I have read in places before that there may be something to this but no one could quantify it.

My question is does the PCM retard timing or something else to reduce power when your feul level drops to a certain point?

I ask because the night before last I was G-Techin out on these FLAT backroads by my house and was running high 14.80s-mid 14.90s in 65deg temps. Tonigth I went to the track and got a bunch of 15.1s@90mph in 85deg temps but as the night wore on and it got coler I started running slower and slower till I was at 15.35s@88.9mph I made zero changes other than my fuel was about on empty on those last runs and by my last run the temp had only dropped to about 72deg or so.

It just felt more sluggish and the wierd thing is if you compare my timeslips I am slower right off the batt, my 60' is slightly worse 2.13 vs 2.16 and everything that follows was slightly slower.

Any ideas or anyone have something similar happen to them?

 

How much time did the truck have to cool down between runs? My first guess would be that even with the cooler temps the transmission fluid and engine coolant were hotter and causing a little knock retard. My car was bad about that. Once the coolant passed 194-6° I would see a little knock and times would fall way off. If I let it cool down my times would come back to the low 13's like normal. On one day I ran anywhere from 13.3's to 13.78 just depending on coolant and trans temps.

I've heard once the PCM sees knock enough times it will shift over to the low octance advance table. That could have been your problem. It's supposed to reset to the high octane table when the fuel sensor registers a refill of over a certain amount which I don't know.

 

Good points. As the night went on the truck had lobger periods of time to cool down, so much so that I actually tried letting the truck run for awhile to get it back up to operating temps because I thought maybe it was too cold. Now the trans may have still been a lil warm but on avg. I had about 45min or so maybe a lil less between runs.

I changed to low octane table with LS1 Edit just for this reason but you may be right about the KR. I have upped my timing a bit but have not been able to dyno tune it to ensure I am getting none and since I was rich before I didn't think it would pose a problem. Like you say though, my truck is prone to KR for no reason. Even bone stock with 91 octane the thing would register 1-2deg KR

I guess the next step would be to install my electric fans, 160deg thermo, and tune the AFR so I know that if I get KR under ideal conditions then it must be false and i can tone down the sensor a bit. What do you guys think?

 

Here's something to consider, maybe the G-tech is off?

 

Reread my first post. I stated that regardless of the G-Tech times I got slower and slower as the night wore on/got colder. I am not comparing my track ET to the G-Tech ET because they were taken on different nights in which the DA may have been different. I hate it when people try and deny the accuracy of the G-tech (within a 10th or 2). I've had the thing since 97' and used it on just about all my friends cars and all of mine (10sec Mustang, 11sec T/A, 12sec Vette, etc etc).

Soo in essence I just wanna know why I got slower as the night got colder which should have made me faster. Was it something to do with low fuel or maybe it was false KR? I'll check the DA for the times I ran and see if it got drasitcally worse later tonight.

 

I'm not sure how to word this correctly but,
I have the wandering gas gauge needle, so it will read empty when its completely full and just bounce everywhere randomly and different times regardless of how much gas is actually in the tank.
If I have a full tank, and the gas gauge bounces down to empty, would it be pulling timing (If in fact this "timing is pulled when fuel is low" is true.)??

 

If it is your sensor in the tank then I would say YES, if this is indeed something that happens in our trucks. The Vette's tend to have the gas guage problems too and most just replace the sensor or it is a bad ground on the sensor.

I am not sure bout the PCM pulling timing but it has been whispered in the F-body forums since I started reading them 3yrs ago but no one can ever truely verify the info.

 

I think that it's the DA getting worse. Humidity tends to get worse as the night goes on (at least here in FL it does) which might impact your times. My truck does the same thing.

You might be getting cooler temperature air, but you're getting less air and more water. I think.

Does this make sense to anyone else but me?

 

Makes total sense bro

When I get home I will check my slips and get the DA for each time and see if it is consistant with my losses.

DA is the main reason i dont even race much in the summer here. The last 3 times I went out since May the DA was over 3000ft. each time and our track is only 50ft. above sea level!