I am trying to put together a general tuning guide for AE and Megasquirt Below is what I have come up with so far. This applies to several versions of Megasquirt.
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There is a lot of confusion regarding the tuning of Acceleration Enrichment in the MegaSquirt family of programs. Over the last two years I have been involved, to a varying degree, in several of these ways to control AE. It is far easier to tune AE if you under the underling causes and effects.
There are a few that we have to consider. (Hang with me here)
1) You may remember from high school physics that water will boil at close to room temperature if you pull enough vacuum on it. Conversely, if you apply pressure you can raise the boiling temp up to 250F or more.
2) Water on the driveway always evaporated faster on a windy day than it does on a calm day.
3) Water on the driveway always evaporated faster on a hot day than it does on a cool day.
4) If you leave a cold glass of beer on the counter water droplets form on the side of the glass, apparently water from nowhere.
5) Cold air has more oxygen molecules per cubic foot that hot air.
6) A VE table as we in the MegaSquirt world is designed to balance the amount of fuel delivered from the injectors with the air flow. The internal math of the MegaSquirt estimates this airflow and in turn calculates the matching injector pulse width. This airflow calculation is based on intake air temperature, RPM, intake manifold absolute pressure (KPA), and RPM.
7) An intake manifold often has hot water in it. This manifold is attached to hot things and is being blown on by constantly changing amounts of hot air coming off of the radiator and from the exhaust system.
8 ) An intake manifold has a constantly changing amount of air properties and the amount of fuel that can condense on the walls.
9) Fuel in the intake constantly collects on the inside walls of the intake.
10) The amount (thickness) of this layer of fuel is constantly changing.
11) The earlier the fuel is injected in the intake airflow, the more area of the intake wall can get wet.
12) Back pressure on the exhaust, weather caused buy high atmospheric pressure at the end of the exhaust system or by trying to flow too much exhaust through the system causes the motor to need less fuel.
13) The injector delivers fuel only after it has opened. The injector opening time consists of about the first 1.0 millisecond of the pulse width. So if MT reports 10 ms pw1 the injector is only supplying fuel for about 9 ms. This is a big deal if you have a cruse pw of say 4ms and your AE added pw is 6ms. The amount of fuel delivered goes from (4-1) or 3ms of fuel to (4-1)+6 or 9ms or three times the fuel. Plenty of fuel to put out the fire.
14) Way too much fuel feels just like way too little fuel from the driver’s seat.
15) Fuel collecting in the walls and floor of the intake behaves much like the water examples above.
A few definitions.
MAP: Manifold air pressure. The units are KPA. The important part to remember is 100 KPA is close to amount of air the world has at sea level. At 6000 ft elevation you have 80% air or 80 KPA. 15 psi of boost amounts to about 200% air of 200 KPA. Most manifolds have on a steady state motor about 40 KPA at idle and 95 KPA at full throttle assuming normally aspirated.
MAPDot: The calculation of the change of MAP/sec An indication of how fast the MAP is changing. Extra and base BG code uses KPA/Sec where MS2 uses percent/sec MAPDot is normaly a better indication of a need for tempary additional fuel than TPS related schemes.
TPSADC: The location of the Throttle Position Sensor as the Squirt sees it.
TPSDot: The rate of change of the TPSADC/Sec but converted on the display of MegaTune to Volts/sec the volts of the TPS go from about .5 volts at idle to about 4.5 volts at full throttle so 4 Volts/sec means the throttle is moving at a rate on going from no throttle to full throttle in one sec. MS2 works with percent/sec.
AE driven fuel percent Gauge: This gauge displays the percent of added fuel taking into account the opening time of the injector. This little advertised is extremely valuable to put into perspective just how much fuel is being added at any given time. The opening time is assumed to be 1ms and is adjustable in the MsnsExtra.ini
TPS Threshold: The value of the TPSDot that activates the AE event
MAP Threshold: The value of the MAPDot that activates the AE event
Note: MS2 allows a mix of MAP and TPS threshold All versions of Extra are all of one or the there, pick one.
So how does all this pull together as it relates to tuning the motor?
Remember that the VE table is designed to deliver the correct amount of fuel at steady state and normally will not be designed to cover for extra fuel required during these transition times in the fuel requirements caused by all these rules stated above. Any motor that is properly tuned to give a stable AFR at steady will have a huge demand for added fuel if the motor is exposed to more pressure in the intake. At low RPM the manifold AMP may jump from 45 KPA to 99 KPA in a very short amount of time. The added fuel is going to wetting the manifold up to the amount of fuel that can now coat it given the newfound pressure in the intake. Any time the opposite happens, fuel has to be pulled from the normal amount fuel required. This is referred to as the Decel setting. 90 is 90% of the normal amount of delivered fuel. The other 10% comes from the fuel that was on the walls of the intake and are now evaporating off the walls caused by the huge decrease in MAP pressure. At high RPM the motor may be running 90 kpa and a punch from ½ throttle to full throttle may only raise the manifold MAP from 90 KPA to 95 KPA. This is a relatively small change in MAP and as such very little AE is required. As you can see from this MAPDot may be a better gauge of the required amount of added fuel. But what do you do if your cam makes it so that you have virtually no workable MAP change to work with and TPSDot does not seem to be a workable solution. What about this thing called decay and where do I ever start on tuning this thing.
Over the last year, several ways to tune AE have evolved.
1) The original BG method: based purely on the rate of change of the TPSDot or the MAPDot. The AE, once activated can remain on for a certain amount of time (or in some versions of code) for a number of ignition cycles. The advantage of engine cycles is that at 4000 RPM the AE time is ½ of the amount of time that AE is active at 2000 RPM. The wider the powerband and the more high strung the motor is the more difficult this method gets to tune both the high RPM and the low RPM requirements. ITB’s make this method almost useless. 5000 RPM v8s with an automatic is doable. All existing codes to date
2) The RPM based AE method: With this method, the RPM of the motor is constantly taken into account. There 4 separate RPM values that can be set. Below the lowest RPM setting the max AE is controlled by the lowest ms cell. Between the lowest RPM cell and the highest RPM cell a linear interpolation happens between the two closest RPM cells. Above the highest RPM the highest RPM ms value is used. Only one threshold is in effect when using the AE scheme, depending on the choice of MAPDot or TPSDot. The AE table on the AE wizard is ignored. Note: This option is on Extra 026h8 and Extra 029M and later. All other versions have known issues.
3) Hybrid BG/RPM based: This is "normal AE/ Non x-tau" option in the MS2 v2.64 and later code. This method is a combination of the two above schemes. The terminology in the this graph and the terminology used on the final release of this scheme may change but the basic idea is that there is the ability to have large AE events at low RPM and less as RPM raises. The base settings would be set at low RPM and taper off to zero AE at some high RPM. This method has advantages and disadvantages compared to the RPM based logic used in the Extra code. I feel it is a huge improvement to the original B&G code. By setting both RPM values higher than your redline the result will be AE just like the original B&G code. Very promising.
Please not that the red lines that go from the upper left to the lower right of the graph are not necessarily linear but to draw them that way is too confusing.
4) X-Tau: This method is by far the most elaborate and potentially difficult to understand and tune but very promising. The just of the scheme is that virtually all of the inputs listed above are in the algorithm. It takes into account the airflow of the air in the intake (based on RPM), the pressure in the intake (MAP), the temp of the intake walls (coolant temp and air temp) and fudge factors for all of the other stuff. My fear is the numbers of variables involved and if we will ever be able to get a handle on the setup. This will be in the 2.62 and later codes yet to be released. This is used buy many OEM motors.
Tuning the Extra RPM Based AE
Tuning is a fairly simple procedure. As with all MS installs, the very first thing to do is sort out ALL input signals to the Squirt. IAT, CLT, RPM, MAF if installed, TPS and MAP signals must be all very stable. The goal of all of these AE methods is to have a very flat AFR during all transitions of the throttle. Once perfected, the motor will be very smooth throughout the entire power band including throttle stabs, shifts and throttle roll-ons. I always try to run MAPDot if I have a reasonable idle MAP to work with.
This all assumes that you have a tuned VE map. If using RPM based AE, start by setting the lowest of the four bins close to the RPM that your motor idles at. Say 800 RPM. The next cell to set is the highest RPM bin. This is the highest that you are likely to ever activate AE, say 80% of your redline. This should be close to the RPM that your motor drops to after a 3-4 shift. The two other RPM settings should be evenly spaced from the two extremes. For our example 800, 2000, 3000 and 4500 RPM. The second RPM number is where your motor will most likely need the greatest amount of AE in terms of total ms and percent on the AE Driven Fuel Percent gauge. I normally tune the base AE settings by getting on a stim and temporarily set the AE time to 2 sec and the decay to No Decay. Note that you will need to guess the MAP at say 85 KPA for this test. I use a syringe to apply 85 KPA to the Squirt. Play with the ms of AE numbers until from the lowest RPM thru the second RPM setting get close to a flat 150% AE through this range. The 150% is just a guess on my part but it is a place to start. Now go to the third RPM with the stim and set the third ms of AE to get about 115% Next go to the highest RPM and set the highest ms to get about 103% on the gauge. Set the AE time to .7 sec and the decay back on. If you do not own a stim then the numbers can be guessed at. Now start data logging, drive the car locked in second gear and put the motor at 2000 RPM, stab the throttle and have a friend watch the o2 gauge and the AE driven Fuel Percent. Play with the time and ms of the second RPM cell till you get good throttle response and most likely the best response will be when the o2 remains close to about stoch or a little richer. Lean holes are never the correct answer. Now try applying the throttle with a lighter stab. This ends up a compromise. Once a balance is set go to the third RPM and play with just the ms of the third cell to get good response. The highest RPM cell will be for high RPM (redline) shifts. Last go to idle and tune the ms of the lowest cell to get a reasonable off idle response from the motor. Note that as the RPM climbs, the AE will get stepped down by either the timer and the decay function or slammed down by the rising RPM.
See pages 101 thru 120 of 591 on... http://fordfuelinjection.com/files/GUFB.pdf
for lots more light reading on the subject.
Have fun tuning.