For an external wastegate I have just switched to a small gas pressure regulator and love it! The bottom port of the wastegate gets full boost pressure as per normal while the top port has regulated pressure fed via T off the normal supply line. I can run an 8psi wastegate spring and add say approx 12psi to the top of the wastegate and have 20psi boost. Adjusting is extremely simple - just turn the pressure regulator dial for more or less boost.
On the internal wastegate turbo I had last I loved the ebay boost T thingy, you know the ones consisting of a threaded T piece with two hose tails and an adjuster bolt tensioning a ball and spring with a bleed hole... Seemed to work a treat! I was using this on my external gate but was concerned about back pressure overcoming the gate too easily.
These happen to be the only two forms of manual boost control I have tried, but they worked for me.
Cheers,
Jason
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