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Everytime you T a line, the response is diminished by the length of the line added compared to it's volume. Some devices require very fast responses so it is best to keep them solo.
Some things that diminish response:
1. longer lines at the same diameter (adding to a line via 'T' makes it longer)
2. incorrectly 'T' diameter line matching
3. improper vacuum source port diameter matching
4. Incorrect vacuum line diameter for the 'gas/air cavity' of a device
There is a right way and wrong way to use 'T' for vacuum sources.
It is possible to run a single vacuum line with no 'T' and still do it 'wrong' and suffer poor response. So lets look at some typical ways these things can happen...
if you increase the length of a line(such as by 'T'ing in an additional length of line) the response will diminish. If you continue adding length (lets say increase length to infinity) the response is completely lost at some point, there is no longer any signal at the other end. So response diminishes as a function of length and can be diminished to the point of nothingness when lines get long enough.
Therefore, keep your vacuum lines short as possible.
Next, the response will diminish if the line is too small of a diameter for the length you are using. Long, thin lines will suffer significant delay in signal for reasons above (they are basically too long).
So, make sure to use large enough diameter vacuum lines for more critical jobs, make sure their diameter matches the source port or smaller, and is still large enough for that length of line.
Never 'T' a large line onto a small line source. Its okay to go from big to smaller but not small to big, which should be intuitive at this point.
?Why does a large line attached to a small vacuum source make response awful? Because the large line has a lot of gas molecules to 'move' before the vacuum signal can be achieved there.
The 'response' is going to be slow when a small vacuum source can only remove a small number of gas molecules per unit time from 'too large' of a line.
Therefore, it is important to match the vacuum source's "hole" size with the internal diameter of the vacuum hose, and that as that hose approaches devices it may be reduced slightly in diameter to improve signal response but not increased at any point or lengthened unnecessarily.
Now lets consider a bypass valve for a moment
Inside the valve is a diaphragm, above which exists a space and there is a nipple leading out of that space for a vacuum line
The vacuum line is supposed to carry a vacuum signal to the bypass valve's above-diaphragm area in order to remove the gas molecules there suddenly in order to create a strong vacuum with which to pull the bypass valve open.
So lets consider two situations with identical source vacuum:
1. Source vacuum port is 3/8" and vacuum line is 3/8" and bypass is 3/8"
2. Source vacuum port and bypass is 3/8" but the vacuum line is only 1/4"
For #1 when the engine's source port is drawn down to 15" of vacuum this signal moves along the 3/8" line quickly and arrives at the bypass valve, and the 3/8" diameter is large enough to pull most of the gas molecules from the diaphragm area with sufficient response to activate the bypass valve quickly and fully.
For #2, the line is smaller so it is much easier for the source port to pull a vacuum signal through the small diameter line, so the signal may actually arrive faster to the bypass. However, because the line is so small, it can only move the gas molecules out of the diaphragm area at a much slower rate, so the bypass valve opens more slowly.
So we see that even when we increase vacuum signal 'response' to a device in question by using a smaller line of sufficient short enough length, if that device has an enormous cavity of gas molecules to move through a small nipple and into that small diameter line, response will suffer/slow and the device will not longer function properly.
For this reason it is my recommendation you put devices with large air cavities such as bypass valve and wastegate type of objects on their own, large diameter vacuum source ports with large lines.
Small devices with small cavity such as electronic boost controller signal nipple can use a smaller line because there is only a very small number of gas molecules to evacuate (or that need to enter) the device's pressure sensor area.
This all might seem very nit-picky but the truth is there can be night and day difference to the way devices act when they are tied correctly to the engine pressure source/response. I've solved so many Greddy profec B Spec-II boost controller 'issues' just by telling them to use a larger dedicated vacuum line that I've lost count. I've 'fixed' many bypass valves by moving them close to the compressor as possible (Borg warner puts them ON the compressor in some of their turbos now) and using a large dedicated vacuum line. The devices often malfunction without the owner even being aware- they don't realize that slight compressor surge before the bypass opens, or the delay/overshoot response from the boost controller is because of their vacuum line routes/diameter.
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