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Discussion Starter #1
I mean atmospheric or recirculating ...
Been considering the way the dump valve works and I have a couple of postulations for discussion.
On a twin piston dump valve even on idle the valve is firmly closed.It only opens when
1. there is positive pressure in the hose
2.there is a vacumm diferential between the plenum and the hose
If this is the case then on light throttle openings the valve will remain closed (as it should ) now if the compressor wheel wants to spool up and it pushes air through faster than the engine pumps it away then the hose pressure ncreases but eventually the compressor wheel will slow stall

Now consider what would happen with a recirculating (Single Piston ) valve

This will open as above with the addition on idle it will open as there will be a pressure differential across the throttle flap and will close gradually as the pressure difference disappears
Now in this case on partial throttle the turbo pushes as before but this time the valve opens venting the excess air back into the system
But...In this condition the turbo is still spinning and on WOT there will be significant pressure and spool up is already there
so in theory throttle response should be better with a simple recirculating valve ..or am I missing something ?

I happen to have one to hand so I will give it a go tonite ...
 

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OK, I didn't understand one word of that but I've always been under the impression that recirculating was better as the venting was directed back into the turbo helping with the spool up.
 

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Interesting... but I'm not sure I follow the logic. If you watch the calibrated boost gauge, at part throttle you can see that the inlet pressure is boosted above pure vacuum, meaning that the turbo is already spinning and making a contribution. Given the difference in turbo speed between light and WOT/full boost is so great, I shouldn't think a small increase in part throttle spin speed (if indeed your idea works) would yield a significant reduction is spool up time to a significant level of boost.

That said, it's got to be worth a try...
 

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Discussion Starter #4
at part throttle you can see that the inlet pressure is boosted above pure vacuum[/b]
not quite and hence the discussion
the gauge does not move off zero on partial throttle (usually shows a vacuum)
there is a significant off boost lag (which on a bb turbo should not be there) and a very significant transition from no boost to boost
its like a huge kick in the back side..really savage ..so my thought is that the compressor has stalled because of the inability to vent excess pressure (at small throttle openings ) and if we bleed off a little it will keep it turning..
should help the drivability at low speeds too.
 

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Ylee,on my last car (Subaru Impreza) it didnt like a VTA dump valve at all,it caused uneven idle after a while with it fitted and overfuel at high revs,which isnt a bad thing apart from the consumption.

When i went and had it re-mapped, all the tuning companies advised me to remove the Forge VTA valve and fit either the OEM recirc valve or an aftermarket recirc.

The car ran perfect on the OEM item and i promptly sold the VTA unit.The other thing against VTA valves is the 'Saxo Boy ' sound they make,not very classy for a SAAB to be making too much noise.
 

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Discussion Starter #6
As I posted I had a recirc on to fit so I put it on tonight (after one of the bonnet catches giving up the ghost ..luckily I have a spare lying up at the moment )

took it out ..no woosh but i did not see any difference but the valve is set at its lowest
so I will play about with it to see if I can make a difference
 

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i just installed a Greddy dump valve on my friends WRX and it sounds good at low pressure but if its spooling for a while its not as loud...we have tightend and loosend it but nothing works...i have also noticed its idle drop...??

i liked Abbotts BOV on my 900se best sounding one yet!!
 

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OK ylee I'm with you on the problem now... but it doesn't sound right that the turbo has stalled completely- after all there is still a gas flow past it on both the exhaust and compressor side even at low revs e.g. from memory, mine (although not as big as yours
) sits at about -0.55bar on idle but -0.75bar on overrun.

I wonder if there is something not right with your turbo? If you remove the compressor output hose, you should be able to spin it freely.
 

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Discussion Starter #10
It spins freely...
But it does not when it is trying to push air into a dead space methinks
I will give it a good run tomorrow when I can see the boost gauge... it did not feel much different but....
 

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Having both an atmo type and a recirc type, I can say that there does not seem to be any noticeable difference in spool up times with either. Although, initial response upon gas application does seem quicker with the atmo type.
 

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ylee, I think I may be able to explain the problem.

BB turbos are known from going from zero-to-high boost very quickly. The transition RPM wise is much quicker than a conventional turbo, not because of a design fault, but because there is less friction to keep it from being this way.

To explain more thoroughly I'll need to explain a couple points. (No pun intended) In the exhaust housing there are two flow points which make a huge difference to the turbine:

1. The point at which significant pressure drop occurs, and the turbine can power the compressor.

2. The point at which the turbo hits a "brick wall" and cannot flow any more volume of air, and the air must then be compresses to pass through it. The amount of compression required, and indeed the amount your engine usually operates at, is equal to, or greater than, the pressure on the intake manifold.

As the engine pulls air in at low RPM on its own without aid of the turbo pressure, the compressor freewheels, and the flow of exhaust is not high enough to spin the turbo fast enough to move more air than the engine can take in on its own. Larger turbos have larger exhaust sides, which means more exhaust gas is required to cross this threshold. Once the threshhold is crossed, the resultant boost pressure creates more exhaust gas, which creates more turbo speed and intake gas, and the cycle continues until the wastegate is opened, or your engine blows to little bits. (Hopefully the former.) Since a BB turbo can spin up quickly, it takes much less time to cross this threshold.

This is not to say that turbos are either on or off. There is a transition curve between zero and no boost. However, without any friction, this curve only determines where the "runaway" threshold will be, and how much the wastegate must be opened to prevent it. With lots of friction, and inertial mass, the curve may cause the turbo to "ballance out" until the engine reaches a higher rpm and can power out of that friction. But on a BB turbo, especially a well designed GT series garrett with its revised flow characteristics, generally as soon as the turbo begins to force air into the engine, it's runaway cycle has begun.

Think of it this way ylee, your turbo may go from zero to
a little more abruptly than you'd like ... but on a similar sized non-BB turbo it would simply take a higher RPM to get your
. You could always get your software programmed to act like a non-BB turbo, but that would kind of defeat the point.

Dubbya~

p.s. Try not to think of a turbocharged engine as an "engine with an exhaust driven compressor". This may fit a textbook definition, but it is not truly accurate.

A turbo-charged engine is "an efficient hot gas generator for a turbine engine". And like any turbine engine, you have to operate within its flow limits and pressure curves. It's just never going to act exactly like a supercharged or naturally aspirated engine, and any attempt to make it do so only takes away from its functionality.
 

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, the resultant boost pressure creates more exhaust gas, which creates more turbo speed and intake gas, and the cycle continues until the wastegate is opened,[/b]
OK, Adrian I understand that (just) for WOT, but what about partial throttle when you don't have full boost- this bit is missing in your hypothesis and I think it is this area that ylee is trying to get to grips with.
 

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At part throttle you've simply reduced the amount of exhaust gas to drive the turbine. So it is too low to cross the "runaway" threshold, and the turbo doesn't spool more than just enough to freewheel. Closing the throttle is essentially the same (to the turbocharger) as making your engine "smaller". Likewise at part throttle it takes a higher RPM to get the flow to cross the threshold etc. Really the reason the design of a BOV has so little effect on spool up is that exhaust gas flow is far more important to it.

However BOV's make a big difference to spool DOWN rate, so between shifts a good BOV will release the "right" amount of air, a bad one will release too little or too much. Similarly wtih poorly adjusted ones. At part throttle when cruising, the BOV will have little effect on spool up when you hit the
pedal. If it stays open a little the turbo has no load and spins quickly, but none of that air goes into the engine, so it doesn't really help spool up. If it shuts quickly the air all goes to the engine, but the turbo has more load and spools slower.

Anyhow ... if you have boost lag "problems" at part throttle, they can easily be solved by going to WOT. If you have problems at WOT, see previous post.

Dubbya~
 

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Discussion Starter #16
So...
Cannot say it made any difference apart from at high boost where methinks the valve is opening at under 1.5 bars of boost
So I will crank the spring up but I think it is a lost cause..As adrian postulated the BOV has little effect on this particular turbo
The transition from just zero boost to full is incredibly quick except for that bit at the beginning
once it moves into the positive part of the gauge it is away

I just would like my instantaneous response back
(I have it now, I just do not like changing down...)
Ah well...
I will just have to be patient 'till june
 

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On my NG900 I’m using the Forge valve as a blow off valve with the piston facing the atmosphere. During idle the boost gauge reads vacuum and the Forge valve is open, but if I put my hand near the valve I can feel air coming out of it. So during idle there is enough exhaust gas flow for the compressor to create positive pressure. But the throttle plate and IAC keep the boost gauge from seeing the pressure and keep the valve open.

There is a pretty good benefit from this “leak”, it helps to cool the turbo and intercooler. I recently installed an intake air temp gauge and positioned the sensor just before the throttle body. During normal highway driving intake temps are 25-30 F above ambient. During city driving with lots of stopping intake temps are 30-40 F above ambient. Full boost runs will push it to 50-65 above, but max temp is after the throttle is let off. Takes 30-60 seconds to return to normal.

Anyway my valve is allowing the compressor to continuously pull in fresh air when the throttle is off. This cooler air removes heat from the compressor and intercooler more quickly than recirculated air which would be come thermally saturated.
Eric
- New home for my website soon!
 

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Discussion Starter #18
Tried my simple dump valve (bailey )tonight

result ..
spool up much improved as well as low speed drivability
seems to dump boost much more readily
noisier too
Its knocked almost a second off the 40 -100 figure
its just on 8 secs now
why ...
have not worked that out yet ...
 

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Perhaps you had a leak at full boost before?

My stock figure for 40-100 is around 9.5-10.0 seconds with a passenger, and I have quite a bit less
...



Hope you find out what it was!

Dubbya~
 
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