Squirt and Spark: How your ECU Works
By Andrew Comrie-Picard
There’s a photo of me as a six-year old in my family’s driveway.
The look on my face is one of extreme concentration – I have a stethoscope
in my ears and I’m hunched over the V12 that lived (and died) under
the hood of my father’s Jaguar. He is beside me, instructing me on
how to adjust the four Zenith carburetors for balance by listening to the
subtle changes in pitch at the air intake of each one. Didn’t make
the thing any more reliable, of course. But it made you feel you’d
done something anyway.
Over the last 26 years I’ve figured out how most of an automobile
works, and even got good enough at balancing the SUs on my own various MGs
and early Jags that I can take care of it while my girlfriend pops into
the loo at the filling station, which is about how often you have to do
But then they went and invented fuel injection, and then engine management
that unites fuel injection with engine ignition, and then they went and
packed it in a little metal box. All the magic little set screws and plungers
and orifices and weighted distributor advances and points gaps are set into
a microchip, and you can hardly tell which part to hold the stethoscope
Unless you’re one of the few who really understand how a modern Electronic
Control Unit (ECU) works, it is probable that you subscribe to Arthur C.
Clarke’s notion that “any sufficiently advanced technology is
indistinguishable from magic.” If so, read on.
The Computers Are Just Smarter
Modern engine management is not rocket science. Its just doing the same
thing that carburetors and distributors used to do, but better. We know
(and the computer is programmed with) the ideal amount of fuel to mix with
a certain amount of air at various engine speed and load levels. In The
Olden Days the venturis and jets in your carburetor used to approximate
We and the computer also know when the plugs should spark to ignite this
mixture for best burning at different speeds and loads. All of this was
previously the job of your distributor and some kind of advance mechanism,
and again it was approximated. Looking at how approximate the ignition and
mixture used to be, it’s remarkable that old cars ran at all. Certainly
they ran very inefficiently.
Now, thanks to ECUs, we are in a halcyon era of powerful engines with flexible
powerbands and relatively low emissions and high fuel efficiency. That Jag
of my father’s was the fastest sedan in the world when it was introduced.
It put out 250hp from a 5.3 liter V12. Now you have 250hp in a 2005 Subaru
Legacy with a 2.5 liter 4-banger. Internal combustion hasn’t changed;
the computers have taken over.
It’s All About the Sensors
Electronic engine management is all about getting accurate information
from the engine’s environment and then sending out accurate
signals to the control mechanisms of the engine. In a typical system
you have the following input sensors:
1. throttle position of the accelerator
2. air temperature in the intake manifold
3. air pressure (or weight of the air) in the intake manifold
4. a measure of the residual oxygen in the exhaust manifold (the
5. engine rpm
6. camshaft position
7. crankshaft position
8. engine load, usually as a function of manifold vacuum pressure
9. engine coolant temperature
10. about a zillion other things, like “knock” sensing, battery
voltage, ambient air temperature, and whether Hoobastank is still on the
Billboard top 40.
The computer simply takes all this data, compares it to data about optimum
spark and fuel volumes for different conditions, and controls how much fuel
to inject and when to fire the spark plugs.
Modern Mixture and Maps:
We know that there is an ideal air:fuel mixture ratio for each type of
fuel - for gasoline this is 14.7 weight units of air to 1 weight unit of
fuel, a so-called “stoichiometric” mixture. In principle, this
is the mixture point at which all fuel injected will combust with all the
gases in the ambient air. However the combustion is rarely perfect – an
oxygen may never meet a hydrocarbon on the other side of a crowded cylinder
of dancing vapours - so a somewhat leaner mixture (around 16:1) will ensure
that excess air will gather up all of the hydrocarbons in the fuel and so
will produce fewer emissions and better fuel efficiency. On the other hand
a richer mixture will ensure that all the air that can be moved into the
engine will combine with hydrocarbons in the fuel and so for maximum power
you actually want this mixture to be richer - closer to 12:1. Since you’re
more concerned with power under hard acceleration and less concerned on
the overrun (when you lift the throttle), the computer can target leaner
and richer mixture ratios depending, literally, on how hard you stomp on
Maintaining the ideal mixture is the function of your “fuel map” and
it determines how much fuel the ECU will inject during each cycle of the
engine. Essentially the map is a compromise to give you power when you want
it, keep emissions as low as possible, and all the time keep the mixture
within safe parameters to avoid damage to the engine.
How much Air?
Before it can determine how much fuel to squirt in, the main challenge
for the ECU is to know what weight of air is in the intake manifold and
getting sucked (normally aspirated) or pushed (turbo or supercharged) through
the intake valve. Modern ECUs measure this in one of two ways: either by
taking the pressure and temperature of the air in the manifold and calculating
Manifold Absolute Pressure (MAP), or by measuring the weight of the air
(Mass Air Flow or MAF) as it moves into the intake past a heated wire filament
across which the ECU can measure electrical resistance. Older systems used
other means, including various flaps and pulleys (I’m not kidding)
to measure airflow. Now most production cars use MAF, although I use MAP
in my rally car and some manufacturers, including Subaru, are moving back
How Much Fuel?
The main mixture job of the ECU is to control the injectors. Fuel injectors
are little spray nozzles that can be snapped open and shut by an electrical
charge: the amount of time they’re open, combined with the pressure
of the fuel coming in and the size of the injector, determines how much
fuel will be injected. In “multi-point” fuel injection systems
there’s an injector for each cylinder, while in more basic systems
(“Throttle Body Injection”) there’s just one for the whole
So what determines how long the injectors stay open? The ECU senses the
weight of the air, the rpm of the engine, and compares this to the data “map” it
contains about ideal mixture for each load-rpm condition (see “base
fuel delivery” graph). It then decides how long to open the injectors
for and sends an electrical signal to them for the right amount of time.
At idle the injectors open for just a millisecond – long enough to
keep the engine ticking and no more. But mash the throttle and they go towards
the maximum “duty cycle” – theoretically they could stay
open 100% of the time but generally are engineered to give full throttle
by being open about 80% of the time – that is, 80% of the time that
it takes the crankshaft to go around twice. Thus is mixture in
the modern engine determined.
When to Spark?
As engine speed increases, you need to ignite the mixture
in the combustion chamber progressively earlier to get most efficient
ignition. Note that
the “explosion” in each cylinder is really a very fast fire
and that the “fire front” moves through the mixture until burning
is complete. The earlier you can spark the mixture, the more power you’ll
get as there will be more time to burn all the vapour before the exhaust
valve opens. But if you fire the spark too early, you will end up with the
explosion completing too soon and you get an uneven leading edge to the
burn – effectively a separate explosion - called a “detonation.” This
may allow a “hot spot” of the explosion to hit the top of the
piston (normally it is protected by a barrier of other gases at the edge
of the explosion), and this is what a “knock” in the engine
is. All engines knock somewhat, actually, but if you don’t keep it
within an acceptable level you will burn a hole right through the piston.
Don’t ask me how I know.
Higher octane fuel, by the way, burns more consistently at the leading
edge, which is why for most race applications we use fuels with an octane
above 100 while you might use 87 in your street car, and why we can advance
the timing and run higher compression in the race cars. Lead additives in
the fuel prevent detonation, but as you know this is mildly toxic and so
we don’t use it any more. Of course blowing engines is mildly toxic,
So for ignition, you have a second “map” that determines when
to fire the spark plugs, based again on engine rpm and load (see “base
ignition timing” map). In many four cylinder engines you have two
coil packs and each has two plugs attached to it – when the ECU determines
it’s the right time to fire one of the plugs it sends current to one
of the coil packs and two plugs actually fire, although one is redundant
and has no fuel to ignite (creating a “waste spark”). It’s
just cheaper and more reliable than having four individual coils or (horror!)
So the fuel delivery and spark timing maps are the real
essence of what the ECU is doing for you all the time. Thousands
of times per second the
ECU is calculating how much fuel to deliver and when to spark.
But that’s not all. For example, it knows when the engine is cold
and it delivers more fuel and adjusts the spark for smooth running until
the engine warms up. It also knows if the engine gets too hot and it can
drop into “limp mode” to get you home by cooling the explosions
with a richer mixture and later spark.
It also has two other very important sensors that adjust the fuel and spark
maps constantly. The first is the “oxygen sensor” in the exhaust
manifold, which sends information about how much oxygen is left over after
the explosions in the engine. The computer takes this data and constantly
adjusts the fuel mixture to keep this near maximum efficiency (14.7:1) or
maximum power (12:1) depending on settings. Running in this mode is known
as “closed loop” as the ECU is constantly adjusting itself to
get the desired exhaust. There are a couple of interesting consequences
to this: it self-adjusts for different qualities of fuel, and (this is cool)
it even learns to change the entire fuel map for the car as the engine wears
over the life of the car. But the sensor itself is a rather specialized
device and deteriorates over time, which is why the #1 emissions-test failure
is a faulty oxygen sensor. Now you know.
The second common and important feedback sensor is a little microphone
attached to the engine block that can sense “knocking” or detonation
from the cylinders, allowing the computer to adjust the timing backward
(later) to prevent damage to the pistons. A few knocks per 100 cycles will
be OK. More is bad. Note that there can be problems associated with this
sensor: the Mitsubishi 4G63 engines especially in Galant VR4s and Talons
can be afflicted with a “phantom knock” even when the engine
is running properly but goes into limp mode anyway, sapping power. Basically
the microphone is picking up noise from somewhere else in the engine and
sending it to the computer as a knock; many Mitsu people find that the valve
lifters are the source of the noise. Similarly, in a rally car we have constant
barrage of noise from rocks on the chassis and this can be picked up. As
a result, I don’t have a knock sensor.
Opening Pandora’s Box:
So now you have a dangerous amount of knowledge and want to completely
remap your Honda, right? Be careful. It’s not rocket science, but
it’s not model rocketry either. Still, there is good reason to start
fooling around. Modern cars are mapped to provide a reasonable tradeoff
between fuel efficiency, emissions, and power. If you care more about one
of these things you can tune the engine more aggressively towards that.
I’m going out on a limb and guessing you want more power, right?
Basically your typical aftermarket remapping is about richening the mixture
throughout the range towards maximum power (12:1) and away from maximum
efficiency (14.7:1). You’ll use more fuel, but get more grins. Also
you’ll get more emissions, not just because of the residual hydrocarbons
in the exhaust, but because this will cool the exhaust and catalytic converters
are designed to operate within a very narrow temperature window.
In terms of timing, the factory settings are very conservative, presuming
you will run rotten fuel sometimes and that you want the engine to last
for a while. If you’re committed to running higher-octane fuel all
the time and putting your engine at some risk of detonation, then you can
advance the timing a bit and get more power.
How much more power, and how to get it?
Basically you have four options if you start playing with your ECU: 1.
replace or “reflash” the memory chip that contains your fuel
and timing maps. Normally you remove the ECU and send it away for this to
be done; some devices now exist for you to do this on your own. ($50 to
$800) 2. Put a “piggyback” additional board between the ECU
and the sensors/injectors/coils that modifies the signals to and fro and
allows you to effectively remap the engine, although you’re limited
by some of the original ECU’s parameters. ($500 to $1200). Note that
you can reflash the chip at the same time to open the limiting parameters,
and I’ve seen 450hp Evos with this system. 3. Replace the entire board
in your original ECU case and plug the factory wiring harness directly into
it (a “plug-and-play” board) that offers full programmability.
This is what I use in my rally car. (to approx. $2000) Or 4. replace the
ECU with an aftermarket one that will require you to significantly rewire
your car (to approx. $2000).
But the real challenge is not how to get programmability. It’s what
to program. Either you should rely on another tuner’s wisdom and use
their maps (Dinan, Cobb, Vishnu, etc.), or you need to get on a dyno and
burn up some tanks of fuel to find out what mixtures and timings really
produce the most power and torque for your particular engine on your particular
fuel. It’s a time-consuming and precision process and requires some
experience. The bottom line is what you see on the horsepower and torque
curves from the dyno.
There’s another device you need, though, and I’m happy to report
that it is now available to the common man. You need to get information
from that oxygen sensor in the exhaust so that you can monitor mixture while
you’re tuning for power – too lean and you’ll start blowing
engines; too rich and you’ll lose power (and, at the extreme, start
blowing engines). Although you can get little LED readouts for your dash
that appear to tell you about mixture they’re not accurate enough
and you really need a digital device that can log the mixture against load,
rpm, throttle position, etc. Until recently, the main options were air-fuel
meters available from MoTEC ($2200US) or Autronic ($1800US) which is a serious
But there’s a new product on the market that I’ve used and
I love: Innovate Motorsports (www.innovatemotorsports.com) now makes an
affordable fully digital and data-logging air-fuel meter that retails for
$349US. I’ve used it to tune my Evo already and I find it perfect
for the job. Install the sensor in your exhaust and power the device up
and you are already looking at the A/F ratio (or “lambda”).
Wire in up to five additional inputs and you can watch them on screen as
you tune, or log them for up to 44 minutes in the device itself and download
it later (great for road tests). Also you can take the analog output signal
and feed it to your ECU, as I’ve done, so that you can see A/F ratio
against all the other engine parameters you can monitor by hooking up directly
to the ECU. I really like this device, and won’t tune without it now.
I recommend it strongly. It won the SEMA award for “Best New Product – Performance
and Racing” this spring and I can see why.
[grab whatever images you like from here: screen shots there could be good
Any questions? There will be an exam on Friday. Or rather an exam on the
weekend, when we go out racing and see who has more power and who blows
his engine. Now you wish you’d studied, right?
The bottom line is this: ECUs use a group of sensors to gather information
about engine running conditions and use two maps – one for fuel and
one for spark – to control how long to open the injectors and when
to fire the spark. Playing with these yourself is entirely possible, although
it requires some knowledge to get the best out of it. Note that any remapping
is likely to cause you to fail emissions tests and void the warranty of
your car, so you’ve been warned. But who’s thinking about that
when you’re putting out 450hp from 2 liters, right? Best of all: no
Recommended Reading: Forbes Aird, Bosch Fuel Injection Systems, HP Books
For complete version of this article visit Inside Track News