SPA is older stuff, up to 1975. Which in my opinion excludes the best airframes- Atlanta, Aurora, EU1A

See below SPA rules:

1. Age Groups

NOVICE & SPORTSMAN -- All Ages
SENIOR EXPERT -- 0 - 60
SUPER SR. EXPERT -- 60 & Above

2. Aircraft

a: Any model designed and flown before January 1, 1976 is legal
b. Aircraft must be built to the plan form of the plans.
c. Retractable landing gear, flaps etc. are not legal.
d. The contestant is responsible to furnish documentation for any model not on the Legal Airplane List. The president can provide the member with Legal Airplane information.


3. Engines

a. Any side exhaust two cycle engine up to .61 cubic inches is legal
b. Four cycle engines up to .91 cubic inches with no air chamber or super charging or any kind are legal
c. A proper muffler must be used but tuned pipes are not legal d. Pumps may be used but no oversized carburetors can be used.

3. Radios a. Any radio currently approved by the AMA may be used.

With the BPA, Retracts and pipes are not only allowed but encouraged!