Blow-off valves (BOVs) are a very common upgrade for most turbocharged engines, but are they actually needed? BOVs have been labelled essential for a turbo set-up since the start of
turbocharging time; but why?

We wanted to find out if BOVs are essential nowadays, given turbocharger technology has come a long way, or if they’re only for the ‘pssssh!’ noise we all love so much. We know you all have your own favourite tuners and experts in the industry, and we’ve roped in the best in the business to have their say, so you can read them all and make an informed decision of your own.

So, how do they work? 

Before we get the truth behind blow-off valves, it would make sense to get a brief overview of how these noisy suckers work. BOVs contain a valve that allows the pressurized air in your intake piping to vent to either the atmosphere, or back into the pre-turbo intake pipe when the throttle is closed — greatly reducing the risk of damage to the turbocharger, or intake piping, because where else is all that compressed air meant to go? Without a BOV, compressor surge can be a real problem on older, worn-out turbochargers — resulting in premature turbo failure. 

Carl Ruiterman, E&H Motors

Blow-off valves are an essential part of a forced induction system, as they vent off pressure surges between the throttle body and turbo. When an engine is ‘on boost’ and the throttle is closed quickly, a large volume of compressed air can no longer make its way into the intake plenum, and will then look for the next easiest route to move to. This is where a BOV comes in, it creates that passage for the compressed air charge to escape before it reverts back to the turbo and tries to stall the compressor wheel. Compressor-wheel stall is important to avoid, as it puts unnecessary load on the turbo shafts and bearings, and reduces boost response between gear changes.

The optimum BOV system to aid response between shifts is the plumb-back style. This style of BOV installation vents the unwanted air charge back to the entry side of the compressor wheel, which can help keep it spinning. A perfect example of this comes standard in the new BorgWarner EFR range. The BOV is part of the compressor cover, and recirculates unwanted air charge straight back to the compressor wheel at a perfect angle to keep the wheel spinning. This is effectively recycling the unwanted air charge to aid turbo response rather than wasting it. 

Choose your BOV wisely. During our pre-dyno system-pressure checks, we often find the cheaper BOV brands leak, some more than others. Leaking BOVs are a common cause of low power and poor efficiency, as the turbo has to work so much harder to create the same boost. This overheats the intake charge and causes excessive exhaust back pressure.

If you are planning to fit or upgrade one it is well worth spending the extra money and grabbing a BOV manufactured by a reputable company.

Andre Simon, High Performance Academy 

Blow-off valves are not fitted for the reasons most people think. BOVs are usually marketed at improving the boost response in between gear changes, but the reality is not the case. If you look at many forms of professional motorsport, BOVs often aren’t used at all. The main reason they’re used is to protect the turbocharger from the destructive forces resulting from surge which can occur during a gear shift — essentially the pressurized air has nowhere to go. You do need to size them for your application — a BOV suitable for a 200kW Evo, for example, isn’t going to be adequate for a 500kW Evo, as you need to be able to vent a much larger mass of air.

If you have a low- to mildly tuned set-up, going to a larger BOV is not essential unless the factory one can’t hold the boost pressure. A good example is the early Evo BOVs which leak above about 15psi. When it comes to BOV design, I prefer a diaphragm-style BOV as you’re guaranteed the BOV chamber will seal, whereas the cheaper range of BOVs will still work, but you’re relying on a seal that’s dependent on the tolerance between the body of the BOV and the piston — which is fine when the unit is brand new, but over time the tolerances will increase. If there is a leak inside the BOV chamber, it’s possible to affect the boost pressure in the vacuum line to the BOV. This can be a problem if the vacuum line is also connected to a boost gauge, or even the fuel pressure regulator, as these components may see false pressure signals. Basically, you want to find a good quality BOV that isn’t going to leak — you get what you pay for.

Walter Larson, NZEFI

We think any turbocharged application should run a blow-off valve, but there are only two reasons why you should upgrade from the factory unit. The first reason is the customer wants the noise of it venting to atmosphere, and the other is the factory BOV leaks under higher-than-factory boost applications — some do and some don’t. If you aren’t running much more boost than factory, there is no real advantage in changing it. In our opinion, a vent-to-atmosphere BOV shouldn’t be used with a vehicle that has a factory computer and airflow meter. When the BOV vents, it is dumping a whole lot of air which has gone through the airflow meter, which as far as the computer is concerned is the air that has gone into the engine. For example, when you pull up to a set of traffic lights and button off the throttle, the BOV will open, dumping the air out, but the fuel will still be injected into the motor, which can often cause stalling issues before you come to a halt. Some vent-to-atmosphere BOVs run better than others with a factory ECU and air flow meter, such as the genuine HKS SSQV when compared with the common TiAL and Turbosmart units. 

To be honest, people normally have one for the sound over anything else. 

If you run an engine with an aftermarket ECU and no airflow meter, then you’re pretty much free to run whatever BOV you want, plumbed back in or not. What you need to be aware of when purchasing a cheaper BOV is that they often leak quite badly in between the piston and body of the BOV. We’ve pressure tested plenty of intercooler systems with replica BOVs on, and a majority of them leak like a sieve. Whereas with the genuine ones, you tend to never have a bad one. We think BorgWarner turbos with the BOV mounted to the compressor housing is a great idea, and they’re very effective and suitably sized for the turbo already. 

Alltech Diesel & Turbocharger Ltd

A blow-off valve is a pressure-relief device on the engine intake tract to prevent a turbocharger’s compressor from going into what is known as surge. When the throttle is closed rapidly, the airflow is quickly reduced, causing air back-flow instability and pressure fluctuations.

These rapidly cycling pressure fluctuations are the audible evidence of surge. Over time surge can eventually lead to turbocharger thrust bearing failure due to the high loads associated with it.

In some instances there can be so much surge that it can stall the compressor wheel and unwind the compressor wheel nut. The BOV is installed between the turbocharger compressor discharge and the throttle body, preferably downstream of the intercooler (if equipped).

BOVs use a combination of manifold pressure and spring force to detect when the throttle is closed. In modern vehicles, when the throttle is closed rapidly, the BOV vents boost pressure back into the intake tract before the turbocharger and after the air flow meter, as it is air that has already been measured. Some aftermarket BOVs vent to atmosphere to relieve the pressure. Both methods help to eliminate the phenomenon of surge.

Jacky Tse, Jtune 

We tune a lot of cars here at Jtune, and we don’t see any reason you should not run a blow-off valve of some sort. BOVs help a great deal with increasing the turbocharger’s longevity, but nowadays these new-generation ball-bearing turbos aren’t as essential for what people normally install them for — which is turbo lag between gear changes.

The only issue we have with BOVs is when they are installed on a vehicle such as a Nissan Skyline, Subaru WRX, or the early Mitsubishis, which run airflow meters. Vent-to-atmosphere BOVs have a huge affect on these vehicles; the most common are stalling problems at the lights, and running overly rich — often causing backfires and flames. There are a couple of methods to remedy this — the first is running an A’PEXi SAFC, which has a built-in function that can delay the airflow meter signal. By delaying the signal, the car won’t overfuel after dumping the air — and once you’re back on the throttle the ECU will have the correct amount of air and fuel. The second method is by using a BOV which is half vent-to-atmosphere, and half plumb-back. This way, you get the sound and the performance. We recommend running the factory BOV plumbed back in, unless you’re running more boost, or you have deleted the airflow meter. We’ve found HKS, TiAL and GReddy items to be the most reliable — avoid replica items wherever possible. 

Ross Honnor, Dobson Dyno Tune

The main reason to run a blow-off valve is to stop back pressure against the turbo which can cause it to stall, or even bend and damage impeller wheels — in some cases they can actually snap off. There has always been a debate about where the BOV is placed in the system, but our preference is after the intercooler and before the throttle body, but it’s really a personal preference. In some applications such as high-power GT-Rs, we run two BOVs — one either side of the intercooler. The reason for this is to drop the pressure quickly when the throttle is backed off during racing. In other applications, such as some drag cars we’ve built, we don’t run a BOV at all, because our turbo supplier has specified this turbo is strong enough to handle the shock that can occur during surge. In some rally cars we don’t run BOVs to aid throttle response when getting on and off the throttle quickly, but often this shortens turbo life considerably — sometimes three turbochargers are used per season. 

We recommend they be run in all applications, but we generally listen to our turbo supplier, Steve Murch, as to whether or not he thinks the turbo can handle it, as he has had around 1000 years of experience. We find a lot of the time nowadays the BOV is purely cosmetic or for the noise, and this usually determines whether or not it’s plumbed back in. We don’t see any real advantage to running a plumbed-in unit, but when the vent-to-atmosphere unit is open for that short amount of time, it can let in dust and dirt as the valve goes back to its seat due to the vacuum created. During one D1NZ round-up in Whangarei, there was plenty of loose chipseal on the course, and a few turbos were destroyed — in some cases, whole engines. We believe there were five or six engines destroyed at that round — a big learning curve for a few teams. 

Craig Mills, Turbochargers NZ Ltd

The blow-off valve releases the pressure from the turbocharger before the inlet manifold — when the throttle is closed the air has nowhere to go. It stops the air surging and going back through the compressor wheel. It’s designed as a safety feature as far as the turbo is concerned. You’ve all heard the chirping noise WRC cars have when they are constantly lifting off the throttle — that’s because they are not running a BOV, and that air is actually squeezing back through the compressor wheel the wrong way. That’s why their turbochargers have shut-off valves to the oil supply, and are designed as quick-release items. They can simply shut off the oil supply and carry on driving through the stage until the unit is replaced at the next service point.

Most factory BOVs will vent back into the intake, because that air has more often than not already been metered by an air flow meter. So the ECU is supplying fuel to the engine for air that has already been measured. Venting-to-atmosphere, as far as the turbocharger is concerned, is perfectly adequate (unless you are using the turbocharger to compress an air/fuel mixture). Generally speaking, automatics don’t use BOVs, as you aren’t lifting the throttle as regularly, for example as you do when you change gear in a manual. 
A lot of information about where a BOV should be installed is floating around, but generally speaking they’re installed immediately before or immediately after the intercooler — it seems to vary between manufacturers. 

When searching for a BOV, find something that’s of a good quality — mainly replica products are made out of tin foil and borrowed pen springs. Look for something that suits your purpose, for example, don’t run a factory BOV on your Evo making 560kW, as it won’t match the performance levels of the engine. A BOV that is too small may cause the surge effect to take place, damaging the compressor wheel the same as if you were running no BOV. There are several failure outcomes associated with surge, but the main damages are loosening the shaft nut, damaging the compressor wheel, or prematurely wearing the thrust bearing inside the turbocharger. 

We’re currently organizing a set of Turbosmart BOVs for a diesel race truck that Turbochargers NZ Ltd supports, and that’s in aid of preserving the thrust bearing life. With a diesel it’s a bit trickier to run a BOV, as you don’t have a vacuum source as you do with a petrol engine. Boost pressure is applied to both sides of the BOV to hold it closed, and using an electronic sensor that senses throttle position, it opens the BOV when it senses the throttle is closed. 

Warren Overton, PPRE

When you close the throttle, there is a whole lot of pressure in the intercooler and intercooler plumbing that’s got to go somewhere. If there is no blow-off valve, the only escape for this air is back through the turbocharger. When that happens, it slows the turbocharger down considerably and puts a lot of load on that turbo. The exhaust side keeps on spinning as normal, and the cold side doesn’t — this puts massive load on the shaft and the turbine and compressor wheels. I’ve been at a race track in the States when I was testing a drag car which had no BOV, and when it completed the burnout then shut the throttle, it completely snapped the shaft in half. The turbine wheel broke away, made its way through the exhaust system and shot out through the bonnet of the car and landed on the race track, smoking away — the only reason for this was the lack of a BOV.

Performance-wise, BOVs can have a slight negative affect in applications such as drifting. When you’re on and off the throttle all the time, releasing all the charged air can hurt throttle response and give some lag. But as a general rule, you’re better off having one. For experimental reasons, we ran Mike Whiddett’s quad-rotor twin-turbo engine with no BOVs, but after testing we found it was better to run with two Turbosmart units. With vehicles running very large camshafts with lots of overlap, or a rotary running lots of overlap on the port timing, you can have very serious issues if a BOV isn’t used. Basically when the turbo slows down drastically, the exhaust manifold pressure can overcome the intake manifold pressure — forcing exhaust gasses the wrong way back into your intake. With positioning a BOV, there are two theories — after the intercooler, before the throttle body, or before the intercooler and close to the turbo. I prefer to run BOVs after the intercooler, to keep the mass of air moving in the right direction.

This article was originally featured in a previous issue of NZ Performance Car. Pick up a copy of the edition here: