Boost Bleeding Beyond Overboost Shutdown


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.
Return to the Mini-Mopar Turbo Performance page


This page is maintained by Russell W. Knize and was last updated 04/22/99. Comments? Questions? Email

Copyright © 1996-2003 Russ W. Knize