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Will the MegaSquirt work with my stock ignition system?
Last Updated: 12/21/2010
MegaSquirt-I or MegaSquirt-II
pursuing Fuel-Only Implementations of MegaSquirt Engine Management
Systems won't really need this article as we'll be focusing on how to
trigger the MegaSquirt from the stock Crank Angle Sensor, a.k.a. Crank
Trigger or Trigger Wheel, and sometimes from the Cam Angle Sensor as
well in dual trigger applications. For fuel-only implementations
you really only need to trigger the MegaSquirt with tach signal which
can usually be picked up at the (-) terminal of the coil unless you are
running a Capacitive Discharge Ignition, which can put out 400+ volts
here. In this case you can often use the tach output of the CD
Fuel and Spark Control: Standard Distributor Based Ignition Systems
Common -- Complexity 4 or 5/10
EMS: MS-I with MS1/Extra or
(a.k.a. dizzy) based ignition systems are usually the easiest to take control
of, and support for standard distributor based ignition systems was built into
the MegaSquirt (with MSnS-E) and MegaSquirt-II so there is a huge amount of support there.
So what exactly am I calling a 'standard distributor based ignition system'?
Basically this would be a distributor based ignition system, with an internal
crank trigger mechanism based on:
- Variable Reluctor (VR) Sensor
- Hall Sensor
- Optical Sensor
Each of these will have a 'wheel' or 'cam' with a certain number of
'positions' we'll call them. These 'positions' will be read by the
sensor. For a VR Sensor trigger, these 'positions' will be
represented by a toothed wheel. The sensor will generally have two wires,
and will magnetically read the teeth as they pass by. For a Hall Sensor
trigger, these 'positions' will be represented by a shutter wheel with
windows in it. There will be a magnet on one side of the wheel and a
sensor on the other, and the windows and non-windowed areas of the wheel will
interrupt and pass-thru the magnetic field which is read by the sensor.
For an Optical Sensor trigger, there is an LED (not visible light
usually) and a sensor on opposite sides of a flat disc with small slits it in.
The slits are the 'positions' and let the light pass thru and the sensor reads
it. Lastly, points -- this is the system I have the least
experience with, but it's still around. With this type of distributor you
have a cam with a number of flat areas and lobes. These act as the
'positions' in a points setup. The switching arm follows the cam and opens
and closes the points.
here to check out Lance's excellent write up on the different triggering
mechanisms for more information and then come back over to this article to
Here's a site with a great explanation and animation for a 4 cylinder that
will help you to see a full dizzy (distributor) based ignition system in action:
Notice 'Part F'; the item they call the 'Distributor Cam'. This would
probably be more commonly called the trigger wheel, or Crank Angle Sensor Wheel.
(CAS Wheel for short). That animation best represents a VR Sensor setup,
with a 4-toothed wheel. Notice that the teeth are evenly spaced 90 degrees
apart. The distributor spins at cam speed, which is half of crank speed.
Which means that for every complete engine cycle (two complete rotations of the
crank, or 720 degrees) the cam (and any wheel spinning at cam speed) spins one complete rotation, 360 degrees.
So each time a tooth passes the sensor (every 90 degrees of trigger wheel
rotation) the crank has rotated 180 degrees. This works perfect for a
4 cylinder car, with one tooth passing the sensor each time a piston reaches TDC
(Top Dead Center), or a set number of degrees before or after TDC which the
computer accounts for when firing the coil, which is then directed to the
correct plug by the distributor. But hold on, maybe I'm making this sound too difficult
here, it's really very simple to be able to see if your setup will be compatible
in many cases...
- On a trigger wheel that's spinning at cam speed and a 4
cylinder vehicle. You're looking for the four 'positions' to be spaced
evenly, which will be 90 degrees
apart. If they are, you're good to go, and you can trigger a MegaSquirt-I
with MSnS_E, or a MegaSquirt-II with this trigger wheel with no problem!
- Similarly on a trigger wheel that's spinning at cam speed and a 6
cylinder vehicle. You're looking for the six 'positions' to be spaced
evenly, which will be 60 degrees apart.
- Same thing goes for an 8 cylinder, in that case you're looking for the
eight 'positions' to be spaced evenly, which will be 45 degrees apart.
Don't get caught up too much in trying to measure the number of degrees
apart-- you should be able to tell if they are evenly spaced by looking in most
cases. If so the math will be right as 360/4=90, 360/6=60, and 360/8=45.
If it's a Hall or Optical sensor, you'll also check to make sure the windows
or slits are all the same size, or at least had the same leading or trailing
edge. We'll get into that more later. For now we'll assume they're
all the same size, if you can see that yours aren't then read on down the page.
Let me show you some of the different wheels and sensors:
Notice the 8 cylinder wheel has exactly 8 'positions' (teeth in this case)
and they are evenly spaced apart. The two 4 cylinder wheels both
have 4 'positions', and though you can't see all of the wheels due to the angle
of the pictures and the sensor placement, they are also all evenly spaced.
The other thing too look out for with the hall and optical wheel here is whether
or not the 'positions' (either windows or slots) are all the same size.
These all are so there is no concern.
The quick-n-dirty formula for success-- if the
number of 'positions' equals the number of cylinders, they are evenly spaced,
and they are the same size-- it will work with the MS-I/MSnS-E and the
MegaSquirt-II. All three of the above distributors will trigger the MS and
MS-II just fine!
Note: This is based on the wheel spinning at CAM
speed, as the distributor generally does. If the wheel is spinning at
CRANK speed, such as when the wheel is mounted to the crank pulley directly, the
story is different. But we're not there yet....
We'll talk about some of the funkier situations further
down. MANY of you will need to go no further as a LARGE percentage of
distributor based vehicles will fall into the category above, easily fully
Fuel and Spark Control: Other Distributor Based Ignition Systems
There are four other less common types of
wheels you may see when you pop open that hood/distributor.
distributor with inconsistently spaced/sized holes
Somewhat Common -- Complexity 5/10
Compatible EMS: MS-I with MS1/Extra
Often these trigger wheels will work just fine
as well. Remember you are looking for consistent trigger points
(positions) evenly spaced apart. In some cases You'll have all
trigger positions, usually holes on a optical trigger wheel, spaced
evenly, but one will be larger than the rest. An example of this
are Nissan 240sx and 300zx cars (probably others as well). Here's
a wheel used through most of the 90's on the Nissan 240sx (KA24E).
It has 4 holes, evenly spaced, but one hole is larger than the rest.
What's important to
look for here is if the leading or trailing edge of the holes line up to
be evenly spaced apart. So picture this-- the MegaSquirt EMS is
not so much looking just at the hole itself, but at the beginning and
end points of each hole. Where they start and stop. As long
as one or the other lines up evenly all the way around then you're in
good shape and triggering the MegaSquirt will be no problem. The
above wheel will work just fine with the MegaSquirt-II set to trigger
from the 'falling edge'. A
12v pullup on the ignition input is needed as well.
The late 80's Nissan
300zx (VG30E and VG30ET) have a nearly identical trigger wheel inside
the distributor, just with 6 positions evenly spaced, and one larger
than the rest. This works fine as well. Once again set to
trigger on the 'falling edge' and with the
12v pullup on the ignition input.
No wheel inside the distributor, crank or cam
mounted trigger wheels instead (no decoding required)
Less Common --
MS-I with MS1/Extra or MegaSquirt-II
Remembering that what
we really need for a distributor based ignition is one trigger event
(position) passing the sensor for each cylinder, and they need to be
evenly spaced on even-fire engines (which is almost all of them, except
Harleys and few other odd-fire motors). This tells the ECU to FIRE
the ignition once for each cylinder at the right time so that the
distributor can make sure that spark gets to the right place.
Let's say for example you have a distributor without a trigger wheel
inside the distributor at all. The ignition system HAS to have a
trigger wheel somewhere or the ignition would never fire because it
would never know that a cylinder was approaching TDC (top dead center).
1) Trigger Wheel on
the cam: Since the cam runs at the same speed as the
distributor (1/2 of crank RPM) this would need to be identical to what
you'd expect inside the distributor, with the same number of trigger
'positions' as the engine has cylinders, and all evenly spaced.
Usually this would be a toothed wheel and VR sensor. For a 4
cylinder you'd see 4 teeth, evenly spaced. 6 for a 6cyl, 8 for an
8cyl, etc. This would trigger a distributor based ignition system
just like it would if the trigger wheel were inside the distributor
itself. No problems here, no special decoding required.
2) Trigger Wheel on
the crank: Since the crank runs at full engine speed, which is
twice the cam/distributor speed, you only need half as many trigger
positions on a crank mounted wheel (evenly spaced of course). This
is also usually a toothed wheel with a VR sensor and is mounted on the
crank pulley up front, or the flywheel on the back of the engine.
Note it can also be a 'flying magnet' Hall Sensor setup. The
results are the same. 2 trigger points for a 4 cylinder, 3 for a
6cyl, 4 for an 8cyl, you get the picture. This would trigger a
distributor based ignition system just fine, as if it had a wheel with
twice as many positions inside the distributor itself. There are
actually some benefits to this as it takes the cam and distributor shaft
out of the loop which can flex just a bit at high RPM. This gets
it's reading DIRECTLY from the crankshaft, so it will tell you EXACTLY
where the engine is in rotation without that small margin of error
possible with a distributor located trigger wheel. For most of us
this is of no concern as it's a small fluctuation in most cases.
No wheel inside the distributor, crank or cam
mounted trigger wheel(s) instead (decoding required)
Somewhat Common --
MS-I with MS1/Extra or MegaSquirt-II
There is an ignition
system that was often used on European (Porsche and BMW) cars that had
no trigger wheel in the distributor but has what's known as a missing
toothed wheel mounted on the crank (pulley or flywheel) with spaces for
60 teeth, but 2 are missing. This is known as a 60-2 wheel,
pronounced '60 minus two'. There are car with other missing
toothed wheels as well, maybe a 4-1, 20-1, or 36-1. Some of these
you'll see as a part of a stock ignition system, some may have been
custom made to trigger an aftermarket engine management system such as
Both the MS-I with
MSnS-E and the MS-II (firmware 2.6 or later) support a distributor based
ignition system being triggered by a missing tooth wheel. You can
find a ton more information on this here:
distributor requiring decoding by the ECU
Somewhat Common --
Complexity 7 or 8/10
MS-I with MS1/Extra or MegaSquirt-II with
Basically the exact same story as above except
the wheel or wheels are inside the distributor. It could be a
missing tooth wheel, or two wheels that work in conjunction. The
most common is an ignition system used by Toyota and Mazda which
commonly used this arrangement, where there would be a 24 toothed wheel
(evenly spaced, no missing teeth) and a separate 1 or 2 toothed wheel.
Sometimes the 1 toothed wheel would have 2 VR sensors, providing two top
dead center signals. This is also
compatible with the MS-I and MS1/Extra firmware or MS2/Extra firmware. Note
you will need a 2nd VR conditioner (mid-level modification) in order to
receive the second ignition input signal. See our
Zeal Engineering Daughterboard for an easy way to add this. We
recommend MS2/Extra with the dual wheels because of its superior noise
Click here for more information on MS2/Extra with Nippondenso ignitions.
Going Distributorless: Stock Vehicles with the
Ford EDIS or GM DIS Ignition systems
Going Distributorless: Wasted Spark and Coil
On Plug (COP) Systems
MS-I with MS1/Extra or MS2 with MS2/Extra
You can do either wasted spark or COP ignition
with the MegaSquirt-I and MSnS-E firmware though it's probably the most
advanced implementation of the system—it requires fitting a missing
tooth (12-1, 36-1, 60-2, etc) wheel to the crank pulley and modding the
ECU out with extra ignition outputs to drive the COPs. MS2/Extra
supports other types of crank wheels as well -
click here for a full list. I'd recommend
building your ECU from a kit so you'll be familiar with it when it comes
time to mod the unit. The other piece of this is determining the trigger
settings for your motor, the guys on the forums (linked below) can help
Here's the main info on this setup:
MS1/Extra Wheel Decoding
MS2/Extra Ignition Guide
Here's the MS/Extra forums which will be really helpful, both to research
what others have done, and for any questions you have that the above
link doesn't answer:
It's a very sweet, though complex, setup. You can do it cheap and super
effective-- but it's going to take some work, maybe some frustration,
the car being offline for a bit, and then you'll get it going and have a
sense of accomplishment that is simply amazing-- if that sounds like fun
to you then this is your setup.
The rest of the install is much easier compared to this as it's pretty
much the same across all vehicles (sensors, injectors, etc) and you can
use the main MegaManual at
as a guide for all injector and sensor wiring.
Going Distributorless: Wasted Spark
Conversion the easy way, Ford EDIS to the rescue!
MS-I with MS1/Extra or MegaSquirt-II
If you want or need to go distributorless, and
you'd rather stay away from the 'complexity 10/10' option above your far
simpler option is to adapt the Ford EDIS ignition system-- Ford EDIS is
a wasted spark distributorless ignition system that's very easy to adapt
to just about any internal combustion piston engine out there—you just
need to mount (weld or bolt) a 36-1 trigger wheel to your crank pulley
and build a bracket for the VR sensor to locate it next to the trigger
wheel. That's the hardest part and the only fabrication work. The rest
is a bit of wiring. The EDIS system has an EDIS module and a coil pack.
The module is wired to the VR sensor and the MegaSquirt, and also the
coil pack. The MegaSquirt-II is what I would recommend for EDIS control
and will take full control of an EDIS ignition system easily.
Here are some links with more info:
Here's all the parts that are needed and where they can be found if you
wanted to dig them up yourself:
Here's the MS2 manual:
http://www.megasquirt.info/ms2/ It has an EDIS specific
This page will be
continued--- more pics above and more information will be added.... Stay