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All About Boost

There has been much debate recently on Boost Control. Now that we have a range of Chips to choose from I thought a little guide on how the Esprit controls its Turbo Boost would be in order.

The ECU controls the turbo boost pressure in two ways:

1. For Throttle Openings (TPS) between 0 – 90% - Open Loop Mode

During part throttle openings, the ECU uses a Base Duty Cycle table that estimates the amount of bleed-through the wastegate needs to achieve a set amount of boost. It can only do this in open loop because the MAP sensor cannot see what the turbo is producing. I good way of seeing this in action is to plot Wastegate (DC) vs. TPS from a Freescan log. You can see as from the figure below as the TPS rises then the solenoid is open for more time which has the effect of bleeding the boost way from the wastegate actuator this has the effect of “hiding” the true boost from the wastegate actuator and so stops it opening allowing the turbo to develop boost pressures in excess of 1.2 bar ! If you disconnect the solenoid valve from the ECU then you get just the boost pressure equivalent to the wastegate actuator spring pressure. This should be setup to 0.55-0.65 bar no more.
The plots bewlow are from a Freescan log of an S4 with stock memcal code. Max boost is approx 0.8 bar, note the max Wastegate DC is about 50%.

The plots below are from my S4 with Marcus’ Chip #4 giving 1.1 bar max boost. See how the max Wastegate DC is now 60 % and the Wastgate DC is not triggered until 3000 rpm.

2. For Throttle Openings (TPS) Greater than 90% - Closed Loop

The only feedback the ECU gets of the boost pressure is the MAP sensor, because this is behind the throttle butterflies, it is only a true indication of the boost pressure when they are nearly fully open.

If you install a digital boost gauge in your Esprit you will notice high boost pressure during gear changes. It is thought this is due to the dynamics of the whole charge system. I guess the reaction time of the Dump Valve (BOV) and the fact that the turbo is still around 80,000 rpm means that pressure transients will occur. I wonder if the transfer resistance of the ports to the cylinders means that the bores don't see this charge pressure.

3000 RPM Stumble
Owners of SE, S4, S4s with the stock GM ECU code will all notice a flat spot or “stumble” between 2900-3000prm. This behaviour is inherent in all 4-cylinder turbo cars with the GM ECU, “they all do this” is the common response from the dealership (what to they know !). The problem is the standard SE, S4, S4s code has got too higher wastegate opening at low throttle openings around 3000 rpm before the Volumetric Efficiency (VE) of the engine is sufficient enough to cope with the boost pressure. It is an easy re-code to get rid of it and my plot above shows no wastegate action to just over 3000 rpm.

Andy Whittaker for Freescan and much of the insights documented above. He also the guy who solved the 3000 rpm stumble.

13 May 2003

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