise or acceleration conditions there will be no difference.
Gary
Hi Jeff
Here is quote from Bob de Kruyff on April 3 of this year.
"Start Quote"
This is a volatile subject, but here is the way an engineer of the times
sees it. The engines of this vintage used a retarded timing scheme at idle
in order to control NOx emissions. That was accomplished with a fancy name
called ported vacuum which meant very little if any vacuum to the
distributer advance at idle. This caused several problems such as poor cold
start performance and hot engine temps while idling. The tvs will sense hot
engine temperatures and allow full manifold vacuum to the distributer
advance allowing not only a higher idle speed, but more importantly greater
efficiency and much less heat rejection. Other than complying with
emissions, it is a very detrimental feature of the times. Get rid of it and
go to manifold vacuum to the distributer. This has become such a contentious
issue on this forum that even the engineer who developed the system and
promoted manifold vacuum was chased away and resigned from the net because
of the nay sayers.
-------------------------------------------------------------------------
Bob de Kruyff
"End Quote"
I suspect Bob knows more about this than anyone else on the GMCnet. I accept
his word as gospel. My GMC runs manifold vacuum just like all cars before
the pollution 'fixes' of the seventies. If your engine is properly tuned it
should catch on the first revolution, long before there is any vacuum built
up in the intake.
Dave
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List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: Vacuum Advance [message #65894 is a reply to message #65877] |
Sat, 28 November 2009 22:09 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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| rgleas wrote on Sat, 28 November 2009 17:37 |
I think you have explained it better than the book as the manual was very vague. I know the distributor is different from the auto.
The only question I have in your explanation is about the tvs switch. I do know that GM used the same switch or one like it on a lot of engines. If it is wax operated it would seem like it would only be good for one time. Once the temperature melted the wax it would permanently be connected to the source.
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It is suppose to return to normal when it cools. Dick Paterson told me that he has seen a lot of them that no longer work. I did not ask in which position they failed. I took Dick's recommendation, removed mine years ago, and permanently hooked my distributor to the ported vacuum port on the carb.
Actually my PO in California bypassed the TVS before I got the coach. I just removed it from the intake manifold and put a brass pipe plug in it's place. When I later pulled the manifold to install blocking plates, I painted the entire manifold including the plug. The plug now looks like it was installed by GM.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Vacuum Advance [message #65913 is a reply to message #65853] |
Sun, 29 November 2009 08:09 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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The knowledge base on this has been great. (sort of a history lesson documented) I my experience, connecting to manifold vac (ie. more advance at idle) would increase the tendancy to Diesel even though less degrees of throtle opening are required to maintain the same base idle RPM. If you then retard the dist to 'fix' this issue you loose total advance.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Vacuum Advance [message #65915 is a reply to message #65914] |
Sun, 29 November 2009 08:31 |
mdryan
Messages: 27
Registered: March 2008
Location: Seattle
Karma: 0
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Junior Member |
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It's pretty straight forward -
1. Vacuum advance will always draw vacuum upstream of the throttle butterfly - vacuum increases as throttle opens
2. Vacuum retard draws vacuum downstream of the butterfly - vacuum decreases as throttle opens.
MDR
Mark Ryan
1978 Kingsley
Pac. Northwest
[Updated on: Sun, 29 November 2009 08:52] Report message to a moderator |
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| Re: Vacuum Advance [message #65916 is a reply to message #65914] |
Sun, 29 November 2009 08:44 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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| rgleas wrote on Sun, 29 November 2009 08:25 |
The TVS switch being mounted on the return housing is sensing tempature and will not know when the engine is at idle vs. wide open throttle. Logic would dictate that this switch is dealing with a hot vs. cold engine situation. I would also believe that it was an emmissions situation.
Having work on many engines with out all of the smog adaptations less advance on a cold engine will make it run smoother during warm up.
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Yes, but the hot vs. cold is relative in the case of the TVS with cold being anything less than about 225 degrees F. and hot being anything over 225 degrees F.
Basically in a normal running coach engine the TVS never trips since the engine hopefully never gets that hot. This thing sits around 6" away from the thermostat in the water channel. It should see whatever temperature the thermostat is running at (180 to 195).
The TVS only trips in an engine overheat condition. It's purpose was to increase idle speed to try to bring down the engine temperature. Obviously if it tripped when not at idle the engine speed will be controlled by the vehicle operators foot and not the TVS. At idle when the TVS trips the difference in vacuum applied to the advance could be as much as 18". At throttle positions over idle the ported vacuum is already applying vacuum to the advance so the difference between ported and manifold is only a few inches.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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