View Full Version : SR20 Vacuum source

08-19-2019, 09:57 PM
Hi i have a S13 Redtop, Im just wondering, I got MAP , FPR, Boostgauge and Atmospheric BOV. So for the TB top nipples i got MAP and FPR.

I know ppl say dont Tee FPR but i have no other source, Is it ok if i Tee it off to FPR/BOost gauge and BOV Tee off to wastegate actuator?


crash n' burn
08-20-2019, 09:19 AM
Don't T your FPR. Ideally, you don't want to T any of them but that's just the way it goes when there is not vacuum ports. You could always run a vacuum block but your MAP and FPR need dedicated sources directly from the engine for best performance.

Personally, I've always tapped my compressor outlet for a barb fitting to use as my wastegate source. BOV and boost gauge have always gone together. You could even mount the MAP sensor on the strut tower or firewall, tap your lower intake manifold plenum for a barb fitting, then run a hose to the sensor.

08-20-2019, 02:24 PM
I have my map sensor and fpr together at the top left on the throttle body. I bought the quick tap boost nipple that installs onto the silicone coupler for my wastegate. And bov straight to top right on the throttle body

08-20-2019, 05:44 PM
Don't 'T' anything that requires a fast response, like:

-blow off valve
-boost controller signal line input
-map sensor, electronics

it's ok to slow the responses of:
FPR, boost gauge, hobbs switch, very few things

If the fpr is a little behind thats fine, same with gauges. but if the bypass or map is behind the engine won't be tuned right and the compressor will surge waiting

also be aware one of the vacuum ports on sr20 is a 'transient source' it does not provide vacuum when the engine throttle blade is closed.

08-20-2019, 07:49 PM
Drill and tap the intake manifold plenum for the MAP sensor. Or mount it remotely but run the line to the plenum.

sr vegas 240
08-20-2019, 11:29 PM
+1 on the vacuum block. I have a cheap one off amazon cost me 22 delivered next day and i have up to 7 extra vacuum sources.

08-22-2019, 01:25 AM
Thanks for the reply, really appreciated it. About the Vacuum block, isnt that just like another version of a Tee? Thing is i cannot Tee it anymore

So following Kingtal0n advice, I Tee'd off FPR with boost gauge and a MAP vac source by itself on the other nipple. Wastegate going through Boost controller and through the Hotpipe source.

So now that leaves BOV still undecided.

08-22-2019, 01:40 PM
splicing into other sources is fine...would recommend to those who are just nay-sayers...does it affect it? sure...however its all pulling vacuum/boost to the same degree as the port on the manifold, on my sti the factory fpr is on cylinder 4 exclusively...through research and monitoring the vacuum signals it was found there was a lot of variation in signal so an aftermarket company found that splicing into the BOV vacuum line the signal was more consistent and thereby was able to keep the fuel pressure more stable...as also mentioned there is a port that is closed at a certain time...should this be used for the fpr? no as your pressure will not be where you want it when the throttle is closed...the question on the block is yes, its splicing the vacuum in a larger block which also changes the signal, to what degree? will have to see data on it...the bov, tap another port into the manifold or throttle body! i know the greddy/freddy manifiolds have several additional ports for various vacuum needs, just do research on which will give the signal you need/want!

08-23-2019, 02:33 AM
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.

08-26-2019, 10:20 AM
thank you! i have always wondered why people are putting them on the cold side when i see the same on the diesels! I do agree don't splice if you don't have to!