I have my shift light set at 6300 so I know I wasnt reading it wrong.

123 mph
3.5 gear
th350 so 1:1
28" tire
3k stall

My pass the other weekend was around 5500 rpms and 124 mph. Thats about 5 or 6%.

Its an ATI 8" converter not many miles or runs on it. Trans fluid level was good. What could cause this??

 

28" tall tire
Cross the finish at 5500
124 mph
3.50 gears

That works out to 5.31% slip. Nothing wrong with that.

Are you saying that it's now crossing the finish line at 6300 rpm? That would put it at 18% which isn't good. Since MPH hardly changed, you're still making the same amount of power. It's possible the converter is damaged (they don't last forever and can fail). You could also drop the pan so see if anything is in it. The intermediate sprag could have failed.

 

Yea thats what I'm saying. My shift light came on and noticed it was at 6300 rpms. The run before that, my supercharger pipe blew off and the motor ran real rich and wouldn't rev past 4500 rpms, when I shifted to the next gear, it shifted REAL hard. But the next run it seemed fine besides the high rpms at the trap. The converter and trans is just about new.

Something was definitely slipping.

 

My converter that was made for my car without the blower did this when I put the blower on it and added 250hp or so, but was still fine when used with the old 305 later.

 

It was built with the blower in mind.