Found this neato forum through Miata.net, 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.
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.
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 MBC...no 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.
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