This post is just to sum up the results of some interesting discussions about the LNF engines in the GXP and to perhaps act as a primer for newer owners wondering about some of these issues, and how they relate to the claims of advertisers trying to sell them products to enhance performance.
The GM programming is based on torque limiting. There is a table in the program that sets desired power, and the ECM adjusts boost, cam timing, throttle position and ignition timing to hit the targets. It doesn’t matter what you do to the engine in the way of ‘power improvers’, the ECM will always adjust down to the preset power (actually torque) targets. You get 260 BHP and 260 ft-lbs. no matter what you do to the engine.
The programming does not react instantly to changes, but rather learns new inputs over several start/run cycles, so that when you first add a power mod, you may indeed see an increase in output if you put the car on a dyno immediately, but that power will go away as the ECM learns back down to the preset level.
GM offers a second tune, the GMPP upgrade, often called (but not by the factory) the stage 2 tune. This does not eliminate torque management, as that is fundamental to the program. It bumps power and torque significantly to 290 BHP and 340 ft-lbs. This may not be felt the first time you fire up the car, but the learning sequence will take place over several key cycles.
On this tune I defer to Bill Duncan, a GM employee, who explains it thus:
Quote:
There is a calculated torque model that uses all the inputs pressure, temperature, rpm, throttle position, spark advance, cam timing...etc. If it sees that you have an under performing engine, maybe due to the CR being at the low limit then the program will add power until the original power level is acheived. That is the learn up portion. There is also a learn down portion that if the torque model see there is too much torque, usually due to an add on, it will learn out the torque until the original power is restored.
The GMPP Turbo upgrade kit turns off the learn down portion of the torque managing program. Therefore it will not learn out add-ons such as CAI or Exhaust systems.
What most people are mistaking as learning out/Learn Down, are the safetys that are still in the software and still active. These are there to help guarentee the system is durable for the 100,000 mile warranty and parts are not damaged. There is still an overboost protection, there is still a program that is calculating what the turbine speed is and will limit boost if you approach the maximum allowable speeds.
So as an example if you change the turbo and are getting much higher boost you will engage the overboost protection.
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Advertisers do not generally lie about this situation, but they do mislead by advertising the temporary improvements they no doubt record by dyno after installing their product – they just forget to tell you that this improvement is temporary and will go away after awhile. Any mods you buy for the basic factory GM tuned LNF should therefore be done for aesthetic not power considerations. If you like the sound of a free flow exhaust better, that’s great, but don’t be fooled into thinking you are gaining any (permanent) power from doing it. The GMPP tune OTOH would give additional benefits with mods.
To get any improvement in power that would violate factory safety values requires a non-factory tune for the ECM. The disadvantage of this is that it can be used to deny warranty coverage if something goes wrong with the engine – and high output 4 valve VVT engines can be VERY expensive to rebuild, so this is a serious consideration.
After market programming comes in many different flavours, some requiring that you unplug the ECM (be careful, very delicate contacts not intended to be plugged and unplugged often and if you bend/break one a $400 bill), or buy a special tuning unit and software for installation. Some offer a so called ‘stealth’ feature where GM is apparently unable to detect the new program when they plug into your car for service, while others require that you temporarily unload the after market tuning files and restore the GM program if you need to take it in for service.
The feature of all of these after market tunes is that you can at last get some benefit from mods done to the engine. They can’t eliminate the torque management, which is fundamental to the operation of the GM system, but they can ‘fool’ it by replacing the stock values with much higher values such that the system no longer limits torque to any figure realistically attainable by tuning.
With these tunes, and with teh GMPP tune, you can take advantage of mods and actually get some of the power promised by the manufacturers of the parts.
Hope that helps clear up some of the misinformation that keeps appearing regarding these engines.
