Spun out on the offramp!

[mikey]

I had the rocket assist fired up also......forgot to mention that.

[pbanders]

Lack of protection against overpressure is a basic design flaw of the MPS. In part-load operation, the diaphragm is supported by the part-load stop plate, to prevent mechanical stress on the bronze diaphragm. In full-load operation, the diaphragm is not subjected to mechanical stress (other than the flexure required to move the core), as there is still about a 30 mbar negative pressure difference across it.

When the MPS is subjected to positive pressure, due to rotating the engine backwards (i.e. when spinning with the clutch out), the full-load diaphragm moves to the full-load stop position, but there is nothing supporting the bronze diaphragm. As the diaphragm is already likely to be experiencing some degree of metal fatigue due to normal flexing, this high stress often blows out the diaphragm. Once this has happened, the MPS is operating in full-load mode all the time, and you get a very rich mixture. Amazingly, the car will still run, just very poorly.

A quick check of vacuum integrity can determine if the MPS survived a spin. Someone clever who takes their 914 to the track regularly should be able to rig an overpressure protection pop-off into the vacuum hose that leads from the intake manifold to the MPS.

[mikey]

Is there anyway the MPS can be replicated or replaced by a MAP sensor? I know the MPS also compensates for barometric pressure, so I guess that would be kinda difficult. Quite an clever device.

Mike

[Dave at Pelican Parts]

It would have to be a very clever device. You might be able to do it with a solid-state MAP sensor, plus a whole bunch of circuitry to read the sensor and fiddle with other components to produce a variable amount of inductance depending on what the MAP sensor was reading.

Consensus was, last I checked, that it would be a massive PITA and might even cost more than a rebuilt MPS.

--DD

[mikey]

Yes it would be a PITA for sure......and you would still be left with a non-programmable FI system. But I meant that for its time period, the stock MPS was quite a clever device, espicially being mostly mechanical.

[Eric 914]

Those early EFI systems are analog. It's interesting that we now consider almost anything electronic to be digital, but analog electronics can be very elegant solutions to complex problems like fuel injection. The ability to tweek things can be over-rated, it usually is just compensating for systems that don't fit the requirements precisely. Most people that I've talked to with MAP injection are still trying to tune it to get it almost as good as the analog system that they replaced. If only the analog injection systems had an upgrade path to support more engine than the factory produced...

I'm investigating a MAF injection system for my SBC. I'm hoping it will be a more sophisticated digital solution than MAP.

[pbanders]

Originally posted by mikey
Is there anyway the MPS can be replicated or replaced by a MAP sensor? I know the MPS also compensates for barometric pressure, so I guess that would be kinda difficult. Quite an clever device.

Mike

Like Dave said, with a lot of money and time, anything is possible. But a simple swap? Nope, will never happen. Take a look at my ECU analysis web page and you'll see why. The MPS / ECU system in D-Jetronic is symbiotic, it's pretty hard remove either one without killing the patient.

[mikey]

True indeed

[M. Hendrix]

I think you g'd out a ground, or something at the relay board. I think it's electrical..

How does it sound mechanically?

You need new brakes, and to re-aim your lights.




M