BOV (blow off valve) vs DV (diverter valve) - what's the deal, anyway?

[ Caveat - the statements made here and the opinions expressed here come only from my personal experience over the past 15 years, building small displacement forced induction motors.

I've used a number of blow off and/or diverter valves on these built motors. Bottom line - this is my experience only, but in the real world, experience carries much more weight and has more credibility than hearsay, rumors and/or "ricer speak". ]

A BOV and a DV more or less the same, and perform the same function. The difference in terminology is pretty much meaningless. Where people get their undies in a bundle and/or burned is the issue about venting to atmosphere or not.

With a closed, MAF-based system (such as my current motor, a 1.8 litre turbocharged Audi), venting to atmosphere will cause the motor to run rich. Period. Venting a BOV/DV to atmosphere simply to make more noise is one of a number of definitions which puts the owner of that car into the category of "ricer". ( flames > /dev/null )

The ECU meters incoming air via the MAF, and using inputs from the MAF, TPS, MAP (if so equipped), RPM, O2 sensor(s) and who knows what else determines how long to fire the injectors. If any of that metered volume of air is vented to atmosphere, the ECU has no idea that it's lost some air and will fire the injectors long enough to match the volume or pre-metered air. Result? Rich condition.

And just for a minute, consider what happens a non-recirculated valve is open during normal driving. When not making boost, turbocharged motors live in vacuum. Maybe even enough vacuum to hold the BOV/DV open. Ingesting unmetered, and in 99% of the cases which I've seen, unfiltered air. Sound like a good idea?

Some BOVs are engineered to only vent to atmos, some are engineered to recirculate the vented charge. Look here for a picture of 2 valves sitting side by side. One is vent to atmos only, the other is designed to recirculate. Are the BOVs? Are they DVs? Does it really make any difference? (They are BOVs, for what it's worth...)

This BOV is what came stock on my 1995 twin turbo Toyota Supra. And this HKS Racing BOV is what replaced it when the supra was converted to a big single turbo. The supra was also converted from MAF-based to speed density, so venting this guy to atmosphere was OK. ( That motor made around 730 hp at the crank, with a bad tune (mine), after the mods which were done to it. On a 100% stock longblock. )

This DV comes stock on the TT porsche motors, and is also an upgrade to the OEM Audi/VW 1.8t valve. The part number is "0 280 142 110", and it's referred to as the "110" valve. It is the valve which is currently being used on my A4 1.8t motor. It begins to open at 4 inches of mercury worth of vacuum, and is fully opened at 8.5 in HG. Sorry for the blurry pictures, but my digicam doesn't do good closeups. ( The 1.8t motor made 150 crank hp stock, and is currently making around 200 crank HP. )

This Greddy "Type S" BOV is sitting on my bench, and will ultimately end up on my car. I'm missing the "flange eliminator" which bolts up to the bottom of the valve body. Geez - look at that - it recirculates too! ;)

And finally, here is a Sard BOV which I had on my 1993 Mitsubishi Eclipse GSX. As you can see, it's designed to only vent to atmosphere. This motor was converted from MAF to speed density, so venting in this case was also OK. ( That motor made around 450 hp after the mods which were done to it, on a 100% stock longblock. )

Ok. With that out of the way.

As was stated above, DVs and BOVs are in essence the same. It's simply the nomenclature which is different. They both perform the same function, which is to protect the compressor from 'bad things' when the throttle plate snaps shut during the creation of boost pressure.

The charge in the IC plumbing can travel well in excess of 100 mph, and when the throttle plate closes the charge has nowhere to go. A shockwave can bounce off of the throttle plate and travel back through the IC plumbing to the compressor, and if the impact is great enough can stall the compressor, or perhaps even spin it backwards. Which is 'a bad thing' as far as the health of your turbocharger goes. The wheels are spinning anywhere between 100k and 200k RPM. From 100k to 0 rpm in an instant? No thanks.

The BOVs open when presented with some specific measure of vacuum. Vacuum is produced immediately in the intake manifold when the throttle plate shuts, so as soon as the BOV "sees" whatever level of vacuum is required, (say 10 inches of mercury, for example) it opens and some of the compressed charge is vented out of the IC plumbing and back into the intake stream - between the MAF and the compressor. This venting is what protects the turbocharger.

So - use a valve which recirculates the vented charge, unless you've done a speed density conversion or are using a blow-through MAF, which sits in the IC plumbing, somewhere between the compressor and the throttle body. A blow through MAF is most likely in the IC plumbing just before the TB. ( FWIW, the motor pictured in the blow thru MAF image is a 500+ awhp 4g63 1g DSM motor. )

Even these so-called "hybrid" valves - they still cause you to lose some of your pre-metered air. So why even bother with them? People think that the valves are analagous to the OEM valves (which recirculate 100% of the charge), while in fact they are not. Venting any of your pre-metered intake charge to atmosphere isn't the smartest thing to do.

If you gotta have noise, search Ebay for a "Blow Off Valve Simulator". That should do the trick.