Casual boost enthusiasts think blow-off valves are all about sound bites. If the hissing noise garners adorning looks from passersby great, but if it scares cats deep into the back alleys the valve gets a real thumbs up. But there are performance considerations at work here as well.

Does it come down to a classic fashion-versus-function choice? With the fashion-minded being swayed by the hip “Fast & Furious” sound of released boost pressure while the fans of function say any emphasis on acoustics is misguided when the critical focus should be squarely on performance?

We talked with Marty Staggs from Turbosmart, Aaron O’Neal, chief tuner at English Racing, Alex Shen, chief proprietor of SP Engineering, and Reggie Wynn from Turbonetics about the importance of blow-off valves and how it effects your car.

Twin Turbosmart Dual Port BOVs on a GTR - for the best of both worlds.

This decision is not made with your ears and in many ways your car makes the decision for you. Brace yourself, there is a technological rabbit hole to navigate and it encompasses understanding what a blow-off valve does, what type of metering regime your engine employs, and how the operation of your engine is effected by the blow-off valve

The Task At Hand

First and foremost is understanding what a blow-off valve (BOV) does. Combating compressor surge is the sole purpose of a blow-off valve. Compressor surge occurs when the throttle plate is closed; between shifts or during deceleration. The closed plate causes the incoming boost charge to back up behind it. When this boost stacking backtracks into the compressor housing it works against the natural rotation of the compressor wheel, slowing it or even stalling it out altogether, resulting in an oscillating, fluttering sound.


This condition causes turbo lag, hurts overall drivability, and puts undue stress on the bearings and shaft of the center cartridge. A blow-off valve earns its pay by relieving the pressure before it gets to the compressor housing which keeps the turbo spooled and ready between shifts. In the cockpit you benefit from increased acceleration and enhanced throttle/engine response.

A blow-off valve is a simple device internally with a piston, spring, and diaphragm. Boost pressure applied from the top holds the valve closed and then vacuum pulls the valve open when the throttle closes. It’s important to plumb a blow-off valve’s port to a post throttle body vacuum location.

There are two main types of blow-off valves: vented and recirculated. A vented valve discharges into the atmosphere, making the cool sound that is the genesis of this article. A recirculated valve routes the pressure back into the turbo system. Vented BOVs aren’t for everyone. It has nothing to do with sound quality; your engine’s metering system plays a pivotal role on which type of BOV is best for you.

Mass Air Flow Vs. Speed Density

Without getting into the intimacies of the Ideal Gas Law, basic mass air flow systems meter the airflow after it enters the intake system, measuring volume and density so the ECU can calculate fueling requirements. There are two types of mass air flow (MAF) sensors: a vane meter that uses a trap door mechanism to ascertain flow and the modern, more popular hot wire or hot film type. Hot style set-ups use a heated sensing element warmed to a fixed temperature. When air flows past the element is cooled, the sensor works to maintain the element’s base temperature, and the current needed to do so is how the sensor determines airflow. The amount of current being used is converted into a voltage signal, which the ECU infuses into its fuel calculations.

There are two types of mass air flow (MAF) sensors: a vane meter that uses a trap door mechanism to ascertain flow and the modern, more popular hot wire or hot film type. Hot style set-ups use a heated sensing element warmed to a fixed temperature. When air flows past the element is cooled, the sensor works to maintain the element’s base temperature, and the current needed to do so is how the sensor determines airflow.

A turbocharged MAF-controlled engine runs a recirculating BOV because the system is a self-contained closed loop and the computer has determined its fueling strategy based on the flow. Re-inserting the boost into the system will not upset engine operation. Running a vented BOV changes the airflow (by venting it out of the system) AFTER the computer has decided its fuel parameters. The result is a hiccuping horror show of an engine that will barely run.

Speed density systems are more predictive in their approach to engine control. The major components of a speed density system are the Intake Air Temperature sensor (IAT) and Manifold Absolute Pressure sensor (MAP).


Engine speed, throttle position, and coolant temperature are other parameters that go into speed density calculations. An oxygen sensor mounted in the exhaust system relates combustion efficiency to the computer after the fact. The ECU takes all the data, readings, and measurements and selects one of many pre-programmed tables that represent a volumetric efficiency value of the engine. The table has a corresponding fuel table that is employed to run the engine. Since this system is not a closed-loop, pressurized design, losing or venting pressure to atmosphere will not impact performance so it can run either style of BOV.

Engine speed, throttle position, and coolant temperature are other parameters that go into speed density calculations.

Hiss Is It! Making The Jump To Speed Density

Swapping from Mass Air to Speed Density is somewhat common but it’s usually done for more than acoustics, namely big boost, big power builds. Aaron O’Neal is chief tuner at English Racing. The Vancouver, Washington based shop specializes in forced induction and does everything from flash tunes to a far-reaching exotic builds.

O’Neal points to how MAFs incorporate sampling tubes, screens, and honeycombs to properly ‘align’ the air or better control the sampling process. These parts of the sensor can create an air restriction when higher airflow, read higher boost and horsepower, is desired. We are talking about more than a restriction to overall flow but more critically the sensor’s ability to accurately read the flow when the boost is turned way up.

“Removing the MAF(s) from the equation opens the door to numerous advantages,’ says O’Neal. “Since most intakes can move more air than the MAF is capable of reading it is very easy to max out the stock MAF housing. Supra, Subaru WRX/STi, Mitsubishi Evo X, and GTR come to mind in our industry as cars that frequently have this problem. I feel that speed density adds flexibility on the tuning side that you can’t get any other way. Since a MAF car is looking at volume of air moved per second, it gets limited fast, even when you go to a larger MAF housing. A larger MAF housing will begin to deteriorate idle and cruise drivability as the needs for more airflow at peak power go up because it’s too big to read slower/less airflow. With speed density however, the limit, or the key parameter, is boost, and only boost. A turbo at 20 psi will only calculate fuel based on 20 psi and not volume. If you like loud BOVs then speed density is definitely required to keep the car drivable.”

A MAF screen helps straighten the air so it can be better read, though some remove them to increase flow.

“One of the main hurdles of the conversion is not all stock systems can be reprogrammed to run in speed-density-only mode,” says O’Neal. “The Evolutions can do it on a stock ECU with some fancy programming, the earlier DSMs with aftermarket software from ECMLink, and Subarus and GTRs with Cobb Accessports. Of course, these engines can also be converted to speed density with the use of the various stand-alone ECUs on the market such as Motec, AEM, Vi-PEC, etc. For a car like the Supra there are no options other than a stand-alone if you want to switch to speed density tuning. Some cars have the parts and just need programming, others require a MAP sensor to read pressure, possibly an intake, and then the tune.”

The king of power in speed density conversions has been the ETS/English Racing GTR that recently ran a 7.80-second quarter mile at 183 mph. – Aaron O’Neal, English Racing

“For most cars we can convert and tune them on the stock ECU and speed density for under $650 for the first-time tune. Stand-alone cars are significantly more involved because of the price of the aftermarket ECU and additional sensors that need added in as well as the time to tune it. In a given month I would say 80 percent of the cars I tune are either converted to speed density by us or they have made the change before showing up for us to tune.”

“Probably the ‘most out there’ speed density conversion we have done to date is a turbocharged Hayabusa 1300cc street bike motor that has been swapped into a 1959 Saab Land Speed car for use at Bonneville. Making 500 whp with anything other than a stand-alone and speed density wouldn’t have been possible in that particular set-up. But the king of power in speed density conversions has been the ETS/English Racing GTR that recently ran a 7.80-second quarter mile at 183 mph during the Buschur Shootout. It still runs on the stock ECU with the aid of a Cobb Accessport and speed density conversion software they had written into it… it’s stunningly simple yet stunningly fast.”


Alex Shen, chief proprietor of SP Engineering in City of Industry, California is also a proponent of speed density when it’s done for the right reason. “Going speed density affords numerous advantages over MAF tunes such as idle stability, drivability, the ability to tune for unlimited boost, which all leads to more power production, says Shen. “Depending on the car the conversion can be quite straightforward. SP Engineering does a lot of Nissan GTRs and on a stock R35, you will need a speed density loom kit and an IAT sensor. There are plug-n-play kits for the GTR in the $500 to $550 range. With the Nissan I’d say 650 whp is the transition point where a speed density conversion starts to make sense.

Picking A Winner

Their names are many, blow-off valve, dump tube, recirculation valve, bypass valve, diverter valve, and their function is similar but as with anything you need the right tool for the job. Marty Staggs of Turbosmart USA says the key is to match the size of the valve with the flow of the system and when in doubt it’s better to go a little big than a little small. “Picking the best valve comes down to the how much boost you’ll be running and the physical space available for installation,” says Staggs. “Many units have an adjustment screws or swappable springs to tailor valve activation to each particular application. Use the resources of the manufacturer or a trusted tuner to get the right size the first time.

Single port BOVs can be run open or closed loop by changing the fitting type. Dual Port BOVs vent to atmosphere while also recirculating air back into the intake track

Turbosmart manufactures vented and recirculated valves in sizes from 20mm all the way up to 52mm. The 20, 25 and 34mm Kompact series valves are typically used as upgrades and direct replacements for anything using the Bosch style valve. The larger valves are designed for modified and racing vehicles. Reggie Wynn from Turbonetics concurs, “Most street cars producing under 400 horsepower can use a 35mm blow-off valve. If you’re making over 500 horsepower you should consider running a 50mm or larger unit.

Quality billet products will outperform and out-live cast, stamped, and plastic parts every time. Beware of shortcuts.- Marty Staggs, Turbosmart

Turbosmart’s Staggs is quick to point to quality as another key factor. “Quality billet products will outperform and out-live cast, stamped, and plastic parts every time. Beware of shortcuts. Modifying an OEM product is definitely not the way to go. The OEM valves have very specific design specifications and are often times quite fragile, more so if they’ve been heat cycled countless times. Also be aware of counterfeit and knock-off products. The market is flooded with cheap imitations. ANY performance product should be purchased from a reputable and authorized re-seller. This goes for all automotive products. It’s just not worth the risk.”

Wynn from Turbonetics echoed these statements pointing past the wasted time and money of the initial purchase of a bootleg valve to the potential catastrophic engine damage a poorly manufactured valve can cause.

[Turbosmart Dual Port BOV]

Shape Shifters: Dual Port BOVs

Dual Port BOVs offer a best of all possible worlds solution. “Another option would be to purchase a dual port BOV if you were wanting to stay on a MAF, quips O’Neal. “On a speed density car there is no gain one way or another as far as BOV plumbing, but on the MAF car where it is very sensitive to changes in metered air (and therefore administered fuel) the dual-port design allows for more sound to be produced while not venting all of the metered air that a normal vent to atmosphere set-up would release. This will give some of the drivability back to a MAF car and make the part throttle tuning a little easier if the tuner checks drivability in part throttle/low boost situations. Like any vented BOV set-up it is still letting metered air out of the system without pulling fuel so you will get rich blips though not as pronounced as a straight vented set-up.”

Factory vehicles like a WRX can benefit from a dual port blow-off valve to keep the engine from going rich between shifts while helping generate higher levels of boost at high RPM…all while giving off a more pronounced sound.

Staggs relates that the Turbosmart dual-port BOV is the company’s bestseller so plenty of enthusiasts are daring to want it all and getting what they want.

We can wholeheartedly support the mean underhood hiss generated by a stout turbo system as it deals out the pain and have no qualms with a speed density enthusiast pumping up the volume. But we urge MAF owners to look at the big picture before taking the plunge. “Typically we try to encourage our customers to not go vented on MAF cars,” says O’Neal, “but in the instances where ‘No’ is not an acceptable answer, the dual port BOV design is a good compromise.”

The bottom line here is the decision to go for the noisy vented BOV is not one of whimsy, it should be calculated and the benefits, performance and otherwise, should be carefully considered.