lower mph at track??

The last couple of times I've been to the track I haven't been able to get the same trap speed on motor, as I did from say...last year. My mph has dropped by about 2 or 3mph. What could this be from? Should I mess with my timing? It's on 15 degrees btdc right now. I've adjusted it some but didn't really feel or see a difference bumping it up or down by a few degrees. My rebuilt motor has about 20K miles on it...and has only had 2 & a half bottles of nitrous run through it over the past few years. My nitrous system is set up correctly and working properly so I'd like to believe that the juice isn't causing excessive wear in my motor. Plugs look fine. Check engine light isn't even on. I've been told that I'm running a bit too much fuel psi. being that I have 24lb. injectors and still run close to the stock fuel psi. Yes my MAM is calibrated for the 24s. Currently my fuel psi. is set on 38lbs. w/ vacuum line attached. I'm going to take it down to around 32-34psi. and see if that does anything. Could running my car at the track in different weather cause a loss in mph? It was much colder with a lot of moisture in the air the last couple times I went to the track.

[420nitro]

If you think your engine is wearing a little to fast then perform the following tests.
1) Compression test. There should be no more then a 5% difference between the cylinders. I would expect 135-170 psi for your set-up.
2) Cylinder Leak Down Test. Should be no more then 12%. The less the leak down, the better. Not to say that 12% is bad but thats about the most I would like to see. Mine is at 0-2%.
It is also possiable the the difference in air and traction played a role in you MPH.
I would also agree that you are running a little to much fuel pressure. I would say 36-38 psi with NO vacuum signal.
Good Luck

[exgmguy]

It could be something as simple as plug wires, cap, rotor, or worn plugs.
When I was spraying I would put a new set of plugs in every trip to the track.
You can check your wires with a digital volt meter set to ohms. I don't remember the "good wire" number though.

I want to say 1000 ohms per foot? I had a set of the blue Motorsport wires that were only three months old and had way too much resistance. I replaced them with a set of MSD wires.

[84LX89GT]

With the vacuum line connected at 32 psi, it will be close to 40 psi at full throttle (no vacuum) if i remember correctly from the last time i adjusted my fuel pressure.

[82 GT]

Originally posted by Simi Stang
It was much colder with a lot of moisture in the air the last couple times I went to the track.

I would have thought you would make a little bit more HP since the air was cooler.

[jonnyk]

I agree, that FP is quite high. I shoot for 32-34 psi with vacuum at idle, or around 38 without vacuum. You will make more power in cooler air, but less in humid air...so the two effects will cancel each other, but obviously one effect is likely to dominate the other. The only real way to compare is to keep very close records of temperature, humidity, and barometric pressure (and the altitude of the track you run at) of the times that you run, and then figure out density altitude corrected times, and compare those.