MicroSquirt® Introduction

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V3 MicroSquirt® - QuickStart Guide

Number of Squirts and Injector Staging

There are a number of considerations in choosing the number of squirts:

Throttle body injection (TBI) and port injection are completely different in regard to the required number of squirts - so this must always be taken into account. TBI often requires at least half the number of squirts as cylinders fed by the common plenum (ex.)
Increasing the number of squirts alone increases the accel enrichment (since the accel enrichment is per squirt),
The timing of the injection (the 'injection timing delay' under 'Fuel Set-Up/General') can make a big difference to how sensitive the engine is to the number of squirts (since with fewer squirts you might be squirting into the disrupted airflow of the overlap period at low speeds).

More squirts are more likely to help if the engine:

has a low idle rpm,
is tuned to relatively lean (ex. stoichiometric or leaner) idle mixtures,
has high idle advance (say more than 15° to 20° BTDC),
wants larger accel enrichments.

However, if the engine is tuned so that the idle MAP is minimized (likely 13.5:1 AFR or so), and has a higher idle speed (say 800 rpm compared to 600 rpm), then more squirts is less likely to help.

There are definitely some down sides to more squirts:

More squirts makes the tuning more sensitive to the correct opening time, and this is one of the harder things to determine accurately.
More squirts reduces the effective resolution of the req_fuel, accel enrichments, etc. (since they are adjusted in 0.1 msec steps, but on increasingly short pulse widths)
As well, more squirts eats up more of the available time with open/close cycles, reducing the dynamic range of the injectors. So users who have chosen their injectors based on the net horsepower may find they are too small at high rpms and loads. This can damage an engine, and must be avoided at all costs, of course.

So more squirts may help. They may not, however. And this can be a bandage for poor idle or accel tuning. So users should start with 2 squirt/alternating for port injection, and tune it as well as they can.

Then, if a user can't solve some tuning issues, they should try more squirts. But jumping straight to more squirts is probably not a good idea - most often it just masks the need for a richer idle mixture or more accel enrichment.

Example: Suppose you have a 4 cylinder 4-stroke engine. Then in 720° (two revolutions) you have 4 spark events (at 0°, 180°, 360° and 540°, then starting over at 720°=0°). MegaSquirt can only inject on an ignition event, so 1, 2, or 4 squirts/cycle for this engine.

If you have 2 squirts/simultaneous, you have:

crankshaft degrees	0°	90°	180°	270°	360°	450°	540°	630°	720° =0°	...→
cylinder at TDC (not injector)	1		3		4		2		1	...→
bank 1	inject				inject				inject	...→
bank 2	inject				inject				inject	...→

but with 2 squirts /alternating, you have:

crankshaft degrees	0°	90°	180°	270°	360°	450°	540°	630°	720° =0°	...→
cylinder at TDC (not injector)	1		3		4		2		1	...→
bank 1	inject								inject	...→
bank 2					inject					...→

Both are two squirts per cycle, but alternating only has 1/2 as many total squirts per injector (i.e., since only one channel is active per squirt), so the pulse width must be doubled.

4 squirts/simultaneous would look like this:

crankshaft degrees	0°	90°	180°	270°	360°	450°	540°	630°	720° =0°	...→
cylinder at TDC (not injector)	1		3		4		2		1	...→
bank 1	inject		inject		inject		inject		inject	...→
bank 2	inject		inject		inject		inject		inject	...→

There are 4x as many injection events with 4 squirts/simultaneous compared to 2 squirts/alternating, so the pulse width is just 1/4 as long (neglecting the opening times).

The req_fuel calculator will show this - watch the top and bottom numbers as you make changes to the alt/sim and number of squirts. The top number stays the same (it is the 'unadjusted number' for 1 squirt simultaneous), while the bottom 'adjusted' number used for the pulse width calculations changes with the number of squirts and simultaneous/alternating.

Note that the above tables show which cylinder is at TDC at specific crank angles. Which injector fires when depends on which injector bank they are connect to.

If you have any questions or problems that can't be answered from the links above, or a search the MicroSquirt^® manual:

you can ask questions at the MicroSquirt® support forum which is at: www.microsquirt.com Click the links for more information.

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©2011 Bruce Bowling and Al Grippo. All rights reserved. MegaSquirt^® and MicroSquirt^® are registered trademarks. This document is solely for the support of V3 MicroSquirt^® boards from Bowling and Grippo.