hunt4cleanair

Lars
Can you clarify how TVS was used to control spark in the 1979 L48 application? It seems it was used to retard timing to meet 79 emission requirements...A.I.R. was not used on the L48s unless headed for California or High-altitude states. The AIM refers to the switch as TR SP...transmission spark control, an early C3 emission scheme. I reference my post from last week below.

https://www.corvetteforum.com/forums...m-advance.html

ratflinger

You need to call directly for Lars

@lars

hunt4cleanair

calwldlife

I belive the gist of the system disables vac advance until eng is warm and auto trans is in 3rd.
4 speeds I think happens in 4th.

so, you lose the advantage of vac advance at idle

lars

TVS (Temperature Vacuum Switch, or Thermal Vacuum Switch) is not the same as TCS (Transmission Controlled Spark). Neither system actively retards the timing, but both systems prevent the use of vacuum advance under certain conditions, thus limiting the maximum timing the engine sees. This results in the engine running hotter for quicker warm-ups, and the hotter EGT (Exhaust Gas Temp) makes the catalytic converter more effective and lowers certain emissions. The TVS allows vacuum advance once coolant temp indicates that the engine is up to normal operating temperature. Some manufacturers, like Pontiac, used a TVS that allows "ported" vacuum once the engine warmed up, and then switches to full manifold vacuum if the engine goes above a pre-set temperature, since running the vacuum advance off manifold vacuum helps in lowering engine operating temperature. The TCS system, on the other hand, prevents the vacuum advance from operating until the transmission is in high gear in an attempt to reduce emissions in city driving conditions.

hunt4cleanair

So conversely, TVS prevents vacuum advance until the engine achieves its normal operating temperature. The engine runs hotter sooner and simultaneously, the catalytic converter runs hotter reducing exhaust emissions?