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Old 12-08-2006, 06:22 PM   #1
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Default an introduction and boost control question

Found this neato forum through, seems a good place for Miata-specific turbo system info.

I have a `90 Miata with what started as a Bell System III (uses an IHI RHB5 with internal wastegate) with AFPR, intercooler, J&S Dual Monitor, and boost set to 8 psi. It has since been upgraded with a MKI Link ECU, 370cc injectors, MBC, and an Enthuza turbo exhaust. Boost was set to 12 psi. No dyno-tuning had been performed, only street-tuning Rows 1, 2, and 3 using the NBO2 sensor. J&S knock control has been retained due to the need for the MKII Link MAP sensor to run the Link knock sensor.

My goal: a safe, well-tuned, and smooth 12 psi. 200 rwhp would be nice on the dyno, and I'm sure I'll meet that power goal once the car gets tuned.

The problem: after having a BEGi separated-gases downpipe made to fit the IHI turbo (still working on getting it to not rub the underbody, but that's another story) and upgrading to the FMII airbox, I found I was having boost control issues. The turbo would produce up to 15 psi at WOT...not good since my injectors were maxxed at 87% BEFORE this upgrade. That and from reading other people's experiences, 12 psi is powerful yet still fairly reliable...not that I'll get another 100k out of this engine at 12 psi (91k now), but it's plenty durable at that boost setting with good tuning and respectful driving.

Anyways...removed the MBC and plumbed the wastegate actuator into the compressor outlet to rule out the difference. Adjusted the wastegate arm all the way out...still didn't help.

It appears this downpipe flows REALLY well and, as a result, my tiny wastegate path cannot divert enough exhaust gas to keep the turbine speed under control.

Solution: it seems I need to open up this wastegate path to allow more flow. I have not taken measurements on the turbo yet, but approximating the diameter to 0.75" and using sonic velocity into the turbine housing, it seems the diameter of the wastegate path only needs to be enlarged less than 0.25" to provide an increase in flow of nearly 75% (eq. Q=AV). Sound about right?

Another question...if I enlarge this wastegate path less than 0.25", then the flapper door for the wastegate may not fully cover the wastegate path. Are replacement wastegate flapper doors sold or would this be something that would need to be machined?

I'd appreciate some input. Thanks. -Matt
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Old 12-08-2006, 08:20 PM   #2
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is the wastegate dead now? can you shoot compressed air into it and make it open? since boost straight into the can makes no changes then I'd first verify the condition of the can itself.
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Old 12-09-2006, 04:16 AM   #3
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With that many setup changes and no change in your boost problem, I'd suspect that your wastegate may be to blame.

I had one fail on me and it would fully open at about 1psi and wouldn't spool at all. I know you've got the opposite problem, but maybe something inside your actuator is broken and not allowing it to fully open. Only way to troubleshoot that is grab another actuator... used ones can be had for cheap.

Better yet, great time to upgrade to EBC and take full advantage of the Link!
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Old 12-09-2006, 02:08 PM   #4
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Make sure the wastegate is ok and put an EBC on it.

FWIW, i see about 4psi at redline with the wastegate flapper unhooked (laying open), but I can get a solid 12psi with my ebc.
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Old 12-09-2006, 02:58 PM   #5
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Whoops, that was one key piece of information I neglected to include. The wastegate actuator works; blowing compressed air through the can, the wastegate arm swings freely. It is not a maifunctioning wastegate. Stranger things have happened, but the likelihood of reinstalling the turbo after the downpipe was made and having the wastegate fail...

Before the mods, the turbo would spool to 8-10 psi by 3500 rpm then reach 12 slightly past 4000 rpm. Seemed to have no problems building boost. Talking with Corky Bell, maker of my downpipe, he tipped me off on the wastegate flow possibility, and I am inclined to believe him. Reading through Maximum Boost, this seems to be the culprit. This is why I posted here to see if anyone on here has needed to perform this work to their turbo. I don't wish to bother Corky with fixing overboosting while we are still working on getting my downpipe to fit.

On a side note, a boost control solenoid will not work with my MKI Link.
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