Piggyback ECU is an additional ECU use to extend some limitations of
factory setting ECU. Thus, the piggyback ECU has to be physically wired to main
ECU/factory ECU. It allows user to modify stock fuel injection without
replacing the entire ECU. They operate in various ways. Some of them modify the
injector duty cycle control signals as they travel from the ECU to the
injectors. Others modify input data to the ECU (like MAF for example),
effectively “tricking” the ECU into delivering more or less fuel at a given
RPM. Regardless of how exactly the piggy-back works, you’ll want to measure the
results of any modifications you make to your fueling map.
I came across to this article, a simple
explanation how the piggyback ecu works at ::- http://www.enginebasics.com/EFI%20Tuning/Piggyback%20Vs%20Standalone.html
entitled
Piggyback ECU
Piggyback ECU’s are wired to work with the factory ECU. Many times
people ask: “Do I have a stand-alone or a piggy back?” We can answer that
question really easily with another question. Is the factory ECU still in the
car controlling some part of the motor? If the answer is Yes, than you have a
piggyback.
There are two types of logics when it comes to piggy back ECU’s:
1. Intercept the signal from the sensors before the factory ECU, and modify those
signals so the stock ECU is “tricked” into making the vehicle behave the way
you want. For example, if you want the ECU to add more fueling, you would intercept the O2
sensor wire before the factory ECU “saw” that signal and modify it to make the
ECU THINK that it is running lean causing the factory ECU to add fuel.
Another method is to intercept the Maf sensor and tell the ECU there is more air
going into the motor than there really is. Again, this would make the factory
ECU compensate for the extra air coming in and add fueling.
2. The second method is POST ECU manipulation. Some piggy back ECU’s
will modify the signal after it has left the stock ECU. Example. The piggy back
will wire into the fuel injectors after the stock ECU so that you can add or
take away fuel by either raising or lowering the injection time. This way the
stock ECU is seeing all of the sensors information in real time but technically
doesn’t have any control of the fueling that is ACTUALLY going to the fuel
injectors.
Things can really get complicated when you realize that some signal
wires have to be intercepted and modified before the ECU and some after the
ECU. And some cars will work better using one method than the other. Figuring
out the best set-up for your particular vehicle can be a never ending process.
Hopefully you have a little understanding now of how a piggy back ECU works, so
lets talk about the Pro’s and Con’s.
Pro’s of piggy back ECU’s:
1. Can be easier to set-up and tune. We will be putting this as a con as
well because they can also be very difficult to set-up and tune depending on
how hard the stock ECU fights you when you try and make adjustments or the
wiring and manipulation you must do to get the factory ECU to do what you want.
2. Will retain OBD-II compliance. With the factory ECU in the vehicle
functioning as it normally would, the car will still be able to communicate
through the OBD-II port allowing you to pass emissions, scan for codes and
problems, and use the port to run OBD-II gauges.
3. Cost. Piggy backs are usually a third if not a quarter of the cost of
a full stand-alone ECU.
4. Full compliance with all sensors and gauges in the factory dash. With
no modifications necessary to the factory ECU or sensors, everything will still
work just as it normally would.
Con’s of Piggyback ECU
1. Tuning. Some factory ECU’s can be next to impossible to use a piggy
back ECU on. No matter how you try and trick them into doing what you want,
they are too adaptive or too sensitive. Many times you will get the map right
where you want it, and the ECU will “learn” and change so that the perfect map
you just had, is now a map ready to cause catastrophic engine failure. Other
times the car will run great, but the ECU will know something is not right and
pop the check engine light on you, making the vehicle not
emissions compliant. The list could go on. While on paper the piggyback sounds
like a great idea, the reality is it just doesn’t work for every vehicle
platform.
2. Can only control so much. When you don’t have the ability to actually
change the fueling or timing of a car, you can only manipulate what you’re
working with so much. This is why you will often read about how a piggy back
ECU can only tune up to 500cc injectors, or is only good for 400 HP on a
given engine, or you are fine as long as you keep the boost under 18 psi. We’re
just making up numbers there, but you get the idea. You are working inside the
parameters of something that was set from the factory and it can only be
“TRICKED” to work so much.
3. Wiring. Usually you are intercepting signal wires or ECU wires, and
because of this you will have to cut and modify your factory engine harness.
While this isn’t necessarily a bad thing, just remember that things change, and
in the future you may want to bail that piggyback and move to a full
stand-alone. Having a hacked up ECU harness could lead to a messy situation.
