Saxperience - Citroen Saxo Forum - recirculating dump? or run without?

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- - recirculating dump? or run without? (http://www.saxperience.com/forum/showthread.php?t=389634)

the compressed air goes back through the compressor between the blades and the housing, i kow theres not much room but thats where it goes.

we'll just agree to disagree. i'll not worry about your engine you dont worry about mine,

I'm not worried about your car (nor mine that much to be fair!) - I was replying originally because I don't like to see misinformation on a subject that could end up actually damaging someone's engine if heeded.

I'm still waiting for you to link to where lag is reduced by NOT running a dump valve. :)

ive posted it about 5 times. the car made 1.8 bar after a gear change 0.2s quicker without a DV.

According to one of your "mates" who you claim works at garrett. You've provided no link or documentation to back up this claim.

However, garrett disagree. As does the rest of the industry. As per the links I posted, I've backed up everything I've claimed with major players documentation.

I'm still waiting for your links. :)

he isnt a mate, it was a private conversion so its pretty hard to provide a link to that,

Quote:

Originally Posted by williamsvts (Post 5513461)

he isnt a mate, it was a private conversion so its pretty hard to provide a link to that,

So in essence, one guy you know said something to you that you've taken as gospel truth.

The entire turbo industry disagrees (with which I've backed up from all major players, garrett, turbo dynamics, turbosmart, hks, greddy, grahame goode etc).

Don't you think that's a little bizarre? "The whole world is wrong, and I'm right because one person told me something". Are you honestly that naive?

only one of your qoutes say they reduce lag, turbo technics say they are SOMETIMES required on boost above 1 bar, and the rest explain what DV's do. i had come to that conlcusion before i spoke to this person. and from the test he done showed me it made boost quicker.

i'll be honest. i got it all from max power mag.

Just to clear this up one LAST time, here's what Garrett say on the subject:
http://www.turbobygarrett.com/turbob...r/faqs.html#t3

Quote:

What is compressor surge?
The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side.
Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself.

Common conditions that result in compressor surge on turbocharger gasoline engines are:

A compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body

So - we've established NOT running a dump valve causes surge or compressor stall (dictated by the amount of pressure built up in the intake system).

Given that...

Quote:

What is Turbo Lag?
Turbo lag is the time delay of boost response after the throttle is opened when operating above the boost threshold engine speed. Turbo lag is determined by many factors, including turbo size relative to engine size, the state of tuning of the engine, the inertia of the turbo's rotating group, turbine efficiency, intake plumbing losses, exhaust backpressure, etc.

So - we can see a direct correlation between the compressor stalling and lag.

Is that clear enough?

but you already have boost pressure built up in the intake. your starting with postive pressure as opposed to atmospheric.

Quote:

Originally Posted by williamsvts (Post 5513476)

but you already have boost pressure built up in the intake. your starting with postive pressure as opposed to atmospheric.

Agreed. And that positive pressure is effectively trying to turn your compressor wheel the wrong way as the shut TB is forcing air to go back into the compressor housing it's just come out of.

Lets say the turbo is spinning at 50,000 rpm whilst making boost. In your example, the turbo is forced to slow to perhaps 5,000 rpm because of the back pressure on the compressor wheel trying to force the blades the wrong way as the air has to go somewhere - obviously it's losing inertia slowing down like this. With a dump valve, the boost is allowed to escape the intake system, meaning the compressor is NOT slowed down by the air being forced back on it, and it is allowed to continue spinning with it's own momentum, perhaps retaining 80% of it's inertia. The time taken to recover the RPM from this near stall is FAR higher than the time taken the recover from a 20% loss.
(I've plucked these numbers of of the air but just to highlight the examples).

If we look on wikipedia for example:
http://en.wikipedia.org/wiki/Turboch...low_off_valves

Quote:

Turbocharged engines operating at wide open throttle and high rpm require a large volume of air to flow between the turbo and the inlet of the engine. When the throttle is closed compressed air will flow to the throttle valve without an exit (i.e. the air has nowhere to go).
This causes a surge which can raise the pressure of the air to a level which can damage the turbo. If the pressure rises high enough, a compressor stall will occur, where the stored pressurized air decompresses backwards across the impeller and out the inlet. The reverse flow back across the turbocharger causes the turbine shaft to reduce in speed more quickly than it would naturally, possibly damaging the turbocharger. In order to prevent this from happening, a valve is fitted between the turbo and inlet which vents off the excess air pressure. These are known as an anti-surge, diverter, bypass, blow-off valve (BOV) or dump valve. It is basically a pressure relief valve, and is normally operated by the vacuum in the intake manifold.
The primary use of this valve is to maintain the turbo spinning at a high speed.

Ever heard of Guy Croft? Engine building legend? Here's his personal take on it:

Quote:

The dump valve gets rid of surplus pressure when the throttle is closed thus preventing the compressor wheel from stalling. Repeated stall (or near stall) can fatigue the blades on the compressor wheel and a severe stall can burst it. Allowing the air to vent off keeps the compressor wheel spinning and thus enhances the turbocharger response when the throttle is re-opened.

I don't really recommend running a turbocharged unit without one, especially with boost pressures over 10psi or so.

Don't mean to take away from your debate gents. To the OP, I have personally found that the Bosch brand valve is very quiet, nearly silent. Also, have you thought of going with a dual port valve? Best of both worlds, recirculating and atmospheric. Most of the dual ports you can convert to a full atmospheric or full bypass using a plug(which they supply). Just my 2 cents

love it its like verbal tennis :boxing:

I'm just not sure how someone can continue to disagree in the face of total overwhelming evidence to the contrary. I've posted links from industry experts, engine builders, suppliers, tuners and even turbo manufacturers themselves, and yet here we are, still disagreeing. All because one person had a conversation about it. I don't understand. Hell - I hate being wrong, but when faced with the evidence as strong as this, even I would concede ;)

Loving the debate I personally don't know bugger all about turbo stall so I'll keep myself to myself. From reading most of the posts i feel like im getting more inteligent. :clapping: What I want to know is if I remove my dump valve. Will I get the famous flutter noise? If so, anyone want a Bailey dump valve lol.

Quote:

Originally Posted by stevo1600 (Post 5514636)

Yes you will. And it costs nothing. Right up until your turbo fails :p... You'll also loose response on gearchanges - exactly what the discussion is about :p

hopefully will see something different come out of this in the next few weeks

On 8psi though Ross? Will I notice any lag? Will it damage my turbo quickly?

Cheers for the help mate.:y:

So anyway - to clear this up, as William said he spoke to someone who worked at Garrett, I emailed Garrett this AM, and have now got a reply. Here's what they say when asked the direct question about damage and spool up time.

Here's my email to them:

Quote:

Guys

I hope you can quickly help clear up a discussion I’m having with your opinion on the matter.

I understood that adding a dump valve in the intake system will reduce compressor stall and decrease lag. Someone else is claiming that running WITHOUT a dump valve actually improves response (less lag).

Would you be so kind as to clarify the situation for me?

Thanks very much!

And their reply:

Quote:

Hello Ross,

I’m not sure what you mean by “dump valve,” so I’ll address my best hypothesis which is that it is a blow off valve.

If this is what it means, then I highly recommend using one in order to reduce the occurrence of surge in the turbo and significantly increase the life of the turbo. Blow off valves should absolutely be used on gasoline engines.

I would tell the person with whom you seem to be having this discussion the same thing.

Regards,

Garrett Gearhead

I replied:

Quote:

From: Dagley-Cleworth, Ross [mailto:Ross.Dagley-Cleworth@somewhere.com]
Sent: Monday, August 22, 2011 8:49 AM
To: Garrett Gearhead
Subject: RE: Turbo lag / dump valve

Hi again :)

I was under the impression that transient lag was reduced (ie, between shifts) – because of the buildup of pressure in the intake system forcing pressure back on the compressor wheel (stalling it).

Disregarding the damage that could be caused by not running a blow off valve, would you say the “pickup” of boost whilst shifting would be improved or worsened with a blow off valve?

Thanks very much for your thoughts thus far – it’s good to get it from “source”!

Ross

and finally, their reply:

Quote:

Hello,

There will likely be some advantage between shifts to utilizing a blow off, or dump valve. It helps to keep air in the system, while preventing surging of the wheel. Your general argument is correct in that time that the wheel doesn’t have to recover from surging it time that it can be working to produce useful boost.

Regards,

Garrett Gearhead

Hopefully that settles it :)

dam you! I have to budget for a poo valve aswell now lol! oh well no harm done