wastegate dumping. . . been there doing that

[Ken Keith]

Hi Christian,

We are talking about two different devices, doing two different things.

On a turbo car, without a wastegate, the turbo would spin faster and
faster until something blows up.  It's the nature of a turbo.  So, to limit
the speed at which it spins and the boost it produces, some of the
exhaust gas that spins it must be diverted in a controlled way.  That is
what the wastegate does.  It opens up and lets some of the exhaust
gas go around the turbo instead of through it.

The discussion regarding that was what to do with the gases that get
diverted.  In a stock system, they are piped back into the main exhaust
after the turbo.  We were discussing piping it somewhere else, like out
the back of the car in a separate pipe, or just letting it blow out under
the hood somewhere (yuck).

In that way, the wastegate IS a bypass valve, but to use that name
would be confusing, because there is something else called a bypass
valve, and there's already a name for the wastegate. :-)

As for the bypass valve stuff.  This is not absolutely necessary for a
turbo car to have, but it is beneficial.  Using a bypass valve let's the
turbo stay spinning when you close the throttle.  Otherwise the
pressurized air has nowhere to go and the turbo slows down because it
can't pack anymore air into the same space.

On our CIS cars, the air gets recirculated in the intake system, around
and around the turbo.  No (little) new air is being sucked into the intake
system through the airflow meters, so the fuel being delivered is
reduced.

If you let that air out of the system, like with a blowoff valve, new air
would be sucked though the airflow meter, which would cause more gas
to be injected into the motor.  More gas than is necessary for the air
getting put into it.

Cars, such as Japanese cars, that use other methods for regulating fuel
delivery, don't have the same situation.  On those cars, they can let the
excess pressurized air out of the system entirely (and make a cool
whoosing sounds, or an IMO stupid fluttering sound), and it doesn't
screw up the fuel mixture.  This is because they use things like MAP
sensors in the intake manifold, after the throttle body to determine how
much air is being put into the motor, and therefore how much fuel is
needed.

For now, just make sure everything is working well.  After that, a chip
and wastegate spring will put a smile on your face.  Adding a bypass
valve either before or after that is a good thing as well.  No real need to
mess with where the wastegate gases go at this point.  The stock
exhaust is pretty good too, as far as I've heard and experienced.

Maybe after those things above get modded, then a 3" or so exhaust,
and then a separate WG exhaust pipe.  But I'm sure some old parts will
start breaking before you get to spend that much money on cool stuff
like that.

Check out the book Maximum Boost by Corky Bell, and Scott Mockry's
website http://www.sjmautotechnik.com

HTH,

Ken

ps.  As an aside, on a stock car like ours, the boost level that the
wastegate regulates is mostly controlled by a spring that holds it closed
(our ECU's also affects boost).  The wastegate is partially open before
the desired boost level is reached.  So, as you are building boost, it's
leaking past the turbo a bit.  This prolongs the boost-building process a
bit.

This is where boost controllers come into play, they (the kind I like)
keep the wastegate (totally, in some cases) closed until the desired
boost is achieved, or about to be achieved.  There are pneumatic boost
controllers, and electronic ones.  Of the pneumatic ones, most of them
are bleeder types, which trick the wastegate into thinking the boost
pressure is lower than it is.  The other type of pneumatic boost
controller is the one I want to use someday, which keeps the wastegate
completely closed until the desired boost is achieved, instead of just
moving the wastegate's opening point later, at a higher boost.

Schwnnbkr1 at aol.com wrote:
>
>     So it's safe to use a blowoff valve only for the wastegate, because it
> vents to the exhaust anyway. This bleeds off what could be backpressure
> then, but only when you're shifting or slamming the throttle shut? So it's
> unsafe to use a blowoff valve  in the intake system, but safe to use it
> post-turbo in the exhaust system, got that. Now if the bypass valve vents
> boost from post-turbo (cold side) to pre-turbo (at the turbo inlet, during
> shifts and so forth) then wouldn't it be best to have a blowoff in the
> exhaust and a bypass in the intake, or is that just moronically redundant?
> (just a radical idea) Christian Rymer 88' 5000tq