Boost Bleeding Beyond Overboost Shutdown
Description
This is the next level to the modification above. It allows you to
achieve even higher boost levels and requires a more sophisticated system
to keep the air/fuel mixture at safe levels because the computer is no
longer fully aware of what is going on with the engine. Most of these
designs were also conceived and tested by Gus. Most of these modifications
are entirely Gus's design (also referred to as "Gus Bleeds"). Most
of these designs are well documented on Dempsey Bowling's Turbo Performance
Upgrades page and I will not repeat his (or Gus's) efforts here.
I will organize those pages and give my personal comments on them based
on experience. If you're not sure how the turbo and wastegate work,
read my Turbocharger Concepts page.
For some basic information about the parts used to install bleeds, see
Dempsey's
General Information Page. I will assume you have already read
the Increasing Boost With Wastegate Control Bleeds (WCB)
or the Increasing Boost With Wastegate Control Valves
(WCV) page.
Things To Keep In Mind
These designs are meant to suppliment the bleed setups on the WCB
page, or the pop-off valve setups on the WCV page.
Please read that page before going on. These setups allow you to
increase the boost levels of those setups beyond the overboost
shutdown point by modifying the signal from the MAP
sensor. You have several options here on how to do this, but
you must choose wisely and proceed carefully to avoid damaging your engine.
The method that is best for you depends on your setup.
As a general rule, altering the MAP sensor signal will result in a lean
A/F mixture at WOT. You need to compensate for this by increasing
fuel flow to your engine somehow. There are several methods to increase
fuel flow to your engine, and the method you choose will influence which
MAP sensor modification is best for you. It is also vital that you
have a strong fuel system to begin with. See
Diagnosing
Your Fuel System to find out how to test your fuel system. You
should test you system any time that you modify it. The
Engine
Information page describes how the ECU uses the MAP sensor to determine
how much fuel to add to the system.
In order to know where to are at as far as A/F mixture is concerned,
you must install an A/F guage, even if you have an EGT guage.
The EGT guage does not give you as accurate of a picture that the A/F guage
does. See the Choosing Your Guages page
for more important information.
Using a MAP Sensor Bleed
Another way to modify the MAP sensor output is to install a very small
bleed setup on the vacuum line to the sensor. This has the effect
of offsetting the entire signal output of the sensor downward. This
will cause the engine to run slightly lean at low boost, with increasing
leanness as boost increases. By setting the bleed so that the maximum
boost pressure you want gives the MAP sensor an output of around 4.7V (14psi),
you will avoid overboost shutdown at your desired pressure. Should
a problem occur with your boost control causing boost to climb too high,
shutdown will then occur and your engine will be saved. The procedure
for this was written by Gus and is hosted on this site:
Bleeding The MAP Sensor, by Gus
Mahon
Since this design alters the entire response of the MAP sensor's ability
to measure boost, other types of fuel system enhancement are better.
Any combination of these will help.
Upgrading The Fuel System: Fuel Injectors,
by Russ W. Knize
Upgrading The Fuel System: Regulators,
by Russ W. Knize
Controlling Your Fuel System With A MASC,
by Russ W. Knize
Using a MASC
The MASC (Mass Air Sensor
Controller) by TRE, uses a microprocessor
to modify the MAP sensor output to the logic module. It gives you
10 ranges of sensor input with which you can adjust the output. It
works similarly to the graphic equalizer on a stereo system. This
unit gives you the most control over the mixture over the entire load range
of the sensor. This unit comes with a 2.5 bar MAP sensor, or you
can also use the Super 60 3 bar MAP sensor for up to 30psi of range.
It can be yours for a mere $600. See this link for more details:
Controlling Your Fuel System With A MASC,
by Russ W. Knize
Using a Zener Diode
This design uses an electronic part, called a zener diode, to limit the
MAP sensor's output to 4.7V. By chopping off the signal above that
point, the logic module thinks the boost is a steady 14psi. This
has the advantage of avoiding overboost shutdown, but since the logic module
doesn't think the boost is over 14psi, the engine will lean out above this
pressure. Also, this setup completely circumvents the overboost shutdown
feature, so if a problem occurs with boost control, the boost could climb
very high and destroy the engine. Here is the link to the procedure
for this modification:
Installing A Zener Diode On The MAP Sensor,
by Russ W. Knize
The best way to increase fuel flow for this configuration is to install
additional fuel injectors to the intake. This is because the injectors
are set up to turn on at a particular boost level. When the logic
module stops adding fuel because of the limited MAP sensor signal, the
additional injector is turned on by a pressure switch. Here is the
link on how to install additional fuel injectors:
Installing Additional Fuel Injectors,
by Russ W. Knize
Controlling Your Fuel System With A MASC,
by Russ W. Knize
Other modifications, such as installing larger injectors or increasing
the fuel pressure, will cause the engine to run rich when the boost pressure
is below 14psi. This will hurt performance and economy, and can cause
other problems, such as fowled spark plugs and oxygen sensor, or even ruining
the catalytic converter.
This page is maintained by Russell W. Knize and was last updated 04/22/99.
Comments? Questions? Email
minimopar@myrealbox.com.
Copyright © 1996-2003 Russ W. Knize
