Experimental aircraft pilot experienced engine anomalies during test flight over the departure airport and returned; but was informed after landing that a landing clearance was never requested.

Narrative

The airplane was newly built and is in the experimental/exhibition category. The engine has not been producing the manifold pressure that it should. It is supercharged. I contacted the company who built up the engine. They suggested several things for my mechanic to check. My mechanic spent the day working on the engine; and did find a couple of items that needed adjustment. I test flew the airplane. I contacted the Tower first by phone and asked if I could remain over the Class D between 6;000 and 7;000 FT; and they okay'ed that. I took off; and climbed in the pattern; with clearance from the Tower. After takeoff; I noted that the manifold pressure was actually lower than it had been; and became concerned. I climbed to around 5;000 FT. I adjusted the mixture; both lean and rich. I remained right above the pattern (pattern altitude is 4;000 FT). I decided that I needed to land; not knowing if what was happening was going to stay as it was or if the engine might fail due to whatever was going on. On upwind; I called the Tower and said that I wanted to return for landing. I landed; and taxied back to the parking area. The Tower said nothing after I landed; but that is not entirely unusual since they know where I park; there is very little traffic at this airport; and there was no other reported traffic at the time. I called them after I parked to thank them for helping me out (allowing me to stay in the Class D). The Controller said 'we have a question for you.' I said 'sure'. He asked me why I didn't get permission to land. I was kind of shocked; and said that I thought that I did.What could have caused the problem was the following:1. The airplane is new to me. I have about 30 hours in it; and it has some 'unique' systems. The push to talk switch is on the throttle. If you push it up; you are talking on the intercom. If you push it down; you are talking through the aircraft communications radio. I might have pushed the switch up.2. As I turned onto downwind; I was very preoccupied with trying to adjust the manifold pressure by adjusting RPM; mixture; etc. For some reason; I did not wait for clearance to land; but that didn't even occur to me until I spoke with the Controller later. I'm sure I thought that I did; but I obviously did not.3. I had spent most of the afternoon sitting in the cockpit; starting the airplane and checking various engine parameters. It was hot (80 degrees); and I was sitting under a bubble canopy. After a while; I became worried about dehydration; so before I test flew the airplane; I drove to a convenience store; bought 2 bottles of Gatorade; and drank both of them. Nevertheless; I think that I was likely dehydrated to some degree. I could have prevented the incident had I not flown after having a concern about dehydration; and had I not allowed myself to be distracted by my concerns about potential engine failure and my attempts to make adjustments to manifold pressure; RPM; and mixture.