Okay, we went back and tried it again today. We made stiffer mounts for the accelerometer and the optical pick-up. We adjusted the tape width to match the prop rpm, not the engine rpm. We made sure the leads were well away from the spark plug wires. We even replaced the batteries in the Dyna Vibe. The read out was rock steady, but no difference. Still reading 3.5 to 3.7 ips. That kind of vibration should be about shaking the plane apart right? At 4,000 rpm there is no visible motion of the engine. Is it possible the Dyna Vibe is out of calibration?