Increasing Boost With Wastegate Control Bleeds (WCB)

[ Read This! | Single Bleed | Two-Stage Bleed | Multi-Stage Bleed ]


This is the most popular next step in performance modifications, and is safe if done properly.  It is by far the cheapest route to go and there are many options here, so read through them all.  Most of these modifications are entirely Gus's design (often 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 his pages here 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.

A Word About WCBs

There is one thing that is confusing to most people about these types of bleeds.  These bleeds only "take over" control of the wastegate on the turbo, they do not "fool" the logic module into thinking that engine conditions are different.  This gives you control over what level of boost the turbo will reach and how fast it will get there, while allowing the ECU to handle the details of keeping the engine running properly.  The only limitation this presents is that you cannot bleed beyond the overboost shutdown point of you engine (usually around 14.0psi to 14.7psi).

The logic module (whether stock or performance) is designed to bring up boost levels at a slow rate by opening the wastegate at about 3-4 psi.  This is to make the engine response more smooth and managable to the common driver.  As you probably know, the high torque of the engine tends to "kick in" at a certain rpm range.  If the logic module didn't suppress the boost inrease, the engine would "kick" even harder.  This also prevents the boost level from peaking, which is when it momentarily goes over the desired limit.

Taking control of the wastegate does not take away to ECU's ability to safely operate the engine.  If the boost climbs too high (past the overboost shutdown point), the ECU will shut the engine down.  If the engine is knocking, it will retard the timing.  You should retard the base timing some (to about 10 degrees, advanced) and use higher-octane gas, since the logic module would normally reduce boost if the engine was knocking badly.  Since the boost level is not allowed to exceed the shutdown point, the logic module always knows how much air is going into it and therefore will never let the engine run lean (unless you have a weak fuel system - see Diagnosing Your Fuel System).  The Engine Information page describes this further.  You can check the status of your engine during boost by installing an A/F guage, or connecting a volt meter to your oxygen sensor output line.  If the voltage is above 0.8V (8 lights), then you are OK.  Above 0.9V (9 lights) is preferred.

The Single Bleed

This design is the simplest bleed you can do, but it has short comings.  If you are boost bleeding for the first time, then I would advice starting here.  This will orient you to how these bleeds work and you will use the parts from this bleed setup for the other setups.  The main disadvantage to this setup is that it is very prone to "boost peaking".  You can supress this with a larger restrictor or with a boost peak suppressor, but it requires additional hardware (see below).  The other disadvantage is that you get full boost (whatever you set your bleed to) right away.  This can make take-offs rather tricky in first gear, and impossible in slippery conditions.  Here is the link to Gus's procedure on Desmpsey's site:

A Simple Boost Bleed, by Gus Mahon

You can suppress the boost peak by combining the simple bleed with an electro-mechanical boost peak suppressor.  This is my name for Gus's "Overboost Eliminator."  Too see Gus's design for the single bleed and the overboost eliminator together, see Dempsey's page on it:

A Simple Boost Bleed Without Overboosting, by Gus Mahon

There is also another, older design by Gus that uses the boost peak suppressor as the means for boost control.  This design doesn't even use a bleed valve:  the bleed is wide-open.  This design acts in a cycle: the bleed causes the boost to rise and the pressure switch then closes the bleed which causes the pressure to drop.  The pressure switch then opens the bleed again which causes the pressure to rise again.  This cycle takes place rather quickly, but it presents a lot of wear and tear on the pressure switch.  The boost level also "wavers" by about 2-4psi while this system is cycling, depending on your intake setup.  Because of these facts, both Gus and I recommend the above setup over this one.  However if you want to try it anyway, here is Dempsey's page on it:

Overboost Eliminator & Turbo Response Kit, by Gus Mahon

The Two-Stage Bleed

This design gives you more flexibility and control.  By installing two bleeds, you can switch to a lower boost level for take-offs in first gear and to full boost for the higher gears.  This will avoid burn-outs and wheel hop during take-off, but give you full power after that.  Gus's procedure suggests installing a toggle switch, which is fine for those with automatic transmissions.  For those of you with a manual trans, I would advise installing a switch on your shifter or steering wheel.  See the Electronic Engine Performance Controllers page for more info.  Here is the link for the two-stage bleed, which is hosted, with permission, on this site:

Two-Stage Boost Bleed, by Gus Mahon

Again, you can upgrade this design with a boost peak suppressor same as above.  Dempsey has a page on Gus's design for this here:

Two-Stage Boost Bleed With Overboost Eliminator, by Gus Mahon

Multi-Stage Bleeds

By taking Gus's two-stage bleed design and adding additional stages along with a Electronic Engine Performance Controllers, you can give yourself even more precise control over boost and have better take-offs.  The multi-stage boost allows you to have a boost level for each gear.  This is handy for manual and automatic transmissions becuase it will give you maximum power possible for each gear without losing traction.  Here is my design, based on Gus's inital design, for the multi-stage boost bleed:

Multi-Stage Boost Bleed, by Russ W. Knize

Once again, I have added the boost peak suppressor, by Gus Mahon:

Multi-Stage Boost Bleed With Peak Suppression, by Russ W. Knize
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This page is maintained by Russell W. Knize and was last updated 04/21/99. Comments? Questions? Email

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