Wastegate operation on a Mercedes 300D Turbodiesel

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bcc304@gmail.com - 08 Oct 2006 21:35 GMT

Can someone help me to understand just how the wastegate operates on my
83 W123 Benz I5 Turbodiesel.

I assume the following: (correct me if I'm wrong)

The hose going from the intake (high pressure) side of the turbo goes
to the wastegate on the exhaust side of the turbo. I think this
wastegate operates like a pop valve... The turbo boosts to a high
threshold pressure causing the wastegate to open. This allows excess
pressure to escape into the exhaust. Then the turbo slows and the boost
pressure drops to a low threshold causing the wastegate to close.

I did an experiment today to see if my analysis of the system was
correct. I unhooked the hose to the wastegate and held my finger over
it with the engine running. Then I revved it very gently... Just enough
to get the turbo to spool very slightly. I felt a little pressure
indicating boost.

Next I wanted to see just how much power I would lose by unhooking this
hose and driving the car. To my surprise, the engine performance did
not change drastically. I did notice that I had the acceleration was
much more linear. That makes sense because there was no wastegate to
stay closed until the high threshold pressure was reached. Unhooking
the hose simulated having the wastegate open all the time.

I actually liked the performance without the wastegate. That "power
surge" form 1st to 2nd gear was greatly softened. This also feels like
it is not as hard on the transmission during the 1st to 2nd shift.
There is not as much torque at that instant.

I also enjoyed the "psssssh" sound since the hose was venting under the
hood and not into the exhaust anymore.

I am really thinking about getting a boost gauge hooked up to the
intake so I can tweak the boost. I am planning on installing a valve on
this hose and venting the hose into the engine compartment. This way I
can adjust the max boost to about 12 PSI. The smaller the leak, the
more boost and vice versa. I also like a more linear power distribution
and the noise :)

Any input is welcome. I would like to know if anyone else has done
something similar.

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T.G. Lambach - 08 Oct 2006 23:07 GMT

You're close.

There are two components to the power surge:
Turbo boost which is limited by the wastegate to about 10 or 11 psi and,
fuel enrichment based on the turbo's boost. (It takes fuel to make power.)

The hose to the wastegate delivers the turbo's boost pressure, not air
volume, against a spring loaded piston. As the boost overcomes the
spring's load the wastegate valve opens a passage for the exhaust gases
to bypass the turbocharger and be dumped directly into the exhaust. Some
owners have cranked the adjustment to increase the turbo's boost
pressure to about 12 or 13 psi - some turbochargers are easily adjusted,
others easy to adjust but nearly impossible to get back together again.
So don't. Now to fuel.

Look at the very aft end of the motor's intake manifold. You'll find a
"banjo" fitting attached there by a hollow bolt. It's designed to
transmit the turbo's boost pressure over to the injection pump's sensor.
As the boost rises the injection pump adds fuel - beyond the throttle
setting - to give the car better acceleration. To see the difference,
simply remove (and pocket) the hollow bolt and drive the car without the
line to the injection pump connected - that's flat acceleration! And
it's not due to some air escaping, its lack of additional fuel.

That's how it works.

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Richard Sexton - 09 Oct 2006 03:53 GMT

Stu Ritter once drove a car with a welded shut wastegate and disabled the
ovrboost protection. He said it pulled like a 300SEL 6.3 but only drive it
two blocks cause he figured the engine would melt down pretty quickly.

The wastegate is supposed to open at 1.1 bar, most of them come out of the
factory at 0.9 bar so there is a bit of tweaking there while still keeping
it in spec. Clean out the banjo fitting too, it gets clogged easily and
frequently and robs power.

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