Originally Posted by strokey
Whats up guys.
I'm still tuning my engine.
Question about the vacuum advance canister.
I have a distributor in my vehicle very similar to this.
I'm assuming the vacuum canister is very much a typical stock configured part, nothing fancy hear.
At what hg" level would a cannister of this nature start adding advance to the curve.
Is it correct that as vacuum drops the cannister adds less advance to the curve?
weathers gettin nice on the east coast help
Go here and read all about it http://www.corvette-restoration.com/...c_Adv_Spec.pdf
Some things to contemplate:
- Vacuum advance is intended to provide additional advance under light load and cruise where vehicle speed can be high but engine RPM and throttle opening modest. Under these conditions the mixture in the engine has a low density. Remember air and vaporized gasoline are compressible fluids, therefore, their density varies with temperature and pressure. Vacuum is a measure of the lack of pressure, the gauge is read inverse to barometric pressure. In other words, if the atmosphere is at 29 inches of mercury and your engine is idleing at a vacuum of 20 inches of mercury, the "atmospheric" pressure inside the intake is ((29-20)/29))X100 or 31% of barometric pressure)
- Low density mixtures burn slowly, therefore, more advance is required. Cam timing and compression are also conditions that effect mixture density. The longer the cam timing or higher the lift; the lower the bottom end RPM mixture density so more advance is required. The higher the compression ratio, the higher the bottom end density and less advance is required, this also applies to the effects supercharging. Gets complicated quickly, doesn't it?
- The vacuum advance as you read the http attachment has a misnomer in it called "advance start", this is really the drop out point. Engine vacuum is applied against a spring loaded diaphragm. Under high vacuum, the diaphragm is pulled against the spring advancing the ignition timing to it's maximum point. As load or RPM is increased, the assumption is that the throttle blades are opened. this results in a reduction of manifold vacuum, the spring pushes the diaphragm closed reducing the advance. It is also assumed that RPMs increase such that the centrifugal advance picks up as the vacuum drops off. There's a hole in this design, which is RPMs are not necessarily related to engine load. But within this is a safety feature in that when the engine is forced to lug (low RPMs with lots of throttle opening) the lack of vacuum reduces the advance and the lack of RPMs keeps the centrifugal out. This reduces the tendency to detonate but it sure sucks gas.
- Accel, Crane and others sell adjustable vacuum advances which let you tailor the amount and rate to your needs.