Boost gauge location for 6.5TD setup

Does it matter where you tap the hole for a mechanical boost gauge? Do you need to pick the location carefully to avoid berneulli affect from giving low readings?

So reason I ask is that the first place I tried was the port on top of the intake upper used for the EGR from the factory. That actually read vacuum when I rev'd the motor. Inside of that upper had a tube protruding down into the intake so I assume GM used that as a way generating a vacuum to activate the EGR on the civy trucks.

So anyway, I plugged that hole and tapped a new one on the driver's side of the upper. Now I read up to 5lbs of boost but only on WOT while the engine is wound out. So either there's something else messed up or the speed of the air as it pinches through the upper intake at 90 deg. to the brass nipple is lowering the reading.

The air pressure line for the gauge is fine and the pressurized part of the intake system looks good. I also tried some very stiff springs for the wastegate control and it made no difference. Turbo-aft exhaust is all 3" so that shouldn't be too restrictive.

Turbo is a "GM-1" which from everything I've read should spool and boost quick but choke the motor at higher RPMs.....this is seemingly the opposite.

So I'm just wondering if anyone else has run into this. The pics of other installs available seem to have the senders placed just about anywhere there's a flat spot cast so I wouldn't think it matters BUT this is my first turbo install.

Mine is where the volt gauge was in the radio delete.... did you use donut gaskets? Cause the maniflod is set up for 2 inch I belive

5psi at wot with NO LOAD is normal.. were you driving, or in park? also, the wastegate spring needs to be REALLY tight.. I was able to pull 12-14psi out of my GM4 under load..

I was driving it around. Highest I got it was WOT in 2nd gear (manual tranny) uphill. After upshifting to 3rd I ran out of road/speed limit before building any boost. In one test I used a spring so tight that I couldn't compress it any more manually so the wastegate should have been sealed shut for all intents. still made little difference.

Used donut gasket and flange from the same 6.5 setup on pass. side of manifold, left factory pipe section to manifold on drivers side. The 3" reference was for the "down pipe" exhaust after the turbine.

But now that you mention it, I did use flexpipe for a custom crossover. It's painted w/ hi-temp paint, wrapped w/ exhaust wrap and painted again but perhaps it's leaking.

Sounds like I have to mess around with a couple different mounting points for the boost pickup to eliminate that possibility first lest I've been testing it under serious overboost conditions.

Interesting product. I wonder if he's doing that to pickup the "signal" down and away from the airflow from the turbo. Those uppers are pinched down pretty tight where the curved portion meets the polygon section with bolts. I'll probably pick one up to try.

Sorry, about previous post.. I had same problem.. my intake horn had a brass nut and try to use it, however the hole to manifold appeared smaller than nipple opening. So I moved to the center of intake( dead center of the 6 bolt). work but gauge is flaky works intermitently.
I am using a Gm 3 turbo most I have read 12psi I using a heath turbo master also about 3/4 thread showing..

Update on this (if anyone cares):

Pickup location did make a difference. I plugged the spot I drilled and re-used the port in the center of the upper. However, I also added a female/female NPT coupler and a 2" piece of brass tube on the inside. Enough to make the tube stick down halfway into the lowr.
The response when cruising around is more immediate and maybe 1 or 2 PSI higher. Not a scientific data collection but based on driving up and down the same piece of road to test.

My guess is that trying to use a mechanical pickup right after the intake tract gets pinched down had the bernouli principle working against the positive pressure reading you are after.