Quote:
Originally Posted by gmtech921
Question to PTaddict, do you know how far out the gm performance crate motors ECU's are? I have the ls3/525 going into my chevelle and am wondering how much more there could be in it. Have you done any of those set ups yet?
Excuse me if I am hijacking this thread, maybe a P.M. would be better let me know. Thanks.
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I've only done an LS2 and two of the 480 HP LS3s. But it's not really possible to answer your question specifically in any case.
Some background. The GM ECU uses a Mass Air Flow sensor to determine how much air the engine is taking in, and therefore how much fuel to inject to achieve the desired air/fuel ratio. But the thing with a MAF sensor is that it will read quite differently depending on the inlet tract in front and behind it. Change the inlet plumbing, and the MAF sensor will read more or less airflow at different points in the RPM band. If the MAF reads lower than the actual airflow, you end up with a lean condition. (Incidentally, aftermarket air inlets on late model cars nearly always cause the MAF sensor to read less airflow, thereby leaning out the engine at WOT. Some of the HP gains attributed to aftermarket air boxes are almost certainly due to this leaning effect).
GM gives some fairly specific instructions for how the MAF is to be plumbed into an intake tract, but still they can't know the exact configuration. So they program the MAF transfer function to read high, so that if you have a sub-optimal inlet tract, you won't end up too lean (at least, that is my speculation based on what I'm seeing). But if you do have a close to optimal installation, you now end up too rich. On one of the LS3s I tuned, the fuel trims were stuck full lean and the engine was still blubbering on at about 12:1 at idle. The only way to fix this is to retune the MAF transfer function, which fortunately is one of the easier things to tune if you have the right tools (software, wideband O2 meter, chassis dyno).
Anyway, that's why I'd recommend a tune for any retrofit with GM ECU. Hope that all makes sense.