The function of a blow off valve (BOV) is to reduce pressure waves that result when the throttle plate of your engine is suddenly closed. For example, when shifting gears the turbo is still pressurizing the intake piping but without a bov to release this building pressure and with the throttle closed, the air can surge backwards and over time damage the compressor on the turbo. A blow off valve is designed to alleviate this strain by detecting pressure differences in the boosted piping (intercooler piping) and opening up a relief valve that either redirects the air to the non pressurized intake (turbo intake) or to the atmosphere.
The atmospherically vented blow off valve simply dumps the excess, vented air to the atmosphere. Since the BOV will reside on either the hot or cold pipe of your turbo system, venting to the atmosphere can cause problems for cars that are equipped with MAF (Mass Air Flow) sensors in the non pressurized intake. When the BOV actuates, air that has been measured by the MAF is dumped and is no longer present in the intake track. Since the ECU has no way of measuring this reduction in air, it still injects the same amount of fuel into the motor that was calculated by reading the signal from the MAF and therefore, the car will run rich. This will cause the motor to bog after shifts. To negate this issue, a blow off valve’s outlet must be recirculated into the non pressurized intake, after the MAF and before the turbo. This allows the excess air to return to the system, keeping the air fuel ratio in check.
Alternately, switching to a blow-thru MAF will allow cars with MAFs to vent atmospherically. A blow-thru MAF is a MAF placed in the cold pipe of the system so that the MAF reads the air before it enters the throttle. By doing this the BOV can be placed on the hot pipe (before the MAF in the system) and it will no longer affect the reading the ECU gets from the MAF. Also, cars that run MAP (Manifold Absolute Pressure) sensors in the intake manifold generally have no issue venting to the atmosphere as they are not constrained by the air differences in the intake tract that the MAF system is.
Since Nissan motors use MAFs, the BOV will need to be recirculated. I have chosen the HKS SSQV BOV as it is a dual chamber, manifold actuated blow off valve that offers low boost and high boost quick relief. The SSQV will be installed with the recirculation fitting and the outlet of the valve will be plumbed to the intake using over the counter 3/4 inch gas fitting.
Pictured: SSQV BOV, recirculation fitting, weld flange (still in box), 1 inch heater hose for recirculation line, 3/4″ gas pipe for weld on return nipple. Not pictured: small vacuum line.
First, you must decided where to install the blow off valve. Although the BOV can go on the hot pipe, the cold pipe is recommended as the air that gets recirculated will have already passed through the intercooler and will therefore be cooler. This will result in lower intake temperatures than if the BOV vents on the hot pipe.
Now we need to take the weld flange and determine where to locate the flange on the cold pipe. Use a sharpie to mark the location on the pipe and then remove the cold pipe from the car and weld the flange to the pipe. Next cut a circular hole (using a drill press) in the exposed piping inside the weld flange. Use a dremel and sand the inside of the opening as well as the edge. Run your finger along the inside to check for any abrasions.
I placed the weld flange on the pipe by hand and then used a sharpie to mark its location. And my welder is showing me no love at the moment so I am having to pour welds in to ensure that everything seals. Please excuse the mess.
Using a drill press drill a circular hole in the pipe between the flange. Make sure to sand the inside edge of the hole. Note: I welded the inside perimeter of the flange as well as the outside to ensure a tight seal.
Now the blow off valve needs to be installed onto the flange before we reinstall the hot pipe. First remove the outside of the SSQV BOV by removing the four hex bolts pictured below. Now place the rubber o-ring into the groove of the flange. Next place the snap ring around the base of the BOV and press the unit into the flange. (This next part is hands down the most frustrating part of installing this BOV so be patient) Using a pair of snap ring pliers, compress the snap ring while pressing the base of the blow off valve towards the flange, all while pressing the rear of the ring down with say a flat-head screw driver. Expect to spend a while on this. Once the snap ring locks in the channel the BOV will now be attached to the cold pipe. After attachment, take the split screen out of the front of the SSQV’s cover and place the recirculation fitting into the opening. Now place the cover back over the BOV and reinstall the four hex bolts. At this point go ahead and reinstall the cold pipe.
Remove the 4 highlighted hex bolts to remove the cover from the SSQV Blow Off Valve.
As you can see, the snap ring holds the base of the BOV to the weld flange.
The recirculation fitting installed on the SSQV BOV.
Since the SSQV is actuated by a manifold boost/vacuum signal, the nipple on the back of the BOV must be connected to a vacuum source on the intake manifold. On the CA18DET I will connect the BOV to the small nipple just behind the throttle body on the intake manifold. The SSQV instructions state that the filter should be placed between the BOV and manifold and should be installed as pictured below.
As shown, the SSQV is getting its vacuum signal from the small nipple on the side of the intake manifold. The filter is placed on the line.
My car runs a hard induction pipe so I am going to highlight one technique to recirculate the BOV. First, attach the 1 inch heater hose to the end of the BOV and snake it towards the induction pipe. Mark the spot where the hose meets. Now remove the induction pie and drill 3/4 inch diameter hole where the mark was made. Sand the outside and inside of the hole and then cut a 2 inch piece of the 3/4″ inch gas line. Sand the cut side of the gas line remove any debris. Now weld the gas line to the hole on the induction pipe. This is now the proper size nipple to connect the recirculation line to. Finally, connect the recirculation line at both ends and then use clamps to tighten everything down. Viola, a recirculated blow off valve.
Circled in red is the BOV recirculation start and its termination on the induction pipe. Mark the termination point.
The new nipple welded to the induction pipe.
The finished product showing the SSQV BOV recirculated on the CA18DET.