Re: Indy pop-off valves

[PDQSHIP@aol.com]

In a message dated 96-02-29 17:55:18 EST, you write:

>> >
>> ......  The pop off valve is different than a turbo wastegate in that
there
>> is no restriction or slowing of the turbo, it just bleeds boost air to
>> atmosphere, it doesn't bypass exhaust gasses, therefore turbo is spinning
>> full bore all throttle openings......  Taking a race theory of gas or
>brake,
>
>NO, this implies that the turbo is trying to mkae excess boost and that 
>the excess boost is bleeding out through the pop-off valve.  BZZZZZZZT  
>wrongo.
Hmmmm....  Might want to ease the buzzer graydon.....   If you have NO
Wastegate, then there is nothing to slow the turbo down, in fact untill the
pop off slows the motor down, the turbo will be spinning faster......
 Wastegate open = turbo slows, no wastegate, the excess boost IS bleeding off
thru the pop off valve, by definition, graydon....  I think one should read
about Indy cars, me thinks a wastegate is on it (see dave:~), and a pop off
valve is there to prevent wastegate cheating, cuz without a wastegate,
EXACTLY what I described is what would happen.....   A turbo speeds up and
slow down according to how much exhaust gas is BYPASSED, otherwise it will
continue to speed up till the pop off valve hits (specifically, on the very
first WOT).......  And boom theory applies....
>
>If the pop-off valve opens on an Indy motor, the driver VERY quickly gets 
>passed by about three cars.  The pressure inside the manifold drops far 
>below the limit, and takes long enough to rebuild, that a noticeable 
>amount of power is lost, and they are inevitably passed.
How does that work?   Unless there is a delay in the close of the pop off
valve, that is a minimal loss....  As soon as the pop off valve closes the
boost is on again, and you could program the ECU to open the wastegate enough
when that happens to shut the pop off then close the WG again..........
>
>One very clear measure of an Indy motor EFI ECU is its ability to 
>maintain boost _just_ below the CART limit, without invoking the wrath of 
>the pop-off valve.
How would an ECU do that?  By electronic control of the Wastegate, That's
all.....   
>
>The pop-off valve is usually there as a last resort to protect the engine 
>in case the wastegate fails, and creates an overboost condition.  If I 
>were running 20 psi (+/- 1psi) on my 5KCSTQ, I would have a pop-off valve 
>set to about 21.5 - 22 psi, so that if the wastegate failed, the pop-off 
>valve would vent excess pressure, and my motor probably would not be 
>damaged by 1 extra psi of pressure.

There are no pop off valves commonly available for production motors.....
 Wastegate failure I find hard to believe, it is a mechanical system, but
let's explore.....  The computer should retard timing, then/and/or ground the
fuel pump if you go beyond the safe operation of boost, a pop off valve is a
redundancy not needed.....  Look at what would give you overboost mode,
graydon, only a faulty WG freq valve or a mechanical malfunction of the
spring (how, not sure) would do that, and your ECU should take over and
ground the fuel pump .....  I suppose you could mount an additional modified
wastegate that functions as a pop off, but the why question arises......
  And if you are relying on that pop off valve to work, you will already have
turbo failure on an audi, cuz when it pops, there is nothing to slow the
turbo down, in fact you will speed it to oblivion (means wastegate failure
and CPU shutoff failure - your spinning big time baby).....

I told you dave, this wasn't worth mentioning....   

Scott

Wastegate = good
Pop off = bad (downstream of t-body)