With reference to the 1970 Engine Assmebly Manual Section 6 page 32 and 33 and 76:

on page 32 engine type 657 (455 non W30), 4 bbl carb, AMT, no AC, no Y72, does not use the TVS.
It should be noted that this is implied for B and C body cars, which can be confirmed in the parts list for these engines (see page 76).

on page 33 engine type 657, 4 bbl carb, AMT ( A body car)(with or without AC/Y72) does use the TVS (also confirmed on page 76)

I am wondering why the same engine, same transmission, without AC, would have a different TVS requirement based soley on the A or B/C body installations.

I understand the function of the TVS is to provide manifold vacuum to the distributor advance if the coolant temperature goes above 223F, regardless of the state of the TCS solenoid. When not under load or wide open throttle) this would serve to advance the timing. How this creates a cooler running engine at idle or part open throttle at high RPM I do not know, but why would they NOT use it for B/C body applications?

As always your thoughts and comments are appreciated.
Peter

 

These cars used retarded initial timing as a crude way to reduce NOx emissions. Unfortunately that led to inefficient combustion and the potential for overheating. Increasing the vacuum advance at idle by using manifold instead of ported vacuum made the combustion process more efficient and thus cooler, at the expense of temporarily higher NOx emissions.

 

Thanks Joe, I did not know that it would actually make the combustion process occur at a lower temperature (now I am going to have to delve into that!).

But why the difference between A and B/C body setups?
I have a TQ engine, which is the same 455 that was used in the Vista C. It has the same distributor and timing setup as the engine type UN which was used in the 88 and 98. Both are non AC setups.
Why would mine have the TVS and the 88 or 98 not have it?

 

I'm just guessing here, but it's likely that the larger B/C-body cars were less prone to overheating. There might be a couple of reasons for this - one is a larger radiator, the other (and more likely) is better airflow through the radiator on the larger car. Keep in mind that airflow is not just a function of the grille opening. It is also dependent on the free area under the hood and around the engine, since the air needs to be able to exit as well as enter the radiator. The wider cars had more room in the engine compartment and thus better airflow. Deleting an unneeded part on the larger cars would save a few bucks on each of tens of thousands of cars over the production run.