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This article was first published in 2003.
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Open any performance magazine and you'll find dozens of ads for blow-off valves. Ones that go whoosh, ones that screech, ones that are recirculating ("plumback"), ones that are externally venting - and others that are half and half. As we have stated many times, most over-size super shiny blow-off valves (BOVs) do little more for performance than a non-leaking factory valve. However, that doesn't stop lots of people seeing them as a major performance accessory on turbo cars.
Manufacturers fit recirculating BOVs to achieve three outcomes:
- To reduce the noise caused by compressor surge on throttle closure
- To speed-up turbo response following gearchanges
- To reduce airflow metering problems caused by reverse flow back through the airflow meter on throttle closure
Most aftermarket designs are bought to make the loudest noise on gearchanges: a somewhat different priority!
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But what IS a BOV?
A blow-off valve is placed between the turbo compressor outlet and inlet. (Its alternative title - a recirculating valve - is a better description of its function.) The valve opens when a certain amount of manifold vacuum is present.
When your foot is down and the car is on boost, the valve is shut.
But when you close the throttle quickly, the vacuum generated in the intake manifold causes the valve to open, allowing the air still flowing out of the turbo to recirculate back to the intake (or in whoosh-type valves it vents it to atmosphere). This action prevents the sudden build-up of pressure in front of the compressor, which can cause compressor surge as well as abruptly slowing the turbo.
While the two largest plumbing connections join the intake and outlet sides of the turbo compressor, the connections aren't necessarily made right up near the turbo. One connection can be after the intercooler, for example. The smallest diameter plumbing connecting is for manifold vacuum - the trigger that opens the valve. (BOV closure is by spring pressure.)
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Recently I had a need for a BOV - and how that need came about is pretty interesting. On my '88 Nissan Maxima Turbo (a 2-litre V6 import), no BOV is fitted. (The absence of a BOV was standard practice on all turbo Nissans of the era.) The car is an auto, and traditional belief would have it that no BOV is needed - the throttle is kept floored during gearchanges and so the turbo isn't abruptly slowed by the closing of the throttle each time another cog is selected.
But the measurement of two parameters showed that major pressure waves were running round inside the intake system whenever the throttle was abruptly closed. Two undamped (and so very sensitive) pressure gauges are plumbed to the car - one shows normal boost pressure (although - unusually - measured in front of the throttle blade) and the other, the negative pressure (ie restriction) before the turbo.
What was happening was that whenever the throttle was sharply closed, the needle of the boost pressure gauge would flicker rapidly, indicative of pressure waves racing around between the turbo and the closed throttle blade. But what was even more interesting was that the gauge plumbed into place in front of the turbo would also flicker in sympathy! This could only be because the turbo compressor was being rapidly slowed, causing a sudden traffic jam in the air that was previously flowing into the turbo, with negative and positive pressure waves racing back out the inlet.
So in this car- despite it being an automatic - the pressure gauges clearly showed that the turbo was being deleteriously affected when the throttle was closed quickly.
However, given that the Maxima is being modified on a near-zero budget, the new BOV was never going to be a shiny, mega-dollar example. Bosch make a low-cost BOV that is widely used in factory and aftermarket turbo and supercharger kits (its part number is 0 280 142 103), however even the cost of such a valve was beyond budget in this application.
Instead I used a BOV that cost nothing. OK, OK, for the install I had to buy some plastic T-pieces, so it cost about $2...
Making a BOV
If you do a search of wreckers you'll find that a number of cars use valves that add air to the exhaust stream. These air-injection valves comprise a vacuum-operated diaphragm that lifts a plunger off a seat, permitting air to flow. In addition, a series of internal reed valves (light spring leaves) open when the pressure on one side is higher than the other.
The Mitsubishi Colt is one car that uses such a valve, and some Nissan models also use them. In the past I have bought the Colt one, but in this case the Nissan valve was the guinea pig. What Nissan is it from, then? Sorry, but I picked it up at an exhaust shop some years ago when it was on its way to the rubbish skip and have long since forgotten which model it came off. If you're after such a valve I would suggest that browsing at a friendly wrecker is the best way.
Here the Nissan air injection valve can be seen. On top is the vacuum-operated diaphragm, while the two larger diameter plumbing connections can be seen on the left and the right.
The valve comes apart quite easily. On the left is the section of the valve that contained the reed valve block, which is easily removed (the reed valve is not used). The centre section contains the orifice that is opened when the diaphragm operates (ie lifting its plunger), with this vacuum-triggered assembly shown at the far right.
Other than giving it a good clean and removing the reed valve plate, no changes needed to be made to the valve before it undertook its new role in life. The large diameter plumbing is suitable for a 3/4-inch hose.
Installation
I was a little lucky in the case of the Maxima in that suitable plumbing connections were already available. The newly installed intercooler (see "DIY Budget Intercooler Fitment") has a 3/4-inch connection for a throttle bypass hose, while the pre-turbo intake has a 3/4-inch connection for a breather hose. Using 3/4 -inch plastic T-pieces, available for nearly nothing from hardware stores (they're used for irrigation systems - great underbonnet if kept away from direct heat), allowed me to tap into both hoses. Remember that the valve is directional - you want boost applied to the top of the lift-plunger. Manifold vacuum to trigger the BOV was picked up from an existing plenum chamber hose - again it needed a T-piece and I already had one. This hose is arrowed here.
Results
The immediately noticeable result was that there was much less pressure gauge needle movement when backing off the throttle sharply - you could still get the before-compressor pressure gauge to dance, but only when going abruptly from full throttle to zero throttle on full boost. In more normal throttle lifts, the gauge showed that the pressure waves were much reduced.
Getting off and then back onto the throttle showed a slightly quicker boost response as well, although it's not something that's going to make a huge difference to on-road performance times.
Testing of the valve showed that at idle the BOV was open - that is, there was sufficient manifold vacuum that the valve was recirculating at idle. Because it is a recirculating valve this doesn't cause any problems - all combustion air is still both metered and filtered. (Most factory and non-adjustable aftermarket BOVs are open at idle.) However, this characteristic has some implications for the speed with which boost will rise. If the BOV is open for even a tiny amount of the time as the turbo spools up, the rate of boost increase will be slowed as boost is first recirculated.
And so it proved. Measuring the time taken for boost to reach 0.3 Bar (4.4 psi) showed that with the BOV in action, the average of 3 runs was 3.1 seconds, and without it the average was 3.0 seconds. However, that's a minimal change in most people's terms.
Conclusion
It ain't flashy and it won't impress your mates who want to wank over a big shiny BOV. But the modified air-injection valve used in this case cost me nearly nothing and is effective at its primary function - reducing compressor pressure spikes.
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Pressures...
With this type of valve, the vacuum force provided by the diaphragm has to exceed the boost pressure acting downwards on the rubber sealing element before the valve will open. This might delay the valve opening for a moment, however testing with the valve exhausting to atmosphere showed that plenty of air was coming out of the valve on throttle lifts. (Note that this valve isn't suitable for atmospheric venting all of the time, as it is open at idle and so un-metered and unfiltered air is being drawn into it.)
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