MPS- delving into it's secrets?, Update January 3, 2011 Options

[realred914]

if you are running a stock rebuild motor setting the MPS via a Wavetek is the easiest way and works. We have the known settings for the Wavetek to do this.
if you are running a non stock cam you must adjust the inner and outer screws for the proper AFR and slope, then set the WOT stop screw for the WOT AFR (12.5 to one). Using a LM1 hooked up properly doing multiple runs is the only way I know how to do this. On a roller dyno or on the road works fine.

looks like an O2 sensor should be in my plans then???? My motor will be modified, 96 mm pistons, and a Elgin 330-1 cam. I had used the LM-1 system I borrowed once for tunning a triumph carbed car, actually I found the readout to never be stable, so it was an exercise in frustration. maybe a function of the carb not being stable, it was a bicth cuase it was smog controlled car!

how well does teh LM-1 system work on our D-jet cars, is the signal pretty stable when driving? (no dnyo, so i would be tuning on the street)

[Bleyseng]

once you weld a bung in the exhaust in the right place it works great...if you have a laptop and the rest of the wires and adapters you can log it in.

like I said, it takes several runs adjusting the screws ever so slightly to set the AFR correctly. Once you are close you can feel the hp come around along with smooth transition from idle, partload and WOT.

[jk76.914]

Thinking ahead- Since it's now winter, I'm thinking I'll hook up my monitoring rig on my daughter's '99 VW Cabrio. Should be an easy trial- I only need to tap into the O2 sensor, plus add the GM MAP sensor. That'll give me a baseline with a "modern" EFI system, so I can see what the scatter cloud looks like...

Also, thinking about this GM MAP sensor (<$30 at RockAuto): I know you can't practically emulate the inductor in the MPS, but what if one were to use an MPS with no diaphram, and then use the output from a modern MAP sensor for enrichment only? The aneroid cells would get the mixture close, and the ECU would be happy, but full throttle enrichment would be from a separate circuit controlled by the MAP sensor.

Looking at the circuit description in Brad's article, TS2 is the leg of a simple voltage divider. The circuit in the ECU is reacting to the voltage derived to extend the pulse via the PWM circuit. Why not boost this voltage a bit for enrichment, controlled by a couple of op-amps driven by the MAP sensor output? The circuit I'm starting to think about would be adjustable for when the enrichment starts, rate of enrichment vs. decreasing vacuum (slope), and max enrichment limit. Since running with TS2 open or shorted results in too rich or too lean (respectively) to run, seems like this ECU input may provide plenty of range to pull it off.

Maybe a hybrid approach like this can be a path to elimination of the diaphram? What do you think Brad? Others?

[jk76.914]

Made a bit of progress. I received my MAP sensor from RockAuto, and was able to sneak down to the basement and plot the output voltage vs. vacuum. The chart is below and it's unbelievably linear. This is what it's supposed to do, but it still amazed me. The other thing I noticed when I was taking the data was that it is extremely sensitive- one pump on my hand vacuum pump and the needle flys to 14" vacuum. Three pumps and it's all the way up to 27". This is very different from an MPS, and tells me the chamber volume above the "diaphram" is tiny.



There is active circuitry inside, and the module requires +5v power input. The output is volts DC. I had to gin up 5 volt source, which I did using my constant-current battery charger I'm working on and a 7805 regular that I've had kicking around my random electronics parts box since 1977.

Below is a photo of my test setup. Not much to look at, but it did the job.



The analog meter is for the battery charger. I used the DVM on the right for the readings. With the MAP sensor, it'd be easy to make an electrical vacuum gauge, whether analog or digital. Interesting idea.

I've researched the '99 Cabrio, and there are two O2 sensors- one before and one after the cat. Next step is to translate the setup to the car, learn how to use the data logger, and collect some baseline data.....

[rick 918-S]

I'm always facinated when I check this thread. You really have a grasp on this thing. It is so interesting to read and try to figure out what your up to. Let me tell you, I have no clue.. (IMG:style_emoticons/default/confused24.gif) (IMG:style_emoticons/default/biggrin.gif)

[jk76.914]

I'm always facinated when I check this thread. You really have a grasp on this thing. It is so interesting to read and try to figure out what your up to. Let me tell you, I have no clue.. (IMG:style_emoticons/default/confused24.gif) (IMG:style_emoticons/default/biggrin.gif)

Sometimes I drift around a bit, I admit. I'm kind of excited about my track now though- first this tool to visually see, on my laptop, where my AFR is vs. vacuum during normal driving, to tune my MPS.

And then try some things to use a MAP sensor and some simple circuitry to maybe fool the ECU into low vacuum enrichment with a no-diaphram MPS. All the plots I did last winter got me thinking along the lines of separating the contribution of the diaphram from that of the aneroid cell.