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> MPS Tuning Analysis
Not_A_Six
post Aug 15 2020, 10:22 AM
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EDIT: See post #17 for an update that contradicts some of the findings in this OP. See also post #29 for the final results.

I've recently gone down the rabbit hole of D-Jet and MPS tuning and wanted to post what I've learned in the hopes that it may help others. I also hope to start a discussion with some of the experts here in case I've missed something, or there are errors in my analysis.

Background:

With the increased displacement (2056cc) and non-stock cam (Webcam 73) in my engine, I was experiencing Air-Fuel Mixture (AFR) issues across the range of temperature, load, and rpm conditions.

This post concerns MPS tuning. At the moment, I'm also working on modifying component values inside the ECU to change the Volumetric Efficiency curve to better match the non-stock AFR vs RPM characteristics of my engine. If there is any interest in poking around that deep in the bowels of D-Jet, I'll start a thread when I finish. (IMG:style_emoticons/default/blink.gif)

Engine Configuration:

'73 2.0
Displacement: 2056cc
Cam: Webcam 73
ECU: Porsche nnn906021E (Bosch 280000037)
MPS: Porsche 022906051E (Bosch 0280100049) w/ Tangerine Racing tuning kit + spacer ring; tuned to emulate 0280100037
CHT: 0280130017 w/ 270-ohm ballast resistor and steel spacer
Vacuum Hoses routed per @JeffBowlsby (see link below)
Fuel Pressure: 35 psi (at the moment -- still tuning)
PCV: Modern PCV valve routed to plenum
Ignition: 123Ignition PORSCHE-4-R-V-IE, running profile "1" w/ Vac Advance
Timing: 27 degrees at 3500 rpm (w/o vacuum adv/ret)

Equipment used:
Generic handheld vacuum pump/gauge
misc hoses+fittings
AMPROBE LCR55A meter (on 20H scale)
Innovate Motorsports (3837) LM-2 (BASIC) Digital Air/Fuel Ratio Wideband Meter w/ Bosch LSU 4.9
Home-fabricated tailpipe "Stinger" w/ O2 Sensor Bung TIG welded on

Reference Info:
Vacuum Hose Routing
MPS Theory of Operation @pbanders
AFR vs Manifold Vacuum Issue @Demick


Analysis:

Attached Image

As you can see in the "Chart A" sketch above, the MPS Inner Screw (in isolation), Outer Screw+Inner Screw (together), and Stop Plug Screw can be used to tune the MPS's affect on the AFR (via its effect on the Fuel Injection pulse width) vs Manifold Vacuum. As shown, the Inner Screw affects the mixture over the whole vacuum range; the Inner+Outer Screw controls the onset pressure P' where the diaphragm begins to lift off the part-load stop; the Stop Plug Screw controls the final pressure P'' where the diaphragm comes to rest on the full-load stop. On this chart, "up" corresponds to a longer FI pulse; "down" to a shorter FI pulse. For details, see pbanders's excellent link, above.

I had previously thought that the effect of the Inner Screw was similar to that of changing the fuel pressure via the fuel pressure regulator. Namely:

Inner Screw CCW = Increase Fuel Pressure => Richer AFR across entire vacuum range
Inner Screw CW = Decrease Fuel Pressure => Leaner AFR across entire vacuum range

There appeared to be no way to change the slope of the MPS Response curve in the region from 15 In-Hg to P'. And, I was experiencing the same problem that Demick posted about years ago in the thread linked above. Namely, the AFR would become too lean under moderate-load conditions across all RPM's, like this (from Demick's post):

(IMG:http://www.914world.com/bbs2/uploads/post-2-1096387565.jpg)

To fix the lean condition around 6-8 In-Hg, it's really necessary to change the slope of the MPS response curve, not just raise or lower the whole curve.

However, there appears to be hope. It seems that the MPS inductance is not simply linear WRT manifold pressure in the 15 In-Hg to P' region. I speculate that this may be due to non-linearity in the MPS Inductance across its range of movement and/or the effect of the MPS spring. The result is that the curve seems to look more like the "Chart B" sketch above. And, the effect of turning the MPS inner screw isn't really raising or lower the entire curve, but rather shifting the Chart B curve left/right. (Chart B is a really rough sketch, and may even have the convexity backwards. For a more quantitative look, see below.)

I measured the inductance of my MPS at two different inner-screw settings: The red curve is the MPS tuned to pbanders's values corrected for 700 Torr ambient pressure. The blue curve represents an attempt to richen the AFR by turning the inner screw CCW.

Attached Image

As you can see, the slope of the red curve is steeper than the blue curve in this region. Apparently, turning the inner screw CCW has the effect of reducing the slope by raising the right end, rather than raising the whole curve.

I'm surprised that the curves cross around 7 In-Hg and that the blue curve value is less than the red curve value at the left side of the chart. This may be due to the effect of the part-load stop coming into play near P', measurement error, or something else. (IMG:style_emoticons/default/confused24.gif)

The salient point, however, is that adjusting the inner screw affects the slope of the curve, whereas presumably adjusting the fuel pressure does not.

The opens the possibility of tuning the AFR vs Manifold pressure curve by trading off fuel pressure vs inner screw position:

Increase Fuel Pressure + Turn Inner Screw CW => Enrich 6-8 In-Hg region
Decrease Fuel Pressure + Turn Inner Screw CCW = Lean out 6-8 In-Hg region

I experienced the same problem that Demick did (too lean at 6-8 In-Hg across all RPM), and have largely solved the problem by adjusting the fuel pressure up (to 35 psi currently), then tuning the MPS to set the AFR across the entire range of manifold vacuum levels.

I hope this is helpful to somebody. Comments are welcome. If I've made any errors, please feel free to beat me over the head. (IMG:style_emoticons/default/splat.gif)

Cheers. (IMG:style_emoticons/default/beerchug.gif)
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Olympic 914
post Aug 15 2020, 11:16 AM
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I will be watching this closely.

Had considered opening a thread similar with my results/findings

My AFR gauge just took a dump and was returned to Auto Meter for repair.

Similar engine config.

Will try my best to follow your explanations.
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JOEPROPER
post Aug 15 2020, 01:00 PM
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This is a great thread with great links. A lot of really good information. Thanks for posting.
I will probably be adjusting my MPS for my 2056, but wonder how often this is necessary in a stock application. The epoxy leads me to believe that these were adjusted in the factory and were to be left alone after. After 45+ years, I guess some adjustment may be necessary, but were probably not expected to last that long.
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Bleyseng
post Aug 15 2020, 02:21 PM
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With a stock cam and the slight increase in piston size you shouldn't need to adjust the MPS. "Shouldn't" is the key word as always after 45 years your setup should be tested for correct air fuel ratios with a wide band or on a rolling dyno. (IMG:style_emoticons/default/driving.gif)
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Not_A_Six
post Aug 15 2020, 02:35 PM
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QUOTE(Bleyseng @ Aug 15 2020, 01:21 PM) *

With a stock cam and the slight increase in piston size you shouldn't need to adjust the MPS. "Shouldn't" is the key word as always after 45 years your setup should be tested for correct air fuel ratios with a wide band or on a rolling dyno. (IMG:style_emoticons/default/driving.gif)



I agree. The bigger displacement should have a larger appetite for fuel than stock. But with a stock cam, you can likely compensate just by bumping up the fuel pressure a bit, and get it to run "good enough" -- if everything else is working properly. (IMG:style_emoticons/default/smile.gif)

If you want to go down to the next level in the dungeon, you may want to tune the MPS with a Tangerine kit and wideband O2 meter.

If you want to descend all the way to the lowest level of D-Jet hell, you may have to tune the ECU component values to adjust the VE curve at specific RPM's. (Likely unneeded with the stock cam.) (IMG:style_emoticons/default/happy11.gif)
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JOEPROPER
post Aug 15 2020, 05:56 PM
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Webcam 73...
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Not_A_Six
post Aug 15 2020, 06:04 PM
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QUOTE(JOEPROPER @ Aug 15 2020, 04:56 PM) *

Webcam 73...


Your engine setup sounds similar to mine then. I couldn't get mine to run to my satisfaction across temp/load/rpm. And, so began my descent into madness... (IMG:style_emoticons/default/prop.gif)

But, I'm an obsessive engineering geek. I'd start with the fuel pressure and see if you're happy with the results. A little richness can cover up a lot of niggling issues. Good luck. Please post your results.

Cheers. (IMG:style_emoticons/default/beerchug.gif)
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Olympic 914
post Aug 15 2020, 08:01 PM
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On your Blue/Red graph, how many data points did you use? Every( in Hg) ?

I was thinking of mapping my MPS. Was there a reason you did not map it to "0"

Did you correct your numbers to sea level? Could you share the formula? please keep it simple for me.

My location is about 1000 ft alt

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BeatNavy
post Aug 16 2020, 06:19 AM
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Just for fun I'll throw in some of my data and "analysis" from a few years ago. This was when I was trying to tune a 2056 with a stock cam and later a 2056 with a Raby 9950 (or 9550, whichever it is). I think the latter is similar to a Webcam 73, or maybe even the same grind.

I ended up comparing the following for vacuum / inductance numbers for the 043 MPS, which should have been correct for my '74 D-Jet setup (and compatible with the 043 ECU):

1. Anders' numbers for MPS tuning on his website.
2. An 043 "Primary" that I rebuilt using Chris Foley's kit. This was the one I fiddled with the most in terms of tuning and driving.
3. 043 "Spare." This one was also rebuilt, and I used this kind of as my fallback if I screwed up the primary too badly. I also used it to compare how the car ran with the other one.
4. 043 "Reman." This one was rebuilt and tuned years ago by that company that used to perform this service years ago ("Fuel Injection Company" or whatever it is/was). I did not mess with this one.
5. 043 "Stock." Thanks to the yard sale at Jim McLeod's (RIP (IMG:style_emoticons/default/sad.gif) ) a couple of years ago I managed to acquire a working, unmolested 043. Used this primarily to baseline my inductance meter's numbers against those of Anders.

Here are the results in table form. Remember, I was primarily tuning 043 "Primary" and "Spare," so those numbers could be all over the place. I've got the slope in there, but the most relevant slope is that over the "linear region" which I think Anders defines as between 6 and 18 in Hg.
Attached Image

And here's the graph, such as it is.
Attached Image

Here are my lessons learned:

1. I spend a LOT of time on this, and I learned a lot, which was enjoyable. But I got more return out of that than any performance increase or optimization (IMG:style_emoticons/default/smile.gif)

2. Inductance meters seem to vary significantly. I'm not sure why this is (I know mine was a cheap one, not a Wavetek), but I know others have had similar findings. You can see that my stock 043 didn't even get close to Anders' numbers, which I found interesting. It also means you can't simply go by Anders' numbers if you are using those as a baseline.

3. Chasing AFR can be a challenge. Type 4 idle AFR can be very unreliable, and exhaust leaks and/or valve issues can really mess with the numbers. Different exhaust systems / bung location can also impact your values. I guess my point here is that trying to get to an exact AFR down to the tenth can be like chasing your tail. Sometimes it's more important to focus on what the car is telling you (how much that engine "likes" the tune in terms of performance, gas consumption, plug fouling, etc. etc.).

4. I took a tip from a member here (Frank, I believe), and hooked up a "T" vacuum line from my MPS to a vac gauge in the cockpit. That was very helpful for understanding the relationship between "part load," throttle position, and AFR. It gave me a better understanding of when I should be looking for what AFR value. EDIT: in that regard, I should have changed the labels in the table above where 4 in Hg is not "cruise" but "strong acceleration" and 6 in Hg "acceleration."

5. D-Jet has limitations (of course). It works like a champ for what it was intended to do, but of course you can only tease so much out of it either in terms of fine-tuning or performance enhancements. Unless you REALLY get creative or go off the deep end. I've recently moved on to Microsquirt where I can now be ignorant in that arena (IMG:style_emoticons/default/smile.gif)

Just my two cents for the OP who obviously has a much more rigorous scientific and analytical background than do I.

(IMG:style_emoticons/default/beerchug.gif)
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Bleyseng
post Aug 16 2020, 07:14 AM
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Using a Wavetek just like Anders I came up with these numbers on a NOS 043 at sealevel :

15hg-.71
4hg-1.20
0hg-1.42

I found the best way is to tune your MPS to your car using a LM2 O2 setup. I
just hook it up and test drive the car at partload (cruise -4HG) and adjust the MPS to a AFR of 13.7 to 1. Then set the WOT to start at 11 to 1. Idle is adjusted by the ECU knob.

I think these engines are so old and parts are different than a brand new engine's spec's its impossible to expect to just plug in a NOS MPS and want it to be perfect.
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Not_A_Six
post Aug 16 2020, 07:22 AM
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QUOTE(Olympic 914 @ Aug 15 2020, 07:01 PM) *

On your Blue/Red graph, how many data points did you use? Every( in Hg) ?

I was thinking of mapping my MPS. Was there a reason you did not map it to "0"

Did you correct your numbers to sea level? Could you share the formula? please keep it simple for me.

My location is about 1000 ft alt


I measured the inductance and collected data from 0-15 In-Hg, at 1 In-Hg intervals. And, I have a bunch of different MPS "tunes" that I was experimenting with as I played with fuel pressure, ignition timing, etc.

On my graph, I just plotted the data from 5 to 15 In-Hg to show that the slope would change in that region as you turned the MPS inner screw. The red and blue curves diverge below 5 In-Hg due to different tuning in that region.

pbanders's values for the 028100037 MPS measured at 726 Torr (1260 ft elevation) are:

0 in Hg 1.44H
4 in Hg 1.26H
15 in Hg 0.72H

I'm at 2250 ft elevation (700 Torr). So, those values at my elevation are roughly:

0 in Hg 1.39H
4 in Hg 1.22H
15 in Hg 0.70H

The red curve on my graph was tuned to these values, and has data points every 1 In-Hg.

To adjust pbanders's numbers for my elevation, I used the simple straight-line method outlined in his MPS link in my OP. There are more accurate ways to make the altitude correction, but this was easy and good enough for my purposes:

"The simplest way to calculate a correction factor is to compare the absolute pressure of your location in Torr to my absolute pressure, 726 Torr. For example:

Sea level: absolute pressure is 760 Torr. Therefore, the correction factor is 760/723 = 1.05 . So, for a 0 280 100 043 MPS, the 0, 4, and 15 in. Hg calibration values would be, 1.45, 1.24, and 0.75 H, respectively."


All of these values were measured with an LCR55A meter on the 20H scale. That meter is now made by Amprobe instead of Wavetek. pbanders's values were apparently measured with an older Wavetek meter; mine were measured with a newer Amprobe version. I believe the calibration and values are the same for both meters.

It's important to note that the meter type/model and even scale used will likely read different values than those above. So, my numbers in this thread shouldn't be used as any sort of tuning values for any other MPS, in any other car, at any other elevation.

My tuning process was to start with pbanders's values adjusted for my altitude (the red curve), then make adjustments to fuel pressure and MPS from that starting point based on O2 readings while driving under different conditions.

FWIW, my goal AFR tuning values are:

Hot Idle: 12.5-13
Hot Cruise: 13-13.5
Hot WOT: 12.5

This is 45+ year-old d-jet, so those AFR numbers are expected to vary a bit over rpm, vacuum, engine temp, altitude, etc.

Cheers. (IMG:style_emoticons/default/beerchug.gif)
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Olympic 914
post Aug 16 2020, 08:44 AM
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This is great.

I have been hoping others would post there numbers.

Like Rob I have three 043 MPS units with different tunes I adjust but its mostly looking at the AFR numbers and remembering when it whet lean or rich and making the changes.

also using a Amprobe LCR55A meter so I may be able to compare somewhat our numbers.

Will post later and add my 0 4 and 15 figures when I get a chance.

It does seem right now that my numbers are richer than Not a Six

also as stated above my AFR is out for service, though my Butt Dyno tells me its running better.

I really miss looking at the AFR gauge.
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JeffBowlsby
post Aug 16 2020, 09:08 AM
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"2. Inductance meters seem to vary significantly. I'm not sure why this is (I know mine was a cheap one, not a Wavetek), but I know others have had similar findings. You can see that my stock 043 didn't even get close to Anders' numbers, which I found interesting. It also means you can't simply go by Anders' numbers if you are using those as a baseline."

(IMG:style_emoticons/default/agree.gif)

"I think these engines are so old and parts are different than a brand new engine's spec's its impossible to expect to just plug in a NOS MPS and want it to be perfect."

(IMG:style_emoticons/default/agree.gif) FWIW I have had the opportunity to characterize multiple different NOS MPSs. Each has had somewhat different calibration, using the same Wavetek LCR55. Goes to show you there are no absolutes with this stuff. Let the specific engine tell you what it specifically needs using an O2 sensor or dyno. I have found that Brad's (Anders) numbers are lean when compared to calibration readings from my meter, as adjusted for elevation. My calibration numbers are richer than his.

"All of these values were measured with an LCR55A meter on the 20H scale. That meter is now made by Amprobe instead of Wavetek. pbanders's values were apparently measured with an older Wavetek meter; mine were measured with a newer Amprobe version. I believe the calibration and values are the same for both meters."

Who knows, there has never been a comparison and these are delicate numbers.

"I found the best way is to tune your MPS to your car using a LM2 O2 setup."

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post Aug 16 2020, 09:45 AM
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Great thread. Love seeing a logical, methodical approach.
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Not_A_Six
post Aug 16 2020, 09:54 AM
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The main reason that different meters give different inductance readings is that *real* inductors (especially coupled ones like in an MPS) are not very linear devices -- their value is a function of voltage, frequency, etc.

Different meters use different frequencies, and even different waveform shapes (e.g. sine, triangle, square) to measure inductance. They also use different frequencies for different ranges. For example, the LCR55A uses a 1000 Hz frequency for the 200uH to 2H ranges, and 100 Hz for the 20H and 200H ranges.

I wouldn't get too hung up on the absolute inductance numbers if you are tuning an MPS on a particular car. The inductance values are just a starting point, as you'll likely be using AFR rather than inductance numbers to make adjustments. The 15 In-Hg value is pretty important as it relates directly to the part-load AFR. But, the 4 and 0 In-Hg numbers are just proxies for the P' and P'' pressures respectively.

IMO, It's the P' and P'' values that are much more valuable than the 0 or 4 In-Hg numbers.

If you measure a given MPS in 1 In-Hg increments with a given (pretty much any) meter, and plot the results, you can determine P' and P'' from the corners on the curve (which should look something like "Chart A" in my OP).



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Not_A_Six
post Aug 16 2020, 12:49 PM
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QUOTE(BeatNavy @ Aug 16 2020, 05:19 AM) *

Just for fun I'll throw in some of my data and "analysis" from a few years ago. This was when I was trying to tune a 2056 with a stock cam and later a 2056 with a Raby 9950 (or 9550, whichever it is). I think the latter is similar to a Webcam 73, or maybe even the same grind.

I ended up comparing the following for vacuum / inductance numbers for the 043 MPS, which should have been correct for my '74 D-Jet setup (and compatible with the 043 ECU):

1. Anders' numbers for MPS tuning on his website.
2. An 043 "Primary" that I rebuilt using Chris Foley's kit. This was the one I fiddled with the most in terms of tuning and driving.
3. 043 "Spare." This one was also rebuilt, and I used this kind of as my fallback if I screwed up the primary too badly. I also used it to compare how the car ran with the other one.
4. 043 "Reman." This one was rebuilt and tuned years ago by that company that used to perform this service years ago ("Fuel Injection Company" or whatever it is/was). I did not mess with this one.
5. 043 "Stock." Thanks to the yard sale at Jim McLeod's (RIP (IMG:style_emoticons/default/sad.gif) ) a couple of years ago I managed to acquire a working, unmolested 043. Used this primarily to baseline my inductance meter's numbers against those of Anders.

Here are the results in table form. Remember, I was primarily tuning 043 "Primary" and "Spare," so those numbers could be all over the place. I've got the slope in there, but the most relevant slope is that over the "linear region" which I think Anders defines as between 6 and 18 in Hg.
Attached Image

And here's the graph, such as it is.
Attached Image

Here are my lessons learned:

1. I spend a LOT of time on this, and I learned a lot, which was enjoyable. But I got more return out of that than any performance increase or optimization (IMG:style_emoticons/default/smile.gif)

2. Inductance meters seem to vary significantly. I'm not sure why this is (I know mine was a cheap one, not a Wavetek), but I know others have had similar findings. You can see that my stock 043 didn't even get close to Anders' numbers, which I found interesting. It also means you can't simply go by Anders' numbers if you are using those as a baseline.

3. Chasing AFR can be a challenge. Type 4 idle AFR can be very unreliable, and exhaust leaks and/or valve issues can really mess with the numbers. Different exhaust systems / bung location can also impact your values. I guess my point here is that trying to get to an exact AFR down to the tenth can be like chasing your tail. Sometimes it's more important to focus on what the car is telling you (how much that engine "likes" the tune in terms of performance, gas consumption, plug fouling, etc. etc.).

4. I took a tip from a member here (Frank, I believe), and hooked up a "T" vacuum line from my MPS to a vac gauge in the cockpit. That was very helpful for understanding the relationship between "part load," throttle position, and AFR. It gave me a better understanding of when I should be looking for what AFR value. EDIT: in that regard, I should have changed the labels in the table above where 4 in Hg is not "cruise" but "strong acceleration" and 6 in Hg "acceleration."

5. D-Jet has limitations (of course). It works like a champ for what it was intended to do, but of course you can only tease so much out of it either in terms of fine-tuning or performance enhancements. Unless you REALLY get creative or go off the deep end. I've recently moved on to Microsquirt where I can now be ignorant in that arena (IMG:style_emoticons/default/smile.gif)

Just my two cents for the OP who obviously has a much more rigorous scientific and analytical background than do I.

(IMG:style_emoticons/default/beerchug.gif)


BeatNavy, That's awesome data! Thanks.

Your "Anders" values look like they are for 1280 ft el (726 Torr). At what elevation did you measure the others? (I assume your values are raw -- uncorrected for elevation.)

"I spend a LOT of time on this, and I learned a lot, which was enjoyable. But I got more return out of that than any performance increase or optimization"

Yeah! No kidding. I've got way too much time in this so far, but I think it's been really worthwhile. The most valuable stuff for me was 1) Getting a wideband O2 meter instead of trying to tune it blind, and 2) Tangerine Racing's MPS adjustment kit which lets you easily tune the MPS in-car rather than taking it out and fiddling with it on a bench.

"I took a tip from a member here (Frank, I believe), and hooked up a "T" vacuum line from my MPS to a vac gauge in the cockpit."

I've been doing the same thing. Tuning would be much simpler if I had data logging and a couple hours of dyno time. As it is, I have an analog vac gauge and hand-held non-logging AFR meter in the car. I have a 5-mile loop with a nice hill in the middle. I drive a loop, glance at the numbers. Stop, write everything down. Make a small adjustment to...something. Lather, rinse, repeat. (IMG:style_emoticons/default/biggrin.gif)

"I've recently moved on to Microsquirt where I can now be ignorant in that arena"

When I first bought my car, the fact that it had the original D-Jet mostly intact and mostly working was part of its appeal. Had I to do it all over again, I think I would have gone with a Microsquirt as well when I had the engine rebuilt. (IMG:style_emoticons/default/unsure.gif)

Cheers. (IMG:style_emoticons/default/beerchug.gif)
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Not_A_Six
post Aug 17 2020, 04:31 PM
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*** UPDATE ***

I was bothered by the Inductance vs Vacuum graph in my OP (the one with the red and blue lines). I couldn't explain why the red and blue curves crossed at around 7 In-Hg. I also wanted to illustrate how to determine P' and P'' from the graphs. Finally, Olympic914 asked why the graph didn't go all the way to 0 In-Hg.

So, I took some more data. I'm at 2250 ft elevation (700 Torr), and its about 90F in the garage today. The red curve in the graph below shows my actual current MPS tune. The blue curve shows the effect of turning the inner MPS screw (in isolation) one full turn CW. The blue curve is just an experiment to see what would happen to the MPS response -- it doesn't represent any kind of useful tune. I also graphed the data all the way down to 0 In-Hg to show the WOT part of the response.

Attached Image

At first glance, it appears that turning the inner screw CW just lowers the whole curve. But, if you "raise" the blue curve by adding an offset so it starts at .750H at 15 In-Hg like the red curve, the difference in slope becomes clearer:

Attached Image

P', where the MPS diaphram begins to lift off of the part-load stop, and P'', where the diaphram hits the full-load stop aren't readily visible with this particular tune because their effects on the slope are too small to see here.

As to the crossing traces in the OP, data for that graph was taken on different days with different tunes. So, maybe the effect of the outer screw and/or plug affected the values. Maybe it was a difference in temperature or barometric pressure on those days. Maybe it was a measurement error. I dunno. (IMG:style_emoticons/default/confused24.gif)

Finally, the graph in my OP appeared to show that turning the inner screw CW would make the slope of the response curve steeper and thus:

Increase Fuel Pressure + Turn Inner Screw CW => Enrich 6-8 In-Hg region
Decrease Fuel Pressure + Turn Inner Screw CCW = Lean out 6-8 In-Hg region

Today's data shows exactly the reverse -- turning the inner screw CW appears to reduce the slope of the response curve. Hence:

Increase Fuel Pressure + Turn Inner Screw CW => Lean out 6-8 In-Hg region
Decrease Fuel Pressure + Turn Inner Screw CCW = Enrichen out 6-8 In-Hg region

So, I dunno. I may reduce my fuel pressure back to 29 psi and retune the MPS to see the actual in-car driveability effects. If anybody else wants to post data from their own MPS's, that might be useful.


Cheers. (IMG:style_emoticons/default/beerchug.gif)
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Bleyseng
post Aug 17 2020, 04:51 PM
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My readings for a 037 MPS after testing several know good ones were:
0hg-1.52
4hg-1.26
15hg-.74

yes, Seattle is at sea level..

and using a O2 setup get the WOT setting richer than 12.5 at WOT to start as it leans out as you run up the rpms and you don't want to be at 14-15 to 1 at 5500rpms
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Not_A_Six
post Aug 17 2020, 05:04 PM
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QUOTE(Bleyseng @ Aug 17 2020, 03:51 PM) *

My readings for a 037 MPS after testing several know good ones were:
0hg-1.52
4hg-1.26
15hg-.74

yes, Seattle is at sea level..

and using a O2 setup get the WOT setting richer than 12.5 AFR at WOT to start as it leans out as you run up the rpms and you don't want to be at 14-15 to 1 at 5500rpms


Thanks for the data.

Correcting (straight-line) your numbers for my elevation (700 Torr) gives:

0hg-1.40
4hg-1.16
15hg-.68

FWIW, The 0 and 15 In-Hg values are pretty close to pbanders's. It looks like your MPS outer screw is turned more CW than pbanders's as your 4 In-Hg value is a bit lower than his. So, your P' threshold is correspondingly lower (closer to 0 In-Hg).

I have exactly that WOT problem at the moment. I can't maintain 12.5 AFR across the full rpm range, and I'm seeing it spike lean at around 4600 rpm.

But, I just ripped into the ECU and changed the Wfm4 voltage divider in the Speed Compensation sub-circuit to change the VE curve for my displacement/cam. (IMG:style_emoticons/default/blink.gif)

I haven't finished tuning/testing the car with the ECU change yet. I'll start a thread here when I have some results.

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Olympic 914
post Aug 17 2020, 05:57 PM
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The engine configuration I am running basically mirrors Jake Rabys 2056-120 motor

2056 D-jet (of course)
KB pistons 8.6-1 comp
Raby 9590 cam
Ham RS+ heads 42x36 valves (I think)
037 ECU (stock for the ’73)
043 MPS w/stock spacer – rebuilt with tangerine kit. Initial settings to 037 MPS and adjusted from there.
No Ballast resistor
SS HE’s and a Triad muffler,
Auto Meter Wideband AFR. The 02 sensor is in the muffler
Dakota Digital CHT and oil temp
My altitude is 1000 ft. so 760/733 = 1.0368 sea level correction

Tested with a Amprobe LCR55a and a Mityvac vacuum pump.

Now my MPS settings are much richer than the OP’s and Robs, I am NOT running a ballast resistor.
Robs set-up with the 043/044 ECU doesn’t require one and “Not a Six” has one installed. As I understand this resistor (and any resistor under 300 Ohms) richens the mixture but only during the warm-up phase.
The fuel pressure is around 30, not as high as the OP’s. But I will have to check and report exactly what it is since it is an important component.

Recently my MPS developed a leak and I have ordered a new diaphragm kit from Chris.

I ran this MPS (let’s call it “B”) for three years and about 8K miles. I felt it was a little lean, during a steady highway cruise I would see 13.8 – 14.7 AFR
Settings were

0 – 1.51
4 -- 1.34
15 - 0.85

Corrected to sea level

0 -- 1.56
4 -- 1.39
15 - 0.86

So these settings SEEM that they would be way to rich but were not.



I then installed one of my spare MPS units (“C”) and it did come up as too rich across the range.
Settings on “C” were

0 -- 1.51
4 -- 1.38
15 - 0.86

Corrected

0 -- 1.56
4 -- 1.43
15 - 0 89

So I retuned my third MPS “A” and installed it. Unfortunately at this time my Wide band AFR quit so I don’t have any real numbers for it. But it does FEEL better.
Settings on MPS “A” are

0 -- 1.50
4 -- 1.35
15 - 0.83

Corrected

0 -- 1.55
4 -- 1.40
15 - 0.86


When I set the full load stop on the first MPS “B” and Later on “A” it was adjusted it at 4 points from the fully released position. Possibly this contributed to the diaphragm failure of unit “B”.
I may have to look at adjusting it further in.
Also again I will have to check the fuel pressure.
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