Blow Off Valve (BOV)

The Blow Off Valve (BOV) is the next item I installed. Strictly speaking, the stock BOV (or recirculation valve as Nissan calls it) did not need to be replaced as it was working fine at the current level of boost. My main reason for replacing it was simple vanity. I wanted the sound of a BOV venting during gear changes. I admit it, I like rice!

I have read that the stock unit tends to leak at levels of boost not much higher than stock. Normally, a slight leak in the stock unit should not be an issue (beyond the obvious slight loss of boost) as the air is not lost, rather it is recirculated back into the intake stream, downstream from the Air Flow Meter. Thus all the air is accounted for and the correct fuelling is calculated by the ECU.

I had decided that I wanted a pull-type BOV (such as the HKS SSQV) as the design shouldn't suffer from leaking as the valve is positively kept closed until it is required to open.

As always, a tight budget precluded me from buying a new HKS BOV as they typically cost several hundred dollars. Luckily, a second hand Monza SSQV (HKS clone) came up for sale on SilviaWA for the princely sum of $100, already mounted on an S13 pipe! It was a little scratched and the welding of the mount was a little rough, but everything appeared to be in good working condition.

A close look at the device shows that the unit is a pretty faithful copy of the HKS SSQV. So close, that I suspect the genuine HKS accessories would likely fit without trouble!

Installation was a simple matter of removing the original curved pipe and swapping in the pipe fitted with the BOV and connecting the vacuum line. The BOV sits up against the Pod Filter, but doesn't seem to cause any issues. Ideally, I'd prefer it mounted away from the intake, but beggars can't be choosers!

Initially, I left the stock recirculation valve in place as I didn't have caps to block off the hoses that I would have to remove. I eventually was able to locate a rubber cap that was large enough to block off the 25mm recirculation hose, but I would need a larger cap than that for the intercooler end. Having two BOV's wasn't an issue, other than being unnecessary. I eventually found a cap of precisely the right size (the lid off a bleach bottle!) that was sturdy enough to do the job and I was able to then remove the stock unit entirely.

I gave the stock unit to my nephew that runs a small turbo charged car that doesn't normally have a BOV. He was intending to use it as an external venting unit (even machining up a venturi to fit on the outlet), but unfortunately, he found that it continually leaked.

Update Nov 2005

I was having occasional trouble with the blow-off valve not always releasing, so I decided to investigate what the trouble might be. I began to remove the unit from the car, only to discover that the copper tube that connects the vacuum line to the BOV had worked loose. I considered just fixing the problem and leaving it at that, but as I had a bit of time up my sleeve, I decided to tear it down any way and check it's internals before re-installation. As I did it, I thought I would take a few photos of the construction of the valve to show how simple these items are.

The mechanics of the valve are quite straight forward. There is a diaphragm attached to a freely moving shaft, with a large spring that holds the valve closed under normal conditions. The vacuum line is connected to the rear housing. When the throttle is snapped shut, there is a large difference in pressure between the interior of the valve (+ve pressure) and the rear chamber (-ve pressure), which causes the valve to open. This releases the pressure in the front chamber. Once the pressure has been released, the spring will close the valve again.

The valve itself (in the front housing) has two stages. Lower pressure gear changes cause only the central primary valve (the purple coloured piece) to pull away releasing the pressure through a relatively small opening between the primary and secondary (black surrounding piece) valve. High pressure gear changes pulls the valve open further, opening both the primary and the secondary valve. The purpose of the two stages is to reduce the inertia to allow the valve to close quicker on small discharges, reducing the likelihood of stumbling at idle due to unmetered air being sucked in through the open BOV. This photo does not show the rubber seal that normally fits around the exterior of the secondary valve, between the exterior housing and the valve proper. The exterior housing also holds the fins, which have no other purpose other than as a whistle (i.e. a noise maker). Alternate whistles are available (from HKS) to change either the sound and/or the appearance of the unit, as well as an adaptor to convert it from atmospheric venting to plumb-back.