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Re: Powerband loss-what bmw said 2me (archive)

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Posted by Andrew K. on August 13, 1999 at 12:18:43:

In Reply to: Re: Powerband loss-what bmw said 2me posted by marv on August 12, 1999 at 22:33:24:

: Ken--GO FOR IT. Send in the graphs showing the loss. There are MANY posts backing this loss in 540-6.
: I still think it is a quick timing retard that takes a bit to clear. Maybe a knock sensor thinks there is a knock and backs off the timing. I had bad cats on a car once and the car bogged down at higher rpm ALL THE TIME, it was not intermittant.

: I complained to BMW-NA by e-mail about this problem with my early 98 540-6. Sent them post

I doubt BMW is gonna do anything for you guys about this. Looks to me like a knock sensor going off, because the "blip" goes away and the hp/torque curves catch up to where they should be as RPM increases.

Prior experience in hotrodding GM's LT1 V8 engine showed similar problems. Doing any mods to the motor that add noise (eg, headers or exhaust) often times is enough to false trigger the knock sensor and remove any HP gains just made. That led to people trying to "desensitize" the knock sensor. One trick was to put a series resistor inline between the knock sensor and the PCM. That usually didn't work. Another trick was to bypass the knock sensor altogether and hook up a resistor to ground that matched the impedance of the knock sensor, fooling the PCM into thinking "all is quiet". That usually did work, but obviously potentially dangerous (don't even think about running less than 92 octane). The third solution is to have the PCM "desensitized" in software by Brand X aftermarket PCM tuner, reducing the amount of timing it takes out when the knock sensor goes off.

Sux that the 540 seems to have this problem in bone stock condition, however. I'm curious to know if this happens on all 540s or just the 97-98 models that don't have VANOS.


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