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?

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

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.

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.

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

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.

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...

its so the turbo wont back spin

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.

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:

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

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

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 :)

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

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.

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.

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.

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

You are correct Merv.

Thanks, Nathan. I plan to use two of these billet type BOV's... one on the up-pipe from turbo to i/cooler and one post i/cooler.

I don't really know, but I thought it would be nice to get the cooled air back into the system and then it is cooled again.
By placing a blow off / recirc. before the IC you are pumping hot air round the turbo

Just a thought.

Good point actually. I wonder how much of a difference it would make though? Probably minimal?

I was thinking it would be kinder to the turbo and the IC.

Many older turbocharged cars also didn't come with intercoolers, then there is CIS, etc, etc. That really doesn't mean anything in and of itself. At the time most of those cars were built, turbocharging was in it's infancy with gasoline automobiles. Even at low pressure it is still entirely possible and actually more likely (lower than WG spring pressure that is) that you will see compressor surge. At the moment I'm only running 7psi on my Carrera temporarily and I still see surge at partial throttle and building boost pressure below the WG spring pressure. I've seen the same thing on my 951 running around the same boost, so it's not just high boost applications that will cause it. If you are running less boost pressure than the WG spring pressure (such as in partial throttle conditions) all of that pressure has to go somewhere unless you have some way to release it. My 60-1 is dying a slow death because of it. ;) Oh well, it's just an excuse to upgrade. :D

Patkeefe, rubber in your intake (unless there's maybe a bellows/accordion style intake pipe somewhere) isn't going to absorb all or much of the backpressure and you are still likely to see compressor surge.

Just FYI but those plastic bosch BV's aren't all that great. With greater than 14psi they tend to leak pretty badly. Also, over time, due to the relatively light spring rate inside of them they can start to leak which will cause slower boost response. IMO, it's not even worth messing with them when you can just buy a billet BV or aftermarket BV/BOV and have a better unit.

Yeh I know, it was just an example. The one I have is a billet aluminium unit which looks similar. I just wanted to confirm that the side pipe is the inlet ;)

Here's the one I'll use pre and post i/cooler:

http://forums.pelicanparts.com/uploa...1170203459.jpg

I plan to use two because I run max of about 1.3bar, so I want quickest possible venting as I can ;)

Looks to be a Forge BOV(or a good knockoff of it). Forge makes great stuff, I use their products on damn near every turbocharged car here.

This is the BOV I had on my 965 before I sold the car last year. Fully adjustable and adaptable to any fitment necessary. I now have two of these for my current project ;) And yes, that's a quarter right below the BOV.....one of these valves will flow as much as two of the smaller Forge units, but requires some custom-fabbed intercooler/charge piping work to make it fit properly(doesn't come close to fitting stock location) http://i123.photobucket.com/albums/o...x/S5000052.jpg

I know I'm coming in a little late in this thread but here is my own experience with no BOV. I ran with out one for 2 years. Hammered the throttle many a times and heard my turbo stall and chatter occasionally (the sound was just like the 935's I watched at the IMSA races years ago). Never had any issues. I'm not advocating for anyone else to do it, I took the chance on my own. I chose to leave it off for a couple of reasons....1. I got a LOT better throttle response without it. 2. The race cars back in the day (935's) didnt run them. Yeah, I know they rebuilt the turbo's or whatever after every race but from a performance standpoint they were not needed. 3. If I damaged my turbo from not running one then I would buy another turbo AND reconnect the BOV and never do it again! But as it turned out.....nothing happened.

930turbolader, that's the same experiences some of the ricer boys have had with their Japanese imports and running BOV-less... they also experience significantly better throttle response and almost no lag whatsoever.

It's interesting the 935's never used them, especially considering how much boost those monsters use to run :eek: Goes to show how tough the KKK turbo's are I guess :confused:

P.S: Doesn't matter anyway, because tonight I'm installing the BOV. My welder has just called me and told me he's done the job for me ;)

Merv,
I have one now, a Tial I believe. Stephen installed it when he converted my CIS to EFI. Whatever it is, it works pretty good and is hands down better than the 965 set up I had. It does make noise when the pressure is released but I'm used to it now and in fact dont even pay attention to it anymore.

Yeh, I had a TiAL 50mm unit too... I sold it with my old turbo system. It worked great, but it annoyed the hell out of me and my mates would always say "How's the rice cooker going?"... got to me after a while, so I want to go with a quieter stealthy solution :D

I just tested this billet BOV down the FWY and damn it's so much quieter & softer than the TiAL...lt tends to vent fairly quickly too, at the very lift off the accelerator. I like it. I haven't even recirc'd it back to the intake yet and already I'm pleased with the reduced noise levels :cool: