EMAP Sensor Setup: How to Read Exhaust Backpressure Without Killing the Sensor

An EMAP sensor (exhaust manifold pressure, also called a backpressure or drive pressure sensor) tells you what the turbine side of your turbo is really doing. The catch: thread one straight into the manifold and it gets hit with exhaust pulses, heat and moisture all at once. That kills sensors, and when they die they usually fail reading high. The fix is simple. Get the sensor away from the heat with a length of stainless tube and put a backpressure dampening canister between the exhaust and the sensor.

Why measure exhaust backpressure at all?

Boost only tells you half the story. EMAP tells you how hard the engine is pushing to spin the turbine. Divide EMAP by boost (both in the same units) and you get your backpressure ratio, which is one of the best tools you have for judging turbo sizing, turbine housing A/R and wastegate behavior.

• A ratio close to 1:1 means the turbo is moving air efficiently.
• A ratio that keeps climbing toward 2:1 and beyond as boost comes up usually points to a turbine side that is too small for the combo, or a wastegate that can't keep up.
• Watching EMAP pass after pass also shows you things like a sticking wastegate or a cracked manifold before they turn into a broken part.

Why EMAP sensors fail

Pressure sensors are made to read a clean, steady signal. The exhaust side gives them the opposite:

• Heat. Exhaust gas temperature at the manifold will cook the sensor electronics if it is mounted close to the tap.
• Pulses. Every exhaust event slams the sensor diaphragm. Over time that hammering wears it out and makes the reading noisy.
• Moisture. Exhaust carries water vapor. It condenses in the line, sits on the sensor and corrodes it, especially on a car that sits between events.

The same thing applies, to a lesser degree, to dome pressure and manifold pressure. A dome line on a CO2 system sees sharp pressure spikes every time the solenoids fire.

What happens when a sensor fails high

A dead sensor rarely reads zero. It usually reads high, and the ECU believes it:

• MAP sensor: the ECU thinks the engine is at maximum boost and commands fuel to match. The engine floods, stalls and won't run again until the sensor is replaced.
• Dome sensor: the ECU sees far more dome pressure than it commanded, vents it and shuts off the CO2. On the starting line, that is the run gone.
• EMAP sensor: if you use backpressure for logging only, you lose your data. If you use it for any kind of protection or boost strategy, the ECU acts on a number that isn't real.

This datalog was captured during a run, before we released the canisters. The dome sensor fails while the car is on the transbrake. The pressure reading spikes high, the ECU cuts the CO2 and actual boost drops instantly. That is exactly the moment you can't afford a bad sensor.

How to plumb an EMAP sensor the right way

1. Tap the exhaust before the turbine. A weld on fitting on the collector or manifold, ahead of the turbine inlet and the wastegate, gives you a true drive pressure reading. Put it on the top or side of the tube so water and carbon don't sit in the port.
2. Run stainless tube, not rubber line. A few feet of 1/4" stainless lets the gas cool down before it reaches anything that can melt. Route it so it drains back toward the exhaust, with no low spots where water can collect.
3. Add the dampening canister. The canister soaks up the pulses so the sensor sees a smooth, accurate signal instead of a hammer.
4. Mount the sensor away from the heat. Bolt the canister and sensor to the chassis or firewall, not to the turbo or manifold. A 3 port remote sensor mount on vibration damping bushings handles the vibration side and lets you group EMAP, dome and manifold pressure in one place.
5. Wire it to a spare 0 to 5 V input. The sensors we offer are linear 0.5 to 4.5 V, so setup in Holley EFI or any ECU with a free analog input is a simple linear calibration.

The BPS Backpressure Dampening Canister

Our canister was built for exactly this job, and it works just as well on dome and manifold pressure lines.

• CNC machined 6061 T6 aluminum, black anodized
• Standard 1/8" NPT inlet and outlet, so it works with any common pressure sensor
• Stainless mounting bracket and hardware included
• Made in Canada

It comes bare or as a package:

• Sensor option: 0 to 100 PSI or 0 to 150 PSI industrial pressure sensor, linear 0.5 to 4.5 V output. Pick a range above the highest drive pressure you expect; big boost combos usually want the 150.
• Backpressure kit: 3 ft of 1/4" stainless tube, a 90° compression fitting and a weld on compression fitting. Everything you need to go from the manifold to the canister.

Part numbers: U015 (canister only), U015-BP (with backpressure kit), U015-100 / U015-150 (with sensor), U015-100-BP / U015-150-BP (sensor and backpressure kit).

Building the rest of the boost control system

A dome sensor is only as useful as the boost controller reading it. Our high flow dual solenoid boost controller is built for CO2 and on board air, and the boost control kit packages the controller, dampener and CO2 together. For how it all works together, read hot side and cold side boost control integration explained, or browse the full boost control lineup.

EMAP sensor FAQ

Can I thread an EMAP sensor straight into the exhaust manifold?

You can, but it won't live long. Heat, pulses and condensation will kill it, and it will most likely fail reading high. Use a stainless line and a dampening canister and mount the sensor away from the exhaust.

How long should the backpressure line be?

Long enough for the gas to cool before it reaches the sensor. A few feet of 1/4" stainless is typical, which is why our backpressure kit includes 3 ft. Keep it draining back toward the exhaust.

Where should I tap the exhaust for EMAP?

Before the turbine, on the manifold or collector, ahead of the wastegate. Tap the top or side of the tube rather than the bottom.

What is a good backpressure to boost ratio?

The closer to 1:1, the better. A ratio that climbs well past that as boost rises usually means the turbine side is a restriction for the power level.

Do I need a canister for dome and MAP sensors too?

They don't see exhaust heat, but a dome line on a CO2 system sees sharp spikes every time the solenoids fire. A canister smooths those spikes and protects the sensor, which matters most on cars that rely on dome pressure for boost control.

Don't let a cheap sensor end your race weekend. Shop the BPS Backpressure Dampening Canister.