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Discussion Starter #1
Hi All

Just put a recirculating forge dump valve on my T7 9-3. Surprisingly there's been a noticeable decline in the MPG figures. When checking MPG figures I've made sure that I've maintained my old driving style. Most of the driving is done on motorways.

As yet I haven't increased the power - it's still the std 150bhp.

Is this normal?

Thanks

Manders
 

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Whats a noticable decline? I wouldn't have expected any change at all unless its sticking open and you are experiencing low boost pressures thus less performance and are having to put more effort with the acc pedal to compensate...
 

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If you put a dumpvalve on the wrong way round, the pressure will blow the piston open and waste most of your boost.
This can be done, I did it, the inlet and outlet ports are the same diameter and it does matter which is which.
You need to have the pressure side pipe (from pipe on turbo outlet) pointing at the SIDE of the cylinder of the dump valve body. The vent side (back to the inlet to the turbo pipe) must connect to the END of the cylinder.
I only ran it for about 5 minutes (until I realised) but if it was left fitted backwards, your fuel consumption would be grim along with lack of
 

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I know this has been asked before, but there must be a way to put an open air dumpvalve on a T7 system. Abbottracing sells a special open air dumpvalve with twin piston (or twin spring, something was twin anyway lol) that keeps it from sucking in air at idle. Most people say that you can't do this with an mass aiflow system car, but even the new Impreza WRX is frequently modified with an open air dumpvalve, and it is a mass/airflow car. (at least the newer US WRX's are, I didn't believe they were but a friend of mine showed me his sensor, small as heck too. lol) I doubt very much that during the shift the computer takes into account the air being dumped, and even if it did, it would just momentaryly run slightly rich. I think the idle problems are mainly that the recirculating valves stick open a little longer. I'm curious if anyone has tried an dumpvalve on a T7 car designed specifically to dump into the atmosphere. It would be cool to get that loud whoosh.

However ... if you're not interested in the sound, despite a number of people's noted reccomendations for atmospheric dumpvalves, I think they have a slight performance disadvantage. Eric with the 900 claimed the air coming out of the intake pipe was 140 degrees, so he didn't want to recirculate that. But that's the temperature of the COMPRESSED air, much as it heats up when compressed (to well above 140 degrees which is why you need an intercooler) it will cool significantly when decompressed. Think about it. If compressing it to 15 psi heats it up to say 200 degrees, decompressing it from 15 psi to atmospheric pressure should take it back down to about ambient temp right? (not accounting for heat created by inneficiencies in the turbo compressor) Now if it's heated to 200 degrees, and then cooled to 140 degrees and THEN decompressed to atmospheric pressure the temperature of the gas coming out should be below ambient temp! The compression and then radiative cooling of the gasses will work in exactly (not counting for evaporation and condensation) the same way as your air condition works. The gas is compressed which makes it hot, then cooled to close to ambient temp through an intercooler (in your a/c's case the condensor) then decompressed and subsequently cooled when released into the atmosphere (in the a/c's case in the evaporator). Your air con may use the heat absorbtion of evaporation and condensation to grab even more heat from the air going into your car, but the intercooled turbo system should do nearly the same thing, though a lot less efficiently. Also recurculating the gasses reduces the amount of air that has to go through the air filter, since some filtered air is wasted when it's dumped. This too gives more power. Atmospheric dumpvalves may have a number of performance disadvantages, but they sure do sound cool and I want one dangit! lol Oh well ..
 

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Sorry, SJ, I would disagree there. I have a non recirculating valve and the end of that goes to the pressure side, ie the intake pipe post the intercooler.

On a recirc valve, the side arm goes to the inlet pipe between the AMM and the turbo inlet..
 

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Intersting then why it should work in a different configuration on different models... my C900 has it round the way I describe, as did my 9000... so I assumed all Saabs would be the same.

That kind of knocks the presssure theory on the head though doesn't it?
 

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Originally posted by SJ:
[qb]If you put a dumpvalve on the wrong way round, the pressure will blow the piston open and waste most of your boost.            
This can be done, I did it, the inlet and outlet ports are the same diameter and it does matter which is which.
You need to have the pressure side pipe (from pipe on turbo outlet) pointing at the SIDE of the cylinder of the dump valve body. The vent side (back to the inlet to the turbo pipe) must connect to the END of the cylinder.
I only ran it for about 5 minutes (until I realised) but if it was left fitted backwards, your fuel consumption would be grim along with lack of  
 [/qb][/b]
There are several websites that have tested the standard bosch plastic valve both ways and there is little difference in it's leak point.

Now, after market ones it's a different story...

Andrew
 

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I think we should look at the whole picture of ECU's work to be able to answer the BPV BOV issue. Here is some basic info that needs to be noticed

On a car the ECU should be especially interested on the quantity of the air-mass flow to the cylinders. Knowledge of it is important when deciding how much fuel to inject. Air-mass flow to the cylinders is not measurable directly so it has to be estimated.

Injected fuel can be determined by measuring the air-mass flow into the engine and divide it by the air/fuel stoichiometric ratio. The air-mass flow sensor may
typically be located far from the cylinders, close to the air fillter. Consequently there is a large volume consisting of hoses, intercooler, and intake manifold separating the air-mass flow sensor from the cylinders.Volumes introduce filling and emptying dynamics and a transient in the air-mass flow to the cylinders will therefore deviate from the measured air-mass.

On the older T5 cars air flow to cylinders based on the Speed-density pricible. The speed-density methods uses volumetric efficiency, engine speed, and intake manifold pressure and temperature to determine the air-mass flow to cylinder. A drawback of the speed-density methods is that the intake manifold pressure is subjected to noise. To reduce the intake manifold pressure noise, caused by engine pumping and standing waves, observers for mean intake manifold pressure have been proposed.

If we look at T7 more closely we will see that there are also a MAP and intake manifold temperature sensor present. T7 does not use the MAP to determine pressure, it is there just to "wacth" that the highest pressure is not exceeded.

T7 also monitors the throttle plate angle and of course the engine speed. T7 uses it's air mass controller to boost to target air flow. For an example when wastgate is opened the air mass flow decreases momentarily until the air-mass controller has opened the throttle more. This is done because the throttle controller tries to maintain a constant air mass flow.

So if we go and replace the BPV with a BOV the following will happen when gear is changed during WOT acceleration. When the throttle plate closes both BPV and BOV will open. BOV tosses the air out and there will be an increased air flow through the MAF. I see this will cause some error to the measurement. On the other hand I dont believe the ECU keeps a "history" of the measured air quantity.

Both BPV and BOV are closed at idle on T7 cars (not sure about viggen but 9-5 aero is), so there should not really be a difference which valve is there. But the matter is not this simple because the vac line from TB goes to an ECU controlled valve that controls what the BPV does, independent of TB vac. At idle there is a weak positive pressure.

I believe most of the problems people have faced as bad idle with BOV's are mainly due to the ECU control. BPV does not act the same way as a BOV, there isn't a spring.

For an example there are many people at the Finnish volvo club who have found BOV working on s40 T4 car. T4 is also equipped with MAF.
 

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5 days after installing forge BPV and Abbott high flow delivery pipe, MPG is down from roughly 14.5 to a dismal 12.5 average in suburban driving (zoom to 60mph, stop light, zoom to 45mph, stop light)... 70% of my driving I am either on the throttle (hard no less), braking or sitting still. Top end
is improved and the car is boosting a little farther into the red now. Maybe some boost pressure was being lost with the old Bosch valve...have not tested so can't say for sure. Also switched to BCPR7ES-11 plugs at same time but I don't see how that could reduce my fuel economy. On a side note, is there any point in going to stage 3 with an auto trans? I'm not worried so much about replacing the trans as I am worried the TCM will just squash the added power. Does anyone know how remapped ECMs work with auto TCMs? Does the ECM over ride the TCM or is the TCM somehow tricked into thinking less power is coming than actually is?
 

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Discussion Starter #11
Thanks all.

A noticeable decline is about 6-8mpg. I installed the dump valve in exactly the same way as the existing Bosch unit. So the vuc. hose comes out from the side as per the Viggen engine bay link that Adrian's posted.

I assume that's correct???
 

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Vigge I don't believe the BPV is completely closed at idle. When I tried to run it open, all I had to do to get it to idle properly was have my friend put his hand over it. Clearly there is some leakage, likely because it does not have a spring to hold it shut, and there is some vacuum albeit not much. I do think it "could" cause some sort of error if you just ran BOV instead of BPV. But just not sure that it would. If someone happens to have a Forge open air dumpvalve, try lending it to a friend with a T7 car and see if it works?
 

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Going back to SJ's original comment:

If you put a dumpvalve on the wrong way round, the pressure will blow the piston open and waste most of your boost.            
 [/b]
I've been thinking about this and whilst Vigge's post indicates a different set up for T7 cars:

because the vac line from TB goes to an ECU controlled valve that controls what the BPV does, independent of TB vac[/b]
The "traditional" arranagment for a BPV or BOV is that the vac line is fed from the throttle body. This is fed to one side of the piston in the valve. The other side of the piston is connected to the intake, post intercooler. Under boost conditions there is only a small pressure differential (none at WOT) across the piston and it stays put. When the throttle closes, there is a pressure differential on the piston in the BOV/BPV, which then slides to open up a release path either back in to the intake or to atmosphere. Or, put another way, it's my understsanding that the boost pressure does not normally act to force the piston open under open throttle.

I can see that it would work happily configured the "other way" round for a BPV, the pressure differential across the piston being the intake manifold vacuum vs atmospheric. However as this is a smaller differential than intake manifold vacuum vs boosted intake air, it probably reacts a little bit slower.

@Manders: humour me and try it installed the other way round and see if that makes a difference whilst I think of a reason why it should
 

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Originally posted by Mark E
Intersting then why it should work in a different configuration on different models... my C900 has it round the way I describe, as did my 9000... so I assumed all Saabs would be the same.[/b]
When I got my Forge one from Trent there was a brief discussion as to the year of my 9000 because the dump valve's internals were reversed for certain years. Never heard mention of this anywhere else.
 

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In my defence m'lud....
I found when I had fitted the new one the 'other way round', the car didn't go as well. Checking under the bonnet to see if was working, I disconnected the vent side of the valve from the inlet trunking. I plugged the hole in the inlet to avoid leaks and ran the engine to see if the valve was venting properly (to atmosphere) when the throttle was snapped shut. I couldn't tell by the usual whoosh sound so I put my hand over the outlet to feel for the dumped air. To my surprise the valve was leaking air all the time, progressively increasing with engine speed. This would be recirculating hot compressor outlet air back into the inlet and also loosing boost pressure. I reversed the valve and it didn't leak and it did dump with the usual whoosh.

The standard bosch valve (fitted correctly) has the pressure applied to the sides of the piston, which can't push the spring open, if applied up the exhaust port the pressure is directly on the end of the piston, against the spring, and can make it leak past the seal.

This was on a 1995 9000 2.3 LPT, but I can't see why the valve would work differently on other models???
 

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Originally posted by Adrian W:


However ... if you're not interested in the sound, despite a number of people's noted reccomendations for atmospheric dumpvalves, I think they have a slight performance disadvantage.  Eric with the 900 claimed the air coming out of the intake pipe was 140 degrees, so he didn't want to recirculate that.  But that's the temperature of the COMPRESSED air, much as it heats up when compressed (to well above 140 degrees which is why you need an intercooler) it will cool significantly when decompressed.  Think about it.  If compressing it to 15 psi heats it up to say 200 degrees, decompressing it from 15 psi to atmospheric pressure should take it back down to about ambient temp right?  (not accounting for heat created by inneficiencies in the turbo compressor)  [/b]
Welp, I think that's the main issue we're dealing with here, considering that 70% compressor efficiency is deemed to be on the acceptable side of things. That's still a lot of heat.

And then there's a good few application where there's nowhere to put the valve after the IC, eg Impreza et al. I'm using a BOV that
s developed for a (classic) Impreza WRX rally car, as the fifth injector already was where the BOV would normally go on my car...

Then in general, there is something I consider to be a flaw in people's logic in that everyone seems to go out on the assumption that 100 percent of the diverted air will stay inside the system if you plumb it back into the intake before the turbo. As the throttle plate is shut, however, the flow path goes the other way. Saabine's open air filter very loudly indicates which way the airflow goes on each shift. In fact, it is so d*mn loud you can only hear the BOV itself when on (coming from) high boost in 3rd and up.
 

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Originally posted by Eric van Spelde:
[qb]Then in general, there is something I consider to be a flaw in people's logic in that everyone seems to go out on the assumption that 100 percent of the diverted air will stay inside the system if you plumb it back into the intake before the turbo. As the throttle plate is shut, however, the flow path goes the other way. Saabine's open air filter very loudly indicates which way the airflow goes on each shift.     In fact, it is so d*mn loud you can only hear the BOV itself when on (coming from) high boost in 3rd and up. [/qb][/b]
Speaking about the whooosh with BPV, here is a video when I ran my viggen in a dyno with a cone filter.
Viggen dyno run 8.5Mb
 

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One thing that puzzles me though is that, at least on the cars with open cone filters that I know, the whoosh from the air coming out of the filter seems to be getting louder the harder you're on the throttle. In theory, it should only happen when the turbo does not spin hard enough to create any positive pressure, so the air that is diverted from the turbo-TB path with throtthle plate shut and plumbed back into the system, as well as the air column in the air filter-turbo path, taking the path of the least resistance, goes out through the air filter as long as there is less energy needed to do so than to turn the compressor wheel fast enough to overcome athmospheric pressure. As soon as you're boosting hard though, the compressor wheel should have enough inertia to continue sucking air in with the throttle shut, provided it is not stalled by air pressure banging against and being reversed by the throttle plate, i.e. as long as the BPV or BOV is doing it's job of relasing the pressure in the turbo-TB tract. So following that logic you should only hear the 'shhhhhtt' from the air filter when realesing the throttle from low/medium loads, and not when coming from high boost.

Just thinking aloud, does that make any sense??
 

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Eric, I think you're confusing airflow for sound. While in some circumstances air may very well go the wrong direction through the filter, you can still hear a sound when air is going back into the turbo instead. Sound is a pressure wave, and not "significantly" associated with the flow direction of the air that carries it.
 

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Ack, just read Eric's post before the last one. Also it should note, that if any air goes the wrong way through the turbo it does not go "whoosh" it goes "WHEW WHew whew" and is called "Surge Loading" ... this is very very very bad for the turbocharger. I hear it all the time on Supras and WRX's ... it's what happens when a BPV or BOV is set too tight. Turbo chargers require a certain amount of flow at any given pressure in order to maintain the appropriate flow direction without stalling. When the pressure goes way up, but the flow either remains the same, or goes way down you get compressor surge. Sometimes this can happen even when accellerating with an innapropriately sized turbo, but seems to be common in cars with modified BOVs. Because this is fairly hard on the shaft bearings, and seriously increases turbo lag I would reccomend finding a way to eliminate it.
 
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