2.2L/2.5L Turbo Engine Important Upgrade Information

Description

This page explain somes of the aspects of the Chrysler 2.2L and 2.5L 4-cylinder engine as they relate to performance modifications. It will give you a good understand of how the engine and engine computer work together. See my Chrysler Engine Computer Information page for a very detailed brekadown of how the engine computers work. Here the engine computer will be referred to as the ECU (engine control unit).

Intake Mixture

Normally, the intake mixture on fuel injected engines is controlled by a closed-loop system that uses the oxygen sensor to determine what the air/fuel mixture is. This allows the computer to ensure that the engine is receiving he appropriate mixture for efficiency and emmisions. Too rich a mixture (too much fuel) causes the engine to lose power and have poor emissions. Too lean a mixture (not enough fuel) causes the combustion temperature inside the engine to increase and will eventually cause the engine to lose power as well. As you can guess, it's a precision adjustment that is normally handled by the ECU. A slightly rich mixture is OK; it will cause a lower combustion temperature which is good. A slightly lean mixture is very bad because it will increase combustion temperature which can melt your pistons and exhaust valves. The computer uses the oxygen sensor to prevent this until the engine is floored (WOT). When at WOT (wide-open throttle), the ECU uses a fixed-value table, based off of the MAP sensor reading, to adjust the fuel mixture. The MAP sensor is used because there is no mass air flow sensor on these engines. The ECU takes the barometric pressure (read from the MAP sensor when the baro-read solenoid is activated), the intake air charge temperature, and the intake manifold pressure (from the MAP sensor) to calculate the density of the air entering the engine. It knows how the density and pressure of the incoming air relate to how much air is actually entering the engine and then calculates how much fuel is needed. It is programmed to run the engine slightly rich to add a safety margin. Since it is running the engine rich, it can't use the oxygen sensor as feedback because the oxygen sensor can only tell the ECU two things: "too rich" or "too lean". So the oxygen sensor just says "too rich" at WOT and is therefore useless to the ECU.

Turbo Boost Control

How boost is controlled by the ECU is somewhat complicated, but I will try to explain it here as I understand it. Much of this information comes from a specification book for the sensors and logic modules (not the service manuals). Maximum boost for your car depends on the year and type of engine and transmission you have. The first turbo engine Chrysler released in 1984, which had a mechanically-controlled boost limit of 7 psi. In 1985, Chrysler gave control of the turbo to the ECU. "Maximum boost" was still 7psi, but the computer would allow a limited "peak boost" under certain conditions. Peak boost is achieved by going to wide open throttle (WOT, meaning flooring the gas) and is sustained until the throttle is release some or until the peak boost timer is expired. The peak boost timer gives a maximum of ten seconds of peak boost. For every second you are not at peak boost and WOT, one second is added back to the peak boost timer. This seems kind of silly, but by doing this Chrysler was trying to maintain reliability in its engines and drivetrains. If the ECU sees the boost level go over about 11 or 14 psi (depending on the year, see chart), called an "overboost" condition, then "overboost shutdown" will occur. Overboost shutdown will turn off the spark and fuel injectors, basically shutting down the engine. This is to prevent catastophic damage to the engine. In 1987, Chrysler began producing the Turbo II, which featured an air-to-air intercooler. Because the air going into the engine was cooler and therefore denser, Chrysler upped the boost levels for these engines to a maximum boost of 9 psi with a peak boost of 11 psi. The peak boost timer and overboost shutdown were still in effect. For more details about the four types of turbo engines Chrysler built, visit Dempsey Bowling's page on the Dodge Turbo Engine Differences. Here is a chart which indicates the boost levels for and various model years (my information is limited to 1987):

Year	Engine - Trans	Max Boost	Peak Boost	Overboost
1984	Turbo I - Manual	7	7	11
1984	Turbo I - Auto	7	7	11
1985	Turbo I - Manual	7	9	11.75
1985	Turbo I - Auto	7	9	14
1986	Turbo I - Manual	7	9	14
1986	Turbo I - Auto	9	11	14.25
1987	Turbo I - Manual	7	9	14
1987	Turbo I - Auto	9	11	14.25
1987	Turbo II - Manual	9	11	14

I am not certain about the peak boost levels for the automatic transmissions in 1986 and 1987. These are my estimates based on all available data. For more details about how the ECU works, see my Chrysler Engine Control Unit Information page.

Taking Control Of The Turbo

If you decide to "take matters into your own hands" and take control of the turbo away from the computer, you need to keep in mind that the computer assumes certain things about the engine when it's at WOT. These include air flow into the head, fuel flow through the fuel injectors, etc. If you are just going to increase boost without changing anything else and not exceed the overboost shutdown point, you don't have to worry about this because the computer can handle it. By changing any of these values though, such as porting the head, installing bigger injectors or modifying the MAP sensor signal, you will effect the air/fuel mixture and the ECU won't know any better because it is assuming that these factors are no different.

The worst thing you can do is lean out your engine. The leaner the air/fuel mixture, the higher the combustion temperature is inside the cylinder. Too high a temperature will destroy an engine. This can happen if you port the head and/or manifolds or increase boost (by bypassing the MAP sensor) without compensating by installing larger or more injectors, and/or increasing the fuel pressure. You can also degrade engine performance the other way by installing larger injectors, etc. without increasing air flow into the engine somehow. It's a give-and-take situation and requires patience and some trial-and-error.

To help you know where you are at when doing this, you MUST know what your combustion temperature is doing. The most common way to do this is to install an A/F (air/fuel) guage and you should also install a quality boost guage, while you are at it. You also have the option of installing an EGT (exhaust gas temperature) guage, instead of or in addition to the A/F guage. Which guage is better is the subject of much debate. If you were to ask me, I would tell you to install the A/F guage because it is cheaper, much easier to install, and faster-responding than the EGT guage (read about this debate). Installing both can be educational (you can see how the A/F ratio and boost level effect the EGT), but is rather redundant, especially if you install the EGT guage on the down-pipe (see the Choosing Your Guages page). No matter what guage you install, you should always "read" your spark plugs. See the Reading Your Spark Plugs page for more information on that.

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This page is maintained by Russell W. Knize and was last updated 01/19/99. Comments? Questions? Email minimopar@myrealbox.com.