I took my car to California Speedway completely stock for the second time with the only difference being an Alta Panel Filter and what a shock. Last time I ran my R56 Auto Trans between 14.95 and 15.1 at 90-93MPH. This time I only ran twice and then shut it down because I ran 15.85 and 15.98 at 83MPHH both times. My 60 foot was the same as last time around 2.25-2.35 but the car just died after 40MPH. I don't feel like the car is bad driving around town but maybe it has been gradually getting slower because of a problem and I have not noticed. The weather was 70 degrees and my friend who raced his mustang both times was within 2 tenths he actually went faster so I know it wasn't the track

Does anyone have any idea or had a similar experience

 

Maybe the panel filter wasn't such a good idea then? Maybe your tires are worn? Didn't get enough sleep?
Too bad you didn't bring a stock/ITG/K&N filter along to try also...

 

As Arbut said, maybe it was the tires, different or not as good fuel as last time, or maybe you forgot to push the sport button this time or maybe it was the panel filter and you should go back to the stock one and see if it makes things go back to normal.

 

I have yet to take possesion of my MINI but I do have some 1/4 racing experience with a turbocharged car, namely the Eagle Talon. Anything that can produce ping will adversely affect your power output. This includes warmer weather, lower octane fuel, noises in the engine being registered as ping, and turbulance within the incoming air. This last one I'm not sure how or if it affects the MINI as it did with my Talon. These things probably wouldn't affect a normally aspirated car like your friends mustang as much as it would a MINI S.

When I took my Talon to the track, I would drain out the premium and put in unleaded race gas, 106 octane. I used to use 119 leaded till I realized I was burning out my o2 sensors. I'd put a bag of ice on top of the intercooler and pipes right up to race time.

Good luck with your future runs.