Vacuum to advance
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
It should work fine no matter where you keep it, but the vacuum off the carb changes directly with the throttle, so it's a more accurate source. I could be wrong...that's just my understanding of it.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Manifold vacuum is the same regardless of whether you pull it off the carb or the manifold. The carb does have a single ported vacuum source that you can use if emissions are a concern, but the manifold vacuum sources will generally provide better throttle response, drivability, and lower engine temps.
Racer
Joined: Aug 2003
Posts: 259
Likes: 2
From: Sacramento CA
No. They are totally different in nature. For the most part, manifold vacuum is highest at no load (throttle closed) and drops under load (throttle open). Ported vacuum comes from the throttle venturi and is at zero or minimal at idle and increases with RPM. Some carburators have sources for both but they do different things and are not interchangeable.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
No. They are totally different in nature. For the most part, manifold vacuum is highest at no load (throttle closed) and drops under load (throttle open). Ported vacuum comes from the throttle venturi and is at zero or minimal at idle and increases with RPM. Some carburators have sources for both but they do different things and are not interchangeable.
For a complete description on manifold vacuum, ported vacuum, and venturi vacuum, simply drop me an e-mail request for my "Vacuum Explained" paper:
V8FastCars@msn.com
Lars
Last edited by lars; Jan 2, 2007 at 11:06 AM.
Thread Starter
Intermediate
Joined: May 2006
Posts: 35
Likes: 0
From: Ann Arbor MI
No. They are totally different in nature. For the most part, manifold vacuum is highest at no load (throttle closed) and drops under load (throttle open). Ported vacuum comes from the throttle venturi and is at zero or minimal at idle and increases with RPM. Some carburators have sources for both but they do different things and are not interchangeable.
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Manifold vacuum at the manifold is the same as manifold vacuum at the carb. The only vacuum source that is any different is the single ported vacuum port found on most carbs. You can use the ported vacuum if you do not want vacuum advance at idle. Use any other vacuum source, either at the carb or on the manifold, if you want vacuum advance at idle. Most engines will respond better to vacuum at idle, but you need to see what your engine "wants" and tune accordingly, as long as emissions are not a concern.
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Thread Starter
Intermediate
Joined: May 2006
Posts: 35
Likes: 0
From: Ann Arbor MI
Needed info
Manifold vacuum at the manifold is the same as manifold vacuum at the carb. The only vacuum source that is any different is the single ported vacuum port found on most carbs. You can use the ported vacuum if you do not want vacuum advance at idle. Use any other vacuum source, either at the carb or on the manifold, if you want vacuum advance at idle. Most engines will respond better to vacuum at idle, but you need to see what your engine "wants" and tune accordingly, as long as emissions are not a concern.
Thanks,
Paul
Tech Contributor
Joined: Aug 1999
Posts: 14,380
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From: At my Bar drinking and wrenching in Lafayette Colorado
Hey Paul -
Since I'm at the office right now, send me your request to the following e-mail, and I'll get you the info right away:
lars.grimsrud@lmco.com
Lars
Since I'm at the office right now, send me your request to the following e-mail, and I'll get you the info right away:
lars.grimsrud@lmco.com
Lars
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
No. They are totally different in nature. For the most part, manifold vacuum is highest at no load (throttle closed) and drops under load (throttle open). Ported vacuum comes from the throttle venturi and is at zero or minimal at idle and increases with RPM. Some carburators have sources for both but they do different things and are not interchangeable.
BigBlockk
Later.....
Racer
Joined: Mar 2004
Posts: 484
Likes: 0
From: Guston KY
Not to hijack the thread, but. . .
I have the timing set where I think i need it, but; with the vacuum advance attached at either the ported source or the manifold, I get a ping once the engine is warmed up at light throttle for a second.
For example, if I am cruising, no ping, but; accelerate slightly and pings for a sec. If I floor it, no ping. When I disconnect the vacuum advance, it runs great. right now it's not hooked up.
Any thoughts??
I have the timing set where I think i need it, but; with the vacuum advance attached at either the ported source or the manifold, I get a ping once the engine is warmed up at light throttle for a second.
For example, if I am cruising, no ping, but; accelerate slightly and pings for a sec. If I floor it, no ping. When I disconnect the vacuum advance, it runs great. right now it's not hooked up.
Any thoughts??
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
You might have a vacuum advance control unit (aka "vacuum can") that is either too light (pulls in with very little effort), or one that has too long of a curve (pulls in too many degrees of vacuum advance). You need to custom tailor the vacuum advance to your requirements. Which vacuum advance control unit are you using? What is your total timing? You need to be at 36 degrees total and then use a 16-degree vacuum advance.
I have a paper that covers all of the control units with all the specifications and part numbers. E-mail me for a copy:
lars.grimsrud@lmco.com
Lars
I have a paper that covers all of the control units with all the specifications and part numbers. E-mail me for a copy:
lars.grimsrud@lmco.com
Lars
Burning Brakes
Joined: Jun 2004
Posts: 1,012
Likes: 5
From: Granbury, TX
FWIW, MSD told me to use the vacuum source on the carb. Another question, does total advance include vacuum advance??? I have 36 degress with intial timing of 10 degrees plus mechanical, but when I hook up the vacuum it increases it to ~45. I don't have knock or running problems but is this right?
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
All carbs have 2 vacuum sources: manifold vacuum and ported vacuum. You need to hook up to the correct source and know what you're hooking up to. The manifold vacuum ports on the carb are the same as any manifold vacuum source on the manifold. Ported vacuum is only available on the carb, providing you know which vacuum nipple it is...
Total advance does not include vacuum advance. It is total mechanical advance only. So with 36 total, you should get another 16-or-so with vacuum hooked up, to produce a maximum total combined advance of 52-54 degrees. If you have 10 degrees initial and pull in 16 degrees of vacuum at idle, you'll get around 26-or-so degrees of timing at idle.
Total advance does not include vacuum advance. It is total mechanical advance only. So with 36 total, you should get another 16-or-so with vacuum hooked up, to produce a maximum total combined advance of 52-54 degrees. If you have 10 degrees initial and pull in 16 degrees of vacuum at idle, you'll get around 26-or-so degrees of timing at idle.