BD4 pilot reported on test flight aircraft engine failed to produce full power resulted in fly below published pattern altitude.

Narrative

This was the first flight of this particular aircraft after the issuance of the aircraft's operating limitations into Phase I flight testing. Ground testing over the past three years identified no issues with the engine or prop combination; and high-speed runway testing did not identify issues with the engine or prop combination either. In this particular scenario; run-up testing did not show any abnormalities. Since the aircraft was grounded due to issues with brakes and tires and a correction to the W/B of the stabilator; another high-speed test was performed to verify handling and braking action after the repairs; which did not identify any issues. I proceeded to depart on Runway XX for first flight. I had an abort plan in case the aircraft had an issue; which was to abort at roughly the midpoint of the runway (roughly at the windsock; which is very visible from the runway). In this case; the engine seemed fine and the aircraft accelerated quickly at first and there did not seem to be an issue when approaching the abort point; though in hindsight the airspeed was very low for that distance. By the time I realized that there was a performance issue; I was past a safe abort point. The aircraft was maintaining a small margin above stall speed in this configuration (clean; per EAA guidance on not using flaps for first flight takeoff). I took the aircraft into ground effect and accelerated; full-open-throttle (FOT). I was able to maintain about 1.2x stall; and eventually hit a stable airspeed of about 1.3x stall. However; this aircraft is sloppy at these slow speeds; in the backside of the power curve; and handling is poor. I turned to enter the pattern but was not able to maintain roughly 1.2-1.3x stall and gain altitude at the same time; even though I maintained FOT until short final. I elected to prioritize stability and airspeed; resulting in an altitude well below the pattern altitude at this airport. While I was able to maintain obstacle clearance throughout; I was uncomfortably low; especially on final. I was able to safely land without incident. After an in-depth review of the engine and flight performance data from the EFIS systems; and consultation with the engine manufacturer; it is clear that the engine was not generating full power (perhaps only 65-70%; and possibly lower) at FOT (low in the torque curve); and this was not identified as an issue in ground testing. The big lesson here is that I did not have enough performance data awareness during the takeoff roll. I failed to identify the slow build of airspeed in time to abort the takeoff as per my abort plan. The engine should have been generating nearly if not over 30' of manifold pressure and it was only generating 26.3' at the initial roll and 26.6' at its peak in the pattern. Since this EFIS (Electronic Flight Instrument System) lacks the ability to set situational alarms (aka; generating 3000+ RPM and MFP is below 29.5'; sound alarm); more awareness of MFP at roll is required on the pilot's part.