Indy pop-off valves

["Graydon D. Stuckey" <graydon@apollo.gmi.edu>]

On Thu, 29 Feb 1996 PDQSHIP@aol.com wrote:

> >If I am not mistaken, indy cars used a pop-off valve on the intake
> >manifold as their sole means of boost control. In years past,
> >CART would keep all the valves and distribute them to the teams
> >before a race. All the valves are set to have the same max boost
> >pressure. They all have tamper resistant hardware, so if a team
> >decided to alter the pop-off valve max boost setting, the authorities
> >would know.

This discussion has been skirting the Indy subject a little, and I 
thought it was a good time to jump in here.

	The intake mnanifold-mounted pop-off valve is a final sanction 
against overboosting.  The indy motor electronics also have another 
method by which they control boost.  It used to be an external wastegate 
like on an Audi, but recently, I have not seen the wastegate as obviously 
as it used to be, so maybe they found a better way.  Regardles,, they do 
have a primary boost control method OTHER than the CART-supplied and 
mandated pop-off valve.

> >
> ......  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.

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.

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.

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.

HTH

Later,								
Graydon D. Stuckey								
graydon@apollo.gmi.edu								
Flint, Michigan   USA
'86 Audi 5000 CS Turbo Quattro, GDS Racing Stage II				
'85 Mazda RX7 GS 12A-leaning-towards-a-13B-soon