Fun with D-Jet, a WB O2 Sensor, and a 2056 Options

[Demick]

I've been running around the last few days with a WB O2 sensor and a vacuum gauge hooked up in my car so that I can understand exactly what my engine is doing under various conditions. It has been quite interesting.

First: my engine is a 2056, #73 Webcam, 8.2:1 compression, and D-jet. The MPS was modified slightly to run with the 2056.

So after a few days of driving, and playing with different resistances in-line with the head temp sensor, I've been able to characterize the air fuel ratio as a function of manifold vacuum. Below is a chart.

Even though I am using a modified MPS with a modified motor, I think the characterization is typical of stock d-jet 914's (not in terms of actual a/f numbers, but in terms of the shape of the graph a different vacuum levels).

While the car runs very well as currently adjusted, it is quite obvious that this is less than ideal.

>15 in Hg: This condition is called overrun, or when you are decelerating and dragging the engine along. The engine goes very rich under this condition. This is not really a concern of mine, but it does seem to go overly rich.

15 in Hg: This is about the vacuum level you get when cruising at steady speed down the highway. You can see that the motor is quite rich under this condition (12:1). Ideally, for good fuel economy, I would want to see this number a good bit leaner - like 14:1 or so. Again, not too much of a concern - the motor should run cooler by running rich like this.

10 in Hg: This is the vacuum level when accelerating slightly or going at a steady speed up a small incline. What is very interesting here is that the motor continues to lean out under acceleration all the way until 7 in Hg (which, by the way, is exactly where I had set the MPS diaphragm to come off of it's part-load stop). This condition is definitely not ideal. I would like to see the engine running leanest at steady state cruise, and then slowly richen as I apply acceleration. But just the opposite happens until I get to a medium acceleration level, at which point an abrupt transition happens to richen the motor up for heavy acceleration.

5 in Hg: The transition to richen the motor has happened at this point. From here on to WOT (0 in Hg), the A/F mixture is appropriate. 12.5:1 at WOT is perfect.


So what can be done to correct this? Well, the d-jet system is pretty limited in it's adjustments. But what I plan on doing is leaning out the overall mixture (to run leaner at steady speed cruise). This is done by tightening the inner screw on the MPS. Unfortunately, this will lean out the mixture across the entire range making the mixture at slight acceleration worse. So to compensate, I am going to set the point at which the diaphragm comes off of it's part-load stop to happen around 11 in Hg (stock is 6 in Hg). This is done by loosening the outer screw in the MPS. This will cause the transition to richen the motor earlier (at a higher vacuum level). Then in order to keep the motor from going too rich, the full-load stop will also need to be adjusted to stop the enrichment earlier. This is done by tightening the plug on the MPS.

The possible disadvantage of this is that the diaphragm is designed to operate from about 0 to 6 inHg of vacuum. I will be using it from more like 4 to 11 inHg. So it may stress the diaphragm much more than normal and shorten it's life.

I'll modify the MPS tonight I'll give an update tomorrow on what my findings are......

Demick