With increased boost comes increased torque. CS was concerned enough about the somewhat fragile a525 tranny, that they limited the full boost pressure until like 4000-4500 RPM and up. They needed these powertains to survive long enough to make em through the warranty period. The marketing people decided to try to sell that a good thing, and bragged about it on the window sticker, calling it a "sloped boost curve" (lol). So, adding more HP and torque is a two edged sword. Increased performance with increased load on an already suspected weak tranny. Put enough HP through a a525, and they will come apart, sometimes with spectacular results. I know first hand, as I've busted four of them in 20K miles on a GLHT.
With LM controlled boost, if the detonation sensor detects pinging, it will decrease timing and boost. Also with LM controlling things, the boost pressure will come on more gradual because the wastegate is already starting to open (slowing down building boost) as early as 4-5 PSI. You don't get that big torque spike you get with using a Grainger valve to control boost. With the GLHS module, they neuter it even more, but it prevents the tranny from damage. It also prevents the car from getting wrapped around a tree, making sudden lane changes, etc. when the boost hits hard.
With a Grainger valve installed, the boost will build much quicker (even at part throttle). It does this because the Grainger valve effectively prevents the wastegate from cracking open until near full boost is seen. Resulting in dramatically improved part throttle performance. This is also a two edged sword as the downside is much more load on the tranny because of the increased torque at a lower RPM. The LM no longer has control of boost, so it has lost it's ability to regulate boost if it senses detonation.
Many times when installing a Grainger valve, you will run into another roadblock if you keep increasing boost. The LM will have a preset overboost shut-off. This preset shut-off will vary from one application to another, and even one LM to another. Then to disable or prevent that, you have to select and install a MAP sensor zener diode that prevents the MAP sensor from seeing the actual pressure it is running. This is a very slippery slope as you can get bitten quickly with making this decision to run a zener diode. From there it is all downhill chasin your tail finding the next thing restricting performance without hurting the engine or tranny.
www.fwdperformance.com
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