Have a stock 455. There is a ported vacuum switch (or thermal vacuum switch??) that threads into the water jacket on the intake manifold. Vacuum supplied by carb, and passes through the switch to get to vacuum advance on distributor. At what temp should vacuum open through switch to supply vacuum advance? I have read that the switch opens only when engine overheats which would mean that there is no vacuum advance under normal operation. Am confused....
The switch was part of the TCS. Transmission Controlled Spark Emissions control system. What the system did was shut off vacuum advance in 1st and 2nd gears, then allow ported vacuum to the distributor in 3rd gear. Because the engine used relatively retarded initial timing, and there was no vacuum advance, the engines had a tendency to run hot in heavy stop and go traffic. At 220*, the thermal vacuum switch would route manifold vacuum to the distributor. This would advance the timing 14-20* or so, and speed up the idle to bring temperatures down. Bottom line? Unless you are doing some kind of concourse restoration and want to include the performance and mileage robbing TCS system, put that switch in the parts bin. The engine will run better and cooler without it. This is the 1972 Buick Chassis Manual page on it.
Thanks Larry. I bypassed switch today to see what would happen. I have read your thread on power timing and have a digital timing light. What I have with timing is: Vacuum advance off and plugged: Initial 13, Total advance 31 at 2300 Vacuum advance operational: Initial 36, Total advance 50 at 2500 I am surprised at how much vacuum adds Is data unimportant with vacuum operational? Should I reduce the vacuum advance as mentioned in your thread (as well fine tuning the timing) and move on? Thanks
Yes, reduce the vacuum advance. You need to do that when bringing in the mechanical advance earlier. Aim for 40-45* total at cruise. 50* is usually too much. The engine can surge with that much advance. I would limit the vacuum canister to 10*. Looks like you have 18* or so now.
