On Todd's Frankenstein Triple Turbo build/truck, we tried a BD Spool Flange on the GTX42 turbo mounted on the manifold. It helped spoolup, especially when trying to build boost against a tight converter at high altitude, and it worked well when tuned properly to open at the right time during spoolup to allow the truck to gain boost/power and not get a flat spot in the power curve from the valve opening too late creating unnecessary back pressure or opening too early and letting the truck fall on it's face down low.
Once it was tuned perfect (using regulators), it worked EXCELLENT for about 50 to 75 miles, then it filled with soot, quit actuating smoothly, and eventually stuck half open. We tore it down, cleaned the soot, and once again experienced EXCELLENT results, for about 25 miles, then it filled with soot, quit actuating smoothly, and stuck 1/4 way open. We freed the stuck vein/flap with some external lubricant applied to the flap/shaft and that lasted about 5 miles, then it stuck mostly closed, burned out the center divider of the turbine housing from the diverter valve sending hot exhaust directly at the center of the divided housing, fragments of cast housing went through the EXPENSIVE GTX4202 turbine wheel and ruined the turbo.
After several thousand dollars wasted to replace the GTX turbo, we went back without the BD Diverter Valve and installed a higher stall torque converter and tighter AR turbine housing on the GTX4202 and ended up with acceptable spoolup, nearly identical peak HP potential, and no more melted dividers.
Now this was a somewhat extreme application in that it was used on a 1000+ HP triple turbo setup, but the early failure makes me wonder if the diverter valve would last even 6 months on a 600 HP setup. Maybe with a gas engine with lower soot output it would hold up for a while, but I don't think it will provide reliable service on a sooty diesel engine.
