I probably shouldn't have said idle. Assuming that the alternator is making
the electronics work better, you want output all the way from the lowest RPM
that the engine is operated at during a race. You want your sparks to be
good at the highest pressure inside the engine, and full throttle from low
RPM is one of the peaks (one of many reasons why there's often a stumble
there). My assumption is that even though the systems "work" they are
probably not optimal until the cut off of their internal voltage regulator.
I don't have anything but anecdotal evidence for this, but there are plenty
of stories. Some tuners claim the horsepower lost by driving an alternator
is more than made up by the gain from better ignition.
You don't need much current--and if your battery is fully charged, you won't
have much, but you do want somewhere north of 13 volts.
A car battery that is at ten volts with no load is well and truly toast, but
a brand new battery that is three-quarters discharged and operating under
load can be under ten volts. I use a small sealed gel cell for Peyote, and
the terminal voltage on starting drops to eight volts, and while running at
full charge (less one start cycle) is a 12.4 volts. I suspect Gel Cells
might have higher internal resistance than standard lead acid batteries and
therefore exhibit a higher voltage drop under load. I can't find any
documentation of this and I don't remember how to measure internal
resistance--though I suspect I could figure it out if I wasn't so lazy. When
it's discharged to the point that it won't reliably crank the starter the
running voltage with the MSD 6AL was 10.2V. Ballpark similar conditions
with BPC it's 11.7V. The MSD draws a healthy amount of current. I can't find
my lovely Tektronix snap-on current meter or I would have done some charts
and graphs (yes, I know I'm nuts). I think it got permanently borrowed.
-----Original Message-----
From: owner-fot@autox.team.net [mailto:owner-fot@autox.team.net] On Behalf
Of Randall
Sent: Sunday, September 18, 2005 10:32 AM
To: fot@autox.team.net
Subject: RE: Mallory failure alert
> Here's the nasty
> part--you need to be pretty careful about alternators. In the range of
> RPMs that racing tractor motors reach, the alternator needs to put out
> reasonable voltage at idle
You lost me here, Bill ... why does a racing motor need alternator output at
idle ?
My street engines don't charge worth a darn at idle, and they spend a whole
lot more time idling than I'd expect on the race track ...
BTW all "points replacement" ignitions are not created equal ... Crane
promises operation down to 8 volts, while my old Allison crapped out around
10. But a healthy car battery has to be pretty well drained to even get
down to 10, so no problem with not charging at idle unless you plan to let
the engine idle for several hours.
Randall
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