Home » Public Forums » GMCnet » Vacuum Advance
| 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:
|
Senior Member |
|
|
| 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/
|
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
|
|
|
|
 |
|
Vacuum Advance
By: jwillard on Sat, 28 November 2009 12:46 |
|
|
Re: Vacuum Advance
By: rgleas on Sat, 28 November 2009 12:58 |
|
|
Re: Vacuum Advance
|
|
|
Re: Vacuum Advance
By: rgleas on Sat, 28 November 2009 14:38 |
|
|
Re: [GMCnet] Vacuum Advance
|
|
|
Re: [GMCnet] Vacuum Advance
|
|
|
Re: [GMCnet] Vacuum Advance
By: jwillard on Sat, 28 November 2009 17:51 |
|
|
Re: [GMCnet] Vacuum Advance
By: dmumert on Sat, 28 November 2009 18:22 |
|
|
Re: [GMCnet] Vacuum Advance
By: rgleas on Sat, 28 November 2009 20:06 |
|
|
Re: [GMCnet] Vacuum Advance
|
|
|
Re: [GMCnet] Vacuum Advance
|
|
|
Re: Vacuum Advance
By: rgleas on Sat, 28 November 2009 17:37 |
|
|
Re: Vacuum Advance
|
|
|
Re: Vacuum Advance
|
|
|
Re: Vacuum Advance
By: JohnL455 on Sat, 28 November 2009 18:57 |
|
|
Re: Vacuum Advance
By: JohnL455 on Sun, 29 November 2009 08:09 |
|
|
Re: Vacuum Advance
By: rgleas on Sun, 29 November 2009 08:25 |
|
|
Re: Vacuum Advance
By: mdryan on Sun, 29 November 2009 08:31 |
|
|
Re: Vacuum Advance
|
|
|
Re: Vacuum Advance
|
|
|
Re: Vacuum Advance
|
|
|
Re: Vacuum Advance
By: jwillard on Sun, 29 November 2009 09:56 |
|
|
Re: Vacuum Advance
|
|
|
Re: Vacuum Advance
By: roy1 on Sun, 29 November 2009 19:04 |
|
|
Re: Vacuum Advance
|
|
|
Re: [GMCnet] Vacuum Advance
By: USAussie on Mon, 30 November 2009 01:01 |
|
|
Re: [GMCnet] Vacuum Advance
|
|
|
Re: [GMCnet] Vacuum Advance
By: ljdavick on Mon, 30 November 2009 19:07 |
|
|
Re: [GMCnet] Vacuum Advance
|
|
|
Re: Vacuum Advance
By: mdryan on Mon, 30 November 2009 08:01 |
|
|
Re: [GMCnet] Vacuum Advance
|
|
|
Re: [GMCnet] Barbie GMC on eBay (#140362019346)
By: k2gkk on Sun, 29 November 2009 11:06 |
|
|
Re: [GMCnet] Vacuum Advance
|
|
|
Re: Vacuum Advance
By: fred v on Sun, 29 November 2009 14:23 |
|
|
Re: Vacuum Advance
|
|
|
Re: Vacuum Advance
By: Mike_H on Sat, 12 October 2019 10:21 |
Goto Forum:
Current Time: Thu Jun 27 16:02:06 CDT 2024
Total time taken to generate the page: 0.02972 seconds
|
GMCforum
Search
Help
Register
Login
Home
