Boost Control: We Want Your Questions!

Ever since the advent of turbocharging in performance automotive applications, there have been those looking to exploit the absolute power-making potential of turbo-based forced induction. We all totally get that as well, because boost is addictive and too much is never enough, right?

Of course, when it comes to this sort of thing and the components used to control, deliver and limit boost pressure, there are good products and not-so-good products, and the right way and wrong way of going about things. Reliable information on the subject can be hard to find though, so we thought it would be a grand idea to demystify the ins and outs of boost control with the help of our friends at Turbosmart in another round of Ask The Expert.

As one of the world’s most respected manufacturers of turbo ancillaries, Turbosmart have a wealth of knowledge when it comes to boost controllers, wastegates and blow-off valves, and the guys in the R&D department want to share it! Maybe you’d like to learn about the performance differences between a manual or electronic boost controller? Perhaps you’d like to know how high quality external wategates and those cheap, random equivalents you see for sale on the internet differ? Or it could be something as simple as finding out what a blow-off valve actually does, and why turbo cars have them?

Whatever your turbo boost control related question, add it in the comments section below and next week we’ll pass them over to Turbosmart who will choose a handful to answer for the follow-up feature.

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What are the advantages of bypass valves over blow off valves?


Where is the best position for a BOV? (i.e. before the intercooler, after the intercooler, closer to the throttle body, etc.)

Chris Anderson

Is there any solid method to sizing a wastegate? I know bigger isn't always better, but how do you determine the right size?


What are the benifets/detriments of running a 38mm vs 44mm external wastegate apart from the physical size?


What is the best way, and with what components can help to minimize/eliminate lag?


I would love to know more about the photo in the top left corner.
It appears to be a dump pipe?
I have never seen that type of set up before, how does it work? Is the internal gate removed and the larger, more reliable gate used in its place?


I understand you can blow an engine if you go too cheap. But for a student with a budget, is there a relatively cheap setup one could run?


What is the difference and benefit for one over the other when it comes to internal and external wastegates


Yea I don't get it either. Both the standard dump and the gated dump would be coming from the compressor wheel. The point of a wg is to divert gas around the compressor wheel


Can a decent atmospheric bov close fast enough to prevent after fires (commonly incorrectly called backfires) in a car with a maf without a tune?


Why do some wastegates, such as that on the Audi 90 Quattro IMSA GTO, make a very loud screech off throttle?


It is an evo dump pipe. It works by having a turbine housing with the internal wastegate setup removed and using the external wastegate to control boost. The reason to do this is the external wastegate can control the boost more accurately and bypass more exhaust gas than the traditional internal wastegate.


Simply put... How do thy work?


How does what work?


What ALS system should be used for a blow-through (carbed) turbo? The engine is an inline 4 L head. And should this system be analog or digital?


Will we be seeing variable geometry turbochargers outside of the usual diesel applications and inside the engine bays if street legal race cars anytime soon?


watch tuerck off seasons 2. how do i make my rb26 sound the same?


Would be interested in how to select the proper spring preload for IWG or External. And you opinion on the benefits of either.
Also if i can ask another question,, regarding the location or proximity of the waste gate (external) to the turbo. Is there a benefit in a "Vband" set up over say a stock location, which in my persona case is an evo.,


Aside from the obvious differences in complexity, why are Wastegate based boost management solutions favoured over something like a Variable Vane Turbine?

Or rather, what advantages/limitations do VVT turbos have and why aren't they seen often, if not at all in the aftermarket?


can you explain the difference between open loop and closed loop boost control and also the advantages and disadvantages of either


Just posting to throw support points for this 'ask the experts' section. Coz i'm sure that they will hopefully give a thorough autobiography about the humble snail anyway. But hey, since i'm here, i'm just gonna pop some of the voices in my head anyway. I'm maybe hoping for the turbo specialists to shed some light (or maybe personal opinions) about the legendary anti-lag system (prap-prap rally sounds!), then maybe pro tips for turbo conversion steps from 'virgin' stock engines, and lastly pro and con of turbocharging versus supercharging. Hold stumt!


First of all, I just want to say that these "Ask the Experts" features make for a very interesting read. Great job, SH crew, and keep them coming!

My question regards the development process:  how much of it is based on calculations and simulations and how much on actual, real-life tests?

robert armantrout

I drive a 2013 Fiat 500 Abarth and I love the power delivery.  However, the factory built the car with two side mounted intercoolers and a lot of intercooler piping.  Why would the car be designed this way and would utilizing a FMIC be more efficient and a worthwhile investment in making a durable and long lasting system?


o2fst4uo How does turbo work?
Air comes in, turbo spins at high speed, magic happens, engine gets more power. The end.


Age old question...which is more reliable... Electronic Boost Controller vs. Manual Boost controller, and why? 

Additional question:

What determines the proper heat / airflow setup for a conventional sized turbo and how do you combat heat in general (Using applied theories) ?


Is there any advantage to blocking off the bov? Or is it all about noise?
How does a boost controller actually work if the wastegate works off a spring?
If there is a major boost leak ( for example intercooler piping coming off), would this lead to overboosting or would the wastegate still open?


Tbh google could answer these ones for you


how do you reduce duty cycle on a stock wastegate?


What's the benefit of twin wastegate set up? Thanks in advance.


J_W97 I work with VGV (variable guid vanes) jet engines as a designer. Complexity IS the reason why you don't see them much in the aftermarket.


What level of analysis goes into your designs? Non-linear CFD? There are a lot of small companies making parts that "do the job" so what do you do to make sure that your parts "do the job" better? I'm talking about more than fit and finish here. What makes your stuff "performance" instead of just "aftermarket"?


Are supercharges more reliable than turbochargers? what are the pros/cons of boosting a naturally aspirated car?


Hi, I'm thinking about replacing my Trust BOV, with a Turbosmart Raceport BOV, couple of questions: 1) if i did install the TS BOV would I need to retune the car for the new piece of equipment to reap any benefits? Would you recommend retuning after install? Would it be worth installing the BOV controller? Would this give additional performance increases? The car has a build EJ25/20 engine, FMIC with a GT3082R running high boost and putting down over 400awkw. Looking for every advantage I can get. Any information appreciated


Have you done any experimenting with wastegateless systems, (only running a bov or pop off valve) such as nispro was doing a few years back. They were saying things like the turbo is not slowing/speeding up constantly as in a wastegate system. Also they dismissed the myth of over heating the air with a properly sized system.


Can you shed some light on wastegate sizing in relation to engine size and power output. i.e, does a high hp motor need a bigger wg compared to the same motor with a low hp output?
Also, on a twin scroll manifold/turbo setup running an external wastegate. Will a feed from each scroll into one wastegate be ok, or to do it properly should you run two wastegates, one for each scroll?


What is the best layout for sequential turbo applications?
Any advice on motorcycle turbocharging?


stefanotrefano by changing it to a 1jz


stefanotrefano If you mean the extra loud noise when the boost comes on, its a waste gate with a screamer pipe. Venting to atmosphere instead of being being plumbed into the exhaust. 

I highly doubt you have a RB26 if you don't know what a screamer pipe sounds like.


robert armantrout Cost vs performance vs packaging.


Whats the most efficient style of dump pipe for boost control, Bellmouth dump pipes, or Divorced/Split dump pipes?


What is the best hose type for any turbo? Default cars have always plastic hose. Should we really switch to a better hose for greater performance?


Are external wastegate setups on factory exhaust manifolds ideal such as the one pictured below or should one stick with a internal turbo/wastegate? Would you prefer a tubular manifold matched with an external wastegate?


What are the temperature ranges an external gate can handle? More particularity the internal diaphragm. I know some are made with silicone which is usually good till 450-500 F, how does this no burn up when the wastegates get up to 1000+F?


@Jay Just buy one of those diesel turbos... Usually they're a lot cheaper secondhand than a Greddy or HKS item


How easy is it to hold 3-6 psi in the system after the BOV cycles? My understanding is most throttle bodies don't have issue holding back that pressure, but is it worth it as far as fighting turbo lag? Or does it just lead to excess wear and replacing more parts

Marcus Vinicius

Why don't manufacturers install blow offs in the original turbocharged cars ?


robzor J_W97 I figured as much tbh. What're the main advantages/limitations then?


I use a cheap aftermarket boost controller to raise my boost pressure from stock (around  8-10 psi) to around 13-14 psi on my SR20det setup. A problem I experience is that if i am driving the car, and boost raises past the 13-14 psi range causing the wastegate to open, the wastegate then acts like stock again, only allowing the system to boost to 10psi before opening again. The only way to achieve 13-14 psi after this happens is to let the boost drop back down below 5-7 psi and then begin again. Is this problem due to the boost controller quality ? would i still experience this with a $100 boost controller?  or should I only rely on a wastegate actuator with a spring built in to get the amount of boost i am looking for ? 

if any other knowledgeable speedhunting readers can answer this question with an intelligent answer and not waste the pro's time that would be very much appreciated as well!!

Thank you for any answers in advance.


I put on a highflow catted down pipe on my '99 STI Impreza.  The boost dropped.  Wouldn't the stock boost solenoid control the wastegate to keep the boost pressure the same as stock?  Japanese mags show boost increases with aftermarket exhausts and internet smartypants' say it decreases usually.  Either way, why wouldn't it be controlled properly by the stock system?



J_W97 robzor Well, the vane angle is optimized for a certain RPM for the turbo. The whole thing is incredibly complicated but basically a VGV/VVT set up basically lets you increase performance across the whole range. It will help spool up and be more efficient at full load. Additionally you can use it as somewhat of a throttle to slow down the turbine though tbh I'm not sure they would use it that way when they have the wastegate that is probably the safer way for the turbine wheel. Mostly it's about efficiency. The problem is it increases the number of components involved at least ten fold.


What are the pros and cons of a blow off valve vs recirculation. I have heard many pointless arguments on forums debating why each is better for which application but mostly it sounds like a lot of smoke from people who really don't know what they are talking about


Does your car have a maf?




swilton75 Yes, that's almost exactly what it is. You remove the internal gate's valve and place the external gate on this flange. It's not 'dumping' though as it returns to the main exhaust. It (the special flange) allows an external wastegate on a formerly internally gated turbocharger.


How can I measure if my BOV is effective enough? I still hear 'fluttering' at certain loads and throttle angles when letting off. Is any amount of flutter bad?

BOV placement: How much of a difference does it really make (ie, from next to the throttle to next to the IC, or integrated in the compressor housing)?

MCM recently did an episode where they painted a charge cooler black and noticed it dissipated a lot more heat when there's little to no crossflow. They used a hairdryer to supply the charge air. This is not nearly the same mass of air as a turbocharger produces, so I don't think the results are realistic. With something that produces the same air mass as your average turbo, the dissipating effect will be much, much less pronounced. I'm right, aren't I?


Marcus Vinicius Lots of manufacturers use recirculating BOVs. The ones that didn't used very low boost levels.


So I have a theory that i can twin turbo a 4g64 using Garrett gt1548 as using the smaller 1000 cc turbo will give each turbo its expected exhaust volumes and its output will be quick like on a motorcycle.. Port the hotsides maybe internal and external wastegates both set the external a few psi above the internals ... Help me out are these theories legit? Or is it all just babble


Ah, removing the valve makes sense. Then the fumes can flow through the housing freely and get blocked by the external wg. Seems kinda overkill


first thanks for another round of "ask the experts".
I only have basic questions that might interesst many out there.Maybe you guys at SH can Forward one or all of them.Thank you guys.

-What can you say about the mounting location of the BOV.(reasons like backpressure built up).
-Piston driven Blow Offs vs Diaphragm - which one is superior and why?
-Blow off valve vs Turbo size.How important is the size of a blow Offs outlet vs the Turbo or driven PSI?

(i know of so called "no bov-Clubs" who ditched the bov to Keep pressure built up instead of releasing it.Does this affect Turbo lifetime (there is tons of discussions on the Forums about yes or no).

thanks for your time.


o2fst4uo  ive read tons about that too.conclusion on my side was - no maf= vent to air safe the cold intake side from "heated air".


nearwood Marcus Vinicius   back in the days they often didnt use them too.i dont recall the cars.but lots had no bov.after customers complaining more and more about strange noises - the used bovs on turboed cars.
i know tons of guys who dont use any a fact there is no hard evidence on the whole web (not even papers from engeneers Floating around) saying that Turbos got shorter life out of this.
but maybe we get some answers from the guys at turbosmart?(i posted a request up there).


MidWest_Surfer VTA before MAF=okay, VTA after MAF, you will probably get a CEL. Technically you can run VTA after maf on the stock ECU, under the right circumstances with some ECU's, but in general, no.


Any advantage to running the wastegate directly off the turbine housing?


How do different layouts of multi turbo systems affect gas flow.
On a V engine for example...

-Left head exhaust feeds left head turbo, right feeds right, both feed the same intake manifold.
-Left head exhaust feeds left head turbo, right feeds right, left feeds left head intake manifold, right feeds right.
-Left head exhaust feeds left head turbo, right feeds right, left feeds RIGHT head intake manifold, right feeds LEFT.
-All cylinders feed larger single turbo.

Jaap van der Lee

Twin turbo a 4 cylinder can help a bit in improving transient response due to using exhaust pulsation better.But decreasing the turbine size also reduce efficency of the wheels which is reducing response again. Best is to use a twin scroll setup as in a EVO turbo. Which has the benefit of keeping the pulses separated in the scroll.

Jaap van der Lee

Basically recirculation heats up the air a bit, BOV not.

Jaap van der Lee

Marcus Vinicius  
1) Noise
2) Sump ventilation is often fed to the compressor in, which can then result in venting oil vapour to ambient (Which is not allowed)

Jaap van der Lee

Depends on power, if you keep it modest, you can stick with the original setup, 
and maybe bore out the wastegate hole a bit. (Keep some valve seat left) 
Otherwise go with the tubular external option. Please take in mind tubular manifold can not bare load.And your turbo needs to be fixed from elswere.


Hi could you go through boost control on a compound charge set ups? Both dual turbo and supercharger / turbo combinations.


CodyPeterson Hi Cody,
I think you hit the nail on the head in your first 7 words, but let me explain what you are seeing.  With a ball/spring boost controller, there needs to be a breather vent on the output side of the boost controller.  It also has to be the correct size.  On the cheaper boost controllers, the vent is often the wrong size, or missing entirely.  Without a vent, here is what happens:
1) Boost rises until set-point of boost controller, where the ball lifts from the seat
2) Pressure travels to wastegate actuator and opens the wastegate
3) Boost falls until ball returns to seat...but without a vent, here's where it goes wrong
4) Pressure gets trapped in the line to the actuator, preventing the wastegate from closing correctly.
--so, boost rises to set point, then drops off
5) Boost stays below set point until pressure leaks out of the line
  So, what I've found in my testing is that most "cheap" boost controllers have no vent, or it is too large or small.  It's actually not an easy thing to get right.  However, you don't have to spend $100 to get results.  The best manual boost controller I've found is a Dawes Devices for $50.  Look it up on Amazon or check out my review on
Hope this helps!


dudesedwah Dude,
  At the risk of sounding self-serving, check out my post, or just go to "The Basics", at
I wrote up a lot of basic knowledge for the guys I drag race with.  You'll find a lot of useful info on boost controllers and how the wastegate works.  But, to answer your basic question, the wastegate will open, as long as the source for the boost control reaches the set pressure point.  For example, if it was tied in at the turbo and the pressure was at your set point (let's say 14 psi), but your intercooler outlet hose blew, so you only had 2 psi in the intake, the boost control and wastegate would operate, until pressure in the whole system fell.  This would not take long, if it is a restriction free path to the engine.  But, in no case would it overboost.  The turbo could overspin, but that is a different subject.  However, if you blew the pressure line off the wastegate OR the boost controller, you could have exactly the situation you describe.  Hope this helps!


_7PM In theory, a manual boost controller has to be more reliable, since it is a simpler system.  Also, if the spring fails, the failure mode is low boost.  The electronic boost control has more things that can go wrong (wiring, solenoid, controller) but that said, the failure mode is usually with the solenoid closed, so low boost.  It really comes down to the following (IMHO)
Pros: Cost, reaction time, area under the boost curve
Cons: Best installation location is close to the wastegate, so no in-car adjustability
Pros: Adjustable from inside the car, can pick different curves
Cons: Cost, Complexity, cannot react as fast, so boost curve is softened to prevent overboost

Heatflow is outside of my professed field of expertise ;-)



J_W97 J,
   I'm going to say it is cost.  At least from the OEM point of view.  The turbo itself is pricier with a VNT, and that adds up over their volumes.  But you do see them on the small displacement engines, where there is hardly any low end torque to begin with and you can't afford to lose any (VAG 1.8T).  By the way, you can still control a VVT with a boost controller (manual is easier, have not personally tried it with an electronic system).  Look up "Nissan Patrol Boost Control" and you'll see an example.  You are usually using the boost control in a vacuum setup, but the principle is similar.



J_W97 J,
 Sorry, I should have scrolled more.  You specifically said aftermarket, so in addition to cost of the turbo +1 on the complexity and controls comments.



If you are new to turbos in general, I'd suggest checking out Corky Bell's book on turbocharging.  He has another on supercharging.  Link here, or just search the author:



Hi I just installed turbosmart dual manuel controller. It works fine on low setting set on 15 lb but when i am trying to set high setting i have turned it way higher and still cant get over 15 lb to me the outlet on tee piece looks too small like a pinhead i have taken away gatebal and spring since i want nothing to spike, i like to get 25 lb , i should say i use gtx3528 on a 2jz , my next option is changing larger tee piece but i just want someones opinion if i am doing the right thing