What is the difference between a vacuum intake modulator that has a vacuum line port on top along with the electrical hookup on top vs. the vacuum modulator that is flat on top? Also I noticed marking next to the vacuum ports are MT, C and D. What do these letter mean?

Thanks,
Shawn

 

It's not a modulator. It's a vacuum switch- DTVS, Distributor Thermal Vacuum Switch. It switches distributor vacuum advance source between ported and manifold vacuum depending on engine temperature.

MT= manifold vacuum source. This hose goes to the intake manifold and vacuum is always present.
D= Distributor. This hose goes to vacuum advance on distributor.
C= carb. This one goes to a vacuum port on front of the carburetor. Vacuum is only present when engine is running above idle speed.

Normal operation is vacuum advance runs off the C port, thru the switch to the D port. If coolant temp gets up around 230 or so, the switch moves to open the MT port to the D port and distributor vacuum comes from manifold vacuum. This should occur only at idle and is designed help cool the engine by advancing the timing at idle.

The switch with electrical connection operates a little differently. It will also have a V port that vents vacuum back to the rear of the carb air horn under certain conditions. The electrical part of it is tied in to the transmission and prevents any vacuum advance to the distributor until the transmission is in high gear.

IMO the only reason to use the electrical DTVS is for a high-point restoration. It's detrimental to performance and gas mileage, and was a stopgap measure for emission control as it reduced NOx emissions. The non-electrical DTVS is actually beneficial since it allows full-time vacuum advance and can help in idle overheating situations.

 

Thanks for the information.