Question regarding an alternator

[dyerkes]

At what RPM does a 911 alternator max out in generating current? I'm assuming that it occurs at something relatively close to idle, and from that point on wards the voltage regulator just ensures that your system doesn't become overcharged.

I'm asking this question because a freind of mine had a catastrophic failure due to lack of fuel while being dynoed, and the mechanic said that the logs indicated that there was a significant drop in voltage at higher RPM's causing the injectors to not be pulsed long enough (highly modified 930 Turbo with TEC3-R EMS, dual plugged, sequential injection), and suggested that this was caused by the fan belt slipping due to lack of adhesion (the fan had recently been powder coated). No one witnessing the car being dynoed noticed the belt slipping, nor the fan turning at a slower rate. If maximum voltage is being created soon after idle, than I don't know how the belt could have been slipping so much, and have been detected by the people watching. FWIF, both the alternator and battery are brand new.

Thanks,
David

[RV Flyer]

David, I tend to think that this failure theory is not very likely. First, the alternator probably produces a good percentage of max output once the engine passes 2,000 - 2,500 RPM; assuming the pulleys on my '78 SC are similar to the 930's, the crank pulley is bigger than the alt pulley, hence the alt turns even faster than crank speed. The big advantage with alternators (versus generators) is that they ramp up and produce more current at lower RPMS.

Second, even with a slipping fan belt (I've had a loose one for a LONG time) the alternator likely is still charging at a pretty good rate.

Third, even with the alt completely off-line, a properly charged battery should still feed the injectors 12.8 volts or better, and its hard for me to see how the injectors could be designed so as to "run dry" (so to say) when getting battery voltage of 12.5+, as opposed to line voltage of 13.5+ when the alt is charging. Of course, it sounds like this injection system is custom/aftermarket, so maybe it is way more sensitive. Also, on a dyno run I cannot imagine it taking so long that the battery would run down....

Hope this helps. I don't have any specific experience on the setup in question, though.

[dyerkes]

Thanks for your assessment.

David