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UPDATE: found my missing horsepower! (archive)
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Posted by Benjamin Treynor on June 10, 2001 at 10:11:32:

As several of you are aware, I've been experiencing dyno woes ever since Dinan installed their cold-air intakes. On the dyno, my HP had dropped from 332 RWHP (with just exhaust) to 290-302 RWHP after the CAI was added. At first I suspected a problem with the Dinan CAI, but after speaking with Steve Dinan personally was convinced that the problem lay elsewhere. I was left with a mystery, however -- on the dyno, even with the DSC fuses pulled as I had previously done, the car ran very rich and made only about 300 RWHP.

Last Friday I brought my M5 up to Sacramento dragstrip for a test 'n tune night to try to get a definitive answer on whether the car was making full power or not. The math is simple enough -- a 3950 lb M5 making 330 RWHP in 60-degree weather should run the 1/4 at 105-106 mph, while one making 300 RWHP should run it at 100-101 mph. I had previously run 13.5-13.8 @ 104-106 at the dragstrip with just the exhaust installed, so I had good baseline figures to compare to.

What I found was very interesting. My best run was 104.3 MPH in 85-degree weather, so my M5 is making all the horsepower it is supposed to. However, this run was accomplished with no wheelspin at launch and no wheelspin in the 1-2 shift. EVERY TIME I HAD ANY WHEELSPIN, THE POWER LEVEL WOULD DROP FOR SEVERAL SECONDS. This was most noticeable during one run with a C4 corvette in the lane next to me. I had a little wheelspin on the launch, and the corvette and I were dead even through first gear, and dead even through second gear. About a second after I shifting into third, I felt the M5 surge forward and left the corvette behind. The 'vette and I had similar shift points, so the most logical explanation for this was that the M5 ECU had decided I was no longer having traction problems and had re-engaged full power. Similarly, when I did a burnout before my launch, 1st gear felt quite sluggish, and the car would wake up again in 2nd. If I had wheelspin in the 1-2 shift, 2nd gear would feel dead and the power would return sometime while in 3rd.

Conclusion: (1) The traction control software has definitely changed sometime in the past year. You can no longer completely turn off traction control, even by pulling the fuses or by the console switch. Based on my dyno results, with traction control off I believe the ECU retards timing and/or richens mixture to reduce power when it senses wheelspin, usually to the tune of 30-40 RWHP. Optimal acceleration in the M5 with this software thus involves avoiding wheelspin on the launch, and babying the 1-2 shift to again avoid wheelspin.
(2) There's nothing wrong with the Dinan CAI or software. I can't state what HP gain is obtained with the CAI, but my car's still making approximately the same power it used to.

I will be speaking with Dinan this Monday about a fix for the traction control disable. I'll post any information I receive.

Cheers,
Ben
'00 M5, Dinan CAI, exhaust, stage 3 suspension & wheels.

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