Understanding MAP Sensor Sizing With Ben Strader Of EFI University

While it may seem simple to some of you, to a large number of people, MAP sensors and their sizing is almost a different language. Ben Strader is here to help not only get you up to speed on what the MAP sensor measures, but to dispel some internet myths, too.

First off, a MAP sensor is the Manifold Absolute Pressure sensor. As the name implies, it measures the pressure in your manifold. However, the key term in the sensor name is “Absolute” because it not only measures positive pressure, as in a boosted situation, but negative pressure – or engine vacuum – as well. They have been used by OEMs since electronic fuel injection became an option, and now, there are both factory and aftermarket options available.

“The most common type [of MAP sensors] are the style made by GM a long time ago,” says Strader of the sensors referred to in the aftermarket as “GM style” (for obvious reasons. “The more modern style of MAP sensor typically has a higher pressure range, a threaded base to simply screw into a port, and overall are nice little sensors, but they still come in all shapes and sizes.”

As Strader points out, the most important difference between the sensors, is what they can actually measure. While they all measure pressure, much like mechanical gauges, each sensor has a specific range of pressure they measure. “Typically, you’ll hear MAP sensors referred to in multiples of ‘bar’; one-bar, two-bar, and so on,” says Strader.

What is Bar?

If you’re reading this in the U.S., you are more than likely used to measuring pressure in PSI or pounds-per-square-inch. However, “bar” is another way to measure pressure. A metric measurement, 1 bar is defined as 100,000 pascal, with the following equivalents:

  • 14.5038 psi
  • 29.53 inHg
  • 0.987 atmospheres

As we know 1 atmosphere is generally rounded to 14.7 psi, and 1 bar is generally rounded to 14.5 psi. A slight difference, but an important one, when you are getting into the larger pressures.

MAP sensors come in all shapes and sizes. On the left is a FAST GM-style sensor, which is popular in the aftermarket, as evidenced by the fact that companies other than GM make GM-style. On the right is an AEM 7-bar sensor, with a more universal threaded end and a ton of resolution.

Sizing Sensors Correctly

So right off the bat, you’re thinking, “I just need to run a sensor that has enough resolution to measure the amount of boost I’m making,” and you’d be partially correct. What you have to remember is that even naturally aspirated, your intake manifold has about 14.7psi (or 0-30 inHg, if you’re looking at a vacuum gauge) of fluctuation. You have to factor that into the size of the sensor you choose.

“A 1-bar sensor can only measure the amount of pressure in the atmosphere. So from an absolute vacuum to about sea-level,” explains Strader. “If you are going to have an engine that is ‘boosted’, it’s going to have pressure that is higher than sea level. So you need a higher capacity sensor, like a 2-bar or a 3-bar.”

As the name implies, a 2-bar sensor would measure about 29 psi, however, once you take into account atmospheric pressure of 14.7 psi, you only have room for about 14 psi of boost, before the sensor runs out of room. In a 3-bar sensor, you would have room for 28 psi of boost, and so on.

Looking at this boost graph as an example, you see that the engine makes almost 35 psi of boost. To figure out what size MAP sensor it would need, take that 35psi, add 14.7 for engine vacuum, and then divide the result by 14.5. That gives you a total minimum pressure range of 3.43 bar. So the minimum sensor size for this engine would be a 4-bar MAP sensor.

“These sensors have gotten really big lately, with all these cars making tons of power. Now you can get a 5-bar, a 7-bar, or even a 10-bar,” Strader says. “So the question becomes, how big of a sensor do you need? The reality is, there’s no need to have a giant sensor that has way more capacity than what your engine is going to see, but there’s also not much downside to it, either.”

This is where we get into the internet myths and misinformation surrounding MAP sensor sizing. “There are a lot of internet experts who talk about how you shouldn’t use a MAP sensor that has way more capacity than you’ll actually be using. They argue, why use a 5-bar MAP sensor on an engine that will only see 20 pounds of boost?” says Strader.

“The reality is that there is so much digital resolution in your ECU, that it can easily work out a fine level of precision when reading the sensor, even if it’s oversized. But if you have a MAP sensor that is right on the edge of its capacity, and you want to change your system and run more boost, you’ll have to replace your sensor.”

Strader goes on to explain that there is so much resolution available in today’s modern ECUs that you could even run a 5-bar MAP sensor on a naturally aspirated engine without any issues. “It doesn’t really matter,” concludes Strader. “You almost can’t overdo it when it comes to sensor size.”

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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