Function of the boost control solenoid

Kunz · Nov 6, 2005, 12:59 AM

I use the Autronic replacement board in my MR. To control boost, I have the Autronic boost solenoid, which is bigger than the stock unit. To make spool-up faster there is a restrictor in the line from the turbo to the T-piece.

To fully understand now the boost-control and maybe make it even a bit better, I would like to know, if I am right with the following:

Under normal condition, such as idle, normal driving, low and middle boost, the boost control solenoid is OPEN, to keep pressure away from the actuator and therefor keep the wastegate closed for fast response and spool-up.
Now if you floor it, boost comes close to the programmed maximal boost level and the solenoid starts to CLOSE now, first partly, and then fully at full boost , so pressure is no longer bleed away, but brought to the actuator and therefor the wastegate opens, which limits the boost.
If you now lift the foot to shift, pressure in the intake manifold falls fast which makes the ECU OPEN the solenoid again immidiatly so pressure in the actuator and the lines to it can bleed away and therefor the wastegate closes fast, which makes again a fast spool-up when opining the trottle again.

Am I right with that?

Thanks a lot!
Kunz

alienauto · Nov 22, 2005, 05:03 PM

You are correct.

Justin
www.alienauto.com

Kunz · Nov 28, 2005, 12:57 PM

OK, I did some work on the car, respectivly on the boostcontrol-unit.

The guy who instaled and tuned the Autronic used the bigger Autronic boost solenoid. To reach boostlevels faster he used a restrictor in the line from the turbo to the T-piece and also one from the T-piece to the actuator.
On one hand this secon restrictor also helped spool-up for the first time, as pressure-increase was slower at the actuator and therefor the wastegate would open later.
BUT when shifting and flooring it again this secon restrictor was not so helpful as the pressure in the actuator could not blow away freely. This lead to a little lag after shifting.

So what I did now:
I replaced all the tubes and restricors, so basicly free-flowing and fast blowing of. To make Spool-up as aggressive as possible I placed a check-valve in the line from the turbo to the T-piece. I set this valve at an opening-pressure of ca. 1.0-1.1Bar. Up to that level no pressure at all is lead to the actuator and therefore the wastegate stays closed. Above that boost-level pressure comes more and more through the valve and is first bled away by the Autronic solenoid.
At the set boost-limit the solenoid starts to close, makes the actuator open the wastegate and limits boost.
When shifting / lifting the foot the solenoid opens immediatly so pressure in the actuator can now freely blow of and so the wastegate closes fast, which again gives fast Spool-up after flooring again.

The result is very good I think.
I set the boost at the same level as before (1.55 Bar peak and 1.5Bar stady). At warm engine Spool-Up is much faster now and some 200-300rpm earlier. Even power will stay about the same, I am now quite a bit faster out of slow corners which makes the car more aggressive but very drivable in my eyes.

Cheers
Andi Kunz