Paul,

the trigger pulse repetition maximum of about 250ms is a build in
feature in the MSD unit. M&W does not have it, that is why it works.
It certainly has nothing to do with battery voltage.

Richard Sohn

I don't understand what you mean about this. "pulse repetition
maximum of about 250ms"

Did you mean 2.50 ms??

If you did I don't think that is right because the rotary racers are
turning up to 11,500 before shifting with MSD. systems. Lynn Hanover
raced his rotary for years at 9300 RPM


133.3 revs per minute. 2.22 revs per second  at 8000 RPM. There are
a thousand ms in each second. That is one rev in 2,222. ms. That is
a long time in a micro computer.

Check my arithmetic.

This is with a 3:1 gear box. Most aircraft people won't turn the
engine that fast.

The MSD 6 is a real old design. Great improvements in switching
transistors have been made in the last few years so it is now
possible to build a system with better specs. Better even than the
M&W. Check out the Mallory CD.

Here is a 3 rotor on the RB dyno with six MSD's

For years it was the standard ignition for rotaries.


Paul Lamar



What I am saying is if the second trigger is not coming at 250 ms
after the first one there won't be a firing. The first trigger never
does fire. Once the time between the first and subsequent trigger is
less then 250 ms it will always fire. I can only say how it is, I
have no idea what is behind that. I could never find anybody at MSD
willing to talk.

Richard Sohn

The MSD is a very complicated circuit. The engineer that designed it
may be dead.

500 RPM is  8.33 rev per second. Or the period is 120 ms. 250 RPM is
twice that period or 240 ms

So what I think you are saying..... unless the starter can turn the
engine over at least 250 RPM the engine won't start.

Obviously hand propping is out of the question.

Does that sound right?

Paul Lamar

Right Paul, that is it.  If you are using one unit per rotor, you have
the same frequency as the single rotor and possibly the same problem.
But there is the M&W to save it.

No, I am not a M&W sales man. It just saved my day with the airplane.

Richard Sohn

You could fix that easy enough with a 555 oscillator. Then it could
be hand prop-able.

You should have contacted me :-) I could wip you up something
'pretty quick :-)

The MSD 6 is now well and truly obsolete.

Paul Lamar

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