Hmmm... The stock cast-iron exhaust manifolds that came with the Citation,
which 3-into-1 instantly and have a single output pipe on each side.
Right-side (formerly front) pipe goes down and curves under the engine,
to merge into the left-side (formerly rear) pipe where it comes down.
I think this is stock Citation exhaust. It ends here with a 5-inch stub
that's about 1-7/8" diameter.

I've plugged a Honda Accord muffler directly into that stub. So this
does sound similar to that short, high-resonance system you had.

The citation had a long pipe on that stub, that went back into a cat
and then into the muffler at the back of the car.

Now that you've pointed this out to me, I wonder if looping a long pipe
forward about three feet, thru a gentle curve, and back to a full-sized
muffler, would make a difference, as your longer exhaust did. Not the
cause of the clutch, you're right, but interesting nonetheless.

Steve

 --------------------
>Steve, what are you using for an exhaust manifold.  I ask because on
>the Porsche engine, with the short exhaust pipe length and the exhaust
>manifolds I originally used, there was an amazingly loud resonance in the
>exhaust system at about the rpms you are having your problem.  Truly
>sounded like >something large and loose banging against something else
>metal.
>
>Now I have the earlier manifolds, with longer distance from exhaust port
>to the place where the pipes 3->1, and the relatively enormous stock
>Porsche muffler, and the problem is completely gone.
>
>Not an explanation for broken clutch disks, but something to consider
>relative to your vibration.