In a message dated 96-02-28 08:11:10 EST, you write:
>Scott, I think that statement needs to be qualified WRT where you are
>'measuring' boost -- upstream of the throttle-plate or in the intake
>manifold.....your statement would be true if you are measuring boost/vacuum
>in the intake manifold and downstream of the throttle-plate -- upstream
>there would still be 'boost' if the turbo is still spinning.....
....hmmmmm not so really.... when you slam the throttle shut, there is no
longer boost in the intake manifold, it actually makes a pressure wave from
t-body flap back to the pressure side of the turbo, causing a phenomenon
called turbo stall or pressure braking of the turbo.... That is what the pop
off valve is intended to prevent.... Esp with IC end caps made of plastic,
or weak IC to T-body hoses.... The pressure spike on a stage II IA box with
a stock k26 is in the 50psi range, more on a modified turbo/more boost.....
Vacuum, glen is instant in the manifold when the throttle shuts, so instant
in fact, that the vaccuum for the pop off valve is located just inside
(downstream) of the throttle plate. That is how it functions in most
Mechanical pop-off valves..... In theory and practice, the boost is measured
at the intake manifold, it really makes no sense to measure it anywhere else,
unless you want to know how fast the turbo is spinning, but the why question
I think where the confusion may lay is in the operation of the wastegate,
let's address that first in racing and production terms, it might help......
A race car builder could care less about where fart air is discharged: To
air or closed loop. So, when you here the air farts under boost conditions,
that's becuz, the builder of the motor vented the WASTEGATE discharge to air
instead of exhaust, some turbulence theory concerning exhaust might apply
(tho gains are prolly minimal). A wastegate diaphram has two pressures
exerted on it when closed, effective spring pressure from the top (spring
rate = spring pressure + Boost/WGfreq valve air) and manifold boost from the
bottom. Manifold boost (the large banjo bolt from the manifold to the WG)
fights the spring pressure on top, and if the MANIFOLD pressure exceeds the
rate of the spring pressure, the wastegate opens, bypassing exhaust from the
turbine (hot) side of the turbo TO either air ( racing-farts) or downstream
of the turbo in the exhaust (production-SBD's?). This slows the spin of the
turbo, reducing boost, until the MANIFOLD pressure is again less than the
SPRING PRESSURE and the wastegate closes again, all exhaust now back to
turbine..... The computer can control/effect the wastegate three ways 1) in
a urq style it retards timing, grounds out fuel pump if Manifold pressure
exceeds a preset level, 2) 86-91 10vturbos --- the computer can do 1, as
well as, add or subtract Turbo pressure air to the top of the wastegate via
the WG freq valve, increasing or decreasing the effective spring pressure on
the top of the WG.... 3) you can control effective MANIFOLD pressure to the
wastegate by regulating the air going to it....
I digress quickly......
However, both the wastegate and the Pop off valve can be "spent" to
atmosphere... So, yes, you can hear gaseous excretions on throttle and off
throttle in any turbo car, but they are two distinct and separate functions,
more specifically the same functions under two different conditions, closed
and open throttle. At closed throttle the Pop off or bypass valve bleeds off
excess air pressure between the throttle flap and the cold side turbo blade,
letting the turbo free spin for less lag on next throttle open (boost). The
Wastegate (and computer) measure air upstream of the t-body (in the manifold)
and bleeds exhaust gasses away from the turbine to slow the turbo, regulating
the amount of boost in the manifold during any pressured throttle condition
(0.0 vacuum thru max boost for any given computer input)....
>That begs another question: is a pop-off valve typically installed at the
>intake manifold side of the throttle-plate?
NO, no function there, that is the wastegate by definition.....
> Also, how are high pressure
>spikes upstream of the throttle-plate addressed?
Pop off valve
> Those that would
>'blowed-up' the IC-to-throttle-body hose or the IC? A second pop-off valve
>located upstream of the TB?
That is the definition of a pop off valve, glen.....
HTH all confused....... There is plenty more to the specifics of all the WG
operating functions, but this is long enough
87 5ktqRS2 X 2