Map output offset

[golgothapro]

I recently upgraded my 16 bit MS (green board) that I've been running since about 02' or 03' with the new 32 bit MS II daughter-board to rid myself of centrifugal advance mechanism issues. I imported my VE table because it worked well enough but; I've also stepped up to an Autometer WB and the IAC option as well as the TSMS. I went through all the calibrations except for thermistors (because I was already using GM ones) but; my map readout is way off. I run multiport with High Z 85lb Accels @ like 23psi (so I tell the required fuel they're 43lb). For the .60 over 1st gen smblk chevy that's like 10ms on the required fuel but; I went to 4 squirts to move the duty cycle from 6 ms down to 4 ms. I found a page for calculating 0 and 5v kPa values for the MPX4250 and calculated 9.13 and 266.41 kPa before I found the 9.3 and 260.9 values I inevitably found in the config manual. I've tried with both values but; no significant change. I'm idle-ing at around 800 RPM but; my Map is showing 70KPa. My present VE map only only goes up to 50Kpa. I was going to change that anyway (cause I have an electric blower system I'm rebuilding) but; my normal vacuum at idle with the cam I'm running is 8"Hg which is about 27KPa (I think). Looking at the old program I can see It was certainly never over like 30-35KPa at idle. I've cross checked for vacuum leaks and the values with a hand vacuum pump equipped with a gauge. No mistake; the output is definitely showing higher than what it actually is. I couldn't find anything about defeating the baro sensor if added or not. Should I have put zero values in for those parameters? What's more is this AFR table. I find lots of stuff about tuning it but; nothing about what it's for. I didn't use to have one. I'm conjecturing it's used with the WB to make a new VE but: how are you supposed to know what AFR is right for your application? You make it from your present VE and then turn around and use it to make a new VE? Oh yea, I checked with engine off and I was seeing about 100KPa. Think my MPX4250 is trashed? I don't think I did anything to it adding the IAC or ign trigger jumpers but; I guess anything is possible. Any constructive suggestions will be appreciated.

[Matt Cramer]

Is the sensor in fact an MPX4250? Any chance you've got a 1 bar in there?

[golgothapro]

Just took the ECM out of the case and looked. The MAP sensor does have MPX4250 printed on it but; with the suffix letters AP. I don't see any physical damage, the 6 solder joints look OK, though I have often wondered if the measly 3lbs of boost I was getting before could have done something to it. I should get more boost than my tranny will handle this time so I was thinking about just replacing it with that 4 bar with the baro from DIY even though I really don't expect to run more than about 8 lbs. Is the code version that which the ini. is named for? I'm using 2.920 if that's the case.

[DonTZ125]

According to the data sheet, assuming a supply voltage of 5.1v, the 0-250kPaA span is represented by 0.204-4.909v. In other words, the full-span voltage is 4.705v with an offset of 0.204v.

[golgothapro]

I'm a victim of my own ignorance. This evening I charted the voltages at 10KPa increments on the bench (picked off R2), compared them with calculated values and was only about 100 to 170mv off in each case. Then I searched and found a chart that equated "Hg vacuum to kPa. I saw then that I've been wrong all along. Just because the dot hovered over 25 to 30 kPa when the engine idled at 8" vacuum with my previous setup doesn't mean that it was logical to assume the two values were equivalent because they're not. According to the chart 8" Hg of vacuum IS about 70 kPa. There's nothing wrong with the program or the sensor. I just need to generate a totally new map for my 357 and give it some head-room for boost. What I didn't know was that inches of Hg pressure is not the same as inches of Hg vacuum and that just served to draw me deeper into the maze. Sorry for the confusion but; if I hadn't posted I probably would have never figured it out so thanks just the same.

[DonTZ125]

You're mixing gauge and absolute units. At standard temperature and pressure, 0 kPaG = 101.3 kPaA = 0 inHg. 8 inHg(v) = -27 kPaG = 74 kPaA. The MPX4250AP used by the MS family reads Absolute pressure. This is done to simplify the math; with all else being equal, a cylinder fill at a MAP of 80 kPa has 20% less oxygen than the same intake at 100 kPa. Trying to do that in Gauge units would just get ugly ...

[golgothapro]

Oh it's already ugly. I stayed up all night making a new map but; I've yet to try it. Basically I just made new pressure intervals with the 12 wide interpolated version of my original 8 columns at the bottom, used the the remaining top 4 rows for boost figures, and made sure the 70 kPa A fell where my idle use to be. By my calculations 200 kPa (which is my highest) should cover up to 15 psi G boost which I think is more than I'll ever use. The old program gave me a fit because the VE table was inverted in ref. to the tuning map and now this one is outputting in kPa A rather than kPa G? Who wouldn't get confused? Thanks for the education cause I sure as heck needed it.