Is this the wrong place? I figured to be on the safe side measure the actual turbo output as it goes into the manifold.
The wastegate location should give you the same reading unless your line to the overboost switch is plugged in which case you would not get an accurate reading anyway.

FWIW, I actually just put a T in the line from the turbo to wastegate and then rigged a piece of 3/8" hose that runs up to the drivers side corner of the windshield, wrapped my gauge in cloth (to protect it from bounces) and layed it right in that corner of the hood so that I can easily see it while driving.

It's a temp install anyway. Just drove 1100 miles like that and it worked fine.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique