Wednesday, March 4, 2020

On "Microsquirting" the Porsche 914

Bosch D-Jetronic

The Bosch D-Jetronic system is pretty cool, especially when you consider it was designed in the 1960s. "Computer"-controlled electronic fuel injection with manifold pressure sensor, intake temperature sensor, crankshaft (well, distributor) angle sensor, and throttle position sensor/switch. It uses constant fuel pressure and flow, so only injection duration needs to be modified to control air/fuel mixture. It measures incoming airflow by monitoring the intake manifold pressure; engine speed, temperature, and other factors are monitored for the purpose of fine-tuning injection duration. Ignition is by a standard cam-driven distributor with an internal D-jet-specific pickup for the crank/cam angle position.

This "speed-density" D-Jet system was used on many cars of the period, including Volvo, Jaguar, Volkswagen, and of course, the Porsche 914 (1.7L and 2L engines only; the 1.8L used L-Jetronic -- "L" for "luft" or "air" controlled, whereas "D-" was for "Druck" or "pressure" controlled. Hey, it's German.)

Bosch D-Jetronic MPS
But D-Jet has limitations. It used analogue circuitry with neither microprocessor nor digital logic; the ECU used around 25 transistors to perform all of the processing. So, it's pretty basic. The lack of processing power and the unavailability of solid-state sensors meant that the vacuum sensor (commonly called the "MPS") was a rather expensive precision instrument, rather like a barometer, with a brass bellow inside to measure the manifold pressure.

Further, with limited (non-existant) processing the fuel pressure was not modulated by manifold pressure, and the injectors were fired only once per 2 revolutions on the engine ("batch injection", half of the injectors firing every revolution).

When it works, it works well enough. The 914 community recognizes that the stock fuel injection provides better driveability and better fuel economy than the "crowbar" replacement with carbs (usually dual big-bore Webers or Dellortos). As time goes on more aftermarket support is becoming available to keep the D-Jet system functioning: replacement D-Jet components are mostly available, with reproduction of key parts such as the TPS/switch and the MPS bellows (with some limited tuning added); an aftermarket ("1-2-3") distributor that incorporates new internal D-Jet trigger points; and vendor support for new wiring harnesses. Other parts are rare failures so there's plenty of used parts available for that.

But the D-Jet's non-existant processing power means that it is not tunable; it cannot accomodate modifications to the engine. 914 owners will commonly modify their engines for performance improvements such as increased displacement, higher compression, and alternate camshafts. When they do this, the D-Jet can't accomodate it and their only induction option is carburetors...crowbars.

Further, while replacement D-Jet components are readily available, they are not necessarily inexpensive. Rebuilding the MPS (the original diaphragms will have failed by now) costs about $450. The 1-2-3 distributor replacement is about $500. A replacement wiring harness - another common failure after 50 years - is $500. Known-good used injectors can be as much as $150 each. It adds up quickly.

Greg's 914s
The D-Jet on my '74 914 was running OK. I had Tangerine Racing rebuild my MPS a couple years ago which resolved a start/run/die problem. I replaced the TPS/switch with a new board, as the transition driveability was suffering. A stumble when tipping in the throttle was lessened with some oil in the center of the distributor shaft; the thought was that the centrifugal advance weights in the distributor were sticking. I am annoyed that the warm-up sequence was poor, because (as WAG'd by the 914 community) the cylinder head temperature sensor was warming up too fast and leaning out the engine before it was actually warmed up (companies offer "solutions" for that.)

But there were other issues. I wasn't getting the fuel economy I expected (it was lower than others were reporting). Plus once I got stranded on the way to work when the car died; turns out that it was a wiring harness problem, resolved by re-terminating some pins (the 45-yr-old wiring harnesses are pretty crunchy).

The final nail in the coffin was last Fall when I was enjoying the great weather with the Targa top off and suddenly the car just would not run worth a crud. Of course, I was an hour away from home. I baby'd it home with it constantly dying and firing back up and just running like hell, but I did get it home. The only thing I figured is that I had just refueled the car (from a local source I've used before); I drained the tank and replaced the fuel from another source and it seemed to resolve the issue. I looked at the fuel and it appeared fine; I tested it for water (it was fine) and for alcohol (it was within the range of E10). I added the old fuel to my near-empty GTI's tank and the car ran fine. I tried it in the lawnmower and it was fine.

So was it fuel, or was it coincidence? Could the car be that sensitive to fuel source? If it is, what if I'm on the road and have to get it from an unknown source? If that wasn't the problem, what was? Wiring again? It was at that moment I realized I just could not trust the engine management system for anything other than drives no farther than my wife or AAA was willing to pick me up (i.e., "not far"). It was time for either parts upgrades or a replacement, and I didn't want to go to carbs.

This decision coincided with my desire to remove the drivetrain over the 2019/2020 winter for some maintenance. The car needed a clutch (it was juddering on take-up and the release bearing was squealing) and I wanted to attack a whole bunch of oil leaks. The transaxle was grunching in a few gears, so I wanted to have that rebuilt. Intake seals and gaskets needed replacing. With the drivetrain out it was the perfect opportunity to consider alternatives to the factory D-Jet system.

So I was going to install some kind of aftermarket electronic fuel injection. But which one?

On to Part Two: "Which Aftermarket Fuel Injection System?"

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