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Hi! So I noticed today that the distributor vacuum control switch on my 72 350 with auto trans had the bottom port plugged with a screw. I drive the car like this (apparently done by the previous owner) and didn't notice this till tonight when I was connecting the new vacuum hoses I got. Remember that I haven't started the car in several years since
i began my restoration. Do I need this entire switch with its ports to work or is there a way to route around it that would result in better Driveability ? The engine is pretty much bone stock. According to the CSM it looks to connect to the modulator and a port on the manifold. Thank you in advance for your help!
Chris
That switch is an early emission control device. It prevents distributor vacuum advance until the engine is up to operating temp in an effort to reduce exhaust emissions.
The connection to the transmission modulator port is merely to access a full manifold vacuum source, which is already there for the transmission modulator. The top port on the switch is for ported vacuum. The switch prevents distributor vacuum advance until the engine is up to operating temperature. If the engine overheats, the switch changes the distributor vacuum advance from ported vacuum to full manifold vacuum, which helps cool the engine by advancing the ignition timing (which also increases the idle speed, causing the fan and water pump to turn faster).
If you want the best operation from your engine, bypass the switch and connect the distributor vacuum advance to either the ported vacuum source or the full manifold vacuum source. I have mine connected to full manifold vacuum as that works better for my setup.
Fun 71, thank you for the detailed message. Could I trouble you for a photo or diagram of how you have your system set up as you described in your last paragraph? I have points and condenser setup. Would your way still work for my car? I appreciate your patience. I sometimes have a hard time understanding vacuum and electrical. Thank you!
Chris
It's very simple. The vacuum hose from the distributor vacuum advance is attached to the hose "T" on the transmission modulator hose, and the port on the carburetor has a cap on it.
Run the line from the modulator manifold port T directly to your vacuum advance. Remove the valve from the manifold and install a pipe hex plug, cap the carb port, and remove the vacuum line to the back of the carb. Then readjust your idle speed.
To clarify a little, the TVS serves two purposes. It provides the Transmission Controlled Spark function that disables vacuum advance unless the trans is in high gear. As noted, this is for NOx control. The electrical connections at the top of the valve operate a solenoid valve that blocks the vacuum port to the distributor when energized. The other purpose is to prevent overheating when idling in traffic for long periods. This is the thermal part of the switch. When the coolant temps exceed a certain level, the actuator in the bottom of the switch open the vacuum valve that changes the distributor advance from ported to full manifold vacuum. This is independent of the Transmission Controlled Spark function. Ironically, plugging that lower port disables what is the most important function of this valve.
What most people fail to realize is that if you simply unplug the electrical connector from the top of the valve but leave all the hoses connected, you now have the distributor running on ported vacuum all the time, with the added benefit of still having the anti-meltdown protection of the thermal sensor to switch to manifold vacuum. Without the electrical connection, the Transmission Controlled Spark function is disabled.
Adding to what others have already provided - adding some diagrams (link below) I created some time ago demonstrating the DVCS deletion and vacuum routing I performed on my '71 CS 350 sbo.
Hi! Okay so I've read over and over what Joe and Fun71 and Vintage Cheif have posted and I truly appreciate all your responses! However I am still a little confused about ported vs full manifold vacuum. Am I understanding correctly that ported vacuum is an inferior form of vacuum meant to curb emmisons and that full manifold vacuum is preferred? And by removing the electrical wiring from the top will ensure full manifold vacuum at all times but may reduce efficiency and increase heat in a stock engine? Please correct me if I'm off with this. And in a stock engine would I better off going Joes route of keeping the tree in but disconnecting the electric or go Vintage chiefs way of removing the whole things and rerouting the vacuum hoses and/or fun 71's way of rerouting the lines? Thank you for your patience and help. And I'm sorry if I recalled this all wrong. It's been a long couple of weeks. You guys are all amazing!
Chris
Do not consider Ported Vacuum to be an inferior form of vacuum meant to curb emissions. The delta between Ported vs. Manifold applies (and much of this has a controversial history relative to stock cam vs. a big cam) to the relative amount of advance timing applied to the engine during IDLE conditions. Much of the controversy centers around (1) throttle plate angle & (2) correctly located sources of ported vacuum. Ported vacuum does not increase with throttle angle - it fully falls off at full throttle exactly like manifold vacuum. In both cases, Ported & Manifold, there is relatively NO vacuum advance at full/heavy throttle. Your source of vacuum (Ported vs. Manifold) applies to advance timing during IDLE conditions. In general, if a ported source is used, there is no additional timing added at idle. If a manifold vacuum is used, you increase vacuum and improve the quality at IDLE. There's like an entire treatise written on this stuff and much controversy.
Ported vacuum has absolutely nothing to do w/ curbing emissions - it is singularly and ONLY a source of vacuum - regardless of ANY emissions controls on the engine. Whether a system uses ported vs. manifold vacuum depends entirely on why you are sourcing ported vs. manifold vacuum relative to an IDLE condition (to slightly advance timing to improve IDLE). Be aware, we're referring primarily to a stock OEM carburetor and stock OEM sbo engine and we're referring to stock OEM ancillary devices sitting on the engine which pull-off and use vacuum e.g. Power Brake booster vacuum, emissions PCV items, DVCS items etc. These emissions systems were first being developed during this time period.
In a nutshell do this. Use either Ported or Manifold vacuum - whichever provides your engine with the best idle condition and throttle response.
Joe, with this statement, are you saying that it might be beneficial to simply disconnect the electric from the top for best performance overall? "What most people fail to realize is that if you simply unplug the electrical connector from the top of the valve but leave all the hoses connected, you now have the distributor running on ported vacuum all the time, with the added benefit of still having the anti-meltdown protection of the thermal sensor to switch to manifold vacuum."
Thank you
Chris
Running ported vacuum is part of emissions. What he's saying is that if you unplug the connector then your disabling the transmission controlled spark portion of the valve and you will get vacuum advance in any gear. The other part of the valve will operate the same. With extended idling, when the temp reaches its predetermined setting, the valve opens and will supply manifold vacuum to the vacuum advance. This does 2 things it will advance the timing so that the lean fuel mixture will burn completely, and it will raise the idle speed and boost the fan air flow through the radiator. Generally the adv timing is what makes it run cooler.
Joe, with this statement, are you saying that it might be beneficial to simply disconnect the electric from the top for best performance overall? "What most people fail to realize is that if you simply unplug the electrical connector from the top of the valve but leave all the hoses connected, you now have the distributor running on ported vacuum all the time, with the added benefit of still having the anti-meltdown protection of the thermal sensor to switch to manifold vacuum."
Thank you
Chris
Disconnecting the connector disables the transmission controlled spark function and allows the vacuum advance to operate the same way it did on the 1969 cars. You get ported vac in all gears and if the car starts to overheat, the thermal vacuum switch changes to full manifold vacuum to advance the timing at idle and reduce the heat. This is a really easy thing to do if you want the car to be optically correct. Simply plug the connector in when you get to a show and pull it off before driving home. Easy Peasy.
Is there a proper way to remove the old switch? It doesn't seem to want to loosen and I don't want to bust it in the manifold by removing it the improper way. Thank you,
Chris
I removed mine of 48 years last year. It was tight. Proper way is to not break it. Soak it well in your favorite penetrating fluid - ATF + Acetone (50:50 mix) works best, IMO. Soak it a couple hours or several days applying small amounts, it doesn't take much - be gentle but firm. I've always found a larger breaker bar (extension) provides better leverage and better overall "feel" when removing stubborn items. BTW, as I recall, the metal is pretty sturdy and rather deep threads.
Last edited by Vintage Chief; September 27th, 2019 at 07:08 PM.
September 16th, 2019, 06:52 PM
