TLDR: you need the 2 or 3 bar MAP sensor (depending on how much boost you run) for the engine computer to reduce the ignition timing as boost increases.

Did you add boost? You will need a sensor that can read pressure that is greater than 1 bar (also known as 1 unit of barometric pressure) of atmospheric pressure. To really understand how all this works, and why you need a 2 bar or 3 bar MAP sensor, we need to define some terms.

The concept that 1 bar equals one barometric pressure stems from how atmospheric pressure at sea level is measured.

• Barometric pressure refers to the pressure exerted by the weight of the Earth’s atmosphere at any given point. This pressure varies based on altitude, weather, and location.
• 1 bar is a unit of pressure that is approximately equal to the atmospheric pressure at sea level, where the weight of the atmosphere pushes down with a force of around 14.7 psi (pounds per square inch), or about 100 kPa (kilopascals).
• The average atmospheric pressure at sea level (1 bar) has been adopted as a convenient reference for pressure measurement. Thus, 1 bar is approximately equal to the pressure of one Earth atmosphere (1 atm).

The MAP sensor in your vehicle reads absolute pressure, meaning it compares the pressure inside the intake manifold to a perfect vacuum. Your MAP sensor would read 0.0 in outer space (which is a near perfect vacuum), 0.3 on top of Mt. Everest, and 1.0 in Miami.

In practical terms, the manifold pressure will be close to 1 bar when the throttle blade on your intake manifold is wide open. Imagine how a wide-open throttle blade easily allows all the weight of the earth’s atmosphere to press against the inside of the intake manifold. Therefore, when the throttle body blade is wide open, the manifold pressure will be 1 atmosphere or 1 bar.  

Now imagine your foot is completely off the gas and the throttle body is closed (or maybe 1% open). Think about the pistons moving down in the cylinder, the increase in the cylinder volume will try to pull in or fill with air, but the closed throttle body blade is a restriction, and the air can’t get by the blade fast enough to keep the pressure equal on both sides of the throttle body blade. So now we have a lower air pressure inside the intake manifold than outside the intake manifold. This my friends, is a vacuum!  

When the engine is under vacuum the MAP sensor will read less than 1 bar (remember 1 bar is 14.7 psi or 100 kPa – anything less than those numbers is considered a vacuum). Typical MAP sensor readings on an LS engine at idle with the throttle body closed are about 4-5 psi or 30-40 kPa. It’s unusual to speak of engine vacuum in terms of psi, but I am doing it here for consistency in this article. Usually, engine vacuum readings would be in units called “inches of mercury” or inHg and a typical value for an LS engine at idle is about 13-14 inHg.

Stay with me, we are almost there. If you add boost to your engine through a turbo or supercharger, it means the intake manifold will have more than atmospheric pressure due to compressed air. The boosted manifold pressure might be 14 psi above the atmospheric pressure (2 bar total pressure), but your engine computer will never know this, because the 1 bar MAP sensor is maxed out.

The point of all of this is that as you add boost it is necessary to reduce ignition timing. If you run the same ignition timing at 1 bar and 2 bar you will likely need a new engine. So you need the 2 or 3 bar MAP sensor (depending on how much boost you run) for the engine computer to be able to reduce ignition timing as boost increases.

The rule of thumb to keep your engine safe is to reduce ignition timing by 1 degree for every 1 psi of boost. This means that at 14psi of boost your engine should have 14 degrees less ignition timing than at 0 psi boost.

So what 2 bar or 3 bar MAP sensor do you choose? You have plenty of options. There is a 3 bar LS3 style MAP sensor that you can find on LSA powered vehicles like the Cadillac CTS-V or ZL1 Camaro. You could use a 2 bar Gen 3 clip-in car style sensor from a Chevrolet Cobalt with the supercharged 2.0 engine. You could use the 3 bar Gen 3 clip-in car style sensor from the Cobalt with the 2.0 turbo engine.

We prefer the older GM 2 and 3 bar “brick style” MAP sensors for boost. They’re cheap and reliable. You can mount them nearly anywhere and run a vacuum line to them. Just make sure you tap into a source of manifold vacuum that is after the throttle body, or you can even drill and tap the manifold if you need to. They look like this, and a quick search on Amazon or eBay will turn up many 2 and 3 bar options. 

One last thing, you will need an adapter to connect these older sensors to your Gen 3 ECU. This one is probably too short to be really useful, but it gets the point across. Also you can always cut and extend the wires.