A turbo by itself does not know how to regulate boost levels. Basically,
a turbo system is a positive feedback loop meaning that the engine's
exhaust spins the turbo which, forces more air into the intake making
more exhaust which, in turn spins the turbo even faster. Without
a way to regulate boost levels the turbo would keep producing higher
pressures until the engine exploded. This is where the wastegate
comes into play. The wastegate attaches onto the turbo header before
the turbo. When you begin accelerating exhaust gas pressure builds
inside the manifold and is forced through the turbo. This pressure
continues to increase as the turbo spins faster (remember the positive
feedback loop). When the desired boost level is reached the wastegate
opens and vents pressure from inside the manifold so the turbo won't
spin any faster.
So how does the wastegate work exactly?
Above is a diagram of a typical external wastegate. Inside the
wastegate is a diaphragm which creates a seal, and a spring
which holds the wastegate closed. Spring rates vary depending on
the amount of boost you want to run, typically they are given in
a "bar" value for example 1 bar would be 14.7psi. This
would mean that in order to open the wastegate you would need to
excerpt a greater pressure than the 14.7psi spring holding the wastegate
closed. In order for the wastegate to work you must have the compressor
reference port hooked up to the compressor side of the turbo,
if you don't have this vacuum line attached than the boost pressure
will not be limited to the set spring pressure; it will build unlimited
boost pressure until your engine is destroyed.
Normally pressure from a spooling turbo pushes against the diaphragm
(though the vacuum line attached to the compressor reference
port) which in turn pushes against the wastegate spring.
When the pressure from the spooling turbo exceeds the spring pressure
the wastegate's plunger opens releasing the excess pressure
through the dump tube into the exhaust after the turbo or
to open atmosphere. Typically, if you use the wastegate to control
your boost levels you will experience a decrease in power and spool
times. Why? Although the spring fully opens at its set spring pressure
it tends to begin opening before reaching the set spring pressure.
This "pre-opening" leaks boost pressure through the dump
tube before max boost pressure is reached resulting in a decrease
in power mostly toward the top end. This can be corrected by using
a boost controller.
Boost controllers serve two functions; increase boost levels beyond
the set wastegate spring pressure and reduce the "pre-opening"
of the wastegate-controlled boost pressure.
A manual boost controller will allow you to increase boost levels
beyond what the wastegate spring is set. How does it work? Below
is a diagram of a manual wastegate.
In order to run a manual boost controller we need to tee off of
the vacuum line which runs from the turbo compressor housing
to the compressor reference port. The manual boost controller
works using a spring and check ball, by screwing the adjusting
screw into the boost controller you put more pressure on the spring
which reduces the amount of airflow through the boost controller
and into the boost controller port. Less airflow means less
pressure will be assisting the spring to keep the wastegate plunger
shut. The pressure in the vacuum line going to the compressor
reference port will equal the pressure the turbo is producing.
A boost controller will allow you to direct some of that pressure
to the top of the wastegate diaphragm creating two opposable
forces. By adjusting the spring pressure of the boost controller
you can vary the amount of boost that the turbo will make before
opening the wastegate's plunger. If you want to run a higher
boost level than the wastegate spring allows you will need a boost
The manual boost controller is a very simple device that can help
you make more power from your turbo setup. Here are three additional
things to keep in mind about wastegates:
Without a line running from the compressor housing to the wastegate's
compressor reference port boost pressures will keep increasing forever.
This will quickly destroy your engine!
Run a wastegate as close to the desired boost pressure as possible
this will help the boost controller handle the pressure better.
You can't reduce your desired boost pressure lower than the spring
forum has become one of the best resources for Saturn performance
on the net. Our members are constantly pushing their cars to new
performance levels. So, log in, share your ideas, and help push
your car’s performance to the next level.
August 2005 TSN will be holding its first ever meet
in Chardon, Ohio. Some of the fastest Saturns in the country will
be attending. Activities will include drag racing, dyno, tech session,
and BBQ. Look for the latest information in the forums.
Jeff and his team Different Racing have big plans
on breaking into some really low quarter mile times this year. Last
year he posted a 12.40 on a pretty healthy nitrous shot. This year
he has a new turbo setup and some serious determination. Visit his
website for the latest news and information. I expect we'll be seeing
some low 12's from him by the end of the year.