I know this comes up from time to time.
I'm re plumbing the vacuum lines to the TVS after detailing under the hood and it occurred to me that as this is a smog motor, perhaps some or all of the vacuum lines to the TVS and the 'Thermal Check and Delay Valve' are unnecessary, and could be plumbed straight to manifold vacuum. The A/T modulator is plumbed straight to constant manifold vacuum via the TVS.

My interpretation of the Emission Control blurb on these two bits of hardware is; (and comments or corrections please..)

Thermal Check And Delay Valve;
Restricts the amount of vacuum the distributor receives when the engine is cold to control exhaust emissions. As soon as the coolant reaches 226F the valve is bypassed and manifold vacuum is provided to the distributor for vacuum advance.

Thermal Vacuum Switch;
Restricts vacuum to the distributor at low coolant temps.

Why can't the distributor vacuum advance be permanently connected to the ported spark port on the carby or directly to manifold vacuum?
My car, '73 455 4v TH400.

From the manual;

TCDV21102014_zpsba425008.jpg
TVS21102014_zps28aea8fe.jpg

My Thermal Check And Delay Valve (tested and working)

P161401_17-10-14_zpsaf31b506.jpg
P161402_17-10-14_zps36dd2bdf.jpg

 

It can be. Many of us have done this and bypassed the switch. I would imagine the manual doesn't state this, because they sold/advertised cars/emissions regulations with the "improvement" from the TVS.

 

It does, and the SEARCH function would have provided the answers...

Again, if you are using PORTED vacuum to operate the vacuum advance, the lack of vacuum advance can cause overheating at idle and low speeds. This is why the factory included a TVS on cars that used ported vacuum. If the engine starts to overheat, the TVS switches the distributor from ported to manifold vacuum. This advances the timing and helps the engine run cooler. If you are running straight manifold vacuum to the distributor, you do not need the TVS. If you are running ported vacuum to the distributor, it is prudent to use the TVS (and a good temperature gauge).

 

Thanks Joe.
Possibly dumb question but why a ported vacuum from the carby? how does that affect performance compared to straight manifold vacuum?
Which one would give better get up and go?

I'm great with spanners, not so good with ignition, timing etc.
Thanks.

 

This debate has been going on for decades. Google "ported vs manifold vacuum" and you'll get pages and pages of posts. Cars used straight manifold vacuum for over half a century, until the first emissions requirements were starting to be formulated in the 1960s. By using ported vacuum, there is no vacuum advance at idle and low speeds. The retarded ignition served to reduce NOx emissions at the expense of reduced mileage and possible overheating at idle. Later the Transmission Controlled Spark system was introduced to lock out the vacuum advance in all gears but high to further reduce NOx emissions. Ultimately, EGR was introduced that performed the same function.

The real world answer is that you should try it both ways and use the one that works best for your setup. The problem is that unless you rework the mechanical and vacuum advance curves, your car may not run better with straight manifold vacuum, as the distributor is set up for ported vacuum.

 

Gee, thanks a heap Joe. This is what I was thinking but wasn't sure.
Of course I was looking for a definitive answer, but kind of knew I posed an open ended question.
I'm not going to get into distributor ignition curves so thanks for sharing your Olds smarts. I'll stick with what I know for now.

 

Mine was connected to a ported source on the carb for over 30 years. I recently switched it to manifold vacuum and haven't really noticed any difference in operation, other than maybe a slightly smoother idle. Typical driving and transition to WOT seem unchanged.

 

Maybe I'm taking this too far, but if someone takes a newer style HEI distributor with vacuum advance and replaces a point distributor, you should have to change the weights; ie recurve it to use it on manifold vacuum, right? Or adjust the timing down to run better with ported vacuum, right?

Like with ported you'd want a couple degrees less initial timing. Just thinking out loud here, and trying to relate the knowledge to my car.

 

No. I have a HEI distributor in place of the original points distributor and the initial is set to to 18º and with 18º mechanical the total is 36º. The points setup was 12º initial and 24º mechanical for the same 36º total.

The book I have shows the original points vacuum canister gave 22º vacuum advance and the vacuum advance can on the HEI was stamped 24º, so essentially equivalent. I made an adjustable stop to limit it to whatever I desire, which currently is 12º. One could also buy vacuum advance canisters with different amounts of advance or even an adjustable canister. There is no need to alter the total advance (non vacuum advance) when using the HEI.