Why Manifold Vacuum ?
Thread Starter
Burning Brakes
Joined: Jan 2006
Posts: 876
Likes: 1
From: Same as our leaders Out of my mind
Why Manifold Vacuum ?
Earlier I posted a thread - Runs Too Hot - and Lars was kind enough to identify some potential areas of interest in getting my 79 to cool down. One suggestion that Lars made was to make sure that my distributor vacuum advance was connected to a manifold vacuum source versus a ported vacuum source. Why would this make an engine run cooler?
P.S. - Lars - You certainly have my attention - and my appreciation. Thanks!
P.S. - Lars - You certainly have my attention - and my appreciation. Thanks!
Race Director
Joined: Nov 1999
Posts: 10,643
Likes: 8
From: Foxfield CO 1970 Convertible
Probably to make sure that your timing is correct. The port source doesn't accurately reflect the engine demand (manifold vacuum), so your timing might be off. If the timing is off, then your engine could overheat...especially if it's too advanced. 
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
First, understand that the only difference between manifold and ported vacuum is at idle. They are exactly the same once the throttle blades expose the ported source. Ported vacuum is absent at idle. Manifold vacuum is full at idle. But, again, they are the same everywhere else.
By hooking up to manifold instead of ported, you will have full vacuum advance at idle. Engines that tend to overheat in traffic will usually benefit from substantially advanced timing at idle. It's also beneficial to run manifold vacuum if you are running a lumpy cam since the increased cylinder pressue from the advanced timing makes the engine's idle more tuneable. Just this can make a car that has a fat idle due to high throttle opening to sustain a stable and proper idle. It allows you to reduce throttle opening and close the transition circuit.
You do not generally have to do any further ignition tuning by just chaning the vacuum advance's source. Pay no attention to what the idle timing is. You are only concerned with total and the centrifugal rate. Base, or initial, or idle timing(they're the same thing) is whatever it turns out to be. And don't worry about hard starting. There is no vacuum until the engine fires so the starter won't labor from overly advanced initial timing.
By hooking up to manifold instead of ported, you will have full vacuum advance at idle. Engines that tend to overheat in traffic will usually benefit from substantially advanced timing at idle. It's also beneficial to run manifold vacuum if you are running a lumpy cam since the increased cylinder pressue from the advanced timing makes the engine's idle more tuneable. Just this can make a car that has a fat idle due to high throttle opening to sustain a stable and proper idle. It allows you to reduce throttle opening and close the transition circuit.
You do not generally have to do any further ignition tuning by just chaning the vacuum advance's source. Pay no attention to what the idle timing is. You are only concerned with total and the centrifugal rate. Base, or initial, or idle timing(they're the same thing) is whatever it turns out to be. And don't worry about hard starting. There is no vacuum until the engine fires so the starter won't labor from overly advanced initial timing.
Thread Starter
Burning Brakes
Joined: Jan 2006
Posts: 876
Likes: 1
From: Same as our leaders Out of my mind
Originally Posted by gerry72
First, understand that the only difference between manifold and ported vacuum is at idle. They are exactly the same once the throttle blades expose the ported source. Ported vacuum is absent at idle. Manifold vacuum is full at idle. But, again, they are the same everywhere else.
By hooking up to manifold instead of ported, you will have full vacuum advance at idle. Engines that tend to overheat in traffic will usually benefit from substantially advanced timing at idle. It's also beneficial to run manifold vacuum if you are running a lumpy cam since the increased cylinder pressue from the advanced timing makes the engine's idle more tuneable. Just this can make a car that has a fat idle due to high throttle opening to sustain a stable and proper idle. It allows you to reduce throttle opening and close the transition circuit.
You do not generally have to do any further ignition tuning by just chaning the vacuum advance's source. Pay no attention to what the idle timing is. You are only concerned with total and the centrifugal rate. Base, or initial, or idle timing(they're the same thing) is whatever it turns out to be. And don't worry about hard starting. There is no vacuum until the engine fires so the starter won't labor from overly advanced initial timing.
By hooking up to manifold instead of ported, you will have full vacuum advance at idle. Engines that tend to overheat in traffic will usually benefit from substantially advanced timing at idle. It's also beneficial to run manifold vacuum if you are running a lumpy cam since the increased cylinder pressue from the advanced timing makes the engine's idle more tuneable. Just this can make a car that has a fat idle due to high throttle opening to sustain a stable and proper idle. It allows you to reduce throttle opening and close the transition circuit.
You do not generally have to do any further ignition tuning by just chaning the vacuum advance's source. Pay no attention to what the idle timing is. You are only concerned with total and the centrifugal rate. Base, or initial, or idle timing(they're the same thing) is whatever it turns out to be. And don't worry about hard starting. There is no vacuum until the engine fires so the starter won't labor from overly advanced initial timing.
Great information! I was wondering about the wide throttle opening at idle and how I am going to have to back it way off when I hook the dist. advance to the manifold vacuum source. (Did that make sense?) I'll be making the changes tomorrow. We are supposed to reach 50 warm degrees here in Jersey tomorrow - big difference from the foot of snow we got on Sunday! I CAN'T WAIT TILL SPRING !!!!
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Verdad Gallardo
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Usually only an issue under load, and the vacuum advance will back off under load to prevent that. If you set the car up for total timing and use a 16-degree vacuum advance unit, you will not be over-advanced. Everything Gerry stated is correct.
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
Here's some data for ya'll...
I just got this off my daily driver- hot from the press..
1970 454 Holley 6211 model 4165 spreadbore, LS5 cam (The car that I bought at Carlisle when I was living in Parsippany NJ after my 71 LS5 was stolen in Franklin Township NJ)
Steady state rpm.... no "blipping" of the throttle:
500 rpm manifold 17 ported 0
750 rpm manifold 18 ported 0
1000 rpm manifold 19 ported .5
1250 rpm manifold 20 ported .75
1500 rom manifold 21 ported 1.5
1750 rpm manifold 21.5 ported 4
2000 rpm manifold 21.5 ported 6
2500 rpm manifold 21.5 ported 9.5
When I blipped the throttle at any point on the curve, manifold dropped to almost zero, and ported jumped to around 15. On the blip release, the manifold went back up, and ported went back down.
Point of connections:
Manifold: a tee on the rear of the manifold.
Ported: The standard Holley ported vac port... small tube on the primary metering block, passenger side.
I just got this off my daily driver- hot from the press..
1970 454 Holley 6211 model 4165 spreadbore, LS5 cam (The car that I bought at Carlisle when I was living in Parsippany NJ after my 71 LS5 was stolen in Franklin Township NJ)
Steady state rpm.... no "blipping" of the throttle:
500 rpm manifold 17 ported 0
750 rpm manifold 18 ported 0
1000 rpm manifold 19 ported .5
1250 rpm manifold 20 ported .75
1500 rom manifold 21 ported 1.5
1750 rpm manifold 21.5 ported 4
2000 rpm manifold 21.5 ported 6
2500 rpm manifold 21.5 ported 9.5
When I blipped the throttle at any point on the curve, manifold dropped to almost zero, and ported jumped to around 15. On the blip release, the manifold went back up, and ported went back down.
Point of connections:
Manifold: a tee on the rear of the manifold.
Ported: The standard Holley ported vac port... small tube on the primary metering block, passenger side.