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Old 04-13-2007, 11:08 PM   #1 (permalink)
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boost taper question

hey, anyone know why the IX's have such bad boost taper? i have tried a greddy profec and got it to hit peak boost at 22psi tapering to 14 psi by redline. then i tried a hallman mbc and i got the same thing. i was planning on getting the forge adjustable wastegate actuator to relieve the problem. any other thoughts on where the taper comes from?
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Old 04-14-2007, 09:53 AM   #2 (permalink)
 
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It's a volumetric efficiency (VE) issue. They flow so much air at higher rpms that the turbo is unable to keep pace with that flow and therefore tapers. It's not "bad" boost taper. It's just a limitation of the turbo. Even so, IXs make way more power mod-for-mod than VIIIs with less taper. Don't worry so much about taper - worry about power delivery. Tapering to 14psi is more extreme than I've ever seen, though, so you may have a boost leak somewhere. Have you done a boost leak test?

The Forge WGA won't "relieve" the problem, because there is no problem with the stock WGA. There is typically more taper on IXs with the stock turbo due to how much that MIVEC head flows, but you should definitely check the system for leaks, since an 8psi drop is as bad as I get it at 6000' altitude. Hmm, isn't Reno around 5000'? If so, then that's exactly the reason right there. The high altitude means lower atmospheric pressure and less oxygen in the air, which means the turbo is even less efficient. My boost tapers from 26 to 16psi, but it used to taper from 26 to 22psi at sea level.





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Old 04-14-2007, 03:25 PM   #3 (permalink)
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yeah i did a boost leak test and came up w/ nothing. and my buddy w/ a IX SE got rid of the problem w/ the actuator. he holds 23 psi with a 1psi drop by redline. i have never seen boost taper as bad as mine. my old 8 held boost way better. its gotta be the new head.
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Old 04-14-2007, 04:06 PM   #4 (permalink)
 
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I do not believe a WGA changed the boost taper for your friend, because the stock WGA isn't why boost tapers. Also, the stock taper simply cannot only drop 1psi like that - it just can't do it unless you force it with an EBC using high rpm gain. Another thing is you have to verify your boost profile using a MAP sensor. Don't base anything off the gauge or your friend's gauge. Anyway, our stock boost taper is not relevant to the WGA. I have a Forge WGA with the 19-24psi spring, and it didn't change my taper at all even at sea level.

Did you see my comment about the altitude? If you're actually in Reno, then there is no way to avoid extreme taper on the stock turbo at that altitude. The turbo can't flow enough to sustain boost at high rpm. It's simply a flow limitation.





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Old 04-14-2007, 04:40 PM   #5 (permalink)
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but i'm telling you, i've driven his car and hes got almost no taper. this is with an accurate electronic boost gauge, granted, its not a MAP signal but i know what i saw.
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