Recently tuned a STi with a built EJ25 running a GTX3576R and a
tial MVS wastegate. In this particular example the MVS wg was vented to atmosphere* While this is not a nissan SR, KA, RB or other turbo engine the boost creep concept still directly applies.
The log above has under 2% duty cycle for the 3 port boost solenoid which means the engine will run at the lowest possible boost AKA "wastegate spring pressure". You can see the white line (boost) climbs quickly then tries to "level off" as the wastegate opens at ~15psi. However do to the MVS wastegate being a bit to small for the EJ25 and GTX3576R turbo combination you can see the boost continues to climb from 15psi up to 20psi by 4800rpm at which point it starts to taper back off as the turbo/engine combo becomes less efficient. 15psi creeping to 20psi or 5psi of boost creep is more than ideal, at most 2-3psi of boost creep would be considered acceptable. Now keep in mind this 5psi of boost creep happened in only 3rd gear, as the engine is loaded up/ran at WOT (wide open throttle) in 4th, 5th and 6th gear the engine will be under load for longer resulting in even more boost creep than we can see happening in the 3rd gear pull. If you look further to the right in the log you can see as the car continues to accelerate in 4th gear the boost creep becomes more severe and boost creeps 6psi over wastegate spring pressure before 4800rpm.
In this case there are 3 options:
- add a second MVS wastegate or replace the existing MVS with an MVR or other 44/45mm wastegate (increase volume of flow through the wastegate/s is the correct way to address a boost creep issue.)
- Lower the spring pressure in the wastegate. This will allow the wastegate to open earlier making it slightly easier to manage the boost creep and keep "peak boost" at the target. (this is a band-aid fix as the setup will still have boost creep it will still creep up just from a lower starting point. Example: 15+6=21psi VS 11+6=17psi. The 3 port boost solenoid is then controled by the ECU to increase boost from 17psi to the target boost however there are still some limitations when it comes to target boost at lower RPM as you MUST still account for the boost creep.)
- recirculate the wastegate dump to the downpipe. (this will only help lower creep 1-2psi) as the wastegate bleeds pressure into the DP the pressure differential on the turbine wheel is decreased even further than if the wastegate was vented to atmosphere.