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Basics of How a Tune Works on the LNF |
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Written by Terminator2
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Wednesday, 05 May 2010 |
The Gasoline Direct Injection (GDI) LNF uses a Bosch E69 GDI ECM. This
ECM is a torque based engine controller containing approximately 2,500
tables that control all engine functions. There are approximately 600
tables that control fueling alone. A typical ECM contains only approx
250 tables total by contrast. A Gasoline Direct Injection (GDI) based
fuel injection system is much harder to control and optimize than a
typical Multiport EFI system which is why there are so many tables In
the ECM that control fueling.
At the heart of this ECM is its torque based control system that uses
driver input via pedal position to determine driver desired torque.
Think of it as 100% pedal is equal to 100% driver desired torque. The
driver desired torque value is fed into the Desired Airload table (DAL)
which uses driver desired torque (pedal position) vs RPM to determine
actual throttle postion. By raising or lower the DALs one can effect
how the car reacts to pedal input from the driver. You can make 50%
pedal 100% actual throttle opening or vise versa by raising or lowering
those values. The DALs are regulated by the Maximum Airload Torque
table (MALT) whch determines the percentage of actual airload allowed
based on RPM. The Combination of the DAL and MALT helps determine
actual throttle postion and does regulate the wastegate to some degree
but does not allow for the desired boost level to be raised above 238
kPa, 34.5 psi absolute or 20 psi guage without modifying another table
that we HP Tuners users do not yet have access to.
The GM stage tune rasies this desired boost ceiling to 285 kPa, 41.5
psi absolute or 27 psi gauge without raising the Desired airloads. The
Superchips Tune also raises the desired boost levels without changing
the desired airloads. Vince's infiniboost tune tricks the ECM into
thinking it is seeing 238 kPa even though it may be seeing up to 300
kPa. We HP Tuners users have to runs external wastegate regulators or
boost controllers to hold the boost levels above 20 psi until they
decide to crack more of the ECM for us.
There is debate as to what is proper as far as what is Air to Fuel
(A/F) mixture is appropriate for the LNF while in Power Enrichment Mode
(PE) at Wide Open Throttle. PE ratios are expressed as Lambda where
1.00 Lambda = 14.69 A/F mixture when using gasoline. The Stock GM Tune
and Stage one tune both use the same PE ratios of 0.95 to 0.87 lambda
or 14.0 - 12.8 A/F. Most Trifecta tunes use 0.80 - 0.84 PE Lambda or
11.8 - 12.4 A/F. The Superchips tune uses uses 0.85 PE Lambda or 12.5
A/F. HP Tunes will vary according to who is doing the tuning. I have
seen anywhere from 11.4 to 14.0 PE A/F used depending on the tuner. PE
A/F is influenced by Power Enrichment VS Time which is better known by
most as the catalyst overtemp protection table or COT Lambda table.
After a certain time at WOT once the ECM models a catalyst temperature
of 1700* the ECM will richen the A/F mixture to as rich as 0.711 Lambda
or 10.45 A/F at WOT. Most change those values to match their PE Lambda
Table or if they are catless they set all values to 1.00 lambda to
disable the extra enrichment.
Ignition and cam timing is another aspect of tuning this ECM as the LNF
has variable valve timing. The VVT is controled by cam phasers on each
camshaft. The LNF has 4 sets of basic timing tables based on load, RPM,
and Camshaft postion in relation to each cam's minimum and maximum
states. Although there are more timing tables these are the main ones
we are concerned with. Timing theories vary according to application
but there is one universal truth. There is a point at which adding more
ignition timing will actually cause torque and conversely HP loss.
There is always a sweet spot where the mixture burns completely and
just at the right time so that maximum cylinder pressure is reached
right as the piston comes to Top Dead Center (TDC). That results in the
the most work possible being performed on the piston (Large Force
Acting on the piston for the most time possible) and hence the most
torque output possible at that RPM. If the mixture burns too long from
running too much ignition timing it will reach peak cylinder pressue
well before TDC and the results will be spark knock and power loss, but
if there is too little ignition timing run the mixture will not have
sufficient time to burn completely and peak cylinder pressure will be
reached after the piston passes TDC and the result will be less torque
produced (Smaller Force acting on the piston for less time) The LNF is
no different. It uses less WOT timing than any other engine I have ever
seen partly do to the efficiency of the Direct Injection which produces
much better fuel atomization and the comparatively lean mixtures GDI
cars are known to run at WOT. Lean mixtures burn faster and hotter and
require less timing because they burn quickly. Rich mixtures burn
slower and at lower temperature and require more timing to burn
completely.
There are over 20 camshaft tables but the only ones most would modify
are the Intake Cam Main Warm table and the Exhaust Cam Main Warm Table.
Many have wondered what the units in those tables are as they are
undefined in HP Tuners. They are based on the camshaft's base circle.
As I understand, the units influence how many degrees from center the
camshaft is based on its base circle. So it is the camshaft itself that
gives meaning to those units. That is why the values in the scanner are
so much different than the values in the tables themselves and why it
is possible to have a negative value in the table but a positive value
in the scanner. Some have found small gains in top end HP by advancing
the intake cam 2-3* in the upper RPM and upper load regions of that
table. When you advance the intake or the exhaust cam you are asking it
to open sooner. The throry behind a slight advance in intake cam timing
is to open the intake valve a little sooner so the cylinder can take in
more air. Exhaust cam tuning theory is the opposite principle. You want
to leave the exhaust valve shut as long as possible so that maximum
cylinder pressure is reached before the exhaust valve opens. The
problem is if you wait too long to open the exhaust valve peak cylinder
pressure already have been reached before the exhaust valve opens. That
means either knock will ensue or the piston will already be past TDC
and the ability to generate peak torque will be lost.
What is "Learn Down" and How Does it Work
written by Terminator2
The "Learn Down" feature. Because the LNF's ECM uses torque based
controls it is much more advanced as far as limiting or adding to the
torque that is desired from it. All modern ECMs that I have seen have
Torque Management (TM) tables, and the LNF is no exception, but its
entire engine management system is built around Driver Desired Torque
vs Actual Torque Output. Driver desired torque is based on Pedal
Position, RPM, Desired Airload, and Maximum Airload Torque. Those
values determine Optimum Engine Torque which calculates engine torque
output based on 1.00 Lambda. The Lambda Efficiency table changes
calculated engine torque based on A/F Ratio expressed as Lambda. Leaner
than 1.00 Lambda will produce less actual torque, and richer than 1.00
Lambda (up to a point) will produce more actual torque output. The
"Learn Down" that most notice occurs when adding mods or driving in
very cold weather they notice the boost pressure values have gone down.
The ECMs torque based controls are to blame for this. If breathing
efficiency is increased either from adding mods or from a denser air
charge the ECM compensates for this by decreasing its Desired Boost
pressure values. It does so because Actual Torque Output is modeled as
greater than Desired Torque output. The GM Stage 1 tune raises the
Desired Boost and Actual Torque output levels but does NOT completely
remove the learn down but it does raise it. Other Tunes work the same
way. They do not completely remove the learn down they merely raise it
to a level that the car cannot acheive in the first place. Think of the
learn down more like a target airload level or target torque output
level. If you add mods or change the charge air density the ECM will
sense that it is reaching its target torque output much more easily
than before. That is not to say that adding mods without a tune will
produce no gains at all. It is possible when adding mods like a catless
DP and an Intake to make more power than stock even without being
retuned. Although the gains are much larger if you retune the car I
have seen evidence that even thousands of miles after adding mods a car
will still dyno higher than stock. I have seen a stock tune LNF with an
Intake and Catless DP make 254 whp and 272 wrtq thousands of miles
after first adding the mods. That car stock on that same dyno would
make closer to 240 whp and 250 wrtq.
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