Vac Line Connections

Boost Gauge and FMU

(1999+ straight throttle body cars)

(If you have the older angled throttle body there is a source of vacuum on the throttle body itself near the alternator. That is the main difference…the rest of the info on this page will pretty much support both the older angled TB cars as well as the newer straight TB car.)

NOTE: It is assumed that you will be using 1/8” rubber/silicone vac hoses for your primary routing. These connections are not only for vacuum but will see positive pressure…”boost”…therefore all connections should be very tight or clamped. To this end you will read were I have used 1/8” rubber vac hoses on 3/16 or even ¼” barbed connections fittings. This creates a very tight seal as the rubber 1/8 ID hose is stretched to fit the oversized fitting. In some cases…as with the 99+ intake manifold vac line elbows they are larger than 1/8” so a multi-sized adapter can be used. To secure vac hose to barbed or especially unbarbed fittings such as the GM style one way check valve I used a couple 4” cable ties as clamps. This works great.

For the ’99 + model years with Straight TB you can connect to a VAC source on the left rear upper intake manifold. There are tow elbows that connect to the main ‘black plastic protrusion’ from the left rear upper intake manifold. Use the front ‘elbow’ (carefully wiggle and pull the elbow connector itself…not the vac line…and remove it from the intake manifold) This is your primary point for vac/boost source.)

This is an example of a multi-sized 3 way T. It can handle hose ID’s of 1/8” to 3/8” and something like this helps make secure connections to the VAC port on the left side rear upper intake manifold as it can transition from hoses with different Interior Dimensions. You will have to splice it in between the manifold sensor port and the stock vac tubing elbow that you just removed. The third end of the T is for connecting the primary VAC line that will be run under the cowl to the drivers side in order to activate the FMU and/or Boost/Vac Gauge.

I drilled a couple of holes in the plastic runner that is at the front of the cowl and under the rubber weather stripping. I then used some 4” cable ties to secure the vac line under the cowl and ran it over to the driver side to meet up with the FMU and boost gauge tubing.

All nice and tidy…good to go.

Now that we have a 1/8” vac line from the intake manifold that has been run over to the drivers side of the engine bay it is time to make another T connection. One end of the T will be for the FMU and the the other end to work the Boost Gauge. The boost gauge I am using shows vacuum as well as boost so I integrated it that way. You may choose to do other wise…but most of what I am about to detail will apply to either method. Lets tackle the FMU first… The FMU supplied with your powerdyne kit is designed to BLOCK the fuel rail return line when the engine is developing POSITIVE BOOST (from the blower) rather than vacuum. With the FMU ingaged and the fuel return line blocked the fuel pressure at the fuel rail will increase. This insures that enough fuel is being supplied to the engine cylinders when the blower is delivering boost. Your vortec FMU multiplies the the boost psi a ratio of 6/1 and then increases the fuel pressure at the fuel rail by that corresponding amount. Therefore, if under normal circumstances your fuel pressure is 50 psi, when you hit +1 psi boost your fuel pressure will rise to approximatly 56 psi…and so on. To insure accurate and relible performance of the FMU (you want reliable performance) a ‘one way check valve’ is required. If you buy a new RKSport kit you should have gotten one. If you have a used kit the former owner may have forgotten to include it. Below is a pic of a one way check valve. You do not need anything fancy…just a basic over the counter unit…. About $4. (Check in the miscellaneous parts bins…near the vac hose adapters and stuff of a good auto parts store.) You will be mounting it BACKWARDS…as it is intended to maintain vacumm in modern EVAP systems such as A/C and cabin climate controls. You mount it backwards because you only want the FMU to see BOOST. The check valve in the pic is not barbed…it does not have barbed ends. To maintain a tight fit for the vac hose I used two cable ties as hose clamps on either side of the check valve. Connect the BLACK end of this check valve to the hose running to the FMU and connect the white end to the T connector that comes off of the vac line you ran under the cowl.

Here is a pic of the one vay check valve hooked inline to an FMU on a supercharged V8. (note that cables ties have not been used yet to secure the check valve connections.)

Optional Check Valve rigging:

If you only want your boost gauge to read BOOST rather than ‘Vacuum and Boost’ you can put the one way check valve before the drivers side T connector. This way both the FMU and the boost gauge will be AFTER the one way check valve.

On to the Boost Gauge:

This will be a multi part guide. It is written backwards…assuming that you already have mounted a boost gauge in the A Pillar and have already run tubing through the firewall and are ready to make a connection to the engines Vac/Boost lines. Actuall boost gauge install will be detailed at the end of this guide.

Ok, your boost gauge probably came with about 6’ of 1/8” nylon tubing and a few adapters. How are you going to get rubber VAC hoses to attach to it? Here you go…

First…the nylon tubing that came with your gauge is CRAP. The nylon may work for you but it kinks VERY EASILY…you may end up with a kink behind your ‘A pillar” once you put the gauge in place, for example. Save the fittings and throw away the nylon tubing. Go to autozone and get a $15 COPPER boost gauge line kit. It will come with 6’ of 1/8” copper tubing and some fittings and compression sleeves. While you are at the parts store look for a ¼” barbed end to 1/8” NPT adapter and pick up a few extra 1/8” compression sleeves…in case you need to make adjustments, and some teflon thread tape. (NOTE: 1/8” or ¼”, for example, do not always denote what the diameter of the threaded end is…it actually means what SIZE range of fitting…it’s a little tricky but you can easily tell the difference between, say..1/8” NPT fittings and ¼ or 3/8” fittings. BARBED ends are always the ACTUAL DIAMETER of the barb and its corresponding ID hose. You will understand when you have a hand full of 1/8” NPT fittings in your hand….)

You will need fittings such as these for hooking up a boost gauge. You need to create a transition from 1/8” NPT fittings to a barbed hose end fitting. The barbed fitting is easily found at any good auto parts store as well as the ¼” female NPT to 1/8” male NPT adapter. (similar to a fitting supplied with the boost gauge itself for connecting to the back of the gauge.) Use thread tape for the threaded connections…buy some extra compression rings…you can only use them once and if you need to make an anjustment it will just be another trip back to the parts store. Take a look at the below pic to see what the fittings look like to create a transition from 1/8” NPT to a Barbed End. (I used a ¼” barbed end because I could not find a 1/8” NPT to barbed end unit….but the rubber 1/8” vac lines will stretch over the ¼” barb with a little soapy water as lubricant.)

(from left to right: A ¼” barbed end to ¼” NPT male. A ¼” Female NPT to 1/8” Male NPT adapter. A 1/8” NPT compression collar. Not shown is a 1/8” compression sleeve.)

With one end of the barbed VAC tee feeding the FMU you have one more open end on your drivers side 3 way VAC T connector. Connect a short section (you decide how long…) of 1/8” rubber vac line to the Vac T. Secure it with small cable ties. Take the other end and (use a little soapy water or…um…spit) force it over the ¼” brass barbed NPT adapter. Congratulations! You have just created and completed a transition from a barbed hose end to an NPT compression fitting.

NOTE: After all that you can also forego the ¼” barbed NPT fitting and fit the rubber vac hose over the threaded end of a male to male 1/8” NPT threaded coupler. You may have one in your kit of fittings. One end is used for the connection to the 1/8” compression tubing with crimp sleeve and threaded collar and the other end is left as it is to have the rubber vac line from the T sliped over it. Again, I suggest using small cable ties as clamps for these types of connections.

Below is a pic of the complete 1/8” NPT copper tubing to vac line rig. (pay no attention to the old fuel line that is slipped over much of the copper tubing. It is there only to protect the copper tubing against kinking while working on the connection, etc….it is not necessary.)

Do not cut the copper tubing with pliers or wire cutters. You need to use a small tube cutter or maybe a hack saw…but that is not suggested…dust…etc. Use a small file or knife tip to de-bur the cut tubing before making a connection.

Another Note: You might also be able to flare the end of the copper tubing in order to directly slip the 1/8” rubber vac line over it and then secure with small clamp such as cable ties. I could not find my flareing tool so I did not get a chance to try it.

Routing the Boost Gauge Tubing

I drilled a hole in the firewall above the main harness connector. You can fit a drill up in there from inside the car while you are down under the dash. You will have to remove your lower and upper dash panels to install the gauges anyway…once they are off you can get a drill in there. Suggest sealing the drilled whole with at least something like rustoleum. Use a grommet as well. A grommet will protect the wires and tubing as well as keep out road noise and engine compartment exhaust, etc.

Once you have a prepped hole in the firewall with the A pillar off and the upper and lower dash knee panels off start one end of the boost tubing down the through the dash near the little side window defroster. Be carefull not to force the tubing around any bends. Kinked tubing will ruin your day. Once you can get a hold of the tubing from below the dash rout it neatly above the break and clucth pedals and then out through the firewall and into the engine compartment. Leave a bit of slack on either end until you figure how long you need the tubing on either side…enough to reach and rout through the A pillar pod and also enough in the engine bay reach your Vac Line connection point.

The pillar pods will create sharp bends in the wires and boost gauge tubing when the gauges are mounted. The hole in the stock pillar where you will have your boost gauge needs to be rather long in order to yield the softest bends in the boost tubing. I suggest ‘pre bending’ the tubing rather than just trying to mash the gauge in the pod. Regardless..it is easier to rout the boost tubing and other gauge wires first…connect them to their respective gauges and then mount the Pillar Pod cluster AND THEN move over into the engine compartment and finish the connections for the boost line and FP sender, etc. This way as the new pillar cluster is being moved into place you can pull the slack out of the wires and tubing as the try to bundle up in the dash, etc.