What are the pro's and con's of venting the blow-off valve into the atmosphere vs. exhaust (post-turbo) vs. routing it back into the header (pre-turbo)?

 

If you route it to the header/turbo and add fuel/spark, you will keep your turbo spooled up on turns/decel. You'll need a check valve so that exhaust won't be sucked into the engine at idle/part throttle cruise.

Easiest would be to the atmosphere via an air filter.

 

Obviously you don't care about emissions, but if you do, it is illegal to have it vent to the atmosphere.

Jon

 

82resto - Please tell me how does venting compressed air affect emissions or make emissions control illegal. A BOV is not even an emission control device.



BTW Monty, GREAT looking setup to follow.

 

Hey,
I wasn't trying to flame or anything. I did read on a turbo retailer's web site ( i cannot remember which one) that having a blow off valve release into the atmosphere as opposed to back into the exhaust, was not legal on street driven vehicles. I agree with you that that makes no sense, because it would just be hot, compressed air, would it not? But, I don't believe that too many emissions laws make sense. If I can find the web site, I will post it on here.

Jon

 

I wasn't flaming anything, I just questioned your facts.


BTW, most emissions laws do make sense.

 

i think you have BOV vavles and wastegates valves confused

a wastegate bleeds off exhaust gasses to control boost output

if you vent a WASTEGATE into the atmosphere allowing exhaust gasses into the air, yes that would effect with emission regulations but with a BOV that just releases compressed air...that wouldn't do anything

 

No, I don't care about emissions...I don't think it would be possible to get a 1200-1300hp car to pass even if I tried.

I am using a blow-off valve on the side of the intercooler end tank as an emergency boost controller in case one or both of my external wastegates fail. Additionally, I am incorporating it to minimize boost spikes ( and subsequent damage to the compressor wheel and throttle body) during sudden throttle closures and between shifts.

Obviously, most custom turbo setups that utilize a BOV just vent it to the atmosophere. My question is what are the pro's and cons of venting it directly to the atmosphere, vs redirecting it back into the header, pre-turbo, or to the exhaust.

I know alot of street guys like to just vent the BOV to the atmoshere because they like the sound that it makes. However, is there any performance benefit of recirculating it through the turbos - i.e. help maintain boost and spool.


73camaro,

Why would I need a filter or check valve. The blow-off valve is closed until the boost pressure exceeds it's spring pressure, at which point compressed air is being forced out of the BOV. Once the boost pressure falls below the BOV's spring pressure it recloses. There should never be a point where air is moving from atmosphere into the BOV.

 

Whoops, you guys were right, I had the two mixed up...

 

monty,

Why would your throttle blades close between shifts ? Still 4L80E auto-trans correct ? IMO Routing it to exhaust could give your wideband 02 a false lean. Routing it to inlet of the turbo is where it goes anyway if you don't have a BOV. With your setup I doubt dumping a spirt of air from initial boost spike is gonna matter much with the 1800 (or so) cfm already going in. I have never seen it done, and if there was any performance to be gained, someone for sure would have done it by now. Poineers always welcome.

 

Thats a good question!! I've never thought about redirecting it back into the header, pre-turbo, or to the exhaust ... I'm not sure if there would be any type of performance increase .. in fact it might be dangereous. I was under the impression the BOV was there to release pressure and prevent the turbo from spinning too fast ... not route pressure back into the engine. From every turbo setup i've ever seen or read about it is vented into the atmosphere. I think thats just the way it should be but I'm not sure.

 

The only reason I would not vent it to the atmosphere is if you're running MAF and the BOV was after the MAF measured the air already. This would cause a rich spike since the MAF already measured the air the BOV released. If you've got a MAP system or the BOV is before the MAF, just let it all vent. KISS - a direct vent will have less piping to get in the way and add weight, FWIW.

Andris

PS - Got the main hoop and halo bent for the roll cage last weekend! It was nice to sit in the Sparco while fitting the hoops. Make some car noises, and pretend I have a running thirdgen!

 

In your statement, you are saying that the BOV is always open when there is boost, regardless of manifold pressure/vacuum.

From the BOV's I have seen, when manifold vacuum is applied to them, they open up, regardless of boost pressure. So when your at idle or part throttle cruise, there is vacuum in the manifold which opens the BOV.

I've only dealt with the stock BOV's and everyone of those are vented into the airbox after the air filter.

 

I never said the BOV is ALWAYS open when there is boost, regardless of manifold pressure/vacuum. That's a flat-out fabrication on your part.

I said "The blow-off valve is closed until the boost pressure exceeds it's spring pressure, at which point compressed air is being forced out of the BOV. Once the boost pressure falls below the BOV's spring pressure it recloses. There should never be a point where air is moving from atmosphere into the BOV. "

If the BOV is set to open at 20 psi, it will stay closed until 20 psi is reached, at which point the boost pressure of 20psi overcomes the spring tension in the BOV.

Obviously, this is overly simplified since we know that, like a non-electronically controlled wastegate, the valve actually begins to open slightly before the maximum boost pressure due to the cracking pressure.

 

What you're talking about is a pressure relief valve similar to the ones used on CART previously to control overboosting. A BOV blows the air that is between the compressor and the throttle body back into the compressor inlet so the compressor does not build a pressure head on the intake throttle. With no BOV the compressor/turbine surges badly and wears on the bearings. No BOV also cause turbo lag between shift IF you lift. WRC's have fuel injectors in the exhaust man. that fire when the trottle is closed to keep up turbine speed and thus no lag. It is ILLEGAL to vent any gas into the atmosphere(besides exhaust which is regulated). That is why we have PCV and evap. emissions canisters. It is also very loud to vent compressed air into the atmosphere. This means absolutely D!CK in this application. Vent whatever you want where ever you want.

TT SBCs rock.

 

ATO, I guess I wan't clear in my description, because now after re-reading it, I see where you got the impression I was talking about the pressure-relief valves (Pop-off or Dump valves) that they used to use in CART/F1. The one's you commonly see mounted at the top of the intake plenum.

When I was referring to the BOV, I failed to mention the fact that they are vacuum referenced, in addition to their internal spring.

 

Fabrication on my part, not really, more of a misinterpretation of you incomplete message and facts. And you also later mention that the BOV is vacuum referenced. Now if engine vacuum exceeds the spring pressure, then the BOV will open.

It appears that both engine vacuum and boost pressure is used to unseat the BOV. I have only worked with factory BOV's and engine vacuum will unseat them. That is why ALL factory BOV's run the output to filtered air.

Aftermarket BOV's may behave differently.

BOTTOM LINE. If engine vacuum unseat the BOV, the output needs filter air because the engine will be sucking in dirty air under idle and part throttle conditions.

 

73camaro,

I'm not trying to be argumentative.


On these OEM applications you're referring to - you're saying that the BOV's discharge is run to filtered air. Are they running it back to the intake side through the air filter then back through the engine, or some kind of secondary filter then out into the atmosphere. If it the first way, through the air filter and engine, they are most likely doing that for emissions and noise reasons, even though the BOV's is simply discharging compressed air.

The vacuum reference on the BOV is similar in function to the vacuum reference on a EFI fuel pressure regulator. Like a fuel pressure regulator, it contains an internal spring as it's primary measure of providing resistance against the boost pressure, but it does use manifold boost/vacuum as a reference. However, under vacuum, the BOV is closed, no air passes through the BOV from the atmoshere into the BOV. It's a one way valve only, from the pressurized intake out to atmosphere, or wherever you chose to discharge it.

That was my original inguiry that started this thread. Emissions and noise requirements/concerns aside, are there any benefits to routing the BOV discharge back to the headers and through the turbo. I am in the finishing stages of compelting my twin turbo setup, and I was wondering if anyone has experimented with this. The only benefit I can think of to routing it back through the turbo is that it may make a small improvement in either spooling the turbo or maintaing it's current spool.

The only reason I can think of to route the discharge of the BOV to the primary exhaust system is for noise muffling reasons, such as when the intercooler is located in the passemger area, as in air/water applicaitons in NMCA/NSCA and NMRA race cars.

I know most people simply discharge it into the atmosphere, both becuase it's simple and because they like the sound.

 

I am not try to be argumentative either, but we have a difference of opinion.

R U talking about an aftermarket BOV?

Here is how I understand how a BOV works (picture format).



With boost in the turbo piping and intake manifold (WOT) pressure is about the same on the BOV's seat (red) and the spring's chamber (blue). So the spring is holding the BOV closed.

With boost in the turbo piping and intake manifold in vacuum (throttle closed) boost in the piping pushes against the seat (red). Vacuum will compress the spring chamber. Both actions open the BOV

Again, on the factory ones, (and I have seen this with my own eyes) if you apply a vacuum to the spring chamber, it will pull away the seat and open the BOV, regardless of pressure in the turbo piping.


The factory BOV's run their outlet to the "after filter" side of the airbox, or downstream from the filter.



As for the pressure regulator on the EFI system, that is in place to give a constant pressure drop between the fuel injector inlet and outlet.

 

I like your drawing .

Yes, I'm talking about aftermarket BOV's, such as those offered by Vortehc, HKS and the like. I'm sure their function and design is the same as OEM's, but their final installation is different since most custom applications aren't as concerned with emissions or noise abatement, but rather total overall performance.

It is obvious we are talking about the same thing, I think the only difference is in our experiences. My experience is limited to aftermarket applications only that are designed for high pressure/high horsepower. In these cases, the BOV's internal springs present enough tension that engine vacuum/pressure on the reference line would not be enough to open the valve by itself.

 

Ok let me see after reading all of this if I can make any sense about it.

Operation: The BOV has many types of configurations available and the japanese have used them by many different names on their vehicles. Most common application is a 300ZX in the US, Because this is a MAF system the BOV is called a recirculation valve. The boosted air is recirculated to the front of the turbo because the 300zx uses a MAF system. The air is redirected because the ECU has already measured the fuel to the amount of air that has passed through the MAF. If this air was to be released to atmospheric pressure the engine would go to a rich condition and causes stalling problems.

The spring pressure inside the BOV is able to keep the seat sealed on a BOV during normal operation or boost by the vaccuum line on the unit itself. During boost the line is boost pressure of the manifold and allows the diaphram to help hold the spring pressure on the BOV closed. When the throttle is released vaccuum is present at higher than idle readings (using 18 inches of vaccuum as a reference for idle, off throttle would be close to 20-25 inches) This momentary high vaccuum condition pulls the piston or diaphram off its seat and releases the pressure. In large boost applications (20-40 PSI) a standard BOV would not be able to hold the pressure because the internal springs on the valve itself cannot hold the pressure against it. For an application as large as you are talking about (1000+ hp) I would highly reccemend a GReddy Type R BOV, this unit has a piston close to 2 inches wide and releases a lot of pressure quickly, is rebuildable, and has a variety of springs for different pressures. They retail for close to $250.00 but can be found on E-bay for a little less.

TC

 

Monty, I have the Vortech Mondo BOV, and it comes from Vortech set with a "neutural" balance. Vortech recommends that you do not adjust the spring pressure. It is stated by Vortech, that with equal pressure on the manifold vacuum sorce, and the "valve", it will remain closed. When there is vacuum on the vac reference port, (like at decel, and light load) The valve will be in an open state. a73camaro, your picture and explaination of a BOV is perfect, and depicts exactly how an aftermarket BOV functions. Good job! BTW. I have been venting my BOV into the atmosphere for a while now, with no problems, but I too run the F.A.S.T, and have no mass air.

 

Thanks for the complements. To burn the cliche - sometimes a picture is worth a thousand words.

And sometimes a word is worth a thousand pictures

Gina Gershon

:hail: