I'm a bit confused with the term 'held' boost pressure. On the manual from MSD it says my T34.48 setup can run a 32 psi (2.2 bar) peak and 26 psi (1.8 bar) held boost pressure, although it might be tailing off at high RPM due to the .48 turbine housing.


In my car I have the possibility to trace the MAP sensor through the revs and I did two runs where I got the below result.





The scale is in kPa, so the 300 kPa peak is 3 bar, which is 2 bar boost. But when I look at the boost after the peak it is slowly dropping and not really a constant value, so I don't understand the 'held' value.


At 5000 rpm it's about 1.7 bar, at 6000 rpm it's 1.45 bar and at 7000 rpm it's 1.35 bar. Is this normal, or should the boost almost be a flat line after the peak? If it should really hold 26 psi (1.8 bar) I am missing quite a bit of boost at higher RPM...


BTW, boost is controlled using the amalvalve.

 

Or am I simply not bleeding enough from the amalvalve, that the wastegate is slowly being pushed open by the increasing pressure in the turbine housing when the revs increase?


I am a little hesistant to increase the bleeding from the amalvalve as I don't want to go beyond the 2.2 bar peak pressure....

 

A quick email to MSD would answer this for you

 

You're right. I've just sent a mail to MSD.