Today I was at Road America for a track day.
MVP Track Time does a great job, and puts on a great event. It's well run and the instructors (at least the one I was with) are very good.
However, on my third session, coming out of turn 13, I noticed a vibration. I immediately backed out of the throttle partially just to get around the last corner to the pits. I didn't hear any weird noises. When I got into pit lane, I up shifted so I was running at lower RPM, and the vibration went away. So I drove back to the paddock keeping the revs fairly low. I let the car cool. After a while I took the car for a drive outside the venue. Across the street was a country road, and I headed down it. I played with speeds and there was no vibration up to 50mph. Then I turned around and played with RPM. Above about 4000 rpm, the engine felt rough, and gets worse at higher RPM (only went to about 5k). Because it felt very consistent and even, it feels like a vibration. Again, as long as I kept the RPM low, the engine felt fine.
So I packed my stuff up, crossed my fingers and started my 2.5 hours home. Luckily, the drive is almost all highway, so I just kept the revs below about 3500 and drove it. I got home with no problems, and by that time I was thinking that maybe I had imagined the problem, or that it was something other than the engine. I needed gas, so I stopped close to home, and on the way home from the gas station, I drove it up into the 4000 rpm range in 1st gear (slow speed). Yup. Same problem.
When I was driving the track, I noticed a vibration in the car a couple of times before this. In a hard turn that I got into a bit hot, I would get a bit of under steer and felt a vibration that I'm pretty sure was from the front tires. When I had the problem, I initially thought it was the same thing, but it didn't go away when I got going straight again. I've been racking my brains over this, and I'm pretty sure it was a different feeling vibration, but I could be wrong.
No fluid leaks, no funny noises. It really just kind of feels like at higher RPM the engine starts running on 5 cylinders or something. It feels like it's running rough, but it's a completely regular, consistent roughness. It's not something where it runs lumpy or anything. It's not like the mixture is out of whack and the car is just running crappy.
So now I don't know what to do. I'm thinking about changing the oil and maybe getting an analysis (unless I obviously find metal particles, then I know it's an internal failure). I don't know if I should take it to my Porsche shop (Fischer Motors) or what. What do you guys think I should do to ID the problem?