Boost levels

Wallace White wallace at
Tue May 8 20:29:18 EDT 2001

Larry -

I'm again late at replying, but here's my understanding of these issues.

The wastegate spring is definitely a "gauge pressure" device. I agree
with your assertion, that it's +0.2 bar above ambient or so. The system
on an '87 5kcstq is supposed to generate 1.4 bar (absolute, because bar
is never measured gauge). I have an Audi document on the "new MC engine"
that touts its ability to generate the same manifold pressure,
regardless of manufacturing variation (e.g. WG spring stiffness). I do
believe that also means the same pressure at all altitudes/ambient
pressures. Not until the Motronic system was added for the 200q20v in
'91 did altitude compensation come in.

I think you're going to have to drive your 5kcstq over to my house (~60
ft above sea level, Menlo Park, CA) to find out if your car is affected
by ambient pressure.

- Wallace
  '87 5kcstq 173k, 1.8 bar QLCC

"Smeins, Larry" wrote:
> What I stated is based on what I have observed and my analysis of what is
> happening.  I posted some of my observations a few weeks back hoping to
> either get confirmation or a "you're full of it" response.  I got no
> response.  What really took me by surprise is it appears that the WGFV duty
> cycle never goes above 50% and I've had that confirmed by two other listers
> that have high boost mods on their cars.  My analysis says that with 50%
> duty cycle the wastegate opens .2 bar above the spring setting which gives
> 1.2 to 1.3 bar at our altitude.  It also corresponds with 1.4 at sea level
> which is the stock boost setting.  According to  Scott's site, the boost
> bible,  the spring is set from 1.2 to 1.3 bar.  I read that as at or near
> sea level.  I am quite confident in my statement about altitude dependence
> of the spring pressure. I've been driving my Audi for over 200k miles at
> this altitude and have never seen above 1.2.    It appears that in stock
> trim boost is limited to 1.4 bar at sea level with or without feedback.  I
> am considering making a duty cycle multiplier circuit to crank up my boost
> to sea level boost pressure.  That would be an electronic Schrapnel Knobben
> that retains the ECM protection features.  A slightly stiffer spring,
> washers may accomplish this, is another possibility.  Presently I'd be happy
> to just get sea level performance.  I don't want to risk melting pistons or
> valves in my 260k miles+ daily driver.
> Is there anybody is this great group that understands the altitude effect on
> our boost?  Reading Bentley gives the impression that absolute boost
> pressure should be constant but that is not what I observe.  It appears I'm
> not the only one seeing 1.2 bar max at 5000 feet.  If I am wrong I have no
> problem with being told that I'm full of excrement just tell me why as well.
> Larry

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