5KT waste gate question...

[Wallace White]

Bill -

Sadly, I can't find my own post about this in the archives, but here it
is from my sent mail, dated 2001/2/21. Hope that helps.

- Wallace
   '87 5kcstq 182k

-------- Original Message --------
Subject: Re: Wastegate, how does it work?
Date: Wed, 21 Feb 2001 07:54:01 -0800
From: Wallace White <wallace at stanfordalumni.org>
To: "Smeins, Larry" <lsmeins at ball.com>
CC: 'Audi' <quattro at audifans.com>
References: <53654725144FD111A24100805F57DEC006E9D00B at aeromsg2.ball.com>

Larry -

I'm not quite sure what's wrong in your case, but here's how an
MC-engine's wastegate works:

The bottom of the wastegate has a valve that sits in the exhaust
manifold (upstream of the turbo's turbine). That valve lifts up,
vertically, to open, allowing exhaust gases to bypass the turbine.

Going up the shaft of that valve, we find the wastegate chambers. It's
like one big chamber split in half by the wastegate diaphragm. The lower
half is always connected to intake manifold pressure--the number you
read on the trip computer's bar gauge. The upper half has a spring
pressing down--the wastegate spring--and it's normally connected to
vacuum.

The spring is sized so that when the intake manifold pressure is about
1.2 bar, the force on the lower side of the diaphragm is greater than
that on the top side. Thus the diaphragm moves up, and the valve down in
the exhaust moves up and vents exhaust pressure. I don't think the
pressure on the exhaust valve contributes much to the force balance...
(is that right?)

Now the electrical part: there is a wastegate frequency valve (WGFV), a
solenoid valve, that connects the upper chamber to either vacuum (the
normal position) or to the intake manifold (the energized position). You
can see that when you connect the top chamber to the intake manifold,
you now have equal gas pressure on top and bottom of the diaphragm but
you still have that spring on top, so the diaphragm is solidly pushed
down and the wastegate valve stays shut.

If things were left in this state, you'd get overboost every time you
stayed on the throttle. Instead, when you are at WOT, the ECU cycles the
WGFV at about 6 Hz, varying the duty cycle towards the goal of reaching
a certain intake manifold pressure (~1.4 bar stock MC-1).

A few things go wrong:
- The diaphragm can tear. This results in overboost, every time.
(Happened to me.)
- One of the hoses can melt shut inside. If it's the top one, this will
also result in overboost, since the pressure in that top chamber is
usually very low. (Also happened to me.)
- The WGFV can stop working. (Hasn't happened to me... yet.)

I hear some people pinch the lower hose to ensure that they _can_ build
boost. With that hose pinched, the wastegate shouldn't crack open at
all.

At overboost (1.6 bar, I think), the ECU does a last-ditch effort to
keep things under control and yanks the fuel pump relay. That's the jolt
you felt.

- Wallace
   '87 5kcstq 170k
===============

Bill Phelps wrote:
> Can anyone give me a quick and dirty explination of how boost is controlled
> in the 5KT? Is it tied into knock or simply boost pressure or a combination
> of both. The Bentley is a bit vauge on how exactly it works. Any idea of
> what the base boost is in absence of the control of the frequency valve (ie.
> just the spring)?
>
> Thanks.
>