Just replaced my stock header with a Gibson GP400 and it fits perfectly but now when I start it it redlines and I've checked every possible issue it could be and I'm well and truly stumped. All the vacuum lines are good, the bolts are torqued in place and everything is flush with the gasket. I have no idea whats going on

 

The header is not the problem. Any possibly that you may have dislodged a sensor during installation?

On another note, how's that header sound?

 

It sounds amazing before it gets up to 6k rpm and starts bumping

 

Did you put the intake back on right?

 

I made sure all the bolts were in. Its all put back together. Im thinking its a sensor problem

 

X2.....Are you positive it's seating right? if the new header flange were thicker than the intake you might have a mongo vacuum leak. (since the washers on the bolts won't pull the intake manifold in) The washers should be flat on there. You are sure the throttle is coming all the way home, and...no major lines off, like the power brake booster?

I don't think it can rev like that if it's not getting air.

 

All the lines are reconnected and theres no gap and all the bolts are tight but the new flange is slightly thicker

 

If the new flange is thicker I don't think the bolts are snugging in the intake manifold. For some reason I had my engine wet last week and I noticed it held a little water in that little space between the gasket and the head. (just a narrow 1/8" space). I went ahead and started it, curious if it would get sucked in by an intake leak. It didn't! (yours might?)

 

I havent moved it and it isnt wet

 

It could one of a few things. Vacuum leak, sensor is unplugged or the throttle cable is hung up cracking the throttle.

 

Your intake has a huge leak at the cylinder head. Climb underneath the rig with a flashlight....you'll see it.

 

What I meant was you could put some water there and see if it gets sucked in. Since you don't want to red-line it like that again, have someone else crank and fire it for just a second. Then again, if the exhaust flange is thicker you already know the bolts are not holding the intake firm.

 

^
This.

 

Yep. The intake manifold seems tight, but it's hanging up on a head dowel.