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-   -   Running without a BOV? (http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/327438-running-without-bov.html)

WydRyd 01-28-2007 10:35 PM

Running without a BOV?
 
Curious, will running the car temporarily without a Blow Off Valve damage a turbocharger, or will it simply affect the spool-up between gear shifts, since the compressor has to spin up to speed again?

I need to know fairly urgently as I need to do a quick work-around and am wondering if it's safe to run BOV'less for a few days?

patkeefe 01-29-2007 06:15 AM

Why do you run a BOV and wastegate? I would imagine the BOV is really just a fail safe for the wastegate against overpressurization, thus should not effect spool. I don't use a BOV on mine, but run much less pressure than you.
Pat

RarlyL8 01-29-2007 06:53 AM

The blow-off valve prevents backpressure pulses from damaging the turbocharger and related plumbing when you chop the throttle (off) while spooled up.
If you stay off boost you'll be OK, if not you risk a harmful situation.

patkeefe 01-29-2007 07:11 AM

OK, I agree with Brian. Bigger problem at higher boost (930 range) than what I run (5-7 PSI). My rubber boots will absorb the pulses.

WydRyd 01-29-2007 11:26 AM

Thanks for confirming... I better just wait till it's sorted then :confused:

Porschefile 01-29-2007 03:06 PM

Patkeefe, the reason you want to run both is because they are complimentary to each other and they both serve a worthwhile purpose. Running without a blowoff valve or bypass valve of some sort is a bad idea as it will slowly damage the turbo over time. How can it damage the turbo? Well basically when you are on boost and you lift off, you'll get compressor surge where the compressor basically stalls due to backpressure in the intake. This can cause excessive wear on the center section and eventually the turbo can develop more shaft play among other things. On something more expensive like a 35r, I'd definitely be using either a BV or BOV.

As far as bypass or blowoff valves affecting spool, they definitely can. If your BV/BOV is leaking then it basically causes a boost leak (well, leaking back into the intake that is, if recirculated) which can lead to increase spool time between gears among other things.

WydRyd 01-29-2007 03:13 PM

I really like the idea of recirculating the air from the BOV back into the intake. I don't really like the "ricey sound" these BOV's make between gear shifts.

Any of you guys seen a turbo running dual BOV's? i.e: one BOV just after the turbo's compressor and a second BOV just after the i/coler's outlet?

What would be the advantage of running two units? Maybe for BIG boost (i.e: 25-30+ psi) applications, needing to vent the reverse pulses quicker? Just curious...

anthony 911 01-29-2007 04:25 PM

its so the turbo wont back spin

RarlyL8 01-29-2007 04:36 PM

EFI engines can vent BOV air into the atmosphere, CIS engines function optimally when all the air is metered by the flapper valve and circulated past the throttle plate.

I would think that having a BOV just past the turbo and just before the throttle would be a nice safety feature in a high boost EFI application.

patkeefe 01-29-2007 04:44 PM

I believe the subject of BOV's and backpressure is certainly interesting, which we should continue as long as Merv doesn't mind the hijack.

My situation is a bit different than the 930 or other higher pressure ratio setup. I have a SC, CIS with an old BAE kit, Rajay T04B. My turbo needs a rebuild anyway, and I'm still shaking the installation out. I have enough rubber and tubing in my system to absorb the pulses, and have been toying with the idea of a bypass back to the inlet side of the compressor for the lift-the-throttle conditions described above. I have something of a modified throttle positioner which came with the CIS, which was evidently an emissions thing to keep the throttle open momentarily, but works pretty well to avoid the pulses...turbo has spooled down by the time the throttle closes.

However, I'm scrapping that throttle positioner when I put in the new IA fuel head and Brian Leask WUR, hence the bypass idea. My concern at my pressure ratio is not blowing anything up, but how can I take advantage of the "pressure pulses" to get spooled up again faster during a shift?

I haven't fully thought this out yet, and I'm open to suggestions and further dialogue.

Pat

WydRyd 01-29-2007 04:57 PM

Quote:

Originally posted by patkeefe
I believe the subject of BOV's and backpressure is certainly interesting, which we should continue as long as Merv doesn't mind the hijack.

My situation is a bit different than the 930 or other higher pressure ratio setup. I have a SC, CIS with an old BAE kit, Rajay T04B. My turbo needs a rebuild anyway, and I'm still shaking the installation out. I have enough rubber and tubing in my system to absorb the pulses, and have been toying with the idea of a bypass back to the inlet side of the compressor for the lift-the-throttle conditions described above. I have something of a modified throttle positioner which came with the CIS, which was evidently an emissions thing to keep the throttle open momentarily, but works pretty well to avoid the pulses...turbo has spooled down by the time the throttle closes.

However, I'm scrapping that throttle positioner when I put in the new IA fuel head and Brian Leask WUR, hence the bypass idea. My concern at my pressure ratio is not blowing anything up, but how can I take advantage of the "pressure pulses" to get spooled up again faster during a shift?

I haven't fully thought this out yet, and I'm open to suggestions and further dialogue.

Pat

I don't mind at all... we're all here to learn so please, hijack away ;)

I'm not sure whether you can actually utilise the back pressure pulses to spool you back up again faster during gear shifts. Then again, a lot of pressure is stored in the intake tract and when these reverse spikes are directed back into the compressor inlet, it may aid it to spin up again quicker?

I certainly can understand how not running a BOV would result in these backward spikes reaching the compressor and slowing it down by a sudden surge of air pressure, especially when running high boost.

I think perhaps running low boost with no BOV might actually result in an "anti-lag" kind of scenario, since all the pressure is still stored in the intake tract and once you open the throttle again, Xpsi is available at the throttle body instantaneously. Again, this might be the case for a low boost application, but for high boost, I'd re-think that theory as it could potentially damage your compressor blade with high pressure boost spikes :eek:

patkeefe 01-29-2007 05:27 PM

Yeah, Merv, I agree we are all here to learn and exchange ideas and knowledge. I have to think about this in greater detail for a bit. I have been somewhat idle on getting the car running for the past couple of months (had to build an addition on the garage for the fleet), and I'm just getting the engine back together now.

I suppose one must consider timing of the throttle closing events:
Throttle closes >
Residual pressure in compressor outlet tube >
Pressure wave (like water hammer at the washing machine) >
Shock wave manifest at the compressor>
but...
Exhaust gas flow slows due to throttle closing >
Throttle re-opens >
Turbo re-spools >

So, how can we make the excess discharge pressure mitigate the shock wave, and use it to our advantage? We have turbos, because it is basically "free" HP.

I kicked the idea of a bladder storage tank around; check valve at the pseudo-BOV inlet, bleed back to the compressor inlet tube at atmospheric plus a skosh pressure, but can't figure the logistics, nor have I done the calculations of the tank sizing. It's kind of crazy, and not fully thought out. I just think that the pressure wave can be utilized to supplement the free HP aspect.

I think about this stuff until someone tells me I'm crazy.
Pat

RarlyL8 01-29-2007 07:52 PM

You need to use something along the lines of a 965 BOV that can be welded into your CIS/turbo plumbing.

When the throttle is chopped the exhaust gasses slow the turbo. The diminishing boost on the pressure side bi-passes the throttle plate when the BOV opens. This same boosted air flows right back into the turbo inlet and continues to cycle across the turbo as the engine slows down. The result is that you are using every bit of intake air energy available to keep the turbine spinning. The decelerating exhaust is what slows the turbo at that point.


http://forums.pelicanparts.com/uploa...1170132645.jpg

BoxxerSix 01-29-2007 08:32 PM

I know people have stated their points here but it's BAD to run without a BOV period. Doesn't matter if you're running .2 bar or 2.2 bar boost, it's bad and will damage your turbos. Even if your wastegate is set at 5psi, when the TB closes and that pressure has nowhere to go it will rise, as everyone knows. However it will continue to rise WELL in excess of 40-60 psi between your compressor wheel and the TB in a blink of an eye. To backspool and stall a compressor wheel that is spinning at 60-120 thousand rpm under full boost takes a massive amount of boost pressure and that's exactly what happens when your BOV jambs closed or is block off

Try this sometime.....ok maybe not but at least vision it :) Block your BOV inlet off and hook a cheap boost gauge to your charge piping or IC and watch it rise after a hard acceleration. 90% of aftermarket turbos are NOT designed to support that type of pressure differential and fail.

Compressor surge damages turbos VERY quickly, especially floating bearing types as it wedges the turbine shaft against the side of the bearings. Remember, floating bearing turbine shafts DON'T actually touch the bearings in the CHRA but rather float on a film of oil at about 15psi of pressure. I've replaced turbochargers that failed from C.S. where compressor buckets(the blades)have folded back or broken right off and even times where the turbine shaft has snapped right clean off right behind the compressor wheel(thrust bearing)


Being a Garrett dealer I'm always advising my customer during a turbo R&R to thoroughly clean all oil lines and passages to and from the CHRA, as well as check the operation of their BOV. Your engines are not diesels and thus need a BOV to maintain turbo life(diesels have no TB's thus no compressor surge). Recirculatory style valves offer no more "spool reduction" than atmospheric styles. They just vent boost air back into the intake tract that has already been used to calculate fuel input, weather it be by a hotwire MAS or a CIS flapper valve. It's dumped back into the intake tract to prevent an overly rich situtation as you let off the gas during shift or coming to a stop. Old DSM's have a problem with people venting bov's to the atmosphere and the stumble/stall due to this phenomenon. Atmospheric BOV's, by manufacture's design, tend to flow more air than recirculatory types and thus are used more often in high perfomance upgrades. Or as stated, multiple BOV's are used to vent enough air fast enough. When you've got twin 90mm compressor wheels forcing 30+lbs of boost into a motor, that can equal quite a bit of compressor surge if a valve went off the deep end :)



Sorry for the long post guys. I go off on a tangent with these things sometimes. Just trying to keep your turbos alive :)

WydRyd 01-29-2007 08:36 PM

No need to apologise, Adam! Thanks for shedding some light on the topic. SmileWavy

Jim2 01-29-2007 10:02 PM

This is my take, and I'm not saying BOVs are bad.

Stock 930s never came with BOVs. Never any damage.

Lots of other early turbo cars also came without BOVs. I know of a dozen or so performance street turbos and turbo race cars which sustained constant use/abuse without failure and no BOV. Garrett, Rayjay, Schwitzer, KKK etc. A good handfull were my own. Note that all were under 20 PSI.

There is a minimal boost spike when the trottle plate is closed. Stock 930 has a fast reacting boost gauge sender mounted upstream of the throttle and I don't think you can see any increase.

Non BOV throttle closure while at boost:
The compressor requires a minimum charge velocity across it's vanes to sustain a given boost pressure, if the charge velocity is not sustained the pressure bleeds off in the reverse direction through the vanes, hence, centrifical compressor. If the throttle closes under boost, the charge velocity instantly stagnates, causing the compressor to cross over to "surge", the charge then reverses back out the compressor. Not pretty, but certainly no 60 psi spikes with our compressors. Maybe a few PSI for a split second before serge kicks in.

The oscilating serge noise resuting from lifting off the throttle can be detected on some installations. After the event described above there is momentarily low pressure downstream of the turbo, yet the turbo is still spinning due to inertia so it recompresses the air until it reaches it's maximum sustainable pressure relating to the velocity of the charge across the vanes, but since the throttle is still closed the charge stagnates again sending the compressor back into surge. Repeated cycles absorb the turbo's inertia.

BOVs alow the turbo to sustain some interia by reducing the charge pressure, but sustaining the charge velocity across the compressor vanes so as to avoid serge.

NathanUK 01-30-2007 06:20 AM

Jim. I really like your project. You have great determination and skills too.

The 930 did not come with a BOV because it has a recirculation assembly instead. Basically a piston opens up and allows boosted air back into the system before the turbo and after the CIS flap. I bet you remember the talk of long and short neck IC's. The short necks mean they are fitted to the recirculation assy. The long necks are fitted in conjunction with a BOV.

David 01-30-2007 06:34 AM

I'm surprised at how much my Tial BOV opens. Anytime I lift it opens. I don't know if it's just the inlet vacuum lifting the valve or if it's the vacuum in combination with the increased pressure with the throttle closed. Either way it vents a lot. No real point to this, just thought I'd mention it.

WydRyd 01-30-2007 11:32 AM

With a BOV such as this one (an example only), which side is the inlet from the charge air pipe and which is the exit, back into the intake (or atmo)?

I'm assuming the side pipe is the inlet from the charge air pipe and the one facing down is the pipe used to recirc back to atmo/intake?

Can someone confirm asap? I have a billet aluminium unit just like this.

http://forums.pelicanparts.com/uploa...1170189126.jpg

NathanUK 01-30-2007 11:38 AM

You are correct Merv.


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