You can read the boost pressure anywhere you can find a place to tap into the intake manifold. The spot shown in the pictures with the other post is fine.
The linked thread showing the boost gauge and "controller" install also contains a bunch of talking about raising the boost pressure, changes to the injection pump, etc. At some point it mentions that making changes to the boost pressure without a boost gauge is wandering in the dark. I'd give one more bit of warning. The boost pressure gauge doesn't tell you anything about what you're doing to the engine. Presuming you can modify the pressure to the wastegate actuator to "fool" it into staying closed longer without adjusting the pump, just because you're not seeing the boost that the factory manuals state (and by the way, that boost pressure should only be tested per the factory procedure, running at full throttle and using the brake to hold the engine at 4,000 RPM long enough to take the reading) isn't entirely safe either. It would presume that the injection pump hasn't been altered in any way and that it was still capable of delivering sufficient fuel to match the higher boost levels. When you give more boost and don't feel increased power, that's a sign you're running lean, not optimum conditions for the engine.
The only way to know you're not cooking the engine with turbo and fueling mods is to install an exhaust temperature gauge. Exhaust temp is everything to a diesel. Keep it in the proper ranges and you'll know you're getting everything the motor has to offer. If it's too low or too high, you're eventually doing damage.
These motors are known for their extreme longevity and strength, so if something is being compromised, you won't know it right away, but eventually it will catch up with you.
If you want to install a boost gauge just to watch the needle move around, there's little or no risk there. But if you want to use it to modify the power delivery from the engine, get yourself an exhaust pyrometer and learn how to let THAT tell you when you've got things set up right.
I agree that to fine tune the power delivery AND take precaution not to cook an engine you need an EGT guage.
However, Just installing the controller and upping the boost will get you no where. Proper adjustments need to be made for fuel delivery. You can jack up the boost all you want but if no extra fuel comes with it you will see no power increase. I made moderate adjustments to boost and fuel and got very good results. Any further adjustments are not safe without an EGT guage.
Making adjustments to the boost without a guage is like wandering in the dark. You don't know where you are starting and you don't know where you are going without a guage.
As far as routing the line: It is just a small teflon tube. I routed mine through the first available hole in the firewall. The hose is very flexible but cannot make sharp bends without kinking.
I have done the same mods. I use an Autometer Ultralite meter and a turbo xs boost gauge. By the time I am finished installing all my gauges, you guys with think I am crazy. I want accurate gauges that will tell me everything that is going on. Trans temperature, EGT gauge, fuel pressure, A/F ratio, as well as a few others. You will need to adjust the ALDA screw as well, to get more fuel into the combustion chamber. As far as actual horsepower numbers, I am not sure that anybody has actually done dyno test, but I would conservatively guess around 20 or 30hp. But the potential is unbelievable with the proper modifications. The Finns have gotten around 450hp out of the factory 603 engines found in the 300sdls, which put out 148hp from the factory if I recall correctly, and those numbers came with very little internal engine mods, just injection pump mods, intercoolers, bigger turbo, and a custom air intake!!
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