Quote:
Originally Posted by ngoma
There is experiential evidence that indicates the dynamic timing action weakens over time (internal vane pump wear, general wear, lower modern ULSD viscosity) and therefore, older IPs need Pressure Control Valve recalibration to restore the desired dynamic timing action.
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Interesting points and discussion on that other thread. Maybe this accounts for why putting on a freshly rebuilt and calibrated IP from a professional fuel shop on these old mechanical IDI engines often seems to really transform the engine, makes it run cleaner and stronger, etc. While with the electronic feedback control motors like TDIs it makes no noticeable difference I have ever seen in how the engine runs with a new pump vs a quarter million mile pump, unless the old IP was seriously broken in some way.
The slow, gradual internal wear over time makes sense as a reason. Experimenting with a pressure gauge or return volume metering system would be very interesting, especially if also trying some fuels of differing viscosity and lubricity too (like ULSD vs biodiesel).
One thing I have imagined would be fun to attempt as a tuning and calibration technique sometime would be to find a way to install an injector needle lift sensor and crankshaft position sensor from a TDI engine, then get an ECU powered up and running on a bare-bones level and the OBD port wired in. With those pieces you would be able to read actual start-of-injection timing and RPM to see what the real world timing curve is, and adjust accordingly. This same method is often used by the guys doing M-TDI conversions to get their mechanical pumps dialed in to mimic what the electronic pumps achieve with computer control, though easier for them since they're starting with an engine that already has the sensors built in and ready to use. I have to imagine it would be more accurate than any other method, since using actual injector opening moment as the timing reference accounts for wear of all the parts in the system that could affect timing, not just the advance piston and vane pump and PRV, etc, but also the head/rotor, injector nozzle, etc.
Of course even more interesting would be to just put a full electronic control system on, with drive by wire pedal and all, and see what kind of performance gains could be had on an old IDI turbo engine simply by optimizing timing and injection quantity with feedback control, plus response to barometric pressure and intake air/coolant temp, etc, like the newer stuff does. But part of the appeal of these engines is their simplicity and non-reliance on computerized systems, so it's hard to see ever really going there.
You're right that we're hijacking RedArrow's thread here. Maybe we should move these posts to a new thread.