This is all very interesting.

If you'll pardon my inexperience and bear with me, I have these observations:

From examination of the BPA and SPA rules, it seems both groups were formed mostly for these reasons:

1) Lower cost than state of the art Pattern equipment
2) Simplicity in airframe set up and contest format
3) affinity for older style designs

I'd like to ask a question which may strike at the heart of the issue discussed in this thread:

Is BPA not concerned with NOISE at all?

The only rule I saw on the BPA site that touched this area was that the plane must have an "effective" muffler. SPA seems to address the noise issue by prohibiting tuned pipes or oversise bore carbs.

As a relative "outsider" (one who does not compete in pattern), I observe the very name "Ballistic" Pattern association seems to celebrate aspects associated with speed AND Noise.

To be honest, I've always liked planes that rip the sky and attract attention. I'm fortunate to fly at a club with no noise restrictions, and for many years I flew a Kaos 60 with a piped rear exhaust 5 port motor and an 11x7 prop. The pipe side area gave it passable KE but I never competed with it.

I'm now wondering when BPA fliers travel to compete, is it assumed that a BPA event will have no noise restrictions at the home field? Perhaps noise may be considered a better discriminator (at least in the eyes of BPA fliers) describing the largest difference between BPA and SPA?

Please enlighten me.
Joe