Map Sensor Trick

Have you always wanted to know what this so called "Map Sensor Trick" is what all these people are talking about? The answer is relatively simple, yet this can be a very effective solution for boosting your N/A motor.

What does it do?

So what's the point of it? Well if you want to turbocharge your N/A motor you will run into problems because of the higher intake pressure, simply because the engine was not designed for running pressure higher then atmospheric.
One of the problems you will (most likely) run into is that your MAP sensor does not "understand" pressure higher then atmospheric and will cut you off.

On the 4AGE 16v engine when the MAP sensor sees the boost it will cut the fuel and throw the Engine Check Light on you, when you let go off the throttle, it will return to normal operation.
Rumors are around that the stock MAP sensor will accept the boost, but this is not true, it will accept so little boost that it is not drivable at all, basically it will accept boost up to around 3 psi, at 20% throttle, even if the boost wouldent rise, just the additional throttle would cut the fuel.

Now, a check valve looks like a very simple solution to this problem, it can be used to make the MAP sensor not see the boost at all! To find out how and why i will first explain some of the basics and then move on to the solution.

Basic Principles - The MAP Sensor

A MAP sensor is one of many sensors used in your engine's EFI system.
The manifold absolute pressure sensor provides instantaneous pressure information to your engine's ECU. This is necessary to calculate air density and determine the engine's air mass flow rate, which in turn is used to calculate the appropriate amount of fuel that should be injected.

An engine control system that uses manifold absolute pressure to calculate air mass, is using the speed-density method. Engine speed (RPM) and intake air temperature are also necessary to complete the speed-density calculation. Not all fuel injected engines use a MAP sensor to infer mass air flow, other methods are the MAF sensor or an AFM.

Basic Principles - Boost

Boost can be described as the pressure above atmospheric pressure that your turbo or supercharger is creating in your engine's inlet manifold, it is usually measured in BAR or PSI.

Basic Principles - The Check Valve

The check valve (or one way valve) is a small mechanical valve, that normally allows air to flow through it in only one direction.
Check valves are two-port valves, meaning they have two openings in the body, one for the air to enter and the other for air to leave.
See pictures 1 and 2 for some examples of check valves commonly used in cars.

Map Sensor from a Supra 7MGTE Engine
As you can see, it is specifically for a turbocharged engine like the 7MGTE
MAP Sensor from a 1st Gen 4AGE 16v, as you can see, it is for Vacuum only.

So what do i do?

Basically all you have to do when you have found a proper check valve is cut the vacuum line going from the inlet manifold to your MAP sensor and place the check valve in between the line.
Make sure you place it in the right direction! Meaning air should be able to flow from the MAP sensor to the inlet manifold and not from the intake to the MAP sensor!

If your car doesn't start after you've placed the check valve, you have put it in the wrong way.

As an extra safety addition, we put in an extra "bleeder" valve, this valve is there in case the normal check valve fails and let's a little boost come through.
The check valve i used was so smooth it sometimes leaked a little boost, with the bleeder valve in place, he would bleed it off into the atmosphere.
It's like a blowoff valve for your MAP sensor :)

The whole construction in position. Blue is the flow direction.
And another view.

The limitations, risks and problems

There aren't really limitations to this way of allowing boost into your engine, as long as the check valve you selected can hold up to the pressure.
(Then again, even if it can't it will leak the excess pressure away via our added bleeder, you did add the bleeder right?)

As far of the risks concerned, there is a substantional risk to this way of boosting your engine, this way you are going above your regular MAP sensor's capacity of measuring and injecting the appropriate amount of fuel, so fuel starvation may occur, you engine will go lean and if you push it far enough it WILL break.

Top View
Single Piston
Close Up, this is what happens when you run lean..


This is a very effective solution for low boost applications up to a maximum of around 5 PSI (0.34 BAR). And as of today, i still run it on my 4AGTE at around 4 PSI of boost.

If you want to go any higher (up to 10 PSI (0.69 BAR)), you can still use this method, but you really should look into your ignition and fuel management first.
(Not that you don't need to look into it on the low boost applications, 5 PSI still is alot of extra air over N/A)

If even that isn't high enough for you, you might want to be looking at a standalone like a MegaSquirt, AEM EMS or Halltech. In most of these applications you lose the original MAP sensor and replace it with a boost-ready one.

by Navi - October 19, 2006

Building the 4AGTE - Part 1