Diesel D
Before "timed" vacumm, high performance engines didn't have vacumm advance. In the mid '60s when we were dumping crankcase gases out road draft tubes, the first timed ports started showing up on Holleys. All of a sudden, nearly every high performance car had a Holley, and vacumm advance. As those engines didn't have a vacumm advance before, every racer ran with them unhooked.
The "timed" port is located just above the trottle blade when closed, and is often partially uncovered when the choke is in operation and the engine on fast idle. It operates with manifold vacumm when the trottle is open far enough, as everything under the throttle will have the same vacumm reading. The port in the venturi on a Holley is the port for the vacumm secondaries.
Problems that occur with maniold vacumm. Idle problems. Most engines won't pass emmissions with manifold vacumm hooked to the dist. Maybe that's why everyone thinks it's an emmissions thing. The problems with meeting emmissions is unburned hydrocarbons. UNBURNED FUEL. Some engines are worse than others and have a noticeable miss. Others just drop a hole every few revolutions and you don't really notice it, but the sniffer does.
Another problem assiciated with using manifold vacumm is what happens when putting the car in gear with auto trans. There is, in most cases, a larger drop in RPM when shifting into gear. The common fix is to raise the idle RPM in neutral, and that creates more "bang" going into gear when using stock convertors. This happens because you are firing it WAY early, and when you put it in gear, the flame front/pressure rise is occuring in the same amount of time, but the engine in gear is idling at a 30% slower pace, so we get more pressure and burn before the piston reaches TDC, the wrong side to do work, and the engine slows more as more energy is used overcoming the negative pressure.
Have you ever noticed the MSD, and owners of, claim better idle quality? MSD made a fortune with the multi spark as it continues to spark until the car fires, overcomming the misfire from too much advance at idle.
Engines that came from the factory in the 30s-60s with vacumm advance had very low intial advance. Ford FH was 0 I believe. My Nailhead Riv was 2. Now people want to combine 12-18 initial with manifold vacumm for 30 or so at idle. Although this will work on some engines very well, most suffer the problems I mentioned above.
The question I have, is why in a situation where the engine is doing "no work" as in idle, do we insist on creating a situation where emmisions and drivability are both compromissed? Is this a Conspiresy to sell MSD units to cure idle problems?
