L-Jetronic Fuel Injection and Distributor question, Intake Manifold Vac Ports to/from distributor |
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L-Jetronic Fuel Injection and Distributor question, Intake Manifold Vac Ports to/from distributor |
Str1974 |
Nov 24 2019, 12:05 PM
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#1
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Member Group: Members Posts: 90 Joined: 23-January 19 From: Dallas Member No.: 22,820 Region Association: Southwest Region |
I have a 1974 1.8 L with a current Bosch distributor (0 231 181 009). I have two vac ports on my throttle body , one that goes to the distributor (vac retard) and one that comes from the distributor (vac advance) back to the throttle body.
I'm replacing my old distributor for a new 123ignition w/Bluetooth adjustment. The 123 dizzy has an optional port on it for a vac line, so therefore one of my vac lines going from and towards the intake manifold would need to be "capped". I'm hearing that I could either chose to cap either the retard line or the advance line or all together cap both ports on my intake manifold and choose to run my new dizzy without any vac lines. I've also read that "The 914 distributor has a centrifugal advance built into it. This mechanism causes the distributor timing to change as the engine's RPM increases. The advancement of the timing is necessary because at higher RPMs, the engine needs to ignite the fuel earlier than at idle. In other words, if there were no centrifugal advance, then at higher RPMs, the full force from the combustion would impact the cylinder when it was already through the major part of it's stroke. This would result in lost power output. The centrifugal advance changes the timing so that ignition occurs earlier. Additionally, the fuel injection may advance or retard the timing based upon the vacuum pressure from the engine." It's this last line from above that has me wondering how vacuum pressure from the engine works if there are no vac lines going or returning from the dizzy to the intake manifold??? Or , am I completely wrong that the vac ports on the intake manifold / distributor has nothing to do with it. I'm still a novice and would like to understand more about this topic, or at least be less confused.... |
Rob-O |
Nov 25 2019, 12:16 AM
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#2
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Senior Member Group: Members Posts: 1,252 Joined: 5-December 03 From: Mansfield, TX Member No.: 1,419 Region Association: Southwest Region |
Giving this some thought I’m not sure that last sentence is accurate. The FI can react to the timing. The FI is using a connection on the coil to determine RPM. That information, along with info from the AFM, the throttle position and the head temp sensor to determine engine load and how to handle injector firing.
But I don’t see how the FI can push engine information the other way and affect timing. |
Junebug |
Nov 25 2019, 03:08 AM
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#3
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Newbie Group: Members Posts: 20 Joined: 7-May 15 From: East Bay Area NorCal Member No.: 18,713 Region Association: Northern California |
I think that info you have was probably published after 1976? Or when the ignition Coil became "Coils" one per cylinder ?
wiki quote: "Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM and load range. One source for vacuum advance is a small opening located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. This is called a ported vacuum. The effect of having the opening here is that there is little or no vacuum at idle, hence little or no advance. Other vehicles use vacuum directly from the intake manifold. This provides full engine vacuum (and hence, full vacuum advance) at idle. Some vacuum advance units have two vacuum connections, one at each side of the actuator membrane, connected to both manifold vacuum and ported vacuum. These units will both advance and retard the ignition timing. On some vehicles, a temperature sensing switch will apply manifold vacuum to the vacuum advance system when the engine is hot or cold, and ported vacuum at normal operating temperature.." copied from: https://en.wikipedia.org/wiki/Ignition_timing |
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