For any of you out there who may have a very good foundation in acoustics, have you ever considered an exhaust nozzle, up close to the cylinder and dumping out the side of the cowling, that would attempt to match the acoustic impedance rho-V of the exhaust gases to the outside air such that the exhaust would flow un-impeded from the cylinder. C.F.Taylor shows that individual exhaust pipes can increase the volumeetric efficiency 5%-7%. That's like getting 7-11 more HP out of an O-320!