Dyno graph analysis

[227bhp]

Max_Torque said:

227bhp said:

Max_Torque said:

Typically a wastegate is a mechanically sprung penny valve. It is held shut by spring pressure and pushed open by both plenum air intake pressure AND preturbine exhaust backpressure. The minimum boost line is set (for any given spring preload) by applying all the plenum air pressure to the air capsule, so the valve is pushed open as early and as much as possible. A "boost controller" works by bleeding off some of that air pressure, so there is less force applied by the air capsule to open the valve. Hence the Max boost line is effectively just the pre-turbine EBP pushing the valve open.

As engine speed increases, and intake massflow increases, then the pre-turbine pressure increases (because more exhaust gas is trying to get out through the turbine). And, if the air capsule pressure is fixed (which it is with a dumb bleed type boost valve) then that rising pressure with engine speed must also act to make boost pressure fall!

So by 'Air capsule' you mean wastegate actuator (WGA) ?

The WG is held shut by a spring in the WGA, if it is being forced open then it is the WGA which is too weak, not the boost controller, no?
You state that the type of boost controller in question somehow bleeds air off, but I can't see how this is possible as it only has two ports, neither of which is vented to atmos.

You have missed the critical fact: Which is the wastegate is being forced open by TWO things, namely:

1) any positive pressure in the wastegate capsule
2) any positive pressure in the exhaust manifold


With a manual boost "control" system, there is no way of compensating for the increasing exhaust manifold pressure (as engine speed increases) so boost will ALWAYS fall with speed when compared to the boost that can be made at a lower rpm.

Yes, but this is a failure of the WGA, it just doesn't have enough power in the spring to hold the WG shut. I can't see how it has anything to do with the boost controller. Until it has the correct amount of air pressure fed to it (to make it open) it doesn't send any signal to the WGA.