Over the years we have dealt with boost creep, or even out of control boost, many times. Nine of ten times it has involved the integral wastegates. As the T25/28/30 Garrett series turbochargers have come into wider use, we seen the growth of compressor flow rates nearly triple yet the integral gate remains the same size. I've no idea what goes through the design guy's heads.
These creep/overboost problems were originally encountered with the IHI turbos as applied to the old Z car and RX7. We solved the problems then and the same techniques have proven successful today.
In my view, the creepy problem stems from a limited flow capability of the gate valve, and the geometry of the turbine inlet in the immediate vicinity.
What we've done from the start of our current designs:
With some historical fear of integral gate creep problems we felt some concern in avoiding the problems at the outset of the new designs. With some past judgments applied, it appeared the streaming of the valve would accomplish what we needed and was only a two minute operation.
While it is often difficult to tell if you have solved a problem or not affected anything, at least we had no reports of over boost for two years.
As with the best laid plans...... we've had two reported incidences of creep in the last month.
What we have now incorporated into every integral gate turbo passing through our office:
Clearly the need was to flow more exhaust gas through the gate's valve, when open, and less through the turbine.
Here the discussion must become a bit vague for fear of telling competitors how to fix their similar problems.
Streamlining, re-routing, directing and expanding requires a 45 minute milling machine operation. Perhaps not the best deal for the company bottom line, but ................
On asking garrett to fix the problem in production with some casting and machining changes, they very clearly stated that old and tired cliché; "we've never seen such a problem, you must have done something wrong."
I bet you've heard that a few times................