DIY FMIC Project....
26 August 2002, 10:44 PM
#662
Subaru Tuning Specialist
Join Date: Jun 2002
Location: 7.74 @179 mph 1/4 mile - road legal
Posts: 6,654
Likes: 0
Received 1 Like
on
1 Post
Dump valves do 4 things
1 Give the turbo an easy life when shutting the throttle on boost
2 Give the turbo an easy life when on part throttle (limiting surge)
3 Increase lag
4 Go woooosh !
So if you have a tough turbo such as one of the TD series........bin the dump valve !!
It's just a centrifugal compressor, like a hairdryer. Put your hand across the front of a hairdryer, what happens ? it increases in speed, why ? because due to the reduced flow the power requirement has dropped and the impeller spins faster.... allow flow and it loads up again.
Same with a turbo, block the dicharge (no dump valve) and it will overspeed/surge. Open the discharge to atmospheric (dump valve) and the work done increases therefore the turbo slows down and pressure is lost (read lag) before the throttle is opened again.
A commonly misused term 'stall' applies to the airflow, not the turbo rotor, when the throttle is snapped shut or high pressure exists at low flow. The turbo rotor spins away merrily doing very little work whilst the air makes 'cho cho cho' noises as it turbulates around the blades.
Hit the throttle again and not only do you have more residual pressure but you also have a turbo running at higher RPM waiting to work (boost) again.
1 Give the turbo an easy life when shutting the throttle on boost
2 Give the turbo an easy life when on part throttle (limiting surge)
3 Increase lag
4 Go woooosh !
So if you have a tough turbo such as one of the TD series........bin the dump valve !!
It's just a centrifugal compressor, like a hairdryer. Put your hand across the front of a hairdryer, what happens ? it increases in speed, why ? because due to the reduced flow the power requirement has dropped and the impeller spins faster.... allow flow and it loads up again.
Same with a turbo, block the dicharge (no dump valve) and it will overspeed/surge. Open the discharge to atmospheric (dump valve) and the work done increases therefore the turbo slows down and pressure is lost (read lag) before the throttle is opened again.
A commonly misused term 'stall' applies to the airflow, not the turbo rotor, when the throttle is snapped shut or high pressure exists at low flow. The turbo rotor spins away merrily doing very little work whilst the air makes 'cho cho cho' noises as it turbulates around the blades.
Hit the throttle again and not only do you have more residual pressure but you also have a turbo running at higher RPM waiting to work (boost) again.
27 August 2002, 12:59 AM
#663
Scooby Regular
Join Date: Nov 2001
Location: Leeds - It was 562.4bhp@28psi on Optimax, How much closer to 600 with race fuel and a bigger turbo?
Posts: 15,239
Likes: 0
Received 1 Like
on
1 Post
got some pics today/.. just pissed... and waiting for the police to come get me 
.... little bit of trouble in the pub 
.... little bit of trouble in the pub
27 August 2002, 01:29 AM
#667
Scooby Regular
Join Date: Nov 2001
Location: Leeds - It was 562.4bhp@28psi on Optimax, How much closer to 600 with race fuel and a bigger turbo?
Posts: 15,239
Likes: 0
Received 1 Like
on
1 Post
did you get a plate.. if not drop me an email and ill sort it this week.
David
David
27 August 2002, 05:15 AM
#669
Scooby Regular
Join Date: Feb 2002
Posts: 73
Likes: 0
Received 0 Likes
on
0 Posts
Quote
It's just a centrifugal compressor, like a hairdryer. Put your hand across the front of a hairdryer, what happens ? it increases in speed, why ? because due to the reduced flow the power requirement has dropped and the impeller spins faster.... allow flow and it loads up again.
Andy. F Incorrect, the whole purpose of the blow off valve is to reduce Lag, with out the blow off valve when the butterfly shuts in the throttle body the air flow past the compressor wheel stops because it cant flow anywhere with a blow off valve the pressure build up can be dumped away and hence allow the compressor wheel to continue spinning at its original rate ready to supply boost again once the butterfly opens.
It's just a centrifugal compressor, like a hairdryer. Put your hand across the front of a hairdryer, what happens ? it increases in speed, why ? because due to the reduced flow the power requirement has dropped and the impeller spins faster.... allow flow and it loads up again.
Andy. F Incorrect, the whole purpose of the blow off valve is to reduce Lag, with out the blow off valve when the butterfly shuts in the throttle body the air flow past the compressor wheel stops because it cant flow anywhere with a blow off valve the pressure build up can be dumped away and hence allow the compressor wheel to continue spinning at its original rate ready to supply boost again once the butterfly opens.
27 August 2002, 05:20 AM
#670
Scooby Regular
Join Date: Feb 2002
Posts: 73
Likes: 0
Received 0 Likes
on
0 Posts
And if you still dont believe me go to this site http://www.turbosmart.com.au/bov_aus.html and read it for your self a blow off valve eliminates turbo back spin which is when the sudden shock wave of air created when the butterfly closes and builds up high pressure between the throttle body and the turbo this high pressure flows back through the compressor wheel slowing it down or in some cases making it back spin.
[Edited by duczz - 8/27/2002 5:34:40 AM]
[Edited by duczz - 8/27/2002 5:34:40 AM]
27 August 2002, 05:27 AM
#671
Scooby Regular
Join Date: Feb 2002
Posts: 73
Likes: 0
Received 0 Likes
on
0 Posts
And Just in case that dosent sink in here is another article about blow off valves and their reason for being there.
A good analogy for what happens in a turbo system when you close the throttle is water hammer in household pipes when you turn the tap off too quickly. The same thing happens when your turbo is spinning at about 100,000 RPM, pumping pressurized air into the engine. When the throttle butterfly snaps shut for a gearshift, that air (which can be traveling at up to 90 meters per second) has nowhere to go except back the way it came and out through the turbo. This is bad for a number of reasons. First, slowing the turbo from maximum RPM to practically zero in less than a second is obviously going to place a lot of strain on the turbo. Secondly, since the turbo has stalled, the lag when you open the throttle again is increased.
The BOV is designed to release excess pressure under these conditions to allow the turbo to freewheel, which both prevents damage and reduces lag. The noise is achieved simply by venting the air externally.
A good analogy for what happens in a turbo system when you close the throttle is water hammer in household pipes when you turn the tap off too quickly. The same thing happens when your turbo is spinning at about 100,000 RPM, pumping pressurized air into the engine. When the throttle butterfly snaps shut for a gearshift, that air (which can be traveling at up to 90 meters per second) has nowhere to go except back the way it came and out through the turbo. This is bad for a number of reasons. First, slowing the turbo from maximum RPM to practically zero in less than a second is obviously going to place a lot of strain on the turbo. Secondly, since the turbo has stalled, the lag when you open the throttle again is increased.
The BOV is designed to release excess pressure under these conditions to allow the turbo to freewheel, which both prevents damage and reduces lag. The noise is achieved simply by venting the air externally.
27 August 2002, 07:25 AM
#673
Scooby Regular
Join Date: Oct 1999
Posts: 3,491
Likes: 0
Received 0 Likes
on
0 Posts
And Just in case that dosent sink in here is another article about blow off valves and their reason for being there.
Andy, I am correct in thinking that your dump valve is somewhere near your knock sensor ?
If so ... how do you cope with the part throttle surge ? Or don't you do part throttle ? 
I tried it once (of course less scientific than quoting articles) and with fast gearchanges there is *definately* less lag without a dumpvalve. Shame the turbo (VF23) wouldn't be able to handle it...
So no, it dosent sink in
27 August 2002, 11:50 AM
#676
Subaru Tuning Specialist
Join Date: Jun 2002
Location: 7.74 @179 mph 1/4 mile - road legal
Posts: 6,654
Likes: 0
Received 1 Like
on
1 Post
Duczz
Lets just agree to differ in opinion then, cos it's just not 'sinking in'
Every mod on my car has been proven by recording it's effect on a 'G' meter and/or pro timing equipment, The G meter graph tells me how much time the boost takes to build between gears.
With NO dump valve fitted I have quicker boost recovery between gears PROVEN FACT.
With NO dump valve fitted I have quicker 1/4 mile times and terminal speed PROVEN FACT.
Don't fall for the sales blurb
Andy
Theo, almost missed that one Yes, it sits on the same shelf
edited to say -
Part throttle surge is still an issue, I'm working on a system of proportioning the boost pressure to the throttle position, variable orifice bleed from the Dawes linked to the throttle looks workable
[Edited by Andy.F - 8/27/2002 12:03:31 PM]
Lets just agree to differ in opinion then, cos it's just not 'sinking in'
Every mod on my car has been proven by recording it's effect on a 'G' meter and/or pro timing equipment, The G meter graph tells me how much time the boost takes to build between gears.
With NO dump valve fitted I have quicker boost recovery between gears PROVEN FACT.
With NO dump valve fitted I have quicker 1/4 mile times and terminal speed PROVEN FACT.
Don't fall for the sales blurb
Andy
Andy, I am correct in thinking that your dump valve is somewhere near your knock sensor ?
Yes, it sits on the same shelf edited to say -
Part throttle surge is still an issue, I'm working on a system of proportioning the boost pressure to the throttle position, variable orifice bleed from the Dawes linked to the throttle looks workable
[Edited by Andy.F - 8/27/2002 12:03:31 PM]
27 August 2002, 12:04 PM
#678
Scooby Regular
Join Date: Nov 2001
Location: Leeds - It was 562.4bhp@28psi on Optimax, How much closer to 600 with race fuel and a bigger turbo?
Posts: 15,239
Likes: 0
Received 1 Like
on
1 Post
LMFAO at andy.. andy have you unplugged / removed the knock sensor so the ecu cant retard the ign. whilst err driving hard..?? and then just manually listen for det??
David
David
27 August 2002, 12:16 PM
#680
Scooby Regular
Join Date: Dec 2001
Posts: 3,418
Likes: 0
Received 0 Likes
on
0 Posts
As to no dump valve stalling the turbo...
Don't believe so as when setting up my Toyota Turbo engine I had a problem which I thought was related to the dump valve..
So I fitted the standard dump valve but the recircluate pipe was no where to be found so I put the dump valve on with a small bit of pipe and a cork clamped in the end.. so it couldn't dump basically and that proved the other dump valve was faulty as it ran fine.. and def no stalling of the turbo as it went like stink.
JGM
Don't believe so as when setting up my Toyota Turbo engine I had a problem which I thought was related to the dump valve..
So I fitted the standard dump valve but the recircluate pipe was no where to be found so I put the dump valve on with a small bit of pipe and a cork clamped in the end.. so it couldn't dump basically and that proved the other dump valve was faulty as it ran fine.. and def no stalling of the turbo as it went like stink.
JGM
27 August 2002, 12:48 PM
#681
Subaru Tuning Specialist
Join Date: Jun 2002
Location: 7.74 @179 mph 1/4 mile - road legal
Posts: 6,654
Likes: 0
Received 1 Like
on
1 Post
Yea, agree JGM, all that stalls is the airflow, not the turbo.
There is a lot of inertia built up in a rotor running at possibly 100k rpm, it's taken a lot of energy to get it there. The small amount of air trapped in the I/C will not 'stall' the rotor. Opening the compressor discharge to atmosphere will however load up the compressor and slow it more than the stalled air could.
When an aircraft wing stalls, its work done falls off dramatically ie no lift. Centrifugal compressor is the same, stall the air = no work done = less decellerating force = more residual RPM = less lag
There is a lot of inertia built up in a rotor running at possibly 100k rpm, it's taken a lot of energy to get it there. The small amount of air trapped in the I/C will not 'stall' the rotor. Opening the compressor discharge to atmosphere will however load up the compressor and slow it more than the stalled air could.
When an aircraft wing stalls, its work done falls off dramatically ie no lift. Centrifugal compressor is the same, stall the air = no work done = less decellerating force = more residual RPM = less lag
27 August 2002, 09:04 PM
#682
Scooby Regular
Join Date: Feb 2002
Posts: 73
Likes: 0
Received 0 Likes
on
0 Posts
Andy.F looks like we are going to have to agree to differ , although I might have an explanation for your negative experinces with blow off valve's, if the Aftermarket valve is too large for the application it is possible that its dumping too much of the boost build up I have heard of this in the past. If this is in fact the case it is better to fit a smaller blow off valve which dosent dump as much rather than go the other extreme and eliminate it all together. I am not judging my comments on the advertising hype but rather on physics. Finally if they really have no purpose other than to make a noise and increase lag then why would million dollar race teams still use them? Its not for turbo protection thats for sure as money isnt an option and turbos are replaced on a regular basis anyway. Hrmm worth thiking about.
27 August 2002, 09:19 PM
#683
Subaru Tuning Specialist
Join Date: Jun 2002
Location: 7.74 @179 mph 1/4 mile - road legal
Posts: 6,654
Likes: 0
Received 1 Like
on
1 Post
Duczz
Not aware that race teams use dump valves ? they tend to use 'pop off' valves which will only relieve pressure above normal full boost.
I notice you keep calling them blow off valves, is this the type of valve you mean ?
I know some (we tend to call them dump) valves have an adjustable spring which allows retention of some boost pressure, I'll meet you 1/2 way and agree that these are better than the factory item which dumps the lot.
This is however only because they retain 'some' boost in the manifold, I still say go the whole way and retain as much as possible though
Andy
Not aware that race teams use dump valves ? they tend to use 'pop off' valves which will only relieve pressure above normal full boost.
I notice you keep calling them blow off valves, is this the type of valve you mean ?
I know some (we tend to call them dump) valves have an adjustable spring which allows retention of some boost pressure, I'll meet you 1/2 way
and agree that these are better than the factory item which dumps the lot. This is however only because they retain 'some' boost in the manifold, I still say go the whole way and retain as much as possible though
Andy
28 August 2002, 12:19 AM
#685
Scooby Regular
iTrader: (1)
Join Date: Mar 2002
Location: Norn Iron
Posts: 2,786
Likes: 0
Received 0 Likes
on
0 Posts
Not sure about the race teams using them, but the pop off valve you refer to is a measure the race governing body uses to ensure nobody uses too much boost ( and hence limits horsepower to an extent )
They will vent to atmos if the boost presssure is too high. They are a sealed unit, and must be fitted.
As for the dump valve arguement, I know of someone who turbocharged his Ka 1.3 ( why??? ), the difference a dump valve made to this car was unbelieveable. The reduction of lag between gearchanges was staggering. The turbo fitted to the car is probably completely the wrong size for the engine, and a total mismatch, but the dump valve did what it claimed.
They will vent to atmos if the boost presssure is too high. They are a sealed unit, and must be fitted.
As for the dump valve arguement, I know of someone who turbocharged his Ka 1.3 ( why??? ), the difference a dump valve made to this car was unbelieveable. The reduction of lag between gearchanges was staggering. The turbo fitted to the car is probably completely the wrong size for the engine, and a total mismatch, but the dump valve did what it claimed.
28 August 2002, 12:35 AM
#686
Scooby Regular
Join Date: Feb 2002
Posts: 73
Likes: 0
Received 0 Likes
on
0 Posts
As far as comparing a compressor wheel to a Aeroplane wing you might as well not bother they are completly different. For one an aeroplane wing is changing its angle of attack all the time where as a compressor wheel is fixed and when you say its the air flow stalling that is incorrect also air cannot stall air is the medium in which the wing stalls when it reaches a certain angle of attack! Hrmm the turbo dosent have to stall it can simply be slowed down by air flowing back past the the compressor wheel when the butterfly closes.
28 August 2002, 12:53 AM
#687
Scooby Regular
iTrader: (1)
Join Date: Mar 2002
Location: Norn Iron
Posts: 2,786
Likes: 0
Received 0 Likes
on
0 Posts
Ive seen a few sheared shafts, and not from knackered bearings, so obviously there must be situations where the compressor is slowed down very quickly. The air has to go somewhere, and if it cant go forward, the only place is back from where it came, and through the compressor, which must slow/stop the wheel turning. Is every case of the compressor wheel explodig/breaking up, to do with overspeeding, or can it stalling cause it too?
Im not saying a dump valve is essential by any means, just that they do allow the pressure to escape, and give the turbo an easier life, like they claim.
Im not saying a dump valve is essential by any means, just that they do allow the pressure to escape, and give the turbo an easier life, like they claim.
28 August 2002, 12:54 AM
#688
Subaru Tuning Specialist
Join Date: Jun 2002
Location: 7.74 @179 mph 1/4 mile - road legal
Posts: 6,654
Likes: 0
Received 1 Like
on
1 Post
First, slowing the turbo from maximum RPM to practically zero in less than a second is obviously going to place a lot of strain on the turbo. Secondly, since the turbo has stalled, the lag when you open the throttle again is increased.
Consider how much energy from the 600+ degree exhaust gases must be used to initially spool the turbo to speed.
Then, do you really think there is enough energy in less than 1 cubic foot of air to halt that 100,000 rpm turbo rotor during a swift gearchange ?
I don't
Stevie, I've always claimed they give the turbo an easier life, read back the thread.
[Edited by Andy.F - 8/28/2002 12:56:34 AM]
28 August 2002, 01:01 AM
#689
Scooby Regular
iTrader: (1)
Join Date: Mar 2002
Location: Norn Iron
Posts: 2,786
Likes: 0
Received 0 Likes
on
0 Posts
That 1 cubic foot ( where did that bit come from?? ) must have some energy, as it makes a lot of noise on the way out. It is also 1 cubic foot of compressed air, which may have a lot of energy.
Compressed air, in the right/wrong circumstances can be very powerful thing.
Compressed air, in the right/wrong circumstances can be very powerful thing.
28 August 2002, 01:05 AM
#690
Scooby Regular
Join Date: Feb 2002
Posts: 73
Likes: 0
Received 0 Likes
on
0 Posts
It dosent take long for the turbo to spool up and it dosent take long for one to slow down either if it is hit by a blast of air returning from the intercooler, this is what does put a high stress on the main shaft.