123 Install Today - GETTING CLOSER..., PROGRESS: BAD MPS |
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123 Install Today - GETTING CLOSER..., PROGRESS: BAD MPS |
FlacaProductions |
Dec 14 2023, 09:50 AM
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#1
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Senior Member Group: Members Posts: 1,733 Joined: 24-November 17 From: LA Member No.: 21,628 Region Association: Southern California |
74 2.0 D-Jet
Heading to the garage now to start the install and my main, unresolved-in-my-brain question is if i need to hook up the vacuum line that is on my current stock distributor, on the 123. |
technicalninja |
Dec 14 2023, 02:40 PM
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#2
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Senior Member Group: Members Posts: 1,827 Joined: 31-January 23 From: Granbury Texas Member No.: 27,135 Region Association: Southwest Region |
I'd hook that pup up to MANIFOLD vacuum only.
On my 123 distributor I'm sure the vacuum port leads to a MAP sensor. Manifold Absolute Pressure... Lots of stuff from the early 70s often had two "types" of vacuum. Manifold and "ported". Manifold is self-explanatory but ported is where it gets strange. Posted vacuum comes from a vacuum port that is covered by the throttle blade at idle. As you open the throttle the vacuum reading will increase sharply and tend to hold vacuum throughout most of the RPM range. It's creating vacuum by the passage of air. The faster the air is passing the higher the value. A little bit like blowing across a coke bottle. That weird noise is caused by differing vacuum at the entrance... This was used MOSTLY for emission control as the old-school method of manifold vacuum to advance diaphragm would cause early burning of the fuel mixture in the combustion cycle and they were trying to INCREASE un-burned HC to the cat for early light off. The airpump was usually part of this operation to also increase O2 content. They wanted FIRE in the exhaust manifold, and this proved to be the wrong way to go in the long run... Hook that up to manifold vacuum ONLY. It has adjustable maps based off of this and you will drive yourself BAT-SHIT crazy trying to make adjustments if your running ported vacuum to it... Hope this helps! Where this gets really weird is at high RPM. Manifold vacuum is near ambient (high KPH 95 or better) and ported vacuum is pulling 20" (low KPH 30 or lower). |
Lockwodo |
Dec 14 2023, 03:16 PM
Post
#3
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Member Group: Members Posts: 203 Joined: 23-December 21 From: Santa Cruz, Californnia Member No.: 26,193 Region Association: Northern California |
I'd hook that pup up to MANIFOLD vacuum only. On my 123 distributor I'm sure the vacuum port leads to a MAP sensor. Manifold Absolute Pressure... Lots of stuff from the early 70s often had two "types" of vacuum. Manifold and "ported". Manifold is self-explanatory but ported is where it gets strange. Posted vacuum comes from a vacuum port that is covered by the throttle blade at idle. As you open the throttle the vacuum reading will increase sharply and tend to hold vacuum throughout most of the RPM range. It's creating vacuum by the passage of air. The faster the air is passing the higher the value. A little bit like blowing across a coke bottle. That weird noise is caused by differing vacuum at the entrance... This was used MOSTLY for emission control as the old-school method of manifold vacuum to advance diaphragm would cause early burning of the fuel mixture in the combustion cycle and they were trying to INCREASE un-burned HC to the cat for early light off. The airpump was usually part of this operation to also increase O2 content. They wanted FIRE in the exhaust manifold, and this proved to be the wrong way to go in the long run... Hook that up to manifold vacuum ONLY. It has adjustable maps based off of this and you will drive yourself BAT-SHIT crazy trying to make adjustments if your running ported vacuum to it... Hope this helps! Where this gets really weird is at high RPM. Manifold vacuum is near ambient (high KPH 95 or better) and ported vacuum is pulling 20" (low KPH 30 or lower). On my '74 2.0 with 123 ignition, I've connected the disty to manifold (not ported) vacuum. But I've not been able to program a MAP curve into the 123 that works well (provides extra advance just when the engine is under load). If anyone has successfully done this, could you share the MAP curve set points you're using? Gotta say though, car seems to run just fine without the vacuum line attached to the disty. |
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