manifold vacuum dist hook up
Thread Starter
Safety Car
Joined: Oct 2000
Posts: 3,593
Likes: 103
From: Southern Indiana
manifold vacuum dist hook up
Well I hooked my dist to manifold vacuum and set at 15deg.
Although the advance is nowhere near 36 at 2500, the car seem to idle at 900rpm good and is smooth.
I have a brand new dist and wires to install along with my headers next week so it will have all the advance.
Anyway what did I do by hooking to manifold vacuum verses no hookup at all.
Im going to keep it this way because it was so easy to do and seems to run cooler and idle better.
Although the advance is nowhere near 36 at 2500, the car seem to idle at 900rpm good and is smooth.
I have a brand new dist and wires to install along with my headers next week so it will have all the advance.
Anyway what did I do by hooking to manifold vacuum verses no hookup at all.
Im going to keep it this way because it was so easy to do and seems to run cooler and idle better.
Drifting
Joined: Mar 2006
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From: Oak Ridge NC
Thread Starter
Safety Car
Joined: Oct 2000
Posts: 3,593
Likes: 103
From: Southern Indiana
Le Mans Master
Joined: Mar 2008
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From: Oxford MA-----You just lost the game!!!!
Yeah, it does. That's what it's supposed to do. For instance, my car , at idle with the vacuum advance disconnected and blocked of, shows 16* of advance on the timing light. With the vacuum advance hooked up, I'm at 32*. The whole purpose of the vacuum advance is to advance the hell out of the timing when there is no load on the engine. Try setting your timing at around 30* with the vacuum advance hooked up to manifold vacuum, then disconnect it and see where the timing is without the vacuum advance, let us know what you come up with. See ya,
Scott
Scott
Team Owner
Joined: Jan 2006
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From: Crossville TN
It doesn't. Because there is no vacuum on the system until the engine is running. But, to run smoothly at idle, with nearly "no load", the ignition has to occur before TDC...so that by the time the fuel charge burrns, the piston is past TDC.
That's why it's called "vacuum advance".
The timing of the engine is supposed to be set at a few degrees BTDC at idle without the vacuum advance; then, the vacuum advance is to be reconnected. Of course, it's always good for the vacuum advance can to be appropriately selected for the amount of vacuum your engine produces at idle. And that's why there are a LOT of vacuum advance cans available...so you can select one that is right for your engine.
That's why it's called "vacuum advance".
The timing of the engine is supposed to be set at a few degrees BTDC at idle without the vacuum advance; then, the vacuum advance is to be reconnected. Of course, it's always good for the vacuum advance can to be appropriately selected for the amount of vacuum your engine produces at idle. And that's why there are a LOT of vacuum advance cans available...so you can select one that is right for your engine.
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Thread Starter
Safety Car
Joined: Oct 2000
Posts: 3,593
Likes: 103
From: Southern Indiana
OK heres what I got today.
With the advance unplugged and the vacuum source plugged, I set the idle timing to 15 BTDC, when I plugged the advance to full manifold vacuum I now have 32 degrees and I put idle at 900.
Engine seems to sound different and run cooler and takes off real good.
Also starts fine.
I know what I did but tell me what I did with the numbers.
Should I mess with it more and put in more timing or just leave it alone?
With the advance unplugged and the vacuum source plugged, I set the idle timing to 15 BTDC, when I plugged the advance to full manifold vacuum I now have 32 degrees and I put idle at 900.
Engine seems to sound different and run cooler and takes off real good.
Also starts fine.
I know what I did but tell me what I did with the numbers.
Should I mess with it more and put in more timing or just leave it alone?
Thread Starter
Safety Car
Joined: Oct 2000
Posts: 3,593
Likes: 103
From: Southern Indiana
I actually prefer ported timing to the advance canister above the carb throttle plates. Just don't see the need for all that timing advance at idle, as really not what it's for.
Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
On the contrary, that's exactly what it's for. Ported timing was emissions related and not needed for normal engine performance.
Le Mans Master
Joined: Mar 2008
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From: Oxford MA-----You just lost the game!!!!
Back in olden times, vacuum advance was connected to manifold vacuum at all times, in order to have the spark timing advanced while no load was on the engine, because that's how engines ran best. With plenty of advance, you can run a nice, lean fuel mixture, because there is more time for the mixture to burn (lean mixture burns slower than rich mixture)
The timed spark port was an emissions gimmick that reduced peak pressure in the combustion chamber at idle, in order to produce less oxides of nitrogen, requiring a richer fuel mixture at idle, increasing fuel consumption.
Scott
Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
OK heres what I got today.
With the advance unplugged and the vacuum source plugged, I set the idle timing to 15 BTDC, when I plugged the advance to full manifold vacuum I now have 32 degrees and I put idle at 900.
Engine seems to sound different and run cooler and takes off real good.
Also starts fine.
I know what I did but tell me what I did with the numbers.
Should I mess with it more and put in more timing or just leave it alone?
With the advance unplugged and the vacuum source plugged, I set the idle timing to 15 BTDC, when I plugged the advance to full manifold vacuum I now have 32 degrees and I put idle at 900.
Engine seems to sound different and run cooler and takes off real good.
Also starts fine.
I know what I did but tell me what I did with the numbers.
Should I mess with it more and put in more timing or just leave it alone?
To check your curve you can push the throttle open till all the mechanical advance is in and note the RPM. 2500 to 2800 is a reasonable RPM for all the mechanical to be in there. If it comes in sooner use heavier springs on the wieghts, if it comes in later use lighter springs. You have a new distributor so I'd think your curve would be alright, but it's easy to check. If you don't have an adjustable timing light you can use a degree tape on the balancer, they are real cheap too.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
You might want to back off the initial timing to 12 degrees ( what you're calling the idle timing).
To check your curve you can push the throttle open till all the mechanical advance is in and note the RPM. 2500 to 2800 is a reasonable RPM for all the mechanical to be in there. If it comes in sooner use heavier springs on the wieghts, if it comes in later use lighter springs. You have a new distributor so I'd think your curve would be alright, but it's easy to check. If you don't have an adjustable timing light you can use a degree tape on the balancer, they are real cheap too.
To check your curve you can push the throttle open till all the mechanical advance is in and note the RPM. 2500 to 2800 is a reasonable RPM for all the mechanical to be in there. If it comes in sooner use heavier springs on the wieghts, if it comes in later use lighter springs. You have a new distributor so I'd think your curve would be alright, but it's easy to check. If you don't have an adjustable timing light you can use a degree tape on the balancer, they are real cheap too.
What we are concerned with here is at high speed cruise, when the mechanical advance is all in because of rpm, and the vacuum advance is all in, because of high vacuum, you don't want the two together to give you more than 50-52 degrees of advance. In the OP's case, he is at 15* @ idle, and vacuum advance gives him 17* more, for a total of 32*. If his mechanical advance provides 20* of advance, he should be all set, but it's best to play it safe until you know for sure.
Scott
Le Mans Master
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From: Oxford MA-----You just lost the game!!!!
Instructor
Joined: Jul 2008
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From: Wertheim Baden-Württemberg Germany
Confused.
I always thought that by having the dis. hooked to ported or metered vacuum was the way to go.When you open the throttle the timing is gradually advanced until the rpm's are high enough for the mechanical advance to set in.
If your hooked to Manifold vacuum,the timing would start to retart as you open the throttle and the vacuum drops and/or as the manifold vacuum drops and the mechanical advance comes up at the same time,it would cancel each other out and the timing advance would never change, or changes would be so slight that it has no real effect.
Clairity please
It's just that I had this very same conversation last sunday with some aquaintences.And in the German C3 Forum alot of debate on what is best. Everybody seems to be divided on the subject.
I always thought that by having the dis. hooked to ported or metered vacuum was the way to go.When you open the throttle the timing is gradually advanced until the rpm's are high enough for the mechanical advance to set in.
If your hooked to Manifold vacuum,the timing would start to retart as you open the throttle and the vacuum drops and/or as the manifold vacuum drops and the mechanical advance comes up at the same time,it would cancel each other out and the timing advance would never change, or changes would be so slight that it has no real effect.
Clairity please
It's just that I had this very same conversation last sunday with some aquaintences.And in the German C3 Forum alot of debate on what is best. Everybody seems to be divided on the subject.
Last edited by Ravoll; Aug 19, 2011 at 08:00 AM.
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
Guess there's always been, and alway's will be a debate on this, but just don't see the advantage of all the timing advance at idle myself. Really more for highway cruising fuel economy. To answer another question, Vacuum and cetrifugal advance don't cancel each other out, they add up. Typical all in highway cruising is about 50 degrees.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Not true. The 'ported' orifice IS just above the throttle plates, as you state. But, it is not a gradual increase of vacuum; as soon as the plate passes the orifice, 'ported' vacuum becomes the same as 'manifold' vacuum. Besides, the purpose of 'ported' vacuum is only to provide a means of eliminating vacuum to the advance can while the engine is at idle {for emissions reduction...and at the expense of your engine's "comfort" and fuel economy}, then convert to normal vacuum advance for off-idle and cruise modes.
If your engine happens to operated better at idle with 'ported' vacuum...well, I guess that's OK. But it is unusual; and I would suspect that there are other tuning problems with the engine to allow that to be the case.
If your engine happens to operated better at idle with 'ported' vacuum...well, I guess that's OK. But it is unusual; and I would suspect that there are other tuning problems with the engine to allow that to be the case.