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BPV and hooting

4K views 40 replies 9 participants last post by  Solid Snake 
#1 ·
I am I correct to assume that hooting noises are due to compressor surge? My stock bypass valve started hooting a few weeks ago and I recently replaced with an ebay BOV. It's a simple jobbie with an aluminum body and a brass plunger, however the spring is extremely stiff. At WOT, the pressure behind the plunger and the pressure in front of it should be equal, right? In that case, it shouldn't take more than a pound or two of spring pressure to maintain boost. I estimate the spring to take 15lbs or more.

The car hoots when take off at WOT and then level off acceleration. The vacuum behind the plunger doesn't seem to be enough to overcome the spring pressure so it doesn't blow off. I expect some boost leakage at partial throttle, which is not an issue as it shouldn't leak at full throttle. Should I try cutting the spring?
 
#2 · (Edited)
Posted by Solid Snake: At WOT, the pressure behind the plunger and the pressure in front of it should be equal, right? In that case, it shouldn't take more than a pound or two of spring pressure to maintain boost. I estimate the spring to take 15lbs or more.
The pressure behind the plunger is = (Current Boost pressure + Spring pressure). The pressure in front of the plunger is, basically, amospheric (14.5 psi). So, at WOT, the plunger is held firmly closed, and no loss of boost should occur at that time.

When you lift off the accelerator pedal, however, the boost in the manifold drops to a partial vaccuum, let's say 4 psi. If the spring was, say, 10 psi, then:
Pressure behind plunger = (4 + 10) = 14 psi, and
Pressure in front of plunger = 14.5 psi
...and the plunger would open and relieve the pressure in the turbo delivery pipe.

However, if your spring was indeed 15 psi:
Pressure behind plunger = (4 + 15) = 19 psi, and
Pressure in front of plunger = 14.5 psi
...and the plunger would never open!

Many BPV manufacturers offer a variety of springs, but I guess this one from eBay just had 1 spring. Before you cut it, however:

-> Make quite sure the BPV was fitted exactly as instructed.
-> Make sure the lubricant you use is light oil, not grease.
-> On the workbench, use your mouth to apply a vaccuum and see if the plunger opens.
 
#3 ·
Say I cut it down to 3lbs.

It idle, I'd have 3+5 vs 14.7 so the valve would be open. This is not a problem as long as the turbo can maintain positive pressure and unfiltered air isn't sucked into the charge pipe through the BOV.

At partial throttle, I might have 3+12 vs 18 so it would still be open.

At full throttle and full boost I'd have 30+3 vs 30 so it would be closed.

I can rev the throttle while looking at the BOV and it does open when the throttle suddenly slams shut. However, if I gun it and then level off the acceleration, it doesn't open and I get surge.
 
#4 ·
I hope I'm understanding what you say, but...

Say I cut it down to 3lbs.
It idle, I'd have 3+5 vs 14.7 so the valve would be open.
The math is OK, but idle is actually about 10 psi because of the low rpm. However, (10 + 3) is still less than 14.7, so that very weak spring would cause the valve to open at idle. So, 3 lbs is too small.

At partial throttle, I might have 3+12 vs 18 so it would still be open.
No, at partial throttle you would have 3+12 vs 14.7 so it would be closed.

At full throttle and full boost I'd have 30+3 vs 30 so it would be closed.
No, at full throttle, you would have 3 + 30 vs 14.7 so it would be very firmly closed.
 
#7 ·
No, at full throttle, you would have 3 + 30 vs 14.7 so it would be very firmly closed.
If I'm boosting at around 16psi, why wouldn't it be 30psi in the charge pipe? It's no longer atmospheric once the turbo spools up.

Anyway, I cut the spring and the compressor surge/hooting is gone. It bleeds off even the slightest amount of boost, so even when I'm driving slowly in a parking garage I can hear faint hissing.

With the engine stopped, the BOV is closed. At idle, it's wide open. If I blip the throttle, it slams shut. At partial throttle, it's partially open and hissing is audible. At full throttle it's fully closed.

I think there may be some leakage as I hit full boost at 2500 RPM. When do you guys reach full boost with your TD04's? It has an interesting side effect though. At high RPMs, the turbo lag is gone. If I hold 3000 RPM in 2nd, air is rushing out of the BOV, but when I nail the throttle, it slams shut and I get full boost instantly. My best guess is this happens because the BOV allows the turbo to maintain speed rather than pumping air against a mostly closed throttle plate.
 
#8 ·
If I'm boosting at around 16psi, why wouldn't it be 30psi in the charge pipe? It's no longer atmospheric once the turbo spools up
That's right, boosting at 16 psi means 30 psi in the charge-pipe/manifold.

That 30 psi goes down the rubber tube to the top of the BPV and, plus the spring pressure, holds the plunger closed. The pressure on the other side of the plunger is always atmospheric no matter what the boost.

With the engine stopped, the BOV is closed. At idle, it's wide open.
With the engine stopped, even a 1-lb spring would keep the valve closed. If the valve is open at idle, that's the equivalent of a vaccuum leak.

Now I'm beginning to wonder whether your BPV works the same way as mine. Anybody else have any comments?
 
#10 ·
That's right, boosting at 16 psi means 30 psi in the charge-pipe/manifold.

That 30 psi goes down the rubber tube to the top of the BPV and, plus the spring pressure, holds the plunger closed. The pressure on the other side of the plunger is always atmospheric no matter what the boost.
We're talking blowoff valves, not wastegate actuators.

Is it installed upside-down?
No, what difference would that make?
 
#14 ·
BOV, BPV, Hooter valve, Atmospheric dump valve, Bypass valve, Diverter valve, Blow-Off valve - they all drain excess boost (either to atmosphere or to the turbo air inlet line) when the driver's foot lifts off the pedal, so as to avoid damage to the turbo from back-pressure.

The operation of the valve (open vs closed) is entirely dependent on how the (currrent boost + spring) pressure compares to atmospheric (~14 psi). So, for a spring with resistance of, say, 6 psi...

Engine conditions........... Boost + Spring = Total ......Valve

Engine OFF.................... 14 ...+.... 6 ...=.. 20 ........Closed
Engine at idle................. 10 ...+.... 6 ...=.. 16 ........Closed
Engine at partial throttle.. 15 ...+.... 6 ...=.. 21 ........Closed
Engine at WOT............... 30 ...+.... 6 ...=.. 36 ........Closed
Pedal lift-off at WOT......... 4 ...+.... 6 ...=.. 10 ........Open

You can see that if the spring was 15 psi, the valve would never open.
 
#17 ·
BOV, BPV, Hooter valve, Atmospheric dump valve, Bypass valve, Diverter valve, Blow-Off valve - they all drain excess boost (either to atmosphere or to the turbo air inlet line) when the driver's foot lifts off the pedal, so as to avoid damage to the turbo from back-pressure.
Not so.
Most systems limit boost using some kind of Blow Off Valve. SAABs don't do that.
The APC system detects Knock and adjusts the Wastegate accordingly. This allows maximum boost for the available fuel, and doesn't waste energy by generating, then blowing off boost.
The only purpose of the Turbo Bypass Valve (nicknamed Hooter valve) is to avoid the "Hoot" noise when the Throttle is suddenly closed at high Boost Pressure. Early Turbos didn't have them at all, they were developed to resolve Customer Complaints about the noise.
Installing an Aftermarket Blow Off Valve will likely decrease performance while making useless Cool Noises.

You can't damage a Turbo with back-pressure.
 
#15 ·
You can see that if the spring was 15 psi, the valve would never open.
Fair enough, it was probably less, but I cut it. It's closed when the engine is off, cracks open at idle and slams shut if I nail the throttle.

Is the hooting really produced by the stock BPV or is it compressor surge? My aftermarket BOV produced surging/hooting as well until I cut the spring.
 
#18 ·
Posted by Jim Mesthene: The only purpose of the Turbo Bypass Valve (nicknamed Hooter valve) is to avoid the "Hoot" noise
OK, if you say so. But what's this?

Blow Off Valve (BOV)
(a.k.a. Compressor Bypass Valve)

What does it do:
The purpose of a blow off valve or compressor bypass valve is to prevent large pressure spikes in the intake pipes when the throttle plate is closed while boosting, preventing the turbo from surging.
 
#21 ·
Engine conditions........... Boost + Spring = Total ......Valve

Engine OFF.................... 14 ...+.... 6 ...=.. 20 ........Closed
Engine at idle................. 10 ...+.... 6 ...=.. 16 ........Closed
Engine at partial throttle.. 15 ...+.... 6 ...=.. 21 ........Closed
Engine at WOT............... 30 ...+.... 6 ...=.. 36 ........Closed
Pedal lift-off at WOT......... 4 ...+.... 6 ...=.. 10 ........Open

You can see that if the spring was 15 psi, the valve would never open.
By the way, the assumptions we've made are for a BOV with a piston area of one square inch. Most BOVs have an area of several square inches, so it's very possible for a BOV with a 15lb spring to be blown open by boost.
 
#26 ·
In SAABs with APC (since 1982), Boost is controlled by the APC. It senses Knock and, using Electronics and a Vacuum/Pressure Solenoid, opens the Wastegate to limit Boost. Most other Turbocharged cars use Blow Off Valves to limit Boost, venting excess pressure.
SAABs don't use or need Blow Off Valves. Adding one only interferes with the APC system. If you have a Trionic system, you can sabotage the integrated Fuel Delivery and Spark control too. Any air vented through a Blow Off Valve will have already been measured by the Air Mass Meter, thus disrupting Fuel Management also.
The only purpose of the Turbo Bypass Valve (nicknamed Hooter valve) is to avoid the "Hoot" noise when the Throttle is suddenly closed at high Boost Pressure. Early Turbos didn't have them at all, they were developed to resolve Customer Complaints about the noise.

This is what I learned when I went to the SAAB Factory Turbocharging School (1985). It's quite different from the systems used in the Race Cars I was familiar with at that time, and in many applications today.
 
#27 ·
Jim Mesthene: The Original Poster replaced a Bypass Valve with a Blow Off Valve, thus defeating the APC.
Now it doesn't work right.
Solid Snake: Jim, what on earth does the BOV/BPV have to do with the APC system?
I agree with Solid Snake on this one, Jim. The only difference with a BOV is that the pressure is vented to atmosphere rather than returned to the turbo air intake. I always understood that this was OK as no Air Mass Sensor was involved. I myself have replaced the original bypass valve with a blow-off (Atmospheric dump) valve, and no problems at all.
 
#28 ·
I may be mistaken, but according to all the recent reading done in tracing issues with boost/hoot/misfires and the rest of De-Gremlin(ing) a recent purchase - the Saab turbo management system is not exactly comparable to the accepted norm of HiPo turbo principles.The APC is the pivotal component that sets it different and possibly making any adjustments accepted by 'mainstream' Hi Po thinking detracts further away from the technology that predates most of the benchracer gospel accepted as the benchmark.As a case in point( again, I may stand to be corrected here) wastegate actuator nor responding to vacuum...assume it may be faulty. Local generic parts store has a <20YO 'guru' I bounce questions off. He confirms that actuator closes wastegate as vacuum drops inline with throttle body/lod pedal being hit! Saab works opposite - actuator moves forward under boost pressure diverted thru APC to govern max boost/protect engine. If I believed the 'guru' -$265 for a T25 actuator and would have no result. I read between the lines of all of the turbo FAQ's and it turns out my hooting/lack of responding to base boost adjustments Et Al is down to a split diaphragm on the BPV. A much cheaper step in the elimination of my personal woes. The initial hooting on getting the car; was down to the 3 tubes off the solenoid misconnected, again the FAQ's solved that but it took time to read and find. If I am mistaken in this, please correct me as I am a newbie to Saabs.... but have a 30+ yr hands on story with most other perfomance stuff(pre computer)
 
#30 ·
I may be mistaken, but.... the Saab turbo management system is not exactly comparable to the accepted norm of HiPo turbo principles.The APC is the pivotal component that sets it different and possibly making any adjustments accepted by 'mainstream' Hi Po thinking detracts further away from the technology that predates most of the benchracer gospel accepted as the benchmark.
Welcome to the Forum.
No, you're not mistaken.
 
#31 ·
In SAABs with APC (since 1982), Boost is controlled by the APC. It senses Knock and, using Electronics and a Vacuum/Pressure Solenoid, opens the Wastegate to limit Boost. Most other Turbocharged cars use Blow Off Valves to limit Boost, venting excess pressure.
I don't know of one car that uses it's BOV to regulate pressure. Some have popoff valves as a last ditch effort to control boost, but that's another story.

When the compressor is spinning many tens of thousands of RPMs and you close the throttle plate, the pressure in the charge pipe surges, but the exhaust pressure needed to keep the compressor spinning is gone, so that pressure leaves the same way it came, backwards through the compressor. This causes huge stress on the bearings. It also causes the compressor to slow down which turns into lag in the next gear.

The noise issue seems secondary at best. The hissing noise the stock BPV makes is just as loud as the hooting.

Saab's APC system really isn't that unique. ECU-controlled boost has been around for a while. Subaru, etc all have their own version.

I have not neutered my APC system at all. It will boost easily into fuel cut unless I've been stuck in a traffic jam.
 
#32 ·
The hissing noise the stock BPV makes is just as loud as the hooting.
If the hissing noise from a BPV was just the boost being released to protect the turbo bearings when the throttle is closed, and the hooting was when the boost is NOT released and surges backwards through the compressor, that would explain many of the seemingly contradictory postings on this subject.
 
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