recirculating dump? or run without?

[loudandproud205]

Rubber chicken over the outlet so it quaaaaaaaaaccckkss instead of psssssstttt

I had to ditch the HKS BOV on the MR2 due to the air not being measured correctly and causing it to run silly rich, so am now back on recirc one and I prefer my old Quuuuuaaaaaaaaakkkk

[Jungle]

Just to add my 2 pence worth...

I dont run a dump valve, purely to get that sexy turbo chatter noise. And so far I havent had a problem with turbo faliure or lag. My mates astra doesnt run any sort of dump valve either and he hasnt had any problems with over 2bar through a GT30.

[tepson]

used to have a starlet gt only running standard boost 0.65 bar and messed about with the dump valve set up loads.
removing the standard bov and blocking it off made it pretty much silent.
but then fitted an air filter and it would chatter. none of which made any difference in spool time and caused no damage to the turbo.

Then refitted the standard dump valve with the air filter still fitted and it would dump louder than with a normal aftermarket bov on.

So basically i think u should just see what your set up is at the time and have a play.

[williamsvts]

take it off. tests have been done and with dump valve you end up with atmospheric pressure in inlet tract, without dump valve you still have positive pressure in inlet. and it wont instantly kill the turbo. dont think norris designs use one on their 1000hp evo so i wouldnt worry about it on a 200hp engine.

[williamsvts]

Originally Posted by loudandproud205

Rubber chicken over the outlet so it quaaaaaaaaaccckkss instead of psssssstttt

I had to ditch the HKS BOV on the MR2 due to the air not being measured correctly and causing it to run silly rich, so am now back on recirc one and I prefer my old Quuuuuaaaaaaaaakkkk

that can only cause it to run rich on closed throttle with the engine slowing. as thats the only time the DV is open, which is the only time the air entering the engine is different to air passing the MAF.

[Rossp]

Originally Posted by williamsvts

take it off. tests have been done and with dump valve you end up with atmospheric pressure in inlet tract, without dump valve you still have positive pressure in inlet. and it wont instantly kill the turbo. dont think norris designs use one on their 1000hp evo so i wouldnt worry about it on a 200hp engine.

I totally disagree.

A dump valve is there to prevent compressor surge. Running low boost this isn't so much of a problem, but higher boost will harm the turbo quickly. Not only that, with a stalled compressor wheel, you're introducing more lag as the turbo has to spin up "from scratch" again.

[williamsvts]

compressors dont stall. i got the informantion from an engineer at garret that done a test on a 2l engine running 1.8 bar, when he shut the throttle with DV pressure went to 0 psi, when he done the same test without DV pressure only dropped to 7 psi. he also said that if a turbo is designd to last 100,000 mile. running without a DV MAY shorten that to 90,000 mile. he said it hasnt been proven that no DV CAUSES turbo failure.

its not my opinion, its what ive been told by someone in the turbo industry.

also on newer ball bearing trubos the load is spread more equal compared to the old journal bearings.

[Rossp]

I still disagree completely I'm afraid.

My information comes from Alan Allard's book on Turbo and Supercharging. He specifically mentions compressor stall. http://www.allardturbosport.co.uk/

General opinion is compressors do stall. Frequently. I've certainly heard it on every car i've had when I've run with an incorrectly setup dump valve, or no dump valve at all. And the effects are obvious when driving. MUCH more lag between gearchanges.

Wikipedia - http://en.wikipedia.org/wiki/Blowoff_valve

99% of all modern turbo cars as standard are fitted with dump valves for this very reason. Your engineer seems to have a different opinion to everyone else, including some very respectable sources (Such as Alan above).

For example: Graham Goode - http://www.grahamgoode.com/dump.htm:

Why do manufacturers fit a dump valve?

It is for a number of reasons. Without a dump valve, when the throttle was closed, the build up of boost pressure would be immense, very similar to closing a quickly flowing water tap. This excess of pressure, (which could be over five times the running boost pressure), would put the components of the system under a great strain. It would try to burst the intercooler and pipework. More significantly, it would put a large strain on the turbocharger itself, firstly stalling the compressor shaft, then trying to force the compressor wheel out of the turbo, against its bearings.
So, the dump valve prevents mechanical damage to the intercooler & turbo pipework, prolongs turbocharger life, and prevents excessive turbo - lag that would be caused by the compressor shaft stalling.

Turbo Technics - http://www.turbotechnics.com/turbo/faq.htm

What is a dump valve ?

A valve which relieves back-pressure between the compressor outlet and engine as the throttle is closed. Only sometimes required on engines producing in excess of 14psi boost pressure.

HKS - http://www.hksusa.com/categories/?id=1062

Blow-Off & Bypass Valves have one goal, prevent compressor surge. Compressor surge occurs when the throttle plate of a turbocharged engine closes and the pressurized air is forced back into the turbocharger’s compressor housing causing the turbo to slow down. Compressor surge not only decreases the spool of a turbo but can also damage its center cartridge.

TurboSmart - http://www.turbosmartonline.com/index.php?id=624

When a gear change occurs, the throttle which allows air to flow into the engine is closed.

This results in a large pressure spike as the turbocharger is still trying to flow air due to the inertia of the compressor and turbine but there is nowhere for the air to go as the throttle is closed. The pressure within the intercooler piping continues to increase until the compressor reaches its pressure limit. When it reaches this limit, it cannot flow the air any more and the built up air pressure inside the intercooler and pipe begins to flow backwards through the compressor which is trying to flow air forwards.

The result of this reversal flow of air is the immediate deceleration of the turbocharger and a high load on the bearings which support the compressor/turbine shaft. At low turbo speeds and low pressures, the deceleration of the turbo and the load on the bearings is low, i.e. a small amount of fluttering at low engine speeds and throttle movements is negligible.

At high turbo speeds and high pressures, compressor surge during a gear change can be damaging to the bearings of the turbocharger as the deceleration rate of the turbocharger is high and the reversal airflow through the compressor is high.

Greddy - http://www.greddy.com/products/turbo...ow-off-valves/

[GReddy] Blow Off Valves [are easily adjustable to] eliminate compressor surge and preventing premature boost leakage, while increasing boost response between shifts.

And finally, Garrett themselves - http://www.turbobygarrett.com/turbob...o_tech101.html

The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.

With respect, your engineer at garrett has it wrong.

[williamsvts]

ive never noticed a difference in lag with and without a DV. all cars without a DV make the noise of compressor surge.

that wasnt his opinion, it was the results from a test he had done.

modern cars are fitted with recircs to keep the noise down.

i will continue to run without a DV untill i see tests that show quicker pickup with one than without.

theres also a few of my mates that dont use them and have never had a problem either.

i dnt see how you can notice loads of lag without, as without you already have boost pressure ready when you open the throttle, without it you need to build all the pressure up from atmospheric.

[williamsvts]

i was looking into this as i want my new engine build to be perfect.
what he said about 'compressor stall'


Stall has nothing to do with turbo speed, its aerodynamic stall i.e. flow separates from the blades creating transient surge (fluttering noise). Unless you're going to run 3bar+ of boost pressure this shouldn't kill the turbo excessively quickly, put the BOV in the bin and be done with it.

so it seems the shaft/blades dont stop spinning.

[williamsvts]

trying to find the email about the tests.

[williamsvts]

When I did tests running with an without a DV logging the boost before the intercooler, car running 1.8bar

With DV, close throttle, pressure drops to atmospheric immediatley, re-open throttle boost build from 0 back to 1.8bar

Without DV, close throttle, pressure drops slowly, change gear and re-open throttle but pressure still at 0.7bar. Boost then builds back to 1.8bar bar at the same rate but from a higher starting point.

My car was pretty low lag and it saved about 0.2s. This is something only the logs showed as on the road you could barely feel the difference.

with what im spending on my new engine/turbo build i dont want to kill it, so if i had getting and information to say that not using a DV will kill it i would be using one.

[maddison_vts]

I never used any form of dump valve on the saxo at 9.5psi and never had an issue with it. As ross said though, it really doesn't matter at mild boost.

My s14a had a re-circ when i bought it and running 22psi you don't hear anything on shut off. I removed the re-circ didn't have any issues apart from it would let a few flames and bangs out when you lift off but this was due to it being afm so it chucked extra fuel in expecting air to be going in with it as it normally would with a re-circ.

There is a lot of debates about it damaging the turbo or not, personally I never use a dump valve of any description as I enjoy listening to the chatter. A good friend of mine works at a well reputed engine tuner and he is dead against it and therefore always runs a re-circ on his gtr. On the flip side though, a guy who's in the same circle of mates has a sierra cosworth running 30psi on a gt30 and he runs no dump valve, so far no problems.

[Rossp]

Originally Posted by williamsvts

i dnt see how you can notice loads of lag without, as without you already have boost pressure ready when you open the throttle, without it you need to build all the pressure up from atmospheric.

The reason is simple physics. When you come off throttle, the boost that's in that pipework tries to take the path or least resistance. During transient throttle positions, that path is into the engine (the engine draws air in, whilst there is pressure from the turbo pushing more air out). When the throttle body is closed, that air still wants to take the path of least resistance - it cannot pass the (closed) TB, so attempts to go back out the turbo compressor, trying to vent to atmosphere via your air filter. The turbo stalls because the air in the intake pipes is fighting the rotational force of the exhaust gasses still coming out of the exhaust and subsequent centripetal forces of the shaft.

The noise you hear (the flutter) is the air being forced across the compressor blades the wrong way.

The lag is caused by the turbo spinning to a STOP by these reverse forces - when the dump valve is introduced, the boost is lost, but the turbo is allowed to continue spinning, improving response.

I have listed many respected sources who have said that a dump valve should be used - including two separate turbo manufacturers (one of those being Garrett who you say your contact works for). Other than your posts, I'm yet to see anyone in a respected area of the industry vote against the dump valve.

In your last post you mention the guy had pressure without the dump valve remaining at 0.7 bar. Of course he did. There was no where for it to vent to. As it can't go down the TB, where do you think it's going to try and go? Reversing into the compressor wheel, stalling it.

I'm interested to see another perspective on the idea, but it's contrary to established and recognised industry practice - including the makers of the very components we're discussing. It's also something I've not come across in the 15+ years I've been modifying turbo cars.

[williamsvts]

the compressor does not stop or stall, its explained above.

thats my whole arguement, how can you have more lag when you have 0.7bar ready, as opposed to 0 bar. in his test it reached 1.8 bar 0.2 sec quicker without a DV, as posted above.

im posting what tests have shown.

and i will continue not to use them till i get proven otherwise.

[Rossp]

Ok William, listen to him. Ignore everyone else

You've ignored the information I've given you - even your own counter-argument about this 0.7bar pressure proves my point - that's 0.7bar that's trying to escape out the compressor housing, via the compressor wheel. That's the reason you get flutter (noise) - you're stalling the compressor wheel.

You honestly want to argue the compressor wheel doesn't stall? Really? Even with all the links I've given you? Turbo Dynamics, Garrett etc? ALL saying it does??

Can you find a single argument by a reputable source, published on the internet backing up your perspective?

You've BEEN proven otherwise. By all the links I've posted. With respect, I think you're being a little blinkered on the subject by refusing to see another perspective. Even that of the turbo manufacturers. You're posting what you've heard from one person. the rest of the INDUSTRY has spoken, as I've highlighted to you.

As the saying goes, you can take a horse to water, but you can't make it drink

[williamsvts]

I haven't been proven wrong, the guy I was talking to explains its the air that stalls not the compressor. I have never see or heard of a turbo dying prematurely due to no DV and have never seen any proof to show that lag is reduced with a DV.

[Rossp]

You understand what compressor stall is... right?

http://en.wikipedia.org/wiki/Compressor_stall

Axi-symmetric stall, more commonly known as compressor surge; or pressure surge, is a complete breakdown in compression resulting in a reversal of flow and the violent expulsion of previously compressed air out through the engine intake, due to the compressor's inability to continue working against the already-compressed air behind it.

Given the above...

This is valid - http://www.turbobygarrett.com/turbob...o_tech101.html

Blow-Off (Bypass) Valves
The Blow-Off valve (BOV) is a pressure relief device on the intake tract to prevent the turbo’s compressor from going into surge. The BOV should be installed between the compressor discharge and the throttle body, preferably downstream of the charge air cooler (if equipped). When the throttle is closed rapidly, the airflow is quickly reduced, causing flow instability and pressure fluctuations. These rapidly cycling pressure fluctuations are the audible evidence of surge. Surge can eventually lead to thrust bearing failure due to the high loads associated with it.
Blow-Off valves use a combination of manifold pressure signal and spring force to detect when the throttle is closed. When the throttle is closed rapidly, the BOV vents boost in the intake tract to atmosphere to relieve the pressure; helping to eliminate the phenomenon of surge.

I really can not explain it any clearer than that. Now thats cleared up, lets address lag (although it should be self explanatory at this stage).

From here - http://www.turbosmartonline.com/index.php?id=497

The secondary function of a BOV is to reduce the “turbo lag” effect between gear changes. Without a BOV, the compressor surge slows the turbine down, which then takes longer to spool up again when the throttle is opened. With a BOV, the excess boost pressure is released, keeping the turbine spinning and thus reducing the turbo lag effect.

TurboSmart even have a nice picture to explain:


Whilst they are singing their own praises (obviously) the principle they're talking about (the peaks and troughs of the boost cycle) are valid

[williamsvts]

the above explains that the airflow changes direction, not the the turbo stops spinning. what happens to the exhaust gasses then if the turbo stops spinning then?

i can understand how your point is valid that the turbo will be spinning faster using a DV than without, but the guy i spoke to about it used datalogging equipment in his engine and recorded a 0.2s quicker spool up, because of still having + pressure in the inlet tract.

even if the turbos life is shortend by about 10% its still not much.

you dont need to explain anything to me. ive got a good understanding of it all and im deciding not to use a DV on my new build.

[Rossp]

Originally Posted by williamsvts

the above explains that the airflow changes direction, not the the turbo stops spinning. what happens to the exhaust gasses then if the turbo stops spinning then?

http://www.turbosmartonline.com/index.php?id=497

Without a BOV, upon shutting the throttle, the pressurised air caught between the free-spinning turbo and the shut throttle is forced back through the turbine blades, this, in turn, forces the turbine to slow down or stall. This phenomenon is often referred to as “compressor surge” or “cavitation” and it places undue load on the turbocharger bearings, significantly shortening the lifespan of the turbo or even causing serious damage.

There is reduced exhaust gasses because you've shut off the throttle (and therefore fuel and air) - the flutter you hear is the reversed airflow trying to turn the compressor the wrong way. It's a flutter because the compressor wheel and the turbine wheel are locked in battle to spin in opposite directions. Eventually the flutter stops - because the turbine wheel wins out once the pressure in the boost system is reduced to the point where the turbo can spin normally.