Q: What is boost creep? Why can't I use a boost
controller to control boost creep or lower boost?
In a perfect world, the wastegate should always control the amount of boost
the turbo can produce. No matter what the set-up is, when the wastegate
is set for 10psi, it should give you 10psi. In reality, when the turbine
section is overrun, boost creep rears it's ugly head.
Please use a good, aftermarket boost gauge to monitor boost pressures; the
stock boost gauge is not accurate enough.
Boost creep is defined as the uncontrollable increase of boost levels.
A boost "spike" is something else that is usually caused by a malfunction of
some sort. Boost creep is due to too much exhaust gas flow through the
turbine section. The stock (FC3S) wastegate is designed to work under
stock parameters - stock intake, stock exhaust, stock engine, etc. Once
you start to change or upgrade those components, engine efficiency will increase,
and thus, engine exhaust gases will increase. Upgrading to a freer flowing
exhaust will almost always increase boost levels. Couple this with the
small Zenki single-port wastegate, and this is a sure recipe for boost creep.
Now, let us step back a moment to explain why boost levels increase when you
upgrade to a freer flowing exhaust system. As mentioned previously, the
stock wastegate was designed to work with stock components. Once these
components are upgraded to increase engine efficiency, more exhaust gases are
produced. The upgrade, freer flowing exhaust system works on the back
end of things by allowing the exhaust gases to exit the engine more efficiently.
By allowing the exhaust gases to exit the engine faster, this causes the
exhaust gases to run through the turbine section faster, thus, boost levels
rise. The wastegate is a purely mechanical device; it does not know how
much boost the turbo is producing, but boost is controlled relative to the spring
pressure inside the wastegate actuator. Due to the increase in exhaust gas
flow from the freer flowing exhaust system, the normally allowable ~6psi of spring
pressure is easily overrun, and a higher boost pressure is allowed - the stock 6psi
of boost no ends up being 10psi...12psi!
So now we end up with more boost - what do we do about it? If the boost is
under control, why not enjoy the added performance? As long as you have enough
fuel for the higher boost levels, you should be fine. If you own a Zenki FC3S
turbo, boost creep is possible with just an aftermarket exhaust system! Porting
the wastegate can help in most cases. Listed below are links to wastegate
porting for both the Zenki and Kouki turbo's.
FC3S Pro: How-To - Zenki Wastegate Porting
FC3S Pro: How-To - Kouki Wastegate Porting
Okay, so how does this all tie in with a boost controller? Some people assume
that a boost controller can lower boost levels as much as it can raise them.
This is wrong; a boost controller cannot lower boost levels from "stock"; if
the turbo already makes 12psi with a boost controller, installing a boost controller
cannot lower it from the base 12psi. Why is this? We need to look at how
the wastegate works to understand why... The wastegate works by pressure.
Once the turbo starts to make boost, the boost needs to overcome the wastegate internal
spring to open the wastegate. Once the wastegate spring is overcome, the wastegate
starts to open. Now, if you want to raise boost, all you need to do is delay the
wastegate from opening. This is easily done by restricting boost pressure going
to the wastegate actuator or bleeding the boost pressure to trick the wastegate
into thinking the turbo is making less boost than it really is. By bleeding off
boost pressure, this delays the wastegate actuator from opening the wastegate itself,
and viola, you get more boost. So how do you get the turbo to produce less boost?
We somehow need to miraculously open the wastegate earlier! To do this, we need
to pressurize the wastegate actuator earlier / faster than the actual boost from the
turbo - this becomes impossible unless you have some kind of pressurize system to
do this or a mechanical actuator! Now you understand why a boost controller
cannot lower boost levels from "stock"...
If all else fails, how do you control boost? Sadly, you need to restrict the
amount of airflow going into the engine or restrict the amount of exhaust gases
exiting the engine. Using a smaller air filter can be an opton, especially if
you're using cone filters. Installing a restrictor plate in the exhaust system
can control boost creep; we've used a 2" opening cut into some sheetmetal then installed
between the downpipe and pre-silencer to drop boost from an uncontrolled 13psi to 14psi
down to a managable 10psi. By drilling two holes for the bolts and lining them up
with the flanges, this is an quick and easy fix if need be.
Questions? Comments? Send email to:
reted(at)fc3spro.com
09/27/2005
