[Mykk]

For your entertainment purposes only. Implement & experiment with this idea on your own build if you'd like, undesired results are on you.



Living at 5500ft elevation with an atmosphere that reads 84kpa on the MAP sensor , the nearest city to the south has an atmosphere of 98kpa and the city to the north has 78kpa atmosphere.

I had an idea to use a MGP (Manifold Gauge Pressure) sensor instead of a MAP (Manifold Absolute Pressure) sensor that way, as far as the EFI is concerned, 100kpa is atmosphere no matter the elevation. Allowing the tune to utilize full resolution of the tables and have access to all the same cells of the tables no matter of elevation.

I fully respect that engineers far more educated and smarter then I universally use Absolute Vacuum in all automotive platforms for a reason. I still wanted to experiment with PSIg on my own:

*note: this experiment can only work if the board has a separate Baro sensor, the Baro fuel table still needs to be fully functional*

First most, find the sensor to use. If my application were N/A, in the line of board mount MPXx----DP Differential Pressure sensors there is one that can read -100kpa to 0kpa. By leaving one of the two ports open to atmosphere it would act as a 1bar MAP sensor referencing the differential to atmosphere.

For my supercharged setup that makes 8-10psi over atmosphere I was looking for a -1Bar/+1Bar sensor. I couldn't find an MPX version of differential pressure sensors that went as low as -1bar and as high as +1bar while connected to the same differential pressure port on the sensor.

I found what I was looking for in industrial Pressure Transducers, specifically the DATAQ 200361 line of sensors. -1Bar/+1Bar, 5v powered, 0.5v-4.5v linear output, conventional 3 pin connector, 1/8"npt threaded port. <1ms sample rate. There is also a -1Bar/+2Bar version, and even one that goes up to 10,000psi... At the time of writing this the DATAQ 200361 sensors cost $80US with shipping. Comparable to a quality MAP sensor.

There is a $20US Amazon no brand version of the transducer that doesn't have a spec sheet, -14.5psi to +30psi (-1Bar/+2bar). Others who have used this transducer in non-automotive applications complain of an atrocious 5ms sample rate. Way to slow for automotive use.







Mind you this is in my hometown of 84kpa atmosphere:



Engine fired up and at idle:





I have yet to take my project out of this altitude to see how well my idea works in the real world in those 98kpa & 78kpa nearby cities... But so far here is my notable list of positives:

• Utilize full resolution and scale of table Y-axis under all atmospheric conditions.

• Consistent load and WOT cell targeting, Example: Target lambda desired .850 @ 100KPA for WOT and target .890 for 80kpa... that target won't change at altitude and otherwise reference the leaner cells at high altitude WOT. Same for ignition timing.

• EFI will match mechanical PSIg Vacuum/Boost gauges.

• Traditional tuning strategies, tuning articles & tuning suggestions online will now apply because now universally no matter where you are, 100kpa is atmosphere.

Here is a neat little perk between the two sensors, both datalogs are raw data with no smoothing:

Previously installed LS1 style 3bar MAP:



DATAQ 200361 Pressure Transducer:



I'll update as Pro's & Con's become evident along the way. Thanks for checking out my project.

[PSIG]

Hmm. Interesting, and I'm trying to establish your goals. I assume you are looking to avoid a lean 'cruise' Lambda target that isn't cruise, but rather WOT at higher density altitude? To confirm my brain is tracking yours (or where my holes are), I'll ask:
What is the functional difference or benefit of a -1 bar/+1 bar sensor at 2.5V atmo, and a 0-200kPa sensor at 2.5V?
Would a code offset to MAP 85kpa/Baro 85kPa on the tables do the same (scale to baro)?
Would testing with a hand pump (pressure and vac) to the sensor prove your function without the drive?

[Mykk]

MAP sensors are PSIA, referencing a fixed value for all engines no matter of operating conditions. PSIG would be referencing the current atmospheric conditions the engine is experiencing.

Yes, a code offset to artificially make MGP using existing MAP and BARO would do the same. LinkECU already does this.

[PSIG]

OK, got that. So, it is a proof-of-concept to use the gauge sensor. With the thousands of boards already equipped with various iterations of MAP and Baro — is the ultimate goal to alter code to scale to Baro range, so existing boards do effectively the same thing?

[Mykk]

... — is the ultimate goal to alter code to scale to Baro range, so existing boards do effectively the same thing?

That would be nice. If MGP is a feature to be added later on, even better so I (we) can use standard readily available over the counter MAP sensors instead of a mail ordered piece of specialty equipment.

Even tuner guru and spokesman Andre Simon likes the use of MGP:

[JHolland]

I don't see where the advantage is, if you have a baro then you are, effectively, calculating MGP anyway. In the quote you have posted it states 'MGP is the difference between baro and MAP". Also remember that your fuel pressure is, probably, referenced to manifold pressure to ensure that your fuel pressure remains x above manifold pressure.

[PSIG]

I can't speak for @Mykk's perspective, but mine is not about fuel control due to baro, but for tuning compliance. In my perspective, an engine at 2000 rpm and wide-open throttle at roughly sea level (±100 kPa), will see tuning in the VE Table and the AFR Table for rich power. However, when wide-open at 85kPa due high density altitude, the tuning is typically set for lean economy, and will not provide the expected AFR for the demand. It is stuck in a different mode. Conversely, an engine tuned at altitude would be delivering rich power AFRs at cruise when at lower density altitude.

Note for a different conversation - the Spark Table is OK for the current AFR, but would be incorrect for an offset, compensated or MGP table; e.g., timing is different for 100 or 85 kPa at the same AFR.

Anyway, this is why I asked about baro table offset, in order to retain the intended fueling scheme under lower barometric pressure (or vice-versa). The artificial (but realistic) AFR Table below may help show this potential issue.

Partial AFR Table baro offset.png (49.99 KiB) Viewed 157 times