Drive pressure is the amount of air that is pushing itself into the turbine housing on a turbo. Boost pressure is just the opposite, it is the amount of air coming out of the compressor housing into the engine. Ideally you want these two numbers to be 1:1. The higher the drive pressure, the less efficient the turbo is working and the more likely failure to happen to the engine and/or turbo.
The way drive pressure is measured is by tapping into the exhaust manifold and using a pressure gauge. Copper tubing at the manifold, coiling it at 5'-6', attaching a nylon tube to it and then nylon to the pressure gauge.
So, to put it as simply as possible; your monitoring both sides of the turbo: turbine (driven by the exhaust), and boost ( created by the compressor side of the turbo, which is spun by the turbine).
Hope that makes sense to you and helps.
In addition to all this, on a stock turbo (what I have) it is virtually impossible to have any high performance out of the turbo without high drive pressures. Tuning plays a key role in drive pressure, but so does picking the right turbo for the application. With my turbo, I could help it out with another turbo, compounding the amount of air coming out of the compressor and into the engine. In this application the drive pressure would likely be about the same, depending on how it's tuned, but I would be creating more boost. I would be getting the ideal ratio closer.
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