WHY does BOV "have" to go back into intake?

[Guest jjohart]

Hi, my local mechanic recommended-without knowledge base- that I remove the hose from my blow off valve on the 83 280ZXT that has repeatedly popped off the connections to the intake, where they simply drilled a hole in the side for it to connect-a real PIA.

Someone already "suggested" that the BOV isn't really to go straight to atmospheric/disconnect in this manner, but they neglected to say exactly "WHY NOT"!!

Can anyone qualify why it is better to link the bov output back into the turbo/intake? I have no idea...seems to me sheer conjecture, though I've heard the output hose will sound really loud and perhaps crude/a bit ragged!

Thanks

83 280ZXT 5 spd

[Brad-ManQ45]

This has been covered many times before, but:

 

It should be routed back before the turbo and after the AFM (or in a mass air system the mass air unit) because the air measuring device in the system has already measured the air.

 

If you just vent it to atmosphere,when you decelerate quickly after boosting, the engine will run very rich and if you have to get on the gas again quickly you lay down a lot of smoke and don't have as quick a response because the A/F ratio is off.

 

This wastes gas, pollutes the air, lowers the responsiveness of the engine and sounds rice.

 

The bling blings that do this are all drain bamaged (get the double entendre?).

[fl327]

Depends on how the BOV is supposed to be setup I would believe.

BOVs that are setup to be recirculated often work to bypass air to the turbo while not boosting as well, and when left open cause a huge air leak and afm and maf cars dont run very well when a air leak that big is present. Aftermarket atmosphere dumping BOVs are setup to do this and should not leak except to vent boost pressure into the air. I ran an HKS ssqv open atmosphere BOV on my turbo setup and it was fine, had no stalling issues, but it would surge a little, I like the HKS unit the best due to it being a pull type diaphram vs. push type, it does NOT vent at idle or cruise whatsoever, which can be an issue sometimes with older blitz and greddy bovs.

[Forrest]

In regards to AFM/MAF, this question has been answered in detail in several topics. Basically, you definitely need to recirculate.

 

However, I have a MAP sensor, not MAF, but if there is a benefit to recirculating I will continue to do that. The Greddy Type S is pretty loud even when recirculated, as it is now. I have heard that it stabilizes pressure. Does anyone have a link to any technical articles on this subject?

[Brad-ManQ45]

This poses an interesting question in fluid dynamics:

 

On one hand you have high speed air being introduced before the turbo, keeping it spinning (faster) if you recirculate. When you do open the throttle to build boost again, air will need to START being drawn through the filter again (if in fact there hasn't been any pushed OUT the aircleaner (or MAS) - I doubt it would be possible for this to happen on an AFM).

 

On the other, you are constantly drawing air through the filter and this pumping (or drawing) slows down the turbo more, but the air is constantly being drawn through the filter. In this scenario, you have a column of air already moving throughout the intake tract before the turbo when you get back on the gas.

 

I'm willing to bet on the recirculating method being more conducive to quicker spool-up, the high speed air and higher compressor speed more than making up for any inertial difference in getting the air moving through the filter again.

 

What's everyone else's take on this?

[Tony D]

remember the stock NISSAN designed BOV for the AFM equipped turbo cars was to dump air into the INTAKE MANIFOLD at the #4 intake runner utilizing a differential pressure check valve.

This works well in that the air is behind the Throttle Body in the Intake Plenum, letting the turbo simply do a slow spindown between shifts or lift-throttle as opposed to a surge or rapid deceleration caused by rising pressure in the intake when not BOV equipped.

 

Forrest, I have my BOV (the stock 280ZXT Piece) dumping straight into the plenum, but with the non-stock BOV (like yours) designed to discharge to atmosphere you are best to discharge it at the turbine wheel in a manner set to pre-spool.

 

Adding a glasspack to the intake tract will muffle both blowoff and intake whine on turbocharger equipped engines. The Japanese would use VW mufflers, motorcycle mufflers, etc to muffle blowoff noises. There was a time in the beginning when "rice" was quiet and efficient.... But that was almost 20-25 years ago now...

 

The archives should have the description on my older setup on the carburetted blow through setup I had before, which was a mandrel bent 3/4" tube with a baloney cut end discharging at the turbine wheel. It was put through my intake piping positioned and welded. It looked very similar to the old 930 Turbo setup if that helps any. Burns Stainless rocks....

 

On the air flow backwards through the aircleaner: on industrial stationary turbines, this is an integral part of the design used to PROLONG FILTER LIFE as the "reverse pulse" usually is designed to blow light dust and swarf off the inlet filter allowing the precleaners to live longer in high dust environments. The pumping loss on reinitializing airflow through the filter is only a factor if you take a long time to get back on the throttle. And in that case it it wouldn't matter. But if you are back on the throttle quickly, the pressure rise in the intake ducting in front of the turbo from the blowoff just dumping statically into the intake ducting (not to prespool the turbo, just 'dumping') may indeed HELP respool pressurization due to a higher "ambient" pressure that may exist at that time.

 

It's easier to pump from 15psi across a turbine with a 3:1 comprssion ratio when starting at say +1 or 2" H2O pressure from blowoff than it would be to the -1 to 2" H2O existing during normal draw. Lower discharge temp would exist for a fraction of a section to be sure. Ideally you wouldn't have any more than -4" H2O during WOT at full load, full boost, full rpm...

[Guest Cronic]

I vent mine to the atmosphere... don't really have any issues with it. It's not "rice" It's however you want to do it. My Z has no problems with it, however some cars, DSM's for example, require this or they will stumble on their faces.

 

A Z however, will not.

[violacleff]

Cronic what setup are u running engine management-wise and what type bov are you using?

[DAW]

I think that when using a MAS or AFM you want to use/acount for all of the metered air. Blowing it out of the system makes it more difficult for consistant overall fuel metering. This is my guess, plus it seems that during a blow-off condition, rerouting blow-off it is the most efficient way to equilibrate the pressure differential across the intake turbine, thus optimize spooling and preserving the momentum so that it's there for the next throttle opening. Why throw out your turbocharging energy instead of using it? DAW

[Guest bastaad525]

I vent mine to the atmosphere... don't really have any issues with it. It's not "rice" It's however you want to do it. My Z has no problems with it' date=' however some cars, DSM's for example, require this or they will stumble on their faces.

 

A Z however, will not.[/quote']

 

 

I dunno... my car stumbles really badly if I vent my 1st gen eclipse BOV. I think there are aftermarket adjustable units that can be set to greatly minimize it, but I doubt you could 100% get rid of it. Also, my car didn't just stumble between shifts, rather, it would run badly almost any time I wasn't accelerating... at cruise or decel, the vacuum in the intake manifold would hold the the BOV open, sucking unfiltered air in and causing a lean condition while cruising... suddenly to a rich condition between shifts. Seems abusive to me

 

By the way, stock dsm BOV's aren't known for their being super loud, and yet, even with mine recirculated you can definately hear the "psssssssssh" sound between shifts when I'm getting on it. That's from inside the car, I can only imagine to other people around me that it's just that much more audible.

[Guest 2slo4u]

Would the extra pressure of blow off air going into the intake tube between the maf and turbine wheel cause the air flap to be pushed backwards for a short period causing a lean condition when the bov releases? Would that be a possible way of harming the air flap on the maf? I hear people talking about backfiring cracking these sometimes.

Doesn't the stock bov vent to the atmosphere? Did Nissan make a boo boo when they designed that? Somebody once commented that the "Blow off valve you are referring to is a relief valve" and now I am getting a little confused. I was posting a thread on another site to instruct others on the simple way to raise the stock BOV/safety relief valve/ whatever, pressure when running higher boost pressures to prevent opening at higher levels.

So does that mean that when you get an aftermarket BOV you need to eliminate the stock one or keep it (since apparently some think they perform different functions)? What the heck?

[Tony D]

on a zxt manifold, there is a BOV (vacuum control valve) AND a relief vavle.

The perform two separate and distinct functions.

One (the BOV, AKA Vacuum Control Valve, ported into #4 Intake Runner) controls pressure rise in the intake manifold after the throttle is snapped shut, relieving pressure from the turbocharger to keep it from a stall condition, and to blow it into the intake manifold, keeping the valve seals from experiencing a rapid spike to total vacuum keeping oil consumption down.

 

The other (the emergency pressure relief valve, the big golden round thing on the top on the manifold near the EGR Valve) is simply an emergbency relief valve to keep the turbo from overpressurising the intake manifold over about 10psi---where the stock fuel map stops! It is a failsafe for the wastegate. If you disconnect the wastegate, and drive hard, you will find the Emergency Relief valve opening wooshwooshwoosh continually to keep manifold pressure at a safe level for the fuel delivery available.

 

They are two different valves, with very different reasons for being there. No, Nissan made no mistakes--but a lot of peole get them confused through lack of understanding of the system and how it is supposed to operate. "Works for Me" and "Properly Engineered" are not mutually inclusive terms in may cases... In many cases, they are mutually exclusive when scrutinized closely, as a matter of fact!

[HeatRaveR]

The other (the emergency pressure relief valve' date=' the big golden round thing on the top on the manifold near the EGR Valve) is simply an emergbency relief valve to keep the turbo from overpressurising the intake manifold over about 10psi---where the stock fuel map stops! It is a failsafe for the wastegate. If you disconnect the wastegate, and drive hard, you will find the Emergency Relief valve opening wooshwooshwoosh continually to keep manifold pressure at a safe level for the fuel delivery available.

[/quote']

 

Does this mean that you can't raise boost pressure past 10 psi because this relief valve will just release the pressurized air? If so, how are a lot of people tuning past the 10 psi level? Is this Greddy relief valve the only aftermarket solution? http://greddy.com/prod_relief_valve.htm

[Muskrat]

Well, if you don't mind loosing the relief valve, your local nissan dealership has an 'aftermarket' solution called a pipe plug. Just tell them which hole you want to block off, they should be able to find you the right part. Failing that, have a look around your local hardware store.

[260DET]

With an aftermarket ECU and MAP sensor you can vent to the atmosphere no problem. Mine has twin Bosch BOV's and makes a ricey PSSSHHT quite loud when going to 15psi boost, think that some Saabs used the Bosch BOV as original equipment.

[nienberg.11]

All right guys, what about this... In a few days I'll be installing a custom turbo setup onto my stock '77 280z motor. I was planning on moving the stock afm between the intercooler and the intake (as close to the intake as possible). With this setup, I don't see what the problem would be with venting my BOV to the atmosphere since the afm will be metering the air flow AFTER it has passed by the BOV. Let me know what you think of this idea, which is actually from a friend of mine who runs Z Performance Racing in NC. He claims to use this setup all the time without problems.

[Guest bastaad525]

nienberg - first I gotta say, I love that sig

 

I had asked about using the AFM in a different location in the line of airflow as well... this was a long time ago so I honestly forget what the oppinion was but I seem to remember there was some reason why it was considered that you would not want to do this. Sorry I can't remember specifically waht the reason is

 

I'm very interested to hear how this goes though, keep us updated.

[nienberg.11]

Bastaad, i never really read your signature before, but now that I do, it sounds like the turbo installation I'm going to be doing on my 280z will be very similar to the setup on your 240z. I also have a BEGI fmu, and will be using stock engine management. I've also got an external walbro 255 lph fuel pump, a nice generic bar and plate ic from ebay, MSD Blaster 3 ignition, and I'm upgrading to 280zxt injectors. I will also be using the stock 280zxt manifold, turbo, and downpipe, as well as a 1st generation eclipse BOV, and manual boost controller. I start work on my istallation tommorow. Do you have any advice?

[Tony D]

"I was planning on moving the stock afm between the intercooler and the intake "

 

Simply stated, "Won't Work!"

 

The AFM is NOT a pressure-flow device, it is designed to work against a vacuum applied to the backside of the throttle plate. The housing is not desigend to work in a pressurized environment.

 

The AFM measures raw airflow into the engine from the atmosphere before ANY compression devices act upon it, simply because it's the easiest and most accurate way to measure without a bunch of sensors.

 

Otherwise, there would be some polytropic head computation algorithim to figure out exactly how much pressureized air was cicrulating through...... We don't need that...

 

So long and short: "Wont Work!"

[240zprace]

you can vent into atmosphere if the BOV is before the AFM/MAF, cause the air has been metered after the BOV... if you put the AFM/MAF before the BOV, it has to vent back, after the the AFM/MAF. if you vent into the atmosphere: ie;

AFM/MAF before BOV, when you floor it, the air that just past the AFM/MAF has been metered..the ecu will compensate fuel to match the air that just been metered., but when you shift, the air is vented out into atmosphere, then you get on the gas again and the ecu dont know where the 5lb of air went , so you would bogg because of the air that vented out was metered already and the ecu dont know that.. and it will be rich. i've ran mines: BOV before AFM, vented into the atmosphere for years and no problems. cause the air is metered and when i shift (vents to atmosphere), new air comes in and goes on and on... BOTTOM LINE IS, if you want to vent to atmosphere= BOV before AFM/MAF... and if you want BOV to recirculate= AFM/MAF before BOV