Blow Off Valves Mythbusted 

In the good old 480p days, MCM was putting out these kinds of videos. Oh how far they've come.

I note that they didn't actually list the reasons for a bov/bypass valve... It has nothing to do with dyno pulls. Use on a diesel would be pointless, as there is no valve to stop/restrict the airflow going into the intake ports of the motor, so there is no pressure wave to damage the turbo, or pop off the intake hoses. (Unless you are doing some ridiculous high speed load, and stopping the motor dead-in which case it won't last long anyway....) The bov/bypass valve was engineered to: 1. Prevent pressure spikes from bending/folding the throttle plate. 2. Prevent pressure spikes from back feeding the turbo compressor, acting as a brake, possibly bending fins, breaking the shaft between the turbine and compressor wheels... 3. Slow the boost pressure drop (pre-throttle), so that you have the turbo still spun up as you shift the gears in hard acceleration. A BOV vents to atmosphere, this creates a momentary rich condition (you can get flames out the tip of the exhaust depending on setup), and requires almost no plumbing. As stated, this can also trigger computer diagnostics, and safety measures, depending on the complexity of the engine computer. This momentary rich condition can mimic that which you get with a carburetor. A Bypass valve, vents to the intake track pre boost device (turbo/supercharger/blower), but post air measurement (if so equipped). This allows the computer to have a better idea of how much air is in the system. And can allow a little (very minor) help in spinning a turbo. Also being that the air escape is part of the closed system, if there is a failure, and it sticks open, or the valve sealing fails, you don't have to worry about dirt and debris entering the intake system, or so much that unmetered air is entering exiting the system. Who should consider them? Those with motors than run boost and have throttle plates. What does it do? Decrease turbo lag, protect against damage from pressure spikes in boost motors with throttle plates. This comment requires no replies. It was likely stated earlier, but frankly I'm not wading through over three thousand comments, given the vitreal and spite I see in the first part of the comment list. This comment is not done in spite, but in education, for those that were hoping for facts.

HAHA that was funny. When I saw them put the battery in the backpack I thought we were going to see a running man with a BOV. lol

why would you not want that stutututut sound?

Damn, i'd barely have been old enough to know what a turbo is when this was released and now it's been recommended by youtube so that I can enjoy it

My horses have blow off valves, but I am perty sure none of them is fitted with a turbo ;) .

Still here and still enjoy the old skool videos like this 😄

me on recommended: youtube recommending me a 10 year old video:seems right

Ok honestly the turbo flutter sound you get with no bov is one of the best automotive sounds out there let's be honest

That's what I was going to say. And actually, depending on your turbo setup, the type of BOV that you're using will actually make a difference. A recirculating blow off valve will help with surge between shifts but also allow you to keep your boost pressure up higher since it puts it back into the system. This surge that occurs when a BOV is not in place can cause a fuel enrichment issue and cause your vehicle to stall, but this is not always the case as it depends on the size of the turbo.

I found this channel from their brz build. Bought a GLI a few days ago, was considering switching to a BOV, and stumbled upon this gem. It's amazing how much some things have changed and how some are exactly the same.

...Love it...I'm going to put a BOV on my wife's vacuum cleaner..

why the dyno with the blow-off valves? the guy just said they only matter when you let OFF the throttle. he said the valves are only ment to maintain spinup between shifts

Perhaps the only MCM episode where they rice a car...

Moog: "let's forget it ever happened" Yet eleven years later, Moog reminded us about this nostalgic blow off valve video. Awesome.

The first part was informative. Thanks for that. The second part about running on a non turbocharged was sill, but I suppose that was the point.

I have 5 BOVs on my non-turbo car. I picked up at least 100hp from each one. :-P

you guys had tears coming out my eyes of laughter when you modified the bicycle.lol I'm subscribing.good job.

I may be 8 years late...but this is an excellent and creative video. BOV on a bike?!! A+ for originality.

I just traded my 2011 Ford Taurus SHO for a 2015 with 10 * more bells and whistles. I was looking at getting a new one since they are discontinued which happened in the past but then they were brought back. Jesus with a credit score of around 780 I was looking at just over $700 a month. I make $29.96 an hour so the money is pretty good but no way could I make a $700 a month car payment LOL. I just really love the sho's it's a fast car but does not raise your insurance. I really wanted to swap out the factory valves with vent to atmosphere unfortunately that would really lower the performance. I wish I could find the ones I was looking at as they recirculated but managed to vent to atmosphere without impacting performance. Great video as usual!

These dudes should work for Top Gear

recircing the BOV is very important if you are running a MAF before the turbo instead of a MAP in the manifold because if you release the air to atmosphere when running a MAF, then you are completely ruining your A/F ratio as you just vented air that the engine is expecting and dumping fuel for. Can have a big effect on 1/4 mile times as your A/F gets messed up for a second at each shift. can make for a nice flame thrower exhaust, though.

Recirc valves are very important on some cars, specifically ones that use the MAF to determine how much air is in the intake system and adjust fueling proportionally. If the blow off valve lets the air out, but the fuel still gets dumped in, you can run extremely rich, which will then cause the short term fuel trim to lean out. Then you get back on the throttle, run lean, blow your motor, and blame it on the tune.

Hey great video you guys are doing a good job keep up the good work my question is do you have to rebuild your motor to install a turbo or will it hold the extra pressure on a Mazda Protege5 2.0 liter

Haha, oh man. I'm here from the most recent video of making the BRZ fast... and oh man have they come far 😂

Holy shit, I'm gunna fit one to my wood chipper for sure! Coolest chipper on the block ;-)

I think it is impresive that mcm is the only channel that have not improved production quality since 2009

Nice explanation and good explanation on why its good to NOT let all the pressure out and not starting from empty ,,Lads need to know its NOT all about noise when it comes to performance : )

dat engine movement doe....

I thought the benefit of a recirculation valve over a bov was seen by slightly quicker spool up AFTER the gear change?

I forgot about this episode.. have me in stitches every time.

this info is awesome and completely relevant to my vehicle, i just got an audi tt and it shares the same motor as the gti.

The reason I want a turbocharged car is FOR that 'stutututu' sound. Why would I want to get rid of it?

Damn near shit myself laughing at the end XD

The sound of a blow off valve in a Subaru sounds amazing plus it saves your turbo for a bit longer which is always nice

Great video. I expect the Golf is going to be throwing fault codes though with an atmospheric dump valve. The ECU expects the air to be returned to the inlet like the stanrdard re-circ.

is it this guy who worked on waaaSaabi?

With a circulated blow off valve, wouldn't you get up to psi faster? Since all the air is being kept in a closed loop system?

That was awesome. When I saw you on the bike I could not contain my laughter

Well gotta support a great Aussie Production!

bom dia sou do BRASIL procuro a muito tempo um barulho de carro turbo para meu carro meu carro nao e turbo gostei muito desta forma que vcs fes este carro nao e turbo nao ne ??? mas um escrito curti pra caramba esta para vlw. ..

"...and forget that it ever happened." LMAO!!!

almost 1 mil subs. Congratz

You guys crack me up! Only just started watching your work, great stuff! If you ever need an English guy with a corsa vxr let me know :-)

Any turbocharging system has a wastegate which relieves the excess pressure. Just because they don't sound cool doesn't mean they are not there.

blow off valves arent going to give you more power if anything theyre just going to improve your shift quality and/or protect your manifold from high pressure

This is the very first episode of MCM I watched 3 years ago.

Very informative. Thank you very much!

*****, that's pretty weird that you have "triple valve springs" in a car with no valves! The RX-7 has a 13B Wankel Rotary engine, so I was just wondering where you managed to install your valve springs at!

Pay more for insurance? I pay less for my 2015 WRX than I did with my Ram 2500 HEMI or even my 2013 Tacoma. I believe i was about $20 less than the Ram but only $6 less than the Tacoma.

i love your guys work. can you do a video on targa tops?

lmfao when he pulled out the bike

105kw =140bhp. The 1.8 turbo (im guessing engine code AGU judging by the cable throttle) sould be running 150 bhp even without a tune. Something isn't quite right there

Everything sounds better with an Australian accent.

Man at 1:42, he really nailed the sound of turbo flutter. Hard to describe without making the noise, lol.

Woke up this morning wondering if my BOV would still work installed on my air intake, essentially releasing the intake pressure when you let off haha, video confirmed it can be done!

The end is hilarious. xD

The reason I have a recirculating BOV on my Subaru is to help maintain boost between shifts, because of my larger aftermarket turbo. I think atmospheric BOVs are stupid and solely for an annoying high pitched purge sound. Obviously it's to blow off the massive surge pressure from the turbo, but be smart and use that surge pressure to hold boost with a recirculating bov

I was surprised at how much the GTI engine rocked while going though the gears. Maybe a test or two on different engine mounts? On Mark I Rabbits/Jettas, the hot thing was to install a poly front motor mount. I even did it on a diesel. On Ford's Taurus (1999-2007) using a short length of 3/8" link chain tied and bolted around the mount and to the subframe kept the engine from rocking.

still one of the most satisfying sounds ever

I can't believe all the people who want the turbo flutter sound. To me, it's right up there with knocking rods, piston slap, and maladjusted valve lash. It's the sound of failing parts.

Australia is such pain in the ass, for car people.

Awesome job guys. Great way to answer keyboard slappers.

That bov on moogs bike is just to funny! lol These guys are hillarious!

Dude lookin like Hugh Grant's younger brother, lol :D

err... i thought blow off value is to help with the response when u on/off/on gas again....dyno is the wrong way to test it isn't it? ... >___>

very informative thanks Gentleman for the video... Could you answer a question please.The video talks od the sound do,,do,do,do,do with air passing back though the turbo .This is a pair of finger eaters , eaters the most insane sound ...Would this set up damage the turbo..?

I love your comedy show. I especially like the challenged of translating Aussie into the Queen's English.

They should make one for my underwear.. So when i am walking down the isle at the store and i Fart it will go pssssssss tututututu..

What i hate is when some people put those damn whistles on their blow off valve, sound silly as hell.

omg i havent laughed that hard from a youtube video EVER! Great video I cant believe I just saw it now!

I once had a 2001 Ford Focus with the 2.0 ztec. I put on a AEM true cold air intake and when I shifted i had enough pressure in the intake that if I was on the gas and let off into the next gear it would make this noise. Would imagine with enough time invested and electronics you could make something that hooked onto the intake that would make it sound like a real blow off valve but it wouldn't be as loud.

I replaced my blow off valve with a modified egg but it broke

hold up. all a blow off valve does is get rid of boost, which helps not put any stress on the motor after you get on it and build up boost with a turbo, im not sure they come on super chargers. on a pro stock pulling tractor, they use them to get rid of the boost thats 40+ lbs. it doesnt give you anymore power in a car. just makes tuners sound "cool"

its all about the sound dude!

You don't lose performance with a BOV, but you do increase your lag between shifts, as compared to a BPV which recirculates the air back into the intake tract and helps keep the turbo spooled.

9:10 LOL, Pushbike? Aussie for bicycle?

If You try to put a BOV on a non turbo car because you want the sound, you're a poser.

You guys are AWESOME!

This is the greatest video on the Internet. Period.

9:06 White S15?

SO FUNNY THESE GUYS.....MYTH BUSTERS FOR CARS

QUALITY IS INSANE

Whether you should vent or recirculate depends on the design of the intake. If the incoming air is measured by the airflow sensor BEFORE it goes through the turbo (like if the airflow sensor is in the air filter box), then you must use a recirculator valve, or else the ECU will be momentarily confused by air that was measured by the airflow sensor but never delivered into the engine, and that will throw off the fuel:air ratio and change the long-term fuel trim in the ECU. You can't just pick and choose which version you want to run, unless you feel like redesigning the entire intake.

Recirculated BOVs don't add power, therefore not making your car faster... Just like how lowering your car doesn't add power. Come on, there's more to speed than just power. Recirculation holds the boost, keeping the compressor ready for the next shift.

i like it when they were ripping up the blow off valve machine! to many ricers got excited!

LOL, that bov in the bike was funny. I always wanted to put a duck caller on my eclipse gsx bypass valve but sadly I never did.

Hey guys! Awesome videos! Very helpful! Can you guys make a video on upgrading cams to a stage 2 and like what should be upgraded along with putting in new cams. I know you need to upgrade the valve springs so you wont lose power.

I wish they would set up mandatory car classes in schools, so every single person who drives a car would have a basic understanding of them. It would make it a lot less painful to talk to people about the subject.

too funny!

What do you reckon the response would be from a correctly turbo'd car where the Air intake is fed from a tank of 100% grade pure medical oxygen, maybe through a K&N arrangement inside a customised air box etc so to eliminate any stray particle's etc?

thanks for making this video just about the engine i was wandering. its the same as my mk2 2.0t tt right?

But Subaru NEED recirculating ones or else they run rich.

The big question now is... Will it fit my honda?

Question: Since the vacuum hose to the bypass (BOV) valve goes to the intake manifold, wouldn't that line also be under boost pressure when the car is under boost, and therefore, serve to assist in keeping the valve closed by providing the same pressure on both sides of the valve, essentially cancelling any boost pressure trying to push the valve open? In other words, it should make no difference how much boost the car is making since whether it's 3 psi or 20 psi at the intake, that same amount is being fed to the BOV piston or diaphragm, therefore equalizing and cancelling the 3 psi or 20 psi that is at the intake and other side of the piston or diaphragm in the BOV.

Amazing. I hope others in 2020 followed the link and watched this vid.

Will this blow off valve fit on my honda? (Mighty car mods joke)

blow off valves don't increase power they release boost when off the throttle letting the turbine free spin and not let boost push the turbine backwards

Great video.

The blow off valve has two purposes. The primary purpose is to prevent turbo surging where high pressure air is forced backward up through the inter-coolers and back through the turbo. This surge can damage the turbo bearings; both shaft bearings and end bearings. The second purpose is to prevent over-pressures that can damage boost feed pipes and inter-coolers.