Why does engine acceleration "stall" at 4k but vehicle acceleration continues ??
 

During a WOT run my engine will reach approximately 4100rpms and "hang" there for well over a second while the vehicle continues to accelerate very strongly (no acceleration loss...just no rpm increase either)

What exactly causes this ?? I have my ideas and as this isn't a "problem" I'm not too worried about it

It sounds like a critical rpm where your torque converter is becoming more efficient and the lost rpms are being reduced. Stall speed or second stall I guess. Not speaking from experience or anything it is just the only thing that it could be.

all gears?

All gears....in second gear it's most prominent but it happens in all of 'em

Well, I think it's in all gears but I don't see 4100rpm in 4th very often unless I'm in a REAL hurry :D

Mine does it, too. I'm worried that it is the torque converter's clutch slipping when it engages.

It's not converter clutch slippage .... the clutch is not engaged during a WOT acceleration run (or shouldn't be anyway..if it is you need to work on your tune badly)

That's my thinking too....

It's set up to engage at full throttle, but it is not locked during shifts.

Why would you not lock it during WOT??? Mine locks in at anything over 50mph at about 3,200RPM during WOT and when it does you better hang on because the truck really picks up speed after that!

Because it's BAD for the clutch !!!....on a stock motor it'll be fine, but you start adding power and you're gonna blow the clutch up !! Maybe I'm alone in this thinking but it seems to work for me....I'm ticking off sub-6 second 0-60's in a 5600lb truck with the converter unlocked