Glenn Coombs

I have read all the threads on the Dawes device and I think that I now understand how it works. It basically replaces the boost control solenoid (BCS) with a pressure switch. This means that the waste gate sees zero pressure until the pressure switch opens, and then the wastegate sees whatever the boost pressure is. Hence no waste gate creep and improved spool up times.

What I don't understand is why the exact same behaviour can't be obtained using the standard ECU and BCS. Isn't the BCS just like the pressure switch, except that it opens and closes on a signal from the ECU rather than the boost pressure ?

Is it not possible to map the ECU so that it holds the BCS closed until the target boost pressure is reached and only then does it open it, allowing the wastegate to operate ?

BRR

The factory solenoid is off a T-piece on the hose to the wastegate, so even when its fully open, there is still some pressure on the wastegate.

The factory solenoid also is not exactly an on/off thing. It pulses on an off around 14 times a second (IIRC) and stays open longer on each pulse to bleed more air and therefore reduce the pressure that the wastegate actuator "sees". This is the duty cycle referred to - 0% and its closed, 100% and its on maximum bleed, but still not zero pressure on the wastegate.

Glenn Coombs

Ah, that explains things. I had assumed that the BCS was positioned inline like the Dawes is:

wastegate ====== BCS ====== turbo output

so that when the BCS was closed the wastegate would see no pressure and when the BCS opened it would allow pressure through to the wastegate.

So it is actually more like this then:

wastegate ====== T-piece ====== turbo output
||
||
BCS

and the wastegate sees full turbo pressure when the BCS is closed rather than when it is open.

Would it be possible to replumb the BCS so that it was positioned inline as in my first diagram ? If so then it should work exactly like a Dawes device except that it opens and closes under control of the ECU rather than at a certain pressure. You'd need to change some of the maps on the ECU as the BCS would have to be open when it is normally shut and vice-versa.

What do you think - would that work ? Or is that just a crazy idea ?


[Edited by Glenn Coombs - 10/3/2002 10:12:02 AM]
To try and line up the BCS in the 2nd diagram.
Argh ! It still doesn't work. How do you draw ascii diagrams and not have the spaces all removed ?

[Edited by Glenn Coombs - 10/3/2002 10:16:19 AM]

Pavlo

If it was as per 1st diagram, there would be nowhere for the air in the actuator to go, so it wouldn't be able to responde very quickly.

The 3 port does almost what you have drawing, except there are 2 options, actuator connected to the high pressure side of turbo, or the low pressure side of turbo.

Paul

Glenn Coombs

That's the whole point isn't it ? When in the closed position the air doesn't go anywhere, allowing the boost pressure to spool up quickly. And then when it opens it allows air to flow through it to the wastegate.

Pavlo

The solenoid isn't generally off during spool up then on when trying to reduce boost. It cycles on/off at about 13 times/second, giving you an average pressure to the actuator.

If you put directly in line with the actuator, you could have the problem that the actuator is pressurised to reduce rising boost, but then the duty cycle is such that there is nowwhere for the air already in the actuator to go, so it remains open too much and the boost drops.

It might work, but it might just go mental in some instances. However you could fix that just by putting a tiny bleed in somewhere to that when the solenoid was fully closed (ie 100% duty cycle) the actuator could still return normal position.

The advantage of the 3 port setup is you can use the pre-turbo vacumm to actually suck the wastegate shut.

Paul

Glenn Coombs

I see what you're getting at now. I guess that the bleed hole in the Dawes performs this function.

Which cars have the 3 port setup on them ? Can anyone recommend any books or web sites that explain this sort of thing in more detail ? I still don't fully understand what the differences in operation are between the 2-port and 3-port solenoid setups.

Pavlo

the pre97 cars have a 3 port solenoid.

And you generally get a 3 port solenoid with 3rd party boost controllers.

You can buy a Delco part for sensible money, no exact details.

They have to be of quite high quality as they need to switch rapidly.

Paul

Glenn Coombs

So do the 3-port solenoids allow for faster turbo spool up ? And if they do, then why did Subaru change to the 2-port ones from 97 onwards ?