SiHethers

I wonder if 1mm might be a compromise. When I got mine over a year ago, it came saying it had the 1.5mm bleed, but it looks more like 1mm in reality. Certainly mine isn't acting just as a bleed valve as spool up is significantly improved compared to normal as is the level of boost held in the higher rpm. I can still achieve max boost at 1/4 throttle opening in fifth gear.

Certainly for a higher boost application a small bleed hole will not be as suitable, with greater chance of overboosting and subjecting the wastegate actuator diaphragm to high boost could potentially increase wear (remember the standard turbo outlet nipple pipe has a restrictor in it to modify the pressure subjected to the wastegate actuator, which opens at 7-8psi). Ideally you only need the wastegate to be closed as the turbo starts to spin, say 0-10psi, with an increasing degree of opening from 10-15psi, at which point the wastegate is fully open. This would give good spool up at lower pressures then a more gradual rise to full boost, helping to kerb overboosting. This would also prevent max boost occuring at very small throttle openings.

Simon

(Edited to say - I don't have any evidence that high boost/small throttle opening does actually cause lean running, but it is a theoretical risk, and was certainly discussed in great detail on Scoobynet as experience with Dawes Devices grew)

[Edited by SiHethers - 5/2/2003 1:24:00 PM]

Jamesn

How many people mod their Dawes with a 1.5mm bleed hole as opposed to the standard 0.5mm one?

J.

Leslie

What does that do for it James?

Les

SiHethers

Jamie supplies them with a 1.5mm bleed for imprezas. If you have a 0.5mm bleed, you can often develop full boost with very light throttle. This has two consequences, one is that it makes part throttle control of your car difficult, especially when cornering, and two there is a risk that your engine may run lean in this situation due to the base fuelling map not expecting to come across a high boost/small throttle opening environment. Spool-up may suffer slightly but drive-ability and safety is improved.

Simon

Jamie Whitfield

Hi lads

Basically after the first year of supplying the Dawes Devices we decided to only supply them in 0.5mm form.

We found that the 1.5mm bleed hold caused the controllers to act more like a bleed valve, and most people preffered the intial surge of boost. Of course if you aren't one of these people you are able to drill the holes out as per our instructions.

I hope this helps a little.

Jamie

www.j-w-racing.com

Jamesn

Jamie,

Surely the bleed hole mod was to protect the engine though?
I do not want to run lean.

James

Jamie Whitfield

Having a 0.5mm bleed hole shouldn't cause the engine to run lean.

Jamesn

Some time ago John banks said that cars with small turbos 98 -on should adjust the bleed valve to avoid part throttle issues

"Avoid full boost part throttle- it is bad for your engine and driveability. Use a 1.5mm bleed hole on a Dawes with a small turbo.
"

Is this not the accepted approach now?
John Banks seems very knowledgeable.

Any input is appreciated pre-install as I will be fitting in a couple of days now that I have a 2nd hand boost guage on the way.


James.

Katana

Depends. If its running rich then its okay. But if its lean then you'll go boom. Get an AFR and you'll know what I mean.

john banks

To some degree it is just opinion, but I did find a lean issue.

Mine ran lean on part throttle with the 0.5mm bleed hole - it did a lean stumble at 1 bar on part throttle, although I never got the turbo to surge (the OEM dump valve stops that because it is quite soft) the ECU did do more knock correction. In addition charge temperatures will go up with excessive part throttle boost and lots of recirculation through the dump valve, which you cannot afford on a TMIC. On my car it made the throttle into an on-off switch and it was horrible to drive. 1.0mm is OK, 1.5mm makes it quite smooth, but yes it does end up a bleed/ball-spring hybrid. 2.0mm is too far and makes it into a non-adjustable bleed valve - even when I removed the ball and spring it was minimum 18 PSI on my car. The bleed hole increase is a cheap way to make a hybrid MBC which most people prefer the driving and safety characteristics of. I never measured the EGT on part throttle as I didn't have the gauge at the time.

Note that the ECU determines the load zone of the map depending on the MAF sensor AND the throttle position sensor. Therefore with excessive part throttle boost on the original map it can and does run lean.

Initial thrust on going WOT is nice in some ways and makes the car feel very quick, but it is just the perception of rate of change of torque, and in bends it can unsettle the car. You could say, well use part throttle, but with the 0.5mm bleed hole you have a VERY non-linear torque:throttle response. This is horrible on a small turbo as you curl your toes at a given RPM and you got from 30 to 40% throttle and you get a massive slug of boost. If you map the original ECU and sort the part throttle fuelling out it can be OK, but it is still nicer to take the edges off the response curve so that it is more consistent at different RPMs. If you part throttle at 4000 RPM the response is very different from part throttle at 2500 RPM. It is too much letting the characteristics of the turbo dictate the way you drive. You can encourage a more linear but brisk response with the ECU by using smoothly but quickly increasing part throttle duty cycles outside the turbo's sweet zone (TD04 is 3500-4500). Using a bleed on the Dawes is a compromise, it makes the torque delivery more OEM and NA, but that is IMHO no bad thing. There is more depth to the car that way.

Also consider a situation of a long up-hill with a full load in the car. If you use part throttle here there is a serious risk of popping something IMHO.

Leslie

A lot of very useful info from all.

Les

SiHethers

I've had a chance to do some deltalogging over the bank holiday whilst driving on the M1. In 5th gear above 3000rpm I found that with the lightest throttle possible, upto 5psi boost showed lambda voltage of around 0.84-0.85, 5-10psi boost showed 0.85-0.87, and over 10psi boost showed >0.87. These were with boost held in the indicated zones with very light throttle opening. My knock link was quiet and there was no ecu knock correction, ie all was well.

This tends to indicate that with whatever size bleed hole I have (roughly 1-1.5mm) everythings reasonably safe. I would say those voltages are pretty ideal seeing as 0.86v is roughly 12:1 AFR. With 15psi+ boost I was getting >0.89v.

Hope this helps

Simon