How Blow Off Valves Work
 

First off I suck at illustrating, so I hope you can picture this in your mind.

1. The goal of the blow off valve is not to just make a cool sound.

The turbocharger makes a freight train of air and shoves it into the engine through the throttle body. When you shut the throttle body to shift gears it's like slamming the door in the train's face. That air/pressure wants to escape. The BOV is a valve usually mounted on the pressurized part of the intake/intercooler pipes that is allowed to open under extreme pressure differentials..i/e vacuum on the engine side and boost on the plumbing side. The BOV valve allows the boost to escape when you let off the throtte/shift gears.

But, but but..I want to keep my boost, right?

Normally yes, but not in this case. Shutting the throttle plate like when shifting gears can cause a shock to the system. I have seen weak throttle plates get bent like a taco shell under the force. I have also seen intercooler couplers blow off, but the most common result is that the freight train of air just bleeds out the turbocharger compressor.

This can (rarely) damage the compressor wheel blades. But what is always does is slow down the turbocharger shaft rpm. As the air is now going backwards out the turbocharger is basically stops the rotation of the turbine..so when you are done shifting and back on the gas again the turbine has to accelerate from low rpm AGAIN before you get full boost. aka more turbo lag.

So how does a BOV solve this? It's a pretty simple part (but there are about 50 different designs)...basically you have a valve held shut by a spring with a hose connected to the engine. Here are the (short list) of conditions that the BOV operates in:

1. throttle shut, zero boost. AKA idle or light cruise. This is when your engine creates the most vacuum. The hose going from the engine to the BOV is actually trying to suck the BOV open at this point. It is the job of the spring to prevent this. I have seen a couple threads on here about bad idles caused by BOV's being sucked open. Get stronger springs people (venders too).

2. throttle open, full boost. The boost pressure in the intake piping is trying to push the valve. But because there is boost in the intake manifold the hose from the engine to the BOV is holding the valve shut. Because the boost int eh piping is basically equal to the boost in the engine there is a net zero effect on the BOV (the springs will take care of any pressure drop differences too).

3. throttle shut, full boost. This only happen when you shift gears or let of the gas after making some boost. This is when the BOV goes to work. There is now pressure in the intake piping pushing the valve open. There is also vacuum in the engine sucking the valve open..the two forces are strong enough to overcome the spring and the valve opens. Once the pressure is released the spring pushes the valve shut.

Ok, so the BOV open to dump excess boost when I shift gears/let off the gas..but how does that reduce lag?

It allows the turbo to "freewheel" during shifts so that the turbine rpm does not drop (as much) between gears. This means that when you grab that next gear and floor the throttle the turbo is already spinning at significant rpm.

For you guys that have never driven a turbo car (but have driven a manual transmission) imagine it like this:

Without a bov, it's like letting the engine rpm drop all the way back to idle between shifts. It's a jerky experience. That is what the poor turbo goes through without a BOV.

BOV's are not safety features that are going to save your motor in the case of an overboost. That is not their job, don't expect them to do it.

I hope this helps explain. If you have questions just ask.

Aaron
ATS Racing.net

Thanks for this! This is great.

I've always known what the job of the BOV is but never how it works specifically.

My Evo was slightly modded when I purchased it years ago and it had a BOV. Eventually the BOV failed and I brought it to Mitsubishi where they replaced it with the stock recirculating valve.

How does this differ? My best guess is that it's basically a BOV that dumps boost pressure to the piping on the intake side of the turbo rather than venting to atmosphere.

Is this correct?

Ah yeah, recirculating valves....They work fine, probably even better, but they don't make the cool sound.

So in my first post I mentioned needing a stiffer spring to keep the valve shut at idle..well that stiffer spring also slows down the response of the BOV. This reduces some of the benefit from running a BOV.

Why does recirculating matter? Because of the sensors used to calculate how much air is going into the motor.

The FRS/BRZ uses a MAF (Mass air flow) sensor to physically measure how much air is going into the motor. It uses that number to calculate how much fuel to inject. A BOV is a controlled leak that can mess up the fuel injection calculations..basically (a made up example here) the MAF says "5" are going in, but "2" leak out the BOV so the engine only gets "3"..but the ecu was told "5" so it injects enough fuel for "5" and the engine runs too rich.

Recirculating the output of the BOV to the unpressurized spot just before the turbo can fix this...if "2" leak out back into the same intake system it's really like they never left. The air was dumped out of the pressurized part (after turbo) into the unpressurized part (pre-turbo) but never totally left the intake system.

Mounting the MAF after the turbo (like Full Blown and P&L do) also helps to fix this problem.

Aaron
ATS Racing.net

Also, any time a VTA valve is open under engine vacuum (like idle or engine braking) your not only sucking in un metered air, but also un filtered air... something that always bugged my about running vta on my evo with the AEM (speed density).

Its purpose is to prevent compressor surge...undersizing or malfunction can have catastrophic consequences

Gem, I don't think compressor surge means what you think it does. :)

- budi

oh god he said it.

Gem and Budi are both right.

There is another surge. But a BOV won't do squat to fix the other kind of surge.
So way way back in my first post I mentioned slamming the door in boosts face causes air to exit backwards through the turbo compressor side.....

There are some situations in normal driving where this same situation can happen. It is usually a fairly low rpm, a higher gear, and almost always a low throttle position. Here is when it has happened to me:

daily driving. 3rd gear. about 40 mph, 3000rpm ~25% throttle. Accelerating slowly up a gently hill/freeway entrance.

The combination of the gearing, and the load caused by the up hill make the turbo work REALLY GOOD. It wants so bad to force that boost in my motor. But I'm just trying to get on the freeway like a normal person so I have the throttle mostly shut preventing that boost from going into my motor...well it has to go somewhere so it backfeeds out the compressor side of the turbo. The BOV is designed to open up under extreme pressure differences..this one isn't that extreme so the BOV is no help.

How do you fix it?

1. get a smaller turbo compressor wheel. One that won't get so anxious at low speeds. But that hurts maximum potential power.

2. get a larger exhaust housing so the air can flow past through the turbo without applying as much force to the turbine wheel...well that creates more lag.

3. Learn to drive.

Yep. that's it folks, just learn to drive differently. Either stay in a lower gear longer before shifting (lower gears have more mechanical advantage and allow the engine to work less therefore making less exhaust so less energy is given to the turbine).

Or apply more throttle. Let the boost in.

Or apply less throttle..less throttle = less exhaust = less energy to the turbo.

Aaron

On point on all of your points. Sorry I ddint mean to confuse anyone; for those who have trouble seeing both types of surge, in TCs, the wheel is still spinning when you've taken your foot off the gas in each gear, and at ofcourse at redline tho thats where the wastegate mainly comes in, in non overboost designed as built driving.

Speaking of wastegates, BOVs and their joint function for any application where RPMS are not a constant,

This applies next part applies to SCs in general vs TC I reckon, tho its a different can of worms between SC and TC when considering the choice of a BOV,when designers design systems, and this applies to wastegates to some extent too: its easier to get better drivability results out of a smaller BOV(or wastegate) setup. Almost easier to design a smaller system and control it in Turbo-land, is what i am getting at. The bigger systems present faster bigger risks too, on the flip side of the dyno charts we all oogle at.

TCs are more efficient than SC, sure this is good, but if youre not careful,they are almost more efficient to needing more danger level management esp on a motor as sensitive as our high CR breed of DI... they can make more boost than most SC, so you often need a bigger BOV or wastegate that has to act very fast to do its job, kinda, not sure if I am saying that right, bit its often harder to control a set of bigger holes in the system is what i am getting at.
That still didnt comeout right.

I love TCs and have them on two cars in my garage, but the motors were built with CR designed for FI

The 4rth FIX if you wanna call it that is :

With a motor with such HIGH CR, read: danger level thresholds are not as high as we like to think, you can Run a made to order boost plot, ie. an impeller wheel like vortech(Or Bullet, HKS, etc)have have -boost plots so safe some designs are made with no wastegate, a very intentional boost plot that itself has protection 'built in' that with the setup you guys buy will make 9psi @7400, what maybe 10psi @8400 with the supplied pulley? So overboost protection becomes a non issue in some setups. Look at their 1st version of kit, it wouldnt make the high boost some were seeing, no matter how fast you spun it, that made it a VERY safe kit. Same concept now,they 'allow' us to make a lil more air at higher spin speeds, I'm sure as fluid dynamic engineers, one can tailor the exact same type of flow on a TC impeller wheel, but you loose the finite control the SC setup has where spin speed is coupled to crank speed at all times, and you have the addition of gearing thats got its pro's and cons. The trade of for the added safety is final HP and TQ. Because racecar is fine. But this is a production motor we are all working with. Anyways I am getting off topic here and dont want to toot my own horn more than appropriate here..The BOV's threshold in mine is 15psi and we never will approach that. I dont ever plan to see more than 12psi even with wastegate. I could keep this thing at 10 psi all day without a wastegate, and with very little reliance on the BOV even the way rpms are always coupled with air output/rpm, both a boon and a drag, pun intended... I think right above there is where its boom time. Dont flame me, just trying to help out.

My point was that
Dealing with BOVs and Wastegates all depends on the air source, the boost plot and the goals. You can 'ask for trouble' or avoid it depending on the above choices. I've been very careful and held my position on this concept even before I got my current kit, even in my past life, if some of you may recall..

Aaron, thank you for posting detailed real-world explanations of the BOV. Hopefully, you'll post up more technical information.

That said; while we understand that an improperly sized turbocharger can cause surge, as does a BOV with a spring that's too light, how often do you see a BOV equipped with too light a spring vs improperly sized turbocharger when it comes to surge. Said in another way, when is the last time you fixed a surging car with a BOV? :)

On a properly designed turbo system, NOT having a BOV isn't catastrophic, is it? Having one with too light a spring cause drivability issues, but folks *should* know to back off when their car is bucking like a wild bronco under light throttle and high load. If the BOV is stuck open, then the system is fueling the motor way too rich which causes severe issues.

I'm not disagreeing with you Aaron, in fact, I look forward to your new post explaining wastegates! That will be a hoot, I predict. Better yet, if you explain how to properly read a dyno chart, that would be even more fun!

What I do disagree with is Gem's comment that a BOV's purpose is "to prevent compressor surge". I will take the time to answer to his post above.

Gem, the problem with adjusting the turbo itself (impeller design or housing design) to match the desired boost pressure is that the turbo isn't linearly connected with the engine RPM. Turbo's can reach super high shaft speeds very early in the rpm range.

So basically what happens is the turbo will spin unhindered right away and reach max spin rpm early (like say at 3-4k rpm) and because the engine is not ingesting its max flow at 3-4k (vs say 7-8k) it will make X amount of boost. Lets say X is 10 psi for the sake of this arguement.

Ok so you have 10 psi in the 3-4k rpm range and the turbo is physically maxed out. Now what happens when you increase the rpms and the flow through the engine increases? Well since the turbo is maxed out, the turbo can't produce more boost to meet the engine's demands. So boost plummets.

One solution may actually be to use a restrictor like the HKS has on it's compressor. This effectively sets a max airflow through the entire system. I have never seen it done with a turbocharger though.

P.S. Your BOV spring may be 15 psi, but it is still opening whenever you lift off of the throttle expelling boost pressure in the system so that the pressure doesn't slam into the throttle plate.

I agree with you 110%, tho I want to clarify a bit: boost doesnt HAVE TO plummet it can stay level and be held 'flat' if the waste gate allows the impeller to remain at an optimal spin speed.

Its with a solid understanding of these concepts, and untill just recently, 2 turbos in the garage(An Infact I'd love another tc car, either a 911T or a NSXT or a new turbo Z car or supra turbo), that I felt and still feel the SC route is the simplest for our car's platform and possibly the safest for our fa20 motor, by no means will it make the most power, but it may well be the safest option in light of todays posts elsewhere in the forum.

a malfunctioning BOV could cause the results the OP of the boom thread we all just saw, tho he seemed pretty sure his BOV was working fine.

This doesn't apply ONLY to turbochargers, superchargers do not suddenly stop when you lift do they? In fact, supercharger kits ALSO include BOVs do they not?

The wastegate doesn't "come in" at redline. The wastegate opens and bleed excess exhaust energy when target boost has been reached. That can happen at various RPM points, not just at higher RPM levels.

I admit I am too dense and uneducated to understand what you're saying here Gem. Would you enlighten me please?

I think the size of the wastegate is entirely dependent on the pressure levels you're trying to achieve. I don't think you'll put a 32mm wastegate in a kit with a GTX42R. While it may have tangential effect on drivability, it's not really relevant.

You are conflating efficiency with danger? I will BET my children's college funds that you WANT the system efficiency of a properly sized turbocharger in your supercharger. Turbocharger systems are inherently more complex than a supercharger system. As such, there are far more points of failure. So I agree with what you're clumsily trying to say. But it's not the efficiency that is the issue. Nor really is the heat introduced into the system internally. The compressor housing is a heat source, that's true: that's why I am keeping my eyes peeled for those kits that has their turbochargers between the radiator and serpentine belts. But the higher efficiency of modern compressors mean that the amount of heat introduced INTO the system is less than a supercharger. It's only recently that twin screw superchargers comes close to 80% efficiency: compared to some compressors that surpass that level of efficiency.

I agree with you completely here. In fact, I was a HUGE disbeliever that this motor can take as much stress as it has shown. I gladly eat my shoe when tuners left and right are making good power and keeping it together.

Not flaming here, just trying to put out accurate information. I'm not infallible, so I fully admit that I am wrong a lot - just ask my wife. But that's really the difference between a turbocharger and a supercharger kit, especially on a small displacement motor such as this. Without putting the two different kits on the same dyno and the same day, what I will say next is rather academic. But just look at what power levels are achieved on similar boost levels. Boost is boost: if X psi on system A makes 100hp, and 80hp on system B, one can conclude that system B is putting in more HEAT into the system, right? I'm glad for you and everybody who opted for a supercharger kit (centrifugal, twin screw, roots type, whichever), it will never make the highest amount of power or torque, but is far simpler to install, and to tune. Full transparency: the most impressive dyno chart for a street car I have EVER seen is made with a supercharger:

https://www.facebook.com/photo.php?f...type=3&theater

The big difference here is that they have 5.0L and 32 valves to breathe through. Our 4-bangers are far different beasts.

- budi

Youre saying it better than me

I appreciate your clarifications, thanks!