gmsracing

What exactly does the wastegate spring do in a Tial 38mm? For example, if I run in single port mode with a .8 bar spring, does the wastegate start to open at .8 bar regardless of what the cycling valve is telling it to do? Or will it stay closed if it is receiving a signal from the cycling valve?

Jake951

The thing to remember about the WG is that it responds to the pressure reaching its side port when in single-port mode. When the pressure reaches .8 bar, the WG starts to open if the spring is rated at .8 bar. If the cycling valve is inline and is functional, then it can only reduce the amount of pressure reaching the WG, so yes, the WG will respond to what the CV is doing. If the CV is not functional or has been bypassed, then it's a moot point.

lart951

What if instead of the oem CV you have an Accuboost or any sort of MBC?

ANORXIC'51'

Please correct me if I'm wrong on this...but if a mbc is inline than the wastegate will open at the set pressure on the Accuboost/Reliaboost/etc....the 0.8 bar spring is sort of an "insurance" in case the mbc fails, that way 0.8 will be the max boost allowed...

gmsracing

I'm planning to run in single port mode until I can figure out what I'm going to do next. I have a boost creep issue that I'd like to correct now.

Jake951

With an Accuboost or similar controller instead of a CV, you now have effectively two spring-loaded mechanisms in series to determine when the WG opens. The first spring is in the Accuboost. When the pressure input to the Accuboost exceeds its spring pressure, then it opens and starts to pass pressure to the WG. The second spring is the one in the WG. The WG won't open until the pressure it receives from the upstream Accuboost exceeds the WG spring rating. Both spring-loaded mechanisms have to be forced open before the WG can start bleeding excess boost. As you can see the minimum boost you can run is then set by the spring with the higher rating.

In practice your WG spring rating should be no larger than the minimum boost you want to run. If you want to run a minimum of 1 bar, get a WG spring with a 1 bar rating (or slightly less). You can then adjust the boost pressure upwards from 1 bar with the Accuboost. However, you can't adjust it lower. The Tial can be ordered with different spring ratings. I use one with a 1.1 bar spring and I usually run 18 psi boost.

If the MBC fails open, i.e. passing pressure all the time, then you are right...the maximum boost you can run is no more than 0.8 bar. In fact this allows you to operate without a MBC at all...the boost is set entirely by the spring in the WG. However, if the MBC fails closed and not passing pressure, then the WG will never open, which can be very dangerous!

Note: I have assumed the spring rating is the same as the manifold boost pressure, i.e. a 0.8 bar spring gives 0.8 bar boost. In reality this is not quite true. A 0.8 bar spring gives you slightly less than 0.8 bar boost. This is because the banjo bolt, where you tap off to run a control line to the WG, is upstream from the intake manifold. There is a difference in pressure between the two locations because there is some pressure drop through the intercooler and perhaps within the other pipes and intake manifold itself. The banjo bolt always sees more pressure...typically ~3 psi in my experience.

Ryan in SD

Hope you dont mind me asking:

-If I want to run around 15psi in dual port mode what should the spring in the wastegate be?
-If I want to run around 15psi in single port mode what should the spring in the wastegate be?

Does the pressure coming out of the boost controller need to be greater than the spring pressure rating in the WG for the wastegate to open? I assume it does. And if thats the case, I would also assume that having the highest spring rate possible would be the best way to go correct?

Lets say I have 2 cars both running 15psi from the boost contoller, one with the .8 and one with the 1.0bar spring in the tial WG. Does the car with the 1.0bar spring go any faster?

What is the optimal configuration?

Matt Sheppard

Jeez Rob,

You getting chummy w/ the local Subie dealer?

ANORXIC'51'

Ryan: no scientific reasoning behind my theory, but theoretically yes. Since 951 #1's accuboost isn't bleeding until 15psi, then opens the wg at 0.8...he is watching 951 #2's same 15 psi set accuboost continue to creep by, because it is still holding boost, until 1 bar is reached before bleeding off the excess