We screwed in the wastegate adjustment bolt almost completely, which would normally increase nominal boost from 6 psi to, say, 9 psi. The Normal (side) and Deltagate (top) wastegate actuator reference hoses were routed to the turbo compressor discharge located below the air-cooler unit and above the throttlebody. Because the reference source was the compressor discharge rather than the intake runners, we expected the minimum boost possible would be 6-9 psi plus about 3-5 psi of output from the blower, for a total minimum boost in the 9-14 psi range.
Bob hammered the throttle and ripped off a new dyno run. The boost controller and turbo combined to reach a MAP of 230 kPa at 4150 rpm, whereupon MAP "fibrillated" fairly flatly to 5450 rpm. At this point, the boost controller duty cycle climbed steadily to 100 percent in an effort to limit boost, while MAP subsequently climbed from 235 kPa at 5750 rpm to 292.9 kPa at 6500 rpm.
In retrospect, clamping down on the adjustment bolt was clearly limiting the opening travel of the wastegate poppet valve; when the exhaust volume became too high for the restricted poppet valve to bypass, boost crept to 28 psi.
The Catch-22: With too little wastegate adjustment pressure, we can't make boost above 24 psi due to backpressure forcing open the wastegate. With too much adjustment pressure, we can't stop overboost due to severe limitations on the wastegate opening travel. The answer is a stronger wastegate spring.
During the dyno pull, backpressure climbed to as high as 35 psi, as measured on a pressure gauge tapped into the front bank exhaust. Torque was virtually flat at a little more than 400 lb-ft from 4000 to 6100 rpm. Power peaked at 471.6 rwhp at 6500 rpm.
The auxiliary temperature-apparently already heat-soaked prior to the run-climbed from 52.1 to 58.5. Air temperatures exiting the intercooler remained in the 28-30-degrees C range throughout the run, even though we'd forgotten to turn on the tap water to cool the air cooler.
We adjusted the wastegate spring out a bit and mashed the pedal. The turbo compressor had exploded. What was left when the compressor failed at 28 psi boost was the compressor hub, broken off where the shaft emerged through the compressor back-plate, totally stripped of compressor blades, and a bunch of blade shrapnel in the bottom air cooler tank.
Fortunately, the intercooler trapped the metal and kept it from entering the engine. The compressor backplate was hugely distorted and bulging outward toward the center section of the turbo. The turbine side appeared undamaged, and the turbine wheel spun freely as always on the hot side shaft.
OK, time to regroup. We put the turbocharger on a bus for Majestic Turbo in Waco, Texas. We installed the turbo-eliminator pipe that enables the MR6 to run without the turbo installed. Then we drove to Austin, where we pulled the wastegate off the car and installed a secondary inner spring to raise the opening pressure of the wastegate from 6 psi to about 18 psi.
In the meantime, confronted with a scored-up compressor housing, Majestic decided to enlarge it for a T-70 compressor wheel.
Back on the Alamo dyno, the plan was to optimize power in turbo-only mode, and then throw the blower on to see what that did to low-end power. During dreadful early-summer Texas weather (more than 100 degrees with extremely high humidity), the MR6 performed a series of dyno runs to about 450 whp. Datalogs revealed significant compressor surging, with manifold pressure jumping between 18 and 28 psi, and in one case momentarily crashing to atmospheric pressure before immediately building again into the 2-3-bar range.
All centrifugal compressors run the risk of surging if the compressor is loafing at relatively low air volume and high pressure ratio. Unlike a positive-displacement supercharger like the Eaton (in which the design makes it physically impossible for air to move backwards through the blower), turbos make boost by accelerating the air to extremely high speeds with a set of fan blades turning at 100,000 rpm or more.