Megasquirt 3 - Launch control / Flat-shift / 2-step / 3-step


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Intro - Launch/ Flat-shift Settings - Variable Launch Settings - 3-step Settings - Wiring


The Megasquirt-3 Launch control system is a effectively a 2-step limiter and is typically used in drag racing applications for consistent hard launches and/or to build boost with a turbo-charged engine.
The flat shift settings allow full throttle shifts on a clutched vehicle, a switch on the clutch triggers the flat-shift rev limiting (for bikes, see also the bike shifting system that may be more suitable.) It is important to install the clutch switch with adjustability as the exact height has an effect on how well flat-shifting works. Flat shift is unlikely to be useful when running an automatic transmission.
The 3-step is typically used as a burnout limiter and will often be linked to a line-lock.

Drag race example (manual trans)
You pull forward into the burnout box, stand on the brakes and flick on the line-lock. This also enables the 3-step. You now do your burnout and the 3-step limits are applied. After your burnout you pull into stage, depress the clutch (activating the launch switch) and then push the throttle to the floor. The launch settings will hold the engine close to your chosen rpm. When you change gear keep the throttle planted and only use the clutch. Now the flat shift settings come into play and the revs will be limited to those settings. After changing gear and lifting your foot off the clutch, the limiter is disabled. Be sure to also set the normal rev limiter should you miss shift.

Launch/Flat-shift Settings
launch settings

Launch control option
- Off - all of launch if turned off
- Launch - launch only, no flat shift
- Launch/flatshift - launch and flat shift
Input on - where the launch input switch is connected
Launch retard above - when the launch conditions are met, the timing is retarded above this rpm
Retard to - the absolute timing to retard to. i.e. if you enter 5 in here, the timing will be 5 BTDC, -3 means 3 ATDC. Using ATDC timing can be useful for building boost, but beware of the potentially harmful heat it generates in the turbo too.
Launch fuel add/remove - Allows fuelling to be changed while the launch limiter is on. Only for specialist use.
Fuel addition - the raw millisecond pulsewidth change to make (+/-)
Enable launch when TPS above - launch is only active when the throttle is open a certain percentage. Most users can safely set this to a very low number. Setting it too high can cause launch not to activate as expected.
Launch hard limit - when the launch conditions are met, above this rpm the hard limiter methods are used
Limiter method
-None - do not use
-Spark cut - drop sparks, this is the most common limiter used. (Will not work with EDIS ignition.)
-Fuel cut - cuts fuel. Seen by many as risky due to the possibility of partly fuelling.
-Spark and fuel - both
Cut X sparks from Y events - cuts spark in a round-robin manner like a fraction, 4 from 5 is like 4/5 so four fifths of the sparks will get cut.
On an 8 cylinder engine running 1,8,4,3,6,5,7,2,1,8,4,3,6,5,7,2 this would then look like :
*,*,*,*,6,*,*,*,*,8,*,*,*,*,7,*,..... where * is a missed spark event.
The larger the X is number is as a proportion of Y, the greater the limiting effect.
Note that some ignitions with semi-smart ignitors might get "confused" by spark cut or revert to a limp home spark mode instead of dropping spark.
Cut fuel above - this is intended to be used during flat shifting. During the up-shift as the revs are falling, the fuel is cut off until the revs have fallen below this set point. This reduces the raw fuel in the exhaust system that can otherwise produce a loud backfire.
Flat shift arming rpm - this rpm set point is used to differentiate between a flat shift and launch condition.
Typically as you are preparing to start a launch you will be at lower rpms, while for flat shift you operate the clutch near your rev limit. So set the flat shift arming rpm to say 3/4 or your max rpms. i.e. consider what rpms are you doing when you press the clutch and make sure the code will always know if you want the launch limits or flat shift limits.
Flat shift mode retard above - when the flat shift conditions are met, the timing is retarded above this rpm
Retard to - the absolute timing to retard to.
Flat shift hard limit - when the flat shift conditions are met, above this rpm the hard limiter methods are used

Variable Launch Settings
The variable launch works in conjunction with the other launch settings but allows the launch rpms to be varied with a 0-5V potentiometer input.
Variable launch input
-Off - disables the feature and uses the single limits
-One of the analogue inputs - connect the potentiometer here and the limits are variable
minimum setting - With a 0V input, the launch hard limit is set to this rpm
maximum setting - With a 5V input, the launch hard limit is set to this rpm
soft limit lower by - The soft launch limit is this many rpm below the variable hard limit. 1000rpm is typical.

3-step Settings
This is another switch operated limiter that works similarly to launch. It could be used as a burnout limiter and the input switched along with your line-lock.
3 step input
-Off - disables the feature
-One of the switch inputs - picks the input to active the feature
Retard above - when the 3-step is on, the timing is retarded above this rpm
Retard to - the absolute timing to retard to.
Hard limit - when the 3-step is on, above this rpm the hard limiter methods are used

Note! Double check that the jumper from S12C to JS9 is installed on your mainboard. Without it the switch inputs will not work.
This shows the available switch and analogue inputs on the MS3X connector. Other DIY options are available, see the MS2/Extra manuals.
Launch wiring

Typical wiring for an active low (gnd) inputs (Launch in, D/Log in, Tablesw in) for MS3X and the optional variable launch potentiometer input.
For an active high (12V) input (N2O in), see this diagram
Launch i/o

Two options for wiring for 3-step input.
3-step input

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No part of this manual may be reproduced or changed without written permission from James Murray and Ken Culver.