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-   -   Boost tapering at high RPM? (http://www.wrxtuners.com/forums/f52/boost-tapering-high-rpm-453/)

Gerlando 12-31-1969 07:00 PM

I've heard that boost starts to cut back at higher RPMs and I never really noticed this effect, but today I decided to see what people were talking about. I was surprised to see that at about 6000 RPM, boost dropped dramatically from 14psi to about 7psi! Is this normal? I'm thinking my catless uppipe and 3" exhaust might have something to do with it,.. or I have an exhaust leak,... or its normal,..



Anyone out there notice the same thing <IMG SRC="images/smilies/icon_frown.gif">

mikeWRX 12-31-1969 07:00 PM

The Stock ECU and FBC is programmed to taper boost at high RPM's. If your using a MBC then it probably has to do with the TD04 turbo being out of it's efficiency range, or you have an exhaust leak. <IMG SRC="images/smilies/icon_wink.gif">

HRTATAK 12-31-1969 07:00 PM

Hey Mike...are you saying that a MBC wouldn't help....because I was thinking about buying one because it's so much cheaper than an electronic one...

Gerlando 12-31-1969 07:00 PM

An MBC DOES help. I had one on my WRX and set it to 13psi and got it just about anytime I wanted. I threw a CEL so I took it off and never put it back on.



I later found out that having full boost at partial throttle isn't a good thing for EGTs so I'm not putting it back on without an EGT gauge.

HRTATAK 12-31-1969 07:00 PM

hey G...does the turbo hold max boost until redline with the MBC? <IMG SRC="images/smilies/icon_confused.gif"> The CEL won't bother me that much. If it does...I'll take the bulb out of the dash...

<font size=1>[ This message was edited by: HRTATAK on 2001-12-11 21:27 ]</font>

BugEyed 12-31-1969 07:00 PM

Beware, the stock ECU retards the ignition and richens up the fuelling if you run high boost and revs. You will still get some benefit, but not as much as you hoped for.



More importantly, beware the high EGTs that Gerlando mentioned above if you have full boost on part throttle. I suggest that if you use an MBC then you should pick one that allows you to avoid this problem (such as the TurboXS one).



Duncan <IMG SRC="images/smilies/icon_smile.gif">

mikeWRX 12-31-1969 07:00 PM

Oh yeah, a MBC will definitely help. Just get one that combines the ball and spring design with the bleed off valve type. That way you can set the ball and spring to a lower boost for part throttle and set the bleed off one to high boost for full throttle.



Here is a link that shows you how to make one of these type of MBC's.



<!-- BBCode Start --><A HREF="http://home.austin.rr.com/gabedude/boost.html" TARGET="_blank">Low boost/high boost boost controller</A><!-- BBCode End -->



Gerlando, you already have the first part of this, all you have to do is add the needle valve section after the ball and spring MBC and set the ball and spring boost to around 7psi for partial throttle position.

HRTATAK 12-31-1969 07:00 PM

Do you need a needle valve after the ball and spring MBC....or just use a restricter like some of the others.....

<font size=1>[ This message was edited by: HRTATAK on 2001-12-14 14:13 ]</font>

mikeWRX 12-31-1969 07:00 PM

I suppose you could just use a restrictor. The needle valve just adds a lot more adjustment to the amount of restriction. (Thereby offering more adjustment to the max boostlevel.)

HRTATAK 12-31-1969 07:00 PM

Thanx Mike....this is a good MBC for less that $50....It's just going to take some trial and error to adjust...but saving money is always a good thing... <IMG SRC="images/smilies/icon_smile.gif">


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