john banks

I am wondering if the Dawes is quite the on-off wastegate creep killer we thought it was.

Reasons:

1. EBC spool up with a duty cycle that starts out maxxed out and only comes down near to target is quicker to spool up and holds more boost from 4500-6000RPM

2. Xavier (a contact from the French Subaru club) has compared a bleed valve and Dawes running the same boost and found no difference in low or high revs performance. He runs the bleed without the factory boost controller connected.

3. A Superchipped car I was in running similar boost to my car spooled up just as quick.

4. I always thought there was a proportional element to control, which is increased when you increase the bleed hole to make it smoother on part throttle. To get the part throttle sensible I wonder if we have been killing any of the benefits of wastegate creep? The ball sitting on the spring is still going to move before target - just like the wastegate does, so it is not on-off.

5. Xavier pointed out the the Dawes is effectively the same as the wastegate - a bleed controlled by a spring, and suggested there was little value in duplicating it.

Could it be that a bleed valve manages to also keep a lot of pressure off the wastegate until near to target boost as well?

With a turbo outlet connected directly to the wastegate with no restrictor and bleed equilibrium is reached at about 8 PSI. Just how much wastegate opening is there at say 2-4 PSI? I suspect not very much?

So overall I am wondering if you might be able to get the same effect as the Dawes by using a bleed valve instead of the factory solenoid, or else running the solenoid at a continuous duty cycle which is functionally the same? Certainly the failure mode is more satisfactory - the failure mode of the Dawes = (stuck closed) would lead to overboost.

Would just a new restrictor and a fuel cut lifter be a better option leaving the factory solenoid connected with all the benefits that confers?

Would keeping the original restrictor and just replacing the solenoid with a fixed orifice also be a good solution?

Certainly with an electronic solution which is throttle position aware you seem to be able to get the best of both worlds. Perhaps this is also the reason you can hold more top end - you can increase the duty cycle up to max to keep as much boost bled away from the wastegate as possible?

Any thoughts? What I am wondering then in summary is, with a small turbo are we throwing away the on-off benefits by having to increase bleed to sort part throttle out and ending up with not much better than a bleed valve, the gains we do get being offset by part throttle twitchiness?

[Edited by john banks - 3/25/2002 2:59:25 PM]

BugEyed

John

A few ideas.
But this costs significantly more to buy/build in the first place. Also, it takes a while to "tune" the ebc before you see this characteristic without spiking.
The bleed valve is as good - it is fundementally the same idea. However, it will require more regular adjustment to cope with atmosperic changes etc.
Effectively this was running a well tuned EBC. {Edited to say see CC's coment below - this is wrong, it should read Bleed valve not EBC}
Exactly - to get the part throttle response you are compromising the accuracy of the control.
True to an extent, however, it is easier to adjust the Dawes than the wastegate actuator, and the wastegate actuators response is not linear (it is not fully open until about 10psi despite opening initially at 8psi).
I do not believe that this is so - surely the pressure has to build up to make the bleed effective. However, the wastegate actuator will not open until the initial pressure (less the force on the wastegate from the pressure drop across the exhaust turbine) = the spring pressure.
None unless the wastegate penny is massively oversized or the turbine pressure drop is excessive.
And this is what the factory ECU attempts to do, but running different boost levels for different speeds ....
Exactly, with some more effort you can tune it for each speed condition.
That is my reading of the downside situation. To balance it up, don't forget that it is a low cost method of achieving boost control that is simpler to set up than even an expensive EBC and is less sensitive to environmental factors than the "pure" bleed valve.

Duncan



[Edited by BugEyed - 3/25/2002 4:08:49 PM]

Cosie Convert

A bleed valve will cause a proportional pressure drop from the supply, it is just like drilling a hole in the pipe and letting some pressure leak out.
So with 50% bleed 5psi in = 2.5psi out. 10 psi in = 5 psi out. 15psi in = 7.5psi out... and so on. The wastegate actuator will see pressure right from the start of the boost ramp.

Dawes however will hold shut until the spring pressure has been overcome and then it will act like a pressure reducer giving a constant pressure drop. So with a 7 psi tension, 5psi in = 0 psi out. 10 psi in = 3psi out. 15psi in = 8 psi out etc.
Increasing the bleed on a dawes will have a minimal effect on this pressure drop.
Assuming there is no restrictor in the Dawes supply then the ball just opens more in order to retain equilibrium.
The Dawes bleed allows the actuator to close quicker during transient throttle as it dumps the pressure, hence the sharper response feel.

For the same boost setting a Dawes will result in marginally quicker spooling than a bleed due to the wastegate pressure being applied later.
As the exhaust backpressure is also trying to open the wastegate then a few psi on the actuator can be enough to float it.

With the EBC of course, you can have no pressure on the actuator until the last milliseconds then apply full pressure which is good

john banks

So if the wastegate only starts to open at 8 and is fully open at 10 PSI and then we put a restrictor and bleed in to shift this up to say 18 PSI, then the wastegate is only opening from 16 PSI even with a bleed valve?

Cosie Convert

Duncan

How was the Superchipped car running a well tuned EBC ?
I thought they used a bleed valve ?

BugEyed

CC

Your right. I'm confusing their boost clamp with the means of raising the pressure.

John

Exactly! Hence why it is effective.

Duncan

[Edited by BugEyed - 3/25/2002 4:04:22 PM]

Cosie Convert

Dawes 'type' valve flow characteristics



Very linear with increasing flow rate

dowser

Over the weekend I helped convert 2 more 'boost modified' cars to use a Dawes - both owners thought the Dawes improved boost response/performance (although I can't honestly say I noticed any difference - I hadn't driven their cars enough). Both ran 1.5mm bleed holes.

One was a my99 running with no fuel cut and a bleed valve (it was apparently peaking at 1.5 bar and then oscillating ), the other was a my98 running with a hybrid turbo (rs2 compressor wheel ), no fuel-cut and a 'TurboSmart' MBC....set at only 1 bar...it was very nearly as fast as my PPP'ed, 1.2 bar'ed my00 (& considerably faster on track 'cos of his b*stard brakes and suspension ).

I think the Dawes gets points for simplicity, but that if the ecu could cope with volumetric efficiency changes (induction/exhaust mods) better it could perform just as well. Also bear in mind that while a few use the Dawes for 'better' boost control, most use it purely to raise the boost level...& that it does very well for $35

Richard

GavinP

If you put a one-way check valve in the line to the wastegate actuator, what effect would that have ?

john banks

Is a one way check valve (=relief valve?) just another ball-spring? So no boost through for a while, then proportional at the end? So on a small turbo too much part throttle boost and twitchy nightmare throttle response for corners as well as leaner than most of us would like? Is this what you mean?

GavinP

John,

I was thinking more along the lines of using a valve like in the line to the carbon canister - allows air flow one way only.

This would prevent the actuator seeing vacuum.

It would be interesting to see what effect that contributes to the overall Dawes results ?

In theory, it should make very little difference but....

Thanks

Gavin

john banks

Sorry, I'm a bit puzzled. Does the Dawes not stop the actuator seeing vacuum anyway - ball on its seat because of the spring pressure and also the vacuum sucking it on? Or have I missed something (quite likely )??

GavinP

Sorry, didn't explain myself very well....

If you remove the Dawes and fit a one-way valve instead, does this give some of the response benefits ?

Thanks

Gavin

dowser

Gavin

But the Dawes is a one way valve with an adjustable opening pressure, no?

Richard

MattRus

Exactly what it is, a simple, adjustable low pressure relief / check valve. Primarily for use as a relief / crude regulator valve.

Scott.T

iTrader: (2)

As mentioned by me before.... I was running a Bleed B4 fitting the Dawes. The Dawes seemed to give no real improvment, so I revereted back to the Bleed Valve (as it can't get stuck).

The major benefit of the Dawes is that it allows more Boost which owners like. IF they have been running standard boost levels.

This I beleive is why most people are positive about the Dawes as they have experienced that, and that only (although not in all cases).

A Bleed valve only requires constant adjustment if you are determined to reach the same level of boost time and time again. Or if you are running too close to the fuel cut/overboost limit.

john banks

I see what you are saying Scott.

Can't see why a bleed valve would need less adjustment than a Dawes? Does it need more adjustment?

GavinP

I've run with a bleed valve and a Dawes and the bleed valve varies pressure much more depending on atmospheric pressure/temperature.

Spool up is also quicker with the Dawes.

Thanks

Gavin

john banks

Maybe you get away with the part throttle issues with tiny bleed because of your big turbo. Perhaps the advantage is less on the small turbo cars.

GavinP

John,

When I went up to Roger Clark Motorsport, we were talking about turbos and they did a lookup on my car's chassis number: TD04H

Whoops! I don't know how this compares to the TD04L but it is obviously smaller than a TD05H....

Thanks

Gavin

john banks

Did the person that nicked off with the clutch pedal also swap the TD05? I gather the TD04H is rather bigger http://www.3si.org/member-home/jluci...turboguide.htm

I believe mine is a TD04L-13G.

Scott.T

iTrader: (2)

The Dawes is more stable than the Bleed Valve set-up as you seem to get less peaking. But is this also determined by the set-up of the Actuator.

I tightened my actuator by 2-3 turns and the peaking increased. Therefore, I turned it back to where it was origionally.

Bleed Valves get a lot of bad press due their atmospheric pressure variation, but this is really only an issue if you are running too close to the Boost Cut.

Most Bleed's are also lockable so you just set it and forget it IMHO.

Mine (Forge Ally Valve) was open by approx 1.5 full turns and gave about 15psi with 16psi peaks.

Currently it's closed as I have just PPP'd the ECU (MY94), but it might just have to start opening again, now that I'm getting used to the PPP.

GavinP

John,

LOL! - they must have swapped it when I wasn't looking (along with the clutch pedal)

Scott,

I can only comment on my own personal experience but IMHO a bleed valve is an inferior method of controlling boost. I ran with the bleed valve before the Link was fitted and it was set at 14psi and worked fine - but not as well as a Dawes would have done.

Thanks

Gavin

dhorwich

Hi,

These dawes device things people are talking about... they can be bought from a bloke i know who has a celica GT-Four CS from his site, they are nearly identical to the dawes one, i know them as relief valves...

He sells them for £25 inc P+P...

Order them from... www.k1ano.co.uk

Just go to modifications... then relief valve boost controller...!

His email is.. contact@k1ano.co.uk

They make a huge difference.... spool up is alot quicker...