The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. This is caused by the pressure wave from the turbo smacking against the closed throttle plate when you shift, or let off the gas after a boosted run. Compressor surge creates rapidly cycling pressure fluctuations, most times audible. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.
Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract or atmosphere to relieve the pressure; helping to eliminate surge.
After 4-5psi turbos will start to suffer from compressor surge with the lack of a BOV.
Using a BOV is not a problem if you run an aftermarket ECU that uses a MAP sensor, it only affects cars with some kind of air flow meter, like a stock miata would have.
If you are using the stock ECU this next bit pertains to you…There aren't many options.
Your best bet is a bypass valve. A bypass valve is basically a Blow-off valve installed near the throttle body with the air recirculated back into the intake (Between the turbo inlet and the AFM/MAF). This way the computer can still monitor all the air entering the system through the AFM or MAF (won't over fuel during shifts or under fuel due to leaks at idle).
For those of you running quality ball bearing turbo's, you're pretty safe without a BOV until about 10 psi. The ball bearing center sections are much more stout and are capable of handling axial loads, unlike journal bearing turbos. The problem with this is its hard on everything else. Silicone couplers can get popped off, throttle plate bushings can be worn out and loosen up causing all kinds of problems.
If you are determined to run a Blow-Off Valve (vented to atmosphere) you must either be sure it seals at idle or install a check valve (A few websites sell these) on the open end to prevent leaks at idle. Without this, you have created a very large vacuum leak and the ECU will not be able to compensate for it, result: bad or no idle. Also understand you will be running quite rich any time the bov opens (you can destroy your cat, foul spark plugs, etc.) Is the noise worth it? That is up to you.
There is no advantage in running a BOV over a Recirculating Bypass valve or visa-verse. The gains are so minimal that anyone concerned with them shouldn't be wasting their time reading this article, lol.
For more info on bypass valves and how they are plumbed, google bypass valve and look at the images.
The HKS Sequential blow off valve are known to seal at idle as well as some OEM Bosche models. Stay away from ebay bovs, the diaphram used to open it is very poor quailty and they will fail very fast (when bovs fail, they lock shut because the vacuum from the intake is leaking past the torn diaphram instead of opening the valve.)
Again the BOV is case where an aftermarket ECU can change the statements above. Most Stand-Alone units use a MAP sensor instead of an airflow meter. Venting BOVs doesn’t tend to bother these units.