How to get your turbocharged engine boost pressure with iOBD2?

Do you know that if your car has a turbocharged engine, you can get turbo boost pressure with iOBD2 although there is no a boost gauge on iOBD2? It is easy. Simply read the MAP (Manifold Absolute Pressure) value, and then subtract 100Kpa and you will get the boost pressure.

I didn’t get this idea until one of our customers sent me a message. Here is the story.

A few days ago, one of our customers sent me a message on our company Facebook page saying he could not find boost gauge on iOBD2 as he mainly wanted to know the boost pressure of his 2013 Hyundai Veloster Turbo. I said sorry to him that iOBD2 app was designed according to the generic OBD II specification and a support for boost gauge was not demanded in the specification.  The next day he said his Veloster Turbo hit 202 KPa (which is too high) and wanted to confirm if there was problem with the app or his car. This time I was confused. How did he get the 202Kpa as the app did not have a boost pressure gauge? He told me that he got a max MAP value of 221Kpa but had been subtracting 100Kpa, so that made a boost pressure of 121Kpa which is normal. Now I understood that the 202Kpa he got was a MAP value but not a boost pressure value.

Yet I still want to know the theory behind it and the following was what I found from volvoforums.com.

“MAP sensors measure absolute pressure. Boost sensors or gauges measure the amount of pressure above a set absolute pressure. That set absolute pressure is usually 100 kPa. This is commonly referred to as gauge pressure. Boost pressure is relative to absolute pressure - as one increases or decreases, so does the other. It is a one-to-one relationship with an offset of -100 kPa for boost pressure. Thus a MAP sensor will always read 100 kPa more than a boost sensor measuring the same conditions. A MAP sensor will never display a negative reading because it is measuring absolute pressure, where zero is the total absence of pressure (it is possible to have conditions where negative absolute pressure can be observed, but none of those conditions occur in the air intake of an internal combustion engine). Boost sensors can display negative readings, indicating vacuum or suction (a condition of lower pressure than the surrounding atmosphere). In forced induction engines (supercharged or turbocharged), a negative boost reading indicates that the engine is drawing air faster than it is being supplied, creating suction. This is often called vacuum pressure when referring to internal combustion engines.

In short: most boost sensors will read 100 kPa less than a MAP sensor reads. One can convert boost to MAP by adding 100 kPa. One can convert from MAP to boost by subtracting 100 kPa.”

Download the iOBD2 app on Apple Store or Google play.