727 RPM limit

[rr]

I'll climb out on a limb a little. . . It depends.

Ok, that really doesn't help, it is a difficult question to answer. The
software itself will calc and update fuel and spark each 80 times a
second.  So, 80 times a sec., the ign & fuel hardware counters are
updated. As mentioned before, only a byte is used to hold the rpm,
hence the 6375 limit. However, the software will use the end
point in any table lookup, where the index exceeds the table end point.

There is another internal value that the rpm is calc'd from, the drp
counter. It's a 2-byte value, and decreases in resolution as the rpm's
go up. It depends on the # cylinders to how far you can go. The val
is the # of ~65.5Khz clock ticks that occur between ignition ref pulses.
So a twelve cyl eng runs out of drp's before a 4 cyl.

I would venture to guess that the limiting factor would be in the
hardware someplace. How often can you give the ecm a drp and
still get the plug fired before the next drp? It might come down to
the resolution of the hardware counters/timers used for firing the
injectors and plugs.

Figuring you still need some resolution to get the plugs fired at
the right time, figure the clock is ~65.5KHz, 1deg at 2 firings
a rev (4cyl), is (65.5KHz / 2) / 360 = 91 Hz = 11msec. Doesn't
sound right. This would then be 2 firings per rev, 60 revs per
minute for 1hz.

91 * 2 * 60 = 10,920 rpm. Hmmmm, maybe it is correct?

I'm not sure that the math I did is correct. Any out there ever
try to take an ecm to the limit? Maybe even on a bench?

BobR.

p.s. I used (1024 * 64) for the real clock freq.

Dave Williams wrote:

>  Does anyone have a good guess at what the RPM limit of a 727 for a DIS
> four cylinder would be?
>
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