[camaro377]

I had a guy build a th350 up for drag racing. He made it a manual valve body and put all hardened parts in it. From day one it has slipped at 4000 RPM. It only does it in 1st gear. 2nd and 3rd work great. I dont know whether it would be in the trans or the torque converter. I believe it would be in the trans seems how it only does it in 1st gear. The torque converter is a 3800 stall. The motor is a SBC putting out about 600 HP. Does anyone have any answers?

[SSedan64]

It's a Trans problem, if it was the TC it would be in all gears.
Sounds like a pressure leak in Low clutch or Valvebody fluid circuit, possibly Vbody/Seperator plate gasket.
It only slips @4000rpm & above in Low/1st?
Being rpm related, maybe Low clutch Drum or Piston problem.

This show the Low/1st gear Fluid flow w/shifter in Drive position.
http://th350.netfirms.com/pages/7a2-12.jpg & http://th350.netfirms.com/pages/7a2-13.jpg
This probably doesn't apply since you have a Manual Valvebody.

This diagram shows Low/1st gear flow w/shifter in Manual/1st position:
http://th350.netfirms.com/pages/7a2-18.jpg & this explains what's happening http://th350.netfirms.com/pages/7a2-19.jpg
Fluid paths concerning the Governor & Vacuum Modulator in the diagram don't apply as these aren't used with a Manual Valvebody.

[Crosley]

this is interesting since the forward clutch could be a problem since slippage is in first gear... If the forward clutch slips, all gears slip since all the power goes thru the forward clutch.

What type of manual valve body is used?