I have a series 5 13b turbo with 3inch exhaust (mandrel bends) running after market computer (microtech), it is tuned to 12psi but in the top of 3rd and 4th boost drops off back to 7,8psi and would like to know how this happens and how to fix this problem, without going to an external wastegate. I was told that the inlet housing on turbo was to small? please help

 

Yeah, I was goin to say sounds like your turbo is running out of steam. Upgrade to big *** huge small animal sucking turbocharger time!!!

That's my opinion of what could be happening....

 

big problem is that the actuators on the series 5 cars are weak, 10 years of use and abuse can do that i guess. Try putting a spring over you actuator to help keep the flapper door closed.

I had this problem for a long time and am experiencing it slightly now since I made some timing adjustments.

Try the spring first and see how it works, I know it sounds Hokie but its cheaper than upgrading turbines for the while.