Everyone keeps telling me to get a blow-off-valve. But nobody has really told me why. I am still confused as to what its function is. From what I understand is when the blower has already made boost and sent it to the throttle body, but you have let off the gas peddle so now the TB is closed, the BOV releases the boost that otherwise has nowhere to go. Is this accurate? But why? What is it for? Is it for performance? Is it for wear and tear? Stillen's own catalogue, under the SC section, recommends getting one because it "increases driveability". What the heck does that mean?! I asked my local tuner shop and he said a BOV can't hurt but it really plays no roll on an SCed car, only a turbo. He says that SC guys buy it mostly for looks/sound. So, needless to say, I am thouroughly confused. Let me sum up my questions below and any and all answers will be much appreciated. Thanks in advance:

What is the actual function (in English/laymens terms please) of a BOV?
Why get one? (ex. performance, wear and tear, other?)
Which one should I get? (Blitz, HKS, etc.)
Are there any negatives? (ex. Ambient Air Sensor and/or MAS have already measured the passing boost and now it isn't there).
How loud is it?


Thanks guys.

 

hmm,

When your boosting you are forcing air into your engine, when you take your foot off the gas, the throttle plate closes, that pressure backs up against the turbo or supercharger compresser blades. Over time this can damage your turbo/sc by causing undue stress, heat, and wear. Putting a BOV post-compressor, usually on the upper intercooler piping right before the TB releases this excess pressure. That is the primary function of a BOV. a nice side effect however, is that by giving the air a place to go, you can also keep the supercharger spooled longer between gears. This is also why you don't hear the BOV much on an automatic, because you're not letting off the gas every shift.

Feel free to correct me if i'm wrong...

Anyway, I reccomend the HKS over the blitz it sounds much better, I've heard the blitz b4 and it just makes a hissing sound, and I've heard the HKS on BrianV's car it makes a really really nice PSSSSSHHHHFT sound.

I can't really think of any negatives, on an N/A car, the car doesn't register any air flow when the throttle plate is closed anyway. Only thing might be if it gets stuck open or the valve gets busted, you'll start leaking boost. not a common occurance i don't think

 

This guy is right! The BOV helps to eliminate backpressure when you shift. This does two things actually :

1. prevents back pressure which can reduce life of turbo or S/C
2. keeps the compressor blades up to speed helping performance.

One negative I have found (at least on the turbo) is that when you rev high and come off the gas pedal immediately, the engine stalls sometimes. It depends on where you put the BOV I guess. If before the MAF, you may not see this problem if after the MAF, you will probably see this problem.

Oh I have the HKS SSBOV. It's supposed to sound the best! PSHHHT!

Nigel

 

Follow-up questions:

Why isn't the stock Bosch that comes with the SC sufficient?
Does an automatic still need a BOV?
Do the aftermarket BOVs expell air into the atmosphere or back to the air filter?
Why did my shop say only turbos need BOVs?
How loud are these things? Can I hear it from the cabin?

 

Hey Josh

1. Don't know answer to the first Q (Bosch)
2. You only really need a BOV if the TB opens and closes frequently as you accelerate. In an auto, I don't think it is as big a problem as on a 5sp.
3. The HKS SSBOV blows off into the atmosphere, hence the cool PSHHT!
4. Yeah, I only really know about turbos, not sure about S/C. My opinion is that you probably don't need it, it sounds cool! In principle any forced induction engine should have this or a bypass valve.
5. You can hear it from the cabin if you under higher boost (at least on the turbo application)

Sorry, I only know about turbo app. I didn't see anyone I know with a S/C reply so I replied.

Nigel

 

Didn't I told you about the BOV already? You should have all the info you need.


Anyway, in automatic you will only get the PSSSHHHTTT sound when you left off. However, the sound isn't very noticeable. You can adjust the screw to get more sound, but you might also create an air leak. You do need a BOV for the SC regardless. I don't know the exact science to this, but after having talked to numerous well respected members on the board...yeah, get the BOV. IMO, get the Blitz SS BOV because it bolts right on. The other BOVs you might need to do a little bit of custom work for it to fit. I know with the HKS SS BOV you need to weld it on. Hope this helps.

 

Yup! Get one that bolts on. Mine was a real pain to install even after welding the flange, the BOV was a royal pain in the ... to install. But it does sound cool!

 

1. It is but its not very pretty to look at
2. Only if you have the SC kit. Mine is an automagic.
3. With the Bosch unit it recycles it back to the air filter. Aftermarkets expell to the atmosphere.
4. Not true.
5. Only if you let off the gas pedal after stomping on it.

 

the car runs rich for a split second, which can cause stalling problems (mainly at higher speeds). THe MAF sensor reads XXX amount of air flowing in so it tells the injectors to put in XXX amount of fuel. Since the BOV is dumping out XXX amount of air, the air/fuel mixture is not what it is suppose to be (more fuel than air), thus, the rich mixture. The A'PEXi SAFC can be used to tuned the vehicle to make the engine run leaner when you let off the throttle (deceleration) to cure the stalling problems.

the Bosch BOV is cheap and inexpensive. Aftermarket ones have stiffer springs and can release the pressure quicker and more efficient.

 

Phuong, you are correct myfriend.

To answer some of the questions above..

The BOV is suppose to hold air in the system and release it when the TB stops it, so the sudden stop doesn't damage the fins or other pieces on the SC/Turbo.. Some High HP cars require 2 or 3 BOVs b/c they are pushing sooo much air, that if it was not let of the system, it will stop the turbo wheel dead in its track causing serious damage. The Super charger is a little different b/c its belt driven, not Exhaust like a Turbo.


The Bosch BOV may be good enough if you are auto, and running low boost, 9psi or Below.

Automatic cars have a faster throttle response then 5spds, but A BOV is still needed if you are forceing a lot of Air through the induction.

Most After market BOVs blow in the air, I have a HKS SSQ and that has a $15 adapter that with the pipeing it will release back into the intake system.

Your shop said Turbos only b/c they are exhaust driven, Supre charger are belt driven. Less likely to stop a fan thats driven by belt then 1 driven by Air..

The higher the boost level the louder the BOV... I am running about 11psi ( on my supra ) and the sound is just awsome.. the HKS SSQ BOV is a Duel Valve Release desigh. One of the best in the market. Also its a Pull design, not a Push like most out there. Push design actually leak air at high boost.. The Adjustable screw on the back controls the first valve which gives you the initial Sound, like a quick wistle.. So Mine sounds more like Cheeeeuuuuuuu- Pussshhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh . B/c of its duel Valves, you get 2 sounds.. Its cool as hell. also you can get inserts for the BOV to make it even louder or quieter if you want. About $15 a insert....

 

Thanks guys. You have been very helpful in answering my q.s. I really appreciate that. This is what I am gathering. They are most important for turbos, high boost, and stick shifts. I have a supercharger, low boost, and auto. So I am thinking that there is no rush to get one. I guess I'll put it on my purchase-list (huge as it is) and I'll get to it eventually. Thanks again guys.

-Josh

 

I dont think the HKS SS is a choice on the SCs its usually down between the Greddy and the Blitz.. Greddy sounds better... but also with the SC the BOV stays open when ur off thethrottle it kinda sounds odd

 

i have always wonderd if the BOV method will work on other supercharged cars such as a s/c solara if you weld the BOV into the intake piping?????he asked me about it once when he saw a bov on a s/c maxima and i told him i wasnt sure..so i m ight as well turn to the org for answers.....again...

Jon