How To Determine If You Have Mechanical Boost Creep
This document has been created to assist you with diagnosis, trouble shooting techniques, and proper repair procedures for
determining and resolving mechanical boost creep.
First, we need to define mechanical boost creep, which is the forceful overrunning of the turbocharger due to the wastegate's
inability to relieve exhaust gases from the turbine housing. This will result in the turbocharger shaft speed (and boost) increasing with
engine RPM, this is also known as an overrun condition. This phenomenon can easily be seen in a datalog or via a graph plotting boost
along an engine RPM range. If you have your boost control system turned off, or your wastegate duty cycles programmed to 0% in
your ECM; as you run your vehicle at wide open throttle (WOT), you should see that your turbocharger pressure output (boost) rises
up to the point when your wastegate (WG) actuator opens the wastegate valve (bypassing exhaust gases past the turbine wheel) then
boost should remain constant until engine redline. This is known as the base mechanical boost pressure your turbo should generate.
Below is an example of what that normal mechanical boost curve would look like.
This graph was recorded from a turbocharger that was larger than stock (you can derive this by looking at the RPM when the
turbo hits full mechanical boost, ~4400 RPM vs ~2800 RPM for a stock turbo) and this turbocharger had a 1 Bar or 14.5 psiG WG
actuator. Notice how boost levels off and remains constant at ~14 psiG until just before redline when boost barely drops. This drop in
boost is due to the exhaust gas back-pressure prior to the turbo forcing open the WG valve at the higher RPM. This test was performed
with the WGDCs set to 0% in the ECU.
This next dyno graph is going to demonstrate what the boost curve would look like when your turbocharger is demonstrating
mechanical boost creep.
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Notice how the boost curve continues to go up as engine RPM increase. This test was also performed with the WGDCs set @
0% in the ECM. This is a clear demonstration of mechanical boost creep. The flow capabilities of the wastegate system are not
sufficient enough to allow the exhaust gases to by-pass the turbine housing/wheel, which forces the turbo to overrun as engine RPM
increases. This condition can and will cause severe engine damage!
You have several easy steps you can follow to verify that this is a mechanical issue. The first trouble shooting step you will
want to take is you will want to eliminate the electronic components of your boost control system by plumbing a vacuum line directly
from your pressure source to your WG actuator, see below picture.
This will force your boost control system to operate at as low of a boost level as possible. If your boost creep is no longer
present then you will want to look at the various components of your boost control system to make sure they are operating properly
and that the vacuum lines are plumbed appropriately. Further WOT testing will also allow you to determine if the physical flow
capabilities of your wastegate system are sufficient for your application when your system is plumbed this way.
Pre-loading the wastegate actuator arm; adjustment of the wastegate actuator rod (if the rod length is not fixed and
adjustments can be made) will allow proper calibration and some additional tuning as well. All IHI turbochargers have a fixed
wastegate actuator rod that cannot be adjusted, while all MHI & Garrett turbochargers have an adjustable wastegate actuator rod. If the
rod coming out of the wastegate actuator is shortened, it will pre-load the spring inside the wastegate actuator increasing the pressure
level at which the actuator will allow the wastegate valve to open and the total boost pressure that the turbo can generate will increase
(as long as the turbo is still within its efficiency range). This pre-load will also limit how far the wastegate valve can open. Pre-loading
(shortening) the wastegate actuator rod too much CAN POTENTIALLY CREATE A MECHANICAL BOOST CREEP ISSUE THAT
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CANNOT BE TUNED OUT! If the wastegate actuator rod is lengthened, the actuator will decrease the load on the spring and
decrease the pressure level at which the actuator will open and the total boost pressure the turbo can generate will decrease. If the
wastegate actuator rod does not put enough pre-load on the wastegate valve, then you could see boost fluctuations of + or – 2psi even
when the wastegate solenoid duty cycles are constant. If you have a stock turbocharger then you should not have to adjust the
wastegate rod length.
The next trouble shooting step will help you determine if the WG actuator pretension is creating the WG flow restrictions.
You can determine this by detaching the WG valve from the WG actuator rod, see below picture. PLEASE allow the turbocharger to
cool down for several hours before attempting to do this, the turbocharger can be as hot as 1000+ degrees Fahrenheit during normal
operation.
While detaching the WG actuator rod from the WG valve mechanism, please be careful to not lose the retaining clip for it
will need to be re-used. Doing this will allow the WG valve to fully open more than it would when it is connected to the wastegate
actuator rod. If you perform another WOT test and boost creep is still present then you know that the flow capabilities (size) of the
existing WG are not sufficient.
At this point in time you should contact your turbo supplier to inform them that they have not properly sized your wastegate.
Regardless, your turbocharger will need to be removed from the vehicle. Another option is to port your turbine housing using a
Dremel (TM) or other rotary, abrasive tool. Before you go hogging out your WG port, we highly suggest you follow the next few
steps.
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1) With the WG valve mechanism detached from the WG actuator rod, you can easily swing it aside giving you full access to
the WG port in the turbine housing. You should be able to see if the WG port is centered under the WG valve. If the WG port
is not centered with the seated WG valve, use a contrasting paint marker (you can see we used yellow in the pictures) to
carefully mark portions of the turbine housing that can be ported in order to center the port under the valve. This will ensure
even pressure is being applied to the WG valve. The proportion of the porting you complete must be done in small
increments. We suggest your porting does not increase the diameter of the WG port by more than 1mm. The effect of the
forces is through psi, or pounds per square inch, so what you are trying to do is allow for a greater surface area for the
exhaust gases to by-pass the turbine wheel. If you over port the turbine housing, you will not be able to achieve your
boost targets...effectively destroying your turbine housing.
2) With the WG valve mechanism detached from the WG actuator rod, you now have the ability to make sure the movement of
both of these pieces is free and clear. You can pump up the canister on the WG actuator to make sure the rod extends fully
and freely, without kinking or getting caught on something internal to the diaphragm. You should be able to rotate the WG
valve mechanism freely. This movement is usually very loose and easy to do with your fingers.
NOTE: The larger diameter wastegate valve a turbo has, the more difficult it is to stabilize boost pressure as the valve initially opens.
This is also true for greater exhaust gas back pressures created by a smaller A/R on the turbine housing.
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3) An additional step can be taken to increase exhaust flow through the WG port by rounding the leading edge of the turbine
housing leading to the port, see above picture. What you are trying to do is to round the 90 degree section of turbine housing
that leads to the port to increase exhaust flow through the port.
NOTE: The MORE boost you run, the LESS wastegate you need/use. So unless you want to run less boost pressure than stock and/or
have un-tunable boost problems, we suggest that you do not port your WG by hand unless you have properly verified mechanical
boost creep exists. We suggest you leave your WG, the area around it, the turbine housing, etc. alone and tune your boost curve
through the proper means. You should only port your WG area once you have verified that you have a mechanical problem associated
with the WG being too small.
BEFORE YOU REINSTALL YOUR TURBOCHARGER, please be sure to throughly clean and inspect all components
before they are installed on your vehicle. A turbocharger spins at incredible speeds, even during idle, and any debris left in the housing
can destroy the turbo. If your wastegate system is operating properly, you should be able to see your boost characteristics are similar
to the first dyno graph in this article.
We hope this document has been helpful and gives you a better understanding of how the internally wastegated system
works.
V1.02
Copyright © 2008 Cobb Tuning Products, LLC All Rights Reserved.