On Thu, 29 Feb 1996 PDQSHIP@aol.com wrote:
> >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
> 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......
I'm sticking by my guns here. The indy motors do indeed use a wastegate,
controlled by the computer, but the pop-off valve is NOT designed to
bleed off manifold pressure under normal conditions. It's only there to
limit max boost as set by CART. Noone on the team wants that thing to open.
> 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
Sure, it could work that way, but the intent on an indy car is not to use
the pop-off valve to control boost - just to limit boost in case someone
tries to go over the limit.
> 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....
> >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
That's exactly what happens. In the time it takes for the spring to push
the pop-off valve closed, the manifold pressure drops so much that they
lose alot of power.
> 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..........
Not in the time they need that to happen.
> >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
> >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.....
No. not for production motors(that I know of) but all the Japanese
tuning firms like HKS, RS Akimoto, Trust, etc, offer pop-off valves, and
if you read their brochures, you'll see that they are there primarily as
a last resort safety measure in case the boost control systems fail.
Boost control systems are all electronic solenoid based on the Japanese
turbo cars, and especially on the tuner motors. That's because they work
> Wastegate failure I find hard to believe, it is a mechanical system, but
More so on the Quattro than the Japanese turbo systems.
> 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 .....
The popoff valve probably was developed back when carbs were used, and
nobody had an ECU, much less used it to stop the fuel pump. Nowadays,
you are prolly right, the pop-off valve is prolly redundant.
> 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).....
Yeah, but on a very expensive racing engine, you at least save the
engine, and only have to replace the turbo.
Graydon D. Stuckey
Flint, Michigan USA
'86 Audi 5000 CS Turbo Quattro, GDS Racing Stage II
'85 Mazda RX7 GS 12A-leaning-towards-a-13B-soon