MSD MAP Sensor readings

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Map63Vette

Map63Vette

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Was wondering if other people out there using the MSD Hemi 6 could give me a hand with my MAP sensor readings. I've had three different sensors hooked up to the car and the readings just don't seem right. The original sensor that came with the engine only read around 12.x psi pulled out of the intake with the engine not running (to assure it was measuring ambient), but I figured it might have been bad since the old engine dropped an intake valve and may have been pressurizing the intake some. I borrowed a sensor from my parent's 5.7 Ram and it only read around 13.x. I bought a brand new one since my parents wanted theirs back (go figure), but it only reads in the low to mid 13's as well. I was pretty sure ambient was supposed to be in the 14's (I live in Kansas for what it's worth), but I've never seen my sensor read that high. What are other people seeing out of their sensors? Wondering if I have a wonky MSD box or wiring.
 
I don't have that part, but my day job deals with aerospace instrumentation. What does ".x" mean? Does one sensor read 12.9 psia and the other 13.1 psia? If so, that sounds comparable. Since you haven't changed the wiring, that variability is probably inherent in the sensors.

To determine ambient pressure at your location, go to a weather site and find the average barometric pressure. To convert to psia, get freeware program "convert.exe". Where I work ~200 ft elevation, it averages ~14.59 psia.

My guess is that a 1 psi error in a MAP sensor is not a big deal. It is used to calculate a "predicted fuel flow", as a "feed-forward" correction. The O2 sensor is constantly fine-tuning the fuel flow as a "feedback control" to perfect O/F at steady-state. Therefore, the MAP sensor only affects things during transients (acceleration), similar to the extra shot an accelerator pump gives in a carburetor. It doesn't have to be perfect. Indeed, some engine controllers may have a learning algorithm that tweaks the transient correction so it becomes perfect over time, and thus corrects for any errors in the MAP sensor, and more importantly differences in the displacement, cam, and valves.

If anybody has researched modern engine controls more, please chime in. It is an interesting subject and where I want to go with my Dart.
 
If you change the sensor type in the MSD program does it change the reading? I will fire mine up and see what it's reading.
 
The .x is more just filler because I don't remember the exact reading. I think the original sensor was reading around 12.5 if I recall and the borrowed one and new one were both closer to 13.4 or something like that I believe. I was thinking that ambient here was supposed to be closer to 14.5, so being 1 psi off on a guage that doesn't have a big range seemed a little imprecise to me. The MSD just uses the MAP as a sort of electronic version of a vacuum advance (I'm not running fuel injection yet), so it's not too big a deal since like you said the difference in readings is really what matters. I was just wondering if the odd reading might be due to a wiring fault or bad box because even though I thought I had a tuning issue sorted out it keeps coming back. Tried unplugging the MAP sensor and that seemed to fix it, but when I zeroed the MAP advance curve (so it basically ignored the sensor, but I could keep it plugged in to keep from getting junk in the connector), the problem is still there.

Tried unplugging it again today though and it didn't make a huge difference. I really need a dyno or something so I can make a change on the spot and test it as opposed to driving it around, changing something, and taking it out later. I don't think I'm ever getting consistent tests. One time the engine will full out cut on a full throttle hit (which I interpret as way too lean of a pump shot), and other times it will just bog down and work through it (which I assume means too rich). I've tried every hole on the accelerator pump multiple times with no vacuum advance and some times the car works perfectly and takes off like a rocket, but the next time I take it out it acts just like it did before I changed it.
 
I'll have to check trying to change the sensor, never thought of that. I did check the wiring with a voltmeter today and the ground is showing good and it's getting the 5v source power it needs, but I don't really have a good way to check the signal line aside from unplugging the connector at the MSD box and checking continuity on it. The reading seems steady when I plug a laptop in and watch it while driving around, so I don't think it's a flakey signal, but I'm just trying to rule out whatever I can at this point. I've thought I fixed this problem 3 times now, but it never goes away. It's just really annoying having a car that should really perform when you step on it and having it fall on it's face a fair chunk of the time. Hard to impress people when you give them a ride and can't hammer down, lol.
 
I've been using the stock O2's that came with my engine for now. I know that they're narrowband, but it's been giving me a ballpark on what the engine is doing with a voltmeter hooked up. What sort of reading should I see on an A/F gauge right on a full throttle hit? It seems like regardless of where I have the accelerator pump my A/F is always dropping all the way to 0 before coming back up. It doesn't drop for long, but it still does. Was going to check my step-up springs in the Carter carb too. From what the MSD tells me, the car is pulling 20 inches of vacuum at idle and cruise, and the strongest springs that I can find for it are only rated for 16 inches. Regardless, the rods should be pulling up at full throttle regardless of the spring rate because it goes to near ambient, but I wonder if it's not cruising or transitioning well, so it works well one time, but not the other.
 
Map63Vette,

Thanks for explaining the MSD MAP sensor function. I doubt spark advance has anything to do with it stumbling or dying on acceleration since "vacuum advance" is usually pretty small and many racers don't even use it.

I have a Holley 2D Projection on my Newport 383, plus a Holley "Rich/Lean Indicator" box tied to the same "narrow-band" O2 sensor. Not sure why they didn't include a rich LED in the 2D box like in the older add-on O2 controller for the analog box. You do similar by connecting a multi-meter to your O2 sensor. Might be slower to read than an LED unless yours has an analog bar, but sounds good enough since you see the voltage dip near zero. If anyone uses a voltmeter with a needle, make sure it is an amplified type that gets power from +12 V. The older gages with low input resistance would damage the O2 sensor (at least the earliest ones).

My Projection is so erratic that I have much experience with how lean or rich feels. As you say, lean makes the engine feel like it is starved. As you press the pedal, it stumbles. If I turn the Projection knobs it makes a big difference. Just a slight turn and the engine will go from stumbling to pulling hard. If even more lean, when you press the pedal it will give a slight pop and die. Even at that setting, if I pulse the pedal it doesn't die because the "accel pump" function kicks in. BTW, my Lean/Rich box has 3 lights (red = rich, green = OK, yellow = lean). I have only seen the lean light a few times, even when it is stumbling and almost dies.

As you say, when too rich it feels like the engine is bogging, but it doesn't tend to die as you press the accelerator. I don't see any black smoke, but maybe people behind me do. Long ago when I had the original Rochester 2-barrel on the Newport, it once stranded me at a motel when the plugs became fouled with a thin coat of soot. That is what too rich will do. Now I want an O2 sensor on all my cars because I hate "flying blind".
 
I actually built my own AFR meter as well as using the voltmeter. I have a little array of 7 LEDs that I can tune to come on at whatever voltage I want based on how I program them. It's based off an SX microcontroller and uses an A to D chip to translate the analog voltage of the O2 sensor to a number that the SX can interpret. I don't know how varied response curves for narrowband sensors are, but the latest graph I've been using for reference has 0.1 to 0.9v as between 15 and about 14.4 AFR (here's the website if you want to see the graph http://www.ztechz.net/id12.html). I have my LED array set up with 2 rich LEDS, 3 middle, and 2 lean. The lean are set to turn on at .05 or so for the first one and 0.1 for the second (15:1 AFR). The rich ones come on around 0.9v for the first and 0.95v (14:1 AFR) for the second. The middle ones are just spaces evenly along the slope of the middle range.

With the limited tuning parts I have for the carb so far, a .95 main jet with my 71x47 rod is quite lean on cruise (voltmeter says .05, so I don't know just how lean), but with a .98 jet I'm cruising around .75v. The weird thing is sitting in the driveway at temperature, if you slam the pedal from idle with the 98 jet it will bog and then take off, but the smaller you make the pump shot the quicker it will recover. When I stepped down to the 95 jet it still bogs some, but with the smallest pump shot it's just about there. I just find it hard to believe the engine wants to be that lean on a full throttle hit, and if I step down the jets more since it seems to like that it will be WAY lean on cruise. The top end is suffering from the smaller jets too. WOT is only hitting maybe .75v on the meter, nowhere near the 13:1 AFR I'm shooting for, though I can make up for that some with a rod change. Just don't want to throw a bunch of money into carb tuning parts if I'm not going to be able to get where I need/want to be.
 
My Map sensor reads 13.7 before I start it and 6.5 to 7.0 at idle. Not sure if that's normal but that's what mine is reading.
 
Hmm, the 13.7 makes me feel a little better. Checked mine again today while I was messing with the tune and it was 13.2 or 13.4. When running I'm closer to 4-4.5, but I have a stock cam and I know yours is pretty big. Think I may have figured out some of the problem though. Just disconnected the secondaries on my carb and that seemed to help a fair deal. The throttle response was pretty good and I could dial it in nicely with the different pump settings. Got to thinking that the secondaries might be way too rich since I was trying to compensate for the smaller front jets on full throttle. Put the carb back to the stock Carter jetting and the response seems to be good in first gear though it's still a little shakey in the higher gears and the top end power is still a little short. May see if I can find a good Carter kit that has several rods and see what I can do with that. It's a weighted vacuum secondary flap though, so it's not exactly easy to tune when it comes on.
 
Sounds like you're getting close to having a good tune. I have a feeling mine is going end up being in the carb tune. I'm getting an air/fuel ratio gauge put in so I can tune off of that.
Average absolute pressure at sea level should be 14.7 and the higher you get the lower your absolute pressure will be, plus barometric pressure will also change that, so it sounds like your MAP is reading correctly. I found a chart that showed manifold absolute pressure vs vacuum Hg. 14.7 MAP equals 0 Hg vacuum. 7.0 MAP equals 13Hg vacuum, thats about where mine is at. Your MAP is reading 4 to 4.5 which means you are pulling about 16 Hg which should be about right because a stock cam will create more vacuum.
 
There's still just something frustrating about the tune on mine. It's definitely wanting more fuel on the top end, but it feels like it bogs so hard on a hit that I'm choking it. However, when I look at my O2 sensor output on a full throttle hit it doesn't light a single LED, so the O2 sensor says that it's starving for fuel, but when I kick up the pump shot it only seems to make it worse. It drives around great without the secondaries, so I feel like the issue must be in there, but there's really nothing I can tune since it's a weighted flapper door an not the later spring type. I really wish I knew someone around here with a 650 carb I could borrow to just try something completely different.
 
It seems to me your way to lean my wideband O2 on my ram would read an air/fuel of 14.7 at part throttle and 12.2 at wide open. You say your in the 14-15 range seems really lean. I would say get a wideband, and shoot for the numbers I posted based off what you see on the wideband. I have a feeling you will have much better luck.

On the MAP sensor, if your seeing a constant change that correlates to what the engine is doing I would safely say that it's functioning correctly. Like you saidthe actual reading is not as important as the change is. You will have the calibrate the
Box to the sensor based off of the readings your getting.
 
Map63Vette said:
...I really wish I knew someone around here with a 650 carb I could borrow to just try something completely different.
Click to expand...

*raises hand*
let me know if you get to that point. It's a holley vac. secondaries. May need rebuilt though.
 
I think I've boiled it down to the secondaries being the problem. I think the flapper door is opening too soon or the secondary jets are just way too small, because when the secondary linkage is disconnected there are no issues on a full throttle stab, but when I hook them back up it bogs hard. I think it feels like it's too rich, but it might be that it's just so lean that the engine can't pick up. I can't make the pump shot any bigger, and I'm not sure that the transition ports on the secondaries can put out enough fuel to keep up until the back boosters start working. I'm guessing a double pumper might be able to make up the difference, but I don't have one sitting around.

I'm seeing decent AFR on cruise and part throttle (maybe 15:1 cruise and 14.5:1 on part throttle), but it's nowhere close to the 12-13 I was shooting for on full throttle. I figured the narrowband should read lean on cruise, in the middle on part throttle, and rich on hard throttle as a qualitative measurement at least, and the thing will just barely starts flickering my first rich light (which should be around 14:1 or so) toward the last few hundred rpm before the rev limiter when I assume it should pretty much peg the rich side all the way through the rpm range with my foot all the way down.
 
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