Re: Changing the Bomb

[Robert Houk - SMCC Bos Desktop Hardware <rdh@UrQ.East.Sun.COM>]

   ...
   I still believe that whether you dynomometer a Turbo engine at Daytona 
   Beach or the top of Pike's Peak, you'll eventually get the same final HP 
   rating because the wastegate spring doesn't care about relative 
   atmospheric pressure and instead opens only once it is pushed with 
   sufficient force.  This is "maximum boost", as predetermined by the 

While it is possible to have an absolute pressure "reference" (e.g., vir-
tually all "manifold pressure sensors" for the engine computers are of
this configuration, and measure "true absolute pressure"), I have never
seen a non-computer-controlled wastegate of this configuration. Purely
mechanical wastegates are manifold pressure against a diaphram against
a spring. what's on the "other" [spring] side of the diaphram? The
atmosphere (nominally 1 Bar / 14.7 PSI of pressure as a reference). The
spring acts as a "bias" relative to this pressure, so a 10PSI spring
gives you 10PSI in addition to whatever the atmosphere is providing as
pressure on the diaphram. The atmosphere applies a lot less pressure
against the diaphram atop Pike's Peak than in Daytona beach. (Try run-
ning a quick 600yd dash . . .).

How do I "increase" the boost in my UrQ? I "bleed" manifold pressure
into the "other" (spring, atmosphere) side of the wastegate, which 
shifts the wastegate opening point upwards (or downwards) in direct
lockstep with the amount of pressure I bleed into the wastegate. 
If you don't believe me, just hook the wastegate "atmosphere" feed
directly to your intake manifold. You will have "infinite" boost --
the wastegate will *never* open. Hopefully, your ECU will decide that
you have a problem and shut your engine off before you (quite li-
terally) blow it up.

Now, "newer" cars which use the ECU to adjust the manifold pressure in
a "servo" feedback manner will be independent of the relative atmo-
spheric pressure since their sensors are (typically) absolute pressure
reading. But even that depends on whether or not the ECU assumes full
and total control of the wastegate, or just has a "transient overboost"
capability (i.e., normally uses the mechanical spring as the wastegate
control, but can bled back (via "frequency" ["duty cycle" or pulse-modu-
lated] valves) and bias the wastegate for that "few seconds of overboost
that is so satisfyingly advertised.

					-RDH