john banks

Last day or so it has been peaking a good 1 PSI higher than the held boost - 18+ PSI has repeatedly been seen and I had to turn it down so that held is 17 PSI to avoid scaring myself. Fuelling still looks OK and car drives very well.Made sure the breather hole was clear and also checked for hose kinks - one might be slightly. Later I will shorten it some more, and if not improving will disassemble and reassemble the device to see if that makes any difference.

Just that the spikes are about double what they were when it first went in, and the conditions are well within the extremes where it was behaving impeccably. Any other ideas? It is connected with minimum hose from turbo nipple to wastegate.

HarryBoy

Hi John,

I think there is a point where shortening the hoses makes no difference. The Dawes Device is a mechanical device and as such the ball has mass and therefore a certain reaction time to opening and closing. Also, maybe the ball gets 'stuck' slightly and is slower to close than normal.....I note Darren is now producing a 'faster' reacting version, lighter ball/stronger spring??

I am going to experiment with a pressure regulator as in the Autospeed article and use the Dawes just as a boost 'creep' control, hopefully I won't get any spikes at all......


Harry

john banks

Do you have a link to that article Harry?

I mean to shorten them cos its kinking a bit. Funny, just doing my home visits a few mins ago and it is seems a bit better. Half day this afternoon for more fiddling

GavinP

John,

I think Harry means this one:

http://www.autospeed.com/A_0685/page1.html

Thanks

Gavin

john banks

Oh THAT one know from the Link that it is Audi boost control - sad eh? The pressure reg bit sounded interesting - thought was something new. As per the other Dawes post I am going to rig up an op amp and solenoid driver and solenoid to enable FBC at <50% TPS and Dawes at >50% TPS.

Jay m A

iTrader: (2)

Just to put in my thoughts about the ball and spring 'sticking'. Maybe if the device was mounted vertically rather than horizontally then there would be less 'relocating error' when the valve closes. I think there is a case for it being able to shut 'cleaner' when vertical - gravity and all that. But then again that theory gets blown away when going round corners, accelerating etc

Anyway just a thought...

Justin

john banks

Useful thought Justin. Although mine is mounted in line on fairly stiff hose, it could flap around a little. Tightened up a bit now.

Will run with induction kit off also as fed up with noise and also do an ECU reset.

john banks

Sounds a lot better with the induction kit off. Fuels ever so slightly richer on part throttle too. Took apart the Dawes and reassembled - no difference. Changed the hose to the supplied silver stuff, cable tied the controller to keep it still and stretched the spring inside a bit and adjusted and now all is well again with <0.5PSI spiking. It was quite a long way screwed in before - one turn off the lock nut's limit so this might be something to do with it - the spring was possibly too weak for the level of boost I was running? Anyway, works better for now.

Floyd

iTrader: (9)

Is the spring being heated up much and affecting it's elasticity?

Just like a magnet looses it's strength in heat? Or perhaps the ball bearing is expanding more than the brass with heat and sticking?

F

Cosie Convert

Hi guys

Like yourselves I can't resist a bit of gadgetry, especially when it's cheap !
I have not followed this 'dawes' thread for very long so this may be common knowledge......or maybe not ??
Dawes valve lookalike available in RS catalogue Pt No 398-9120 and/or 398-9142. One has a ball valve, the other a very lightweight disc valve. This may well be faster acting/more accurate than the ball valve. I've recently purchased both (around a tenner each) with the intention of making by own boost control valve. In the catalogue they also supply different spring ratings, which just might be what you're after ??

regards

cc

HarryBoy

CC,

These are pressure relief 'pop' valves that open at an adjustable pressure to atmosphere. Not the same as the Dawes Device. Of course you may have devised a clever way of using it......let us know!

Harry

Cosie Convert

Harry

The axial flow valve with the disc just needs a copper pipe soldered on to the outlet port and 'hey presto' you got a dawes for a tenner !

John

I know your issue is mainly with spiking however your steady max boost figure will never be the same each day as long as you use a Dawes reducer. The reason for this is that the efficiency of your turbo will vary depending on atmospheric conditions. Atmospheric pressure, humidity and temperature all have an effect.
This causes a change in exhaust back pressure before the wastegate and hence a variable pressure assisting the wastegate opening action.
Also the current operating temperature of your turbine and casing can influence things, hot turbines are more efficient than relativly cold ones. Conversly cold compressors are more efficient that warm ones.
As you can see the number of variables is quite large, heat soak after a stop can add 50 deg C to the compressor and intake manifolding.
How about working on a closed loop boost pressure controller which 'trims' the boost set by the current pneumatic valve ?
Using the existing MAP sensor output and boost control solenoid would cut costs.

I'll buy it !!

cc

john banks

The fact that it drops boost when hot seems good and protective. I am thinking of closed loop circuits, but I know that the factory solenoid is not quick enough to be satisfactory - esp with increased boost.

brickboy

John B, Dawes devices have been big in the Saab community for a while now. Go to Saabnet.com and search in the "performance" forum, there's lots of stuff there. The current wisdom seems to be ***NOT*** to connect to the turbo output nipple, as this may not be an accurate reflection of true boost pressure seen in the manifold, due to expansion losses etc in the I/C.

Instead, the Saab boys run the Dawes off a nipple on the manifold, which apparently reduces spiking and boost pressure fall-off at high rpm.

I've no experience in using a Dawes, just offering up the fruits of many hours idly surfing the web at work on all things turbocharged ......

john banks

Thanks Brickboy will look with interest. Does make sense what you are saying. The physical distance from the turbo was the factor I was thinking of, but that is a bit stupid as by the time the area just after the turbo is at pressure all of the rest of the system will be. There is a spare plug on the manifold...

brickboy

John, try searching in the Performance forum of Saabnet for the terms 'MBC' and 'boost taper' , those will get you close. Look for posts by Dean, he's the board's evangelist for Dawes and has used one on his vehicle for 18 months+.

Hope it helps you.

john banks

I feel I already know Dean! Anyway, I have been reading all about it. It seems that if you install from the manifold (in a Saab) you don't get so much of a taper at high revs possibly due to avoiding the pressure change across the intercooler. But apparently you get more spiking. Will have a fiddle and see.