Dawes device has a dezign issue...but its an easy fix.
06 November 2005, 11:07 PM
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Dawes device has a dezign issue...but its an easy fix.
Recently fitted a Dawes manual boost controller and was well chuffed with the instant and predicable 1 bar of boost each time I floored it.
However, on day two, mid overtake, when I changed from 2nd to 3rd the expected power was gone (it took about 2 or 3 seconds to build full boost, then I was off again and safe). I found a piece of clear road and tried again....same symptom. Engaged brain briefly, and all became clear
The Dawes device sits between the turbo outlet presure sensing take off, and the turbo wastegate actuator. When turbo presure rises to its preset maximum, this overcomes the springloaded ball valve in the Dawes device and presure is fed to the wastegate actuator to open it and limit boost presure. When you lift off the accelerator to change gear, the ball valve closes and the trapped leg of presure between the dawes device and the wastegate actuator is bled away through a tiny drilled hole in the Dawes device (otherwise the wastegate remains wide open merrily bleeding away most of the boost presure, till the cows come home!).
So what happens if that drilling gets blocked? Well, "the wastegate remains wide open merrily bleeding away most of the boost presure, till the cows come home!". And if its only partially blocked, you will eventually get full boost back. . . . . Which is where the story began.
So I got a fine needle, wiggled it around in the drilling, took the car out and all was well again. However, knowlege is great, so I looked at how power was being delivered. In 2nd to the red line, max boost achieved and held, change to third and hard on it, boost presure rises to maximum over about one to one and a half seconds (way better than the standard boost control system, but still feels a bit sanitised). If you think about it, when you are changing gear, the Dawes device has about half a second to bleed away the presure to the Wastgate actuator (thus allowing it to close before the next application of boost, if it is slow bleeding away the presure, the wastegate will still be partially open [still closing] and boost will be artificially reduced).
All that controls the speed at which this trapped presure is decayed, is the size of the drilling. The size is clearly a dezign compromise, if your running 6psi of boost (0.4 bar) and the drilling is too large, you wont have enough presure to open the wastegate and regulate boost presure. But if your running 14.7 psi of boost (1 bar) as most of us could easily be doing, the small drilling is not bleeding away presure quick enough and the slow build in boost is apparent.
Are you all still with me!!!
Focus...Focus....were nearly there!!
So out came the drill....what size can I drill out the vent hole too? If I get it wrong too much presure will be bled away and the wastegate will not close. In the end I went for the smallest drill I had (39 thou/1mm), threw caution to the wind and went for it (Compared to the original hole its now HUGE!)
On the road I cautiously applied full throttle and watched boost rise and steady at 15psi (a smidge over 1 bar) with a spike at about 15.5psi before it steadied. OK, so Im reasonably safe, lets try it through the gears. Went for it......no problem, if anything boost presure is more under control when driven hard at slightly under 15 psi, with less spiking evident. But the best bit is that period when boost use to slowly build...now its Instant-Full Boost as soon as you floor it (the effect on acceleration is grin inducing!). Checked it on my GTech performance meter, and 0-60 has fallen by 0.15 of a second to 5.3 (and that only requires one gearchange!)
If anyone is brave enough to modify their Dawes device, I cant see why you shouldn't get the same result (but dont point the finger of blame if it doesn't, Im only telling you the sucess I achieved on my car).
Enjoy
Pedro
However, on day two, mid overtake, when I changed from 2nd to 3rd the expected power was gone (it took about 2 or 3 seconds to build full boost, then I was off again and safe). I found a piece of clear road and tried again....same symptom. Engaged brain briefly, and all became clear
The Dawes device sits between the turbo outlet presure sensing take off, and the turbo wastegate actuator. When turbo presure rises to its preset maximum, this overcomes the springloaded ball valve in the Dawes device and presure is fed to the wastegate actuator to open it and limit boost presure. When you lift off the accelerator to change gear, the ball valve closes and the trapped leg of presure between the dawes device and the wastegate actuator is bled away through a tiny drilled hole in the Dawes device (otherwise the wastegate remains wide open merrily bleeding away most of the boost presure, till the cows come home!).
So what happens if that drilling gets blocked? Well, "the wastegate remains wide open merrily bleeding away most of the boost presure, till the cows come home!". And if its only partially blocked, you will eventually get full boost back. . . . . Which is where the story began.
So I got a fine needle, wiggled it around in the drilling, took the car out and all was well again. However, knowlege is great, so I looked at how power was being delivered. In 2nd to the red line, max boost achieved and held, change to third and hard on it, boost presure rises to maximum over about one to one and a half seconds (way better than the standard boost control system, but still feels a bit sanitised). If you think about it, when you are changing gear, the Dawes device has about half a second to bleed away the presure to the Wastgate actuator (thus allowing it to close before the next application of boost, if it is slow bleeding away the presure, the wastegate will still be partially open [still closing] and boost will be artificially reduced).
All that controls the speed at which this trapped presure is decayed, is the size of the drilling. The size is clearly a dezign compromise, if your running 6psi of boost (0.4 bar) and the drilling is too large, you wont have enough presure to open the wastegate and regulate boost presure. But if your running 14.7 psi of boost (1 bar) as most of us could easily be doing, the small drilling is not bleeding away presure quick enough and the slow build in boost is apparent.
Are you all still with me!!!
Focus...Focus....were nearly there!!
So out came the drill....what size can I drill out the vent hole too? If I get it wrong too much presure will be bled away and the wastegate will not close. In the end I went for the smallest drill I had (39 thou/1mm), threw caution to the wind and went for it (Compared to the original hole its now HUGE!)
On the road I cautiously applied full throttle and watched boost rise and steady at 15psi (a smidge over 1 bar) with a spike at about 15.5psi before it steadied. OK, so Im reasonably safe, lets try it through the gears. Went for it......no problem, if anything boost presure is more under control when driven hard at slightly under 15 psi, with less spiking evident. But the best bit is that period when boost use to slowly build...now its Instant-Full Boost as soon as you floor it (the effect on acceleration is grin inducing!). Checked it on my GTech performance meter, and 0-60 has fallen by 0.15 of a second to 5.3 (and that only requires one gearchange!)
If anyone is brave enough to modify their Dawes device, I cant see why you shouldn't get the same result (but dont point the finger of blame if it doesn't, Im only telling you the sucess I achieved on my car).
Enjoy
Pedro
06 November 2005, 11:34 PM
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Cancel the patents
And there was I thinking Id found the Holy Grail.
Searched the forums looking for guidance before I drilled it, but found nothing.
We live and learn
Pedro
Searched the forums looking for guidance before I drilled it, but found nothing.
We live and learn
Pedro
07 November 2005, 11:08 AM
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Hmm do you have to take it apart to do it properly, as i really cant be arsed to set it all back up again.
I have 02WRX running 1.1 bar, with a few other mods, what is the standard size of Dawes and if i did drill it out, what size to go for?
Cheers
Cookie
I have 02WRX running 1.1 bar, with a few other mods, what is the standard size of Dawes and if i did drill it out, what size to go for?
Cheers
Cookie
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07 November 2005, 07:52 PM
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Don't agree
Zen
Have you seen how tiny the standard drilled hole is? Only the tip of a sewing needle would go in, and it was a tiny needle. Id speculate the hole is between 5 and 10 thou of an inch.
Unless your going to ultrasonically clean the the entire inlet system and turbo, the likelihood of a piece of dirt, or even a trace of oil (for which the Impreza is famous for clogging up the standard boost orifice) getting into the drilled hole is high. Dawes recognise this because the fitting instructions tell you to check the drilling is clear once a month.
The original dezign "compromise" I quoted, was that a wastegate set to fully open at low presure (eg: 0.4 bar) needs a small orifice or the operating presure will bleed away without opening the wastegate. But on a scooby running 1 bar there is more than enough presure to do the job even with a larger drilling. Look at the reply regarding a 1.7 bar operating presure which worked perfectly with a 1.5 mm drilling (thats a mine shaft, not a drilling!).
For Cookie
I believe you can take out the dawes device intact and redrill the hole to 1mm (its at the actuator end of the Dawes device), if your worried then start smaller and work up (drills are cheap). But do be carefull with very small drills because you realy dont want to snap one in the hole. This is ten minutes work max!
IMHO you will not need to dismantle or reset the Dawes in the course of doing this. But do be cautious when you first drive the car afterwards, let boost peak and hold in a high gear first to see if the maximum boost has rissen (do a before and after comparisson to be certain). I would be dissapointed if you didn't notice a considerable improvement in the way the car delivers power.
Im eagerly awaiting feedback from someone who does this and gets the grin factor I did.
Pedro
Have you seen how tiny the standard drilled hole is? Only the tip of a sewing needle would go in, and it was a tiny needle. Id speculate the hole is between 5 and 10 thou of an inch.
Unless your going to ultrasonically clean the the entire inlet system and turbo, the likelihood of a piece of dirt, or even a trace of oil (for which the Impreza is famous for clogging up the standard boost orifice) getting into the drilled hole is high. Dawes recognise this because the fitting instructions tell you to check the drilling is clear once a month.
The original dezign "compromise" I quoted, was that a wastegate set to fully open at low presure (eg: 0.4 bar) needs a small orifice or the operating presure will bleed away without opening the wastegate. But on a scooby running 1 bar there is more than enough presure to do the job even with a larger drilling. Look at the reply regarding a 1.7 bar operating presure which worked perfectly with a 1.5 mm drilling (thats a mine shaft, not a drilling!).
For Cookie
I believe you can take out the dawes device intact and redrill the hole to 1mm (its at the actuator end of the Dawes device), if your worried then start smaller and work up (drills are cheap). But do be carefull with very small drills because you realy dont want to snap one in the hole. This is ten minutes work max!
IMHO you will not need to dismantle or reset the Dawes in the course of doing this. But do be cautious when you first drive the car afterwards, let boost peak and hold in a high gear first to see if the maximum boost has rissen (do a before and after comparisson to be certain). I would be dissapointed if you didn't notice a considerable improvement in the way the car delivers power.
Im eagerly awaiting feedback from someone who does this and gets the grin factor I did.
Pedro
07 November 2005, 07:59 PM
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I think you misunderstand. Dawes Devices can be purchased (or used to be) with different sized holes, one was 0.5mm and the other 1mm (maybe even 1.5). The Dawes valve just uses a commonly available pneumatic valve, usually one manufactured by Grainger.
The size of the bleed hole will dicate the type of response you get with the dawes valve, so you can size it according to the dynamics of your turbo. It's not a design flaw, just another variable that needs to be matched to the rest of the system.
Paul
The size of the bleed hole will dicate the type of response you get with the dawes valve, so you can size it according to the dynamics of your turbo. It's not a design flaw, just another variable that needs to be matched to the rest of the system.
Paul
Originally Posted by Pedroskate
Zen
Have you seen how tiny the standard drilled hole is? Only the tip of a sewing needle would go in, and it was a tiny needle. Id speculate the hole is between 5 and 10 thou of an inch.
Unless your going to ultrasonically clean the the entire inlet system and turbo, the likelihood of a piece of dirt, or even a trace of oil (for which the Impreza is famous for clogging up the standard boost orifice) getting into the drilled hole is high. Dawes recognise this because the fitting instructions tell you to check the drilling is clear once a month.
The original dezign "compromise" I quoted, was that a wastegate set to fully open at low presure (eg: 0.4 bar) needs a small orifice or the operating presure will bleed away without opening the wastegate. But on a scooby running 1 bar there is more than enough presure to do the job even with a larger drilling. Look at the reply regarding a 1.7 bar operating presure which worked perfectly with a 1.5 mm drilling (thats a mine shaft, not a drilling!).
For Cookie
I believe you can take out the dawes device intact and redrill the hole to 1mm (its at the actuator end of the Dawes device), if your worried then start smaller and work up (drills are cheap). But do be carefull with very small drills because you realy dont want to snap one in the hole. This is ten minutes work max!
IMHO you will not need to dismantle or reset the Dawes in the course of doing this. But do be cautious when you first drive the car afterwards, let boost peak and hold in a high gear first to see if the maximum boost has rissen (do a before and after comparisson to be certain). I would be dissapointed if you didn't notice a considerable improvement in the way the car delivers power.
Im eagerly awaiting feedback from someone who does this and gets the grin factor I did.
Pedro
Have you seen how tiny the standard drilled hole is? Only the tip of a sewing needle would go in, and it was a tiny needle. Id speculate the hole is between 5 and 10 thou of an inch.
Unless your going to ultrasonically clean the the entire inlet system and turbo, the likelihood of a piece of dirt, or even a trace of oil (for which the Impreza is famous for clogging up the standard boost orifice) getting into the drilled hole is high. Dawes recognise this because the fitting instructions tell you to check the drilling is clear once a month.
The original dezign "compromise" I quoted, was that a wastegate set to fully open at low presure (eg: 0.4 bar) needs a small orifice or the operating presure will bleed away without opening the wastegate. But on a scooby running 1 bar there is more than enough presure to do the job even with a larger drilling. Look at the reply regarding a 1.7 bar operating presure which worked perfectly with a 1.5 mm drilling (thats a mine shaft, not a drilling!).
For Cookie
I believe you can take out the dawes device intact and redrill the hole to 1mm (its at the actuator end of the Dawes device), if your worried then start smaller and work up (drills are cheap). But do be carefull with very small drills because you realy dont want to snap one in the hole. This is ten minutes work max!
IMHO you will not need to dismantle or reset the Dawes in the course of doing this. But do be cautious when you first drive the car afterwards, let boost peak and hold in a high gear first to see if the maximum boost has rissen (do a before and after comparisson to be certain). I would be dissapointed if you didn't notice a considerable improvement in the way the car delivers power.
Im eagerly awaiting feedback from someone who does this and gets the grin factor I did.
Pedro
Humbled
08 November 2005, 12:59 AM
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Thanks for your reply will try to get some drills tom as the hole in mine is tiny, will post update when done
Cookie
Cookie
Originally Posted by Pedroskate
Zen
Have you seen how tiny the standard drilled hole is? Only the tip of a sewing needle would go in, and it was a tiny needle. Id speculate the hole is between 5 and 10 thou of an inch.
Unless your going to ultrasonically clean the the entire inlet system and turbo, the likelihood of a piece of dirt, or even a trace of oil (for which the Impreza is famous for clogging up the standard boost orifice) getting into the drilled hole is high. Dawes recognise this because the fitting instructions tell you to check the drilling is clear once a month.
The original dezign "compromise" I quoted, was that a wastegate set to fully open at low presure (eg: 0.4 bar) needs a small orifice or the operating presure will bleed away without opening the wastegate. But on a scooby running 1 bar there is more than enough presure to do the job even with a larger drilling. Look at the reply regarding a 1.7 bar operating presure which worked perfectly with a 1.5 mm drilling (thats a mine shaft, not a drilling!).
For Cookie
I believe you can take out the dawes device intact and redrill the hole to 1mm (its at the actuator end of the Dawes device), if your worried then start smaller and work up (drills are cheap). But do be carefull with very small drills because you realy dont want to snap one in the hole. This is ten minutes work max!
IMHO you will not need to dismantle or reset the Dawes in the course of doing this. But do be cautious when you first drive the car afterwards, let boost peak and hold in a high gear first to see if the maximum boost has rissen (do a before and after comparisson to be certain). I would be dissapointed if you didn't notice a considerable improvement in the way the car delivers power.
Im eagerly awaiting feedback from someone who does this and gets the grin factor I did.
Pedro
Have you seen how tiny the standard drilled hole is? Only the tip of a sewing needle would go in, and it was a tiny needle. Id speculate the hole is between 5 and 10 thou of an inch.
Unless your going to ultrasonically clean the the entire inlet system and turbo, the likelihood of a piece of dirt, or even a trace of oil (for which the Impreza is famous for clogging up the standard boost orifice) getting into the drilled hole is high. Dawes recognise this because the fitting instructions tell you to check the drilling is clear once a month.
The original dezign "compromise" I quoted, was that a wastegate set to fully open at low presure (eg: 0.4 bar) needs a small orifice or the operating presure will bleed away without opening the wastegate. But on a scooby running 1 bar there is more than enough presure to do the job even with a larger drilling. Look at the reply regarding a 1.7 bar operating presure which worked perfectly with a 1.5 mm drilling (thats a mine shaft, not a drilling!).
For Cookie
I believe you can take out the dawes device intact and redrill the hole to 1mm (its at the actuator end of the Dawes device), if your worried then start smaller and work up (drills are cheap). But do be carefull with very small drills because you realy dont want to snap one in the hole. This is ten minutes work max!
IMHO you will not need to dismantle or reset the Dawes in the course of doing this. But do be cautious when you first drive the car afterwards, let boost peak and hold in a high gear first to see if the maximum boost has rissen (do a before and after comparisson to be certain). I would be dissapointed if you didn't notice a considerable improvement in the way the car delivers power.
Im eagerly awaiting feedback from someone who does this and gets the grin factor I did.
Pedro
08 November 2005, 08:50 AM
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Pedro, I popularised the drilling out the bleed hole to fix the part throttle full boost issue on small turbos, there is a Dawes FAQ I posted in 2002 somewhere. If you go to 1.5 to 2.0mm it will be virtually a pure bleed valve - even if you took the ball out it would have a fairly high minimum boost level.
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