Picked this up from a big truck article on sizing turbo's and exhaust. The article also went on the explain that on the road, a waste gate cuts efficiency due to lost energy to power the turbo.

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Install a turbo boost gauge in the exhaust manifold before the turbocharger. Drill a 29/64" or 7/16" hole in the exhaust manifold and tap the hole with a 1/4" pipe tap. Now install a brass fitting with 1/4" pipe threads and a coupler for 1/4" dia. brass or steel tubing. Use about 3’ of the 1/4" tubing then you can reduce down to 1/8" plastic tubing to go through the fire wall and into the instrument panel and couple the tubing to the boost gauge. Now you have two boost gauges, one for the intake boost pressure and one for the exhaust manifold pressure.

What are these gauges going to inform you of? During cruising speeds on the level the exhaust backpressure should be approximately 66% of intake pressure. While pulling moderate grades the exhaust backpressure will increase more than the intake pressure will however it should still be less than the intake. At wide open throttle the exhaust pressure may be greater than the intake pressure but only by 2 to 3 pounds of pressure.

What does all of this mean? If most of your trucking is on level ground or rolling short hills, its best to have a larger turbine housing to minimize the back pressure. If the majority of your trucking is in the city with a lot of stop and go driving, than a slightly smaller turbine housing is preferred. The smaller housing is good for quick turbo response and instant boost, however on the open road the backpressure may be too high.

 

I know a fellow who plumbed his boost gauge tubing though a three way solenoid valve so he could measure boost and back pressure off the same gauge. You will need at least a 60 psi gauge and a short section of metal line from the exhaust manifold to combat the heat.