Barometric fuelling and timing correction

[ol boy]

The MAP sensor doesn't measure the engine load. It measure the difference of pressure before and after the TPS.

Huh? Is this correct? If so, how is the pressure measured on both sides of the Throttle Position Sensor? I thought I understood Manifold Absolute Pressure, but apparently I don't!

Did I just misunderstand this post? Maybe I shouldn't have sold that QuadraJet!

Ken

He is mistaking the difference between an absolute and differential(gauge) pressure sensor. A differential has a vent to atmosphere which would give the results described (ie. pre/post TPS), The MPXA4250 MAP sensor most of us are using has a sealed internal reference pressure which allows a reading of absolute pressure. We add in the baro sensor to measure the "vented" pressure then calculate the differential pressure (%baro).

Hope your truly confused now!

Later Ryan

[jsmcortina]

I merged all of these discussions together as the previous spread across multiple topics verged on spamming.

James

[Alvarodrpower]

Hi Philip, I am from Ecuador southamerica, I live in Quito, at 3000 meter over the sea, I see your post, but I dont see your email data, I see your web but the contact information is empty, can you help me whit your firmware with barotric correction and ignition retard? my email is alvaro.espinosa@hotmail.com
thanks for your attention

[Philip Lochner]

can you help me whit your firmware with barotric correction and ignition retard?

Hi Alvaro

WOW!! 3000m!!! That is serious altitude!

Are you aware that the standard code does barometric fuelling correction?
Barometric timing correction is more important when you have a high compression naturally aspirated engine. With a low compression engine or force fed engine, barometric timing correction is less of an issue.

I would encourage you to stick with the standard code even though it does not offer barometric timing correction. You can use table switching (with a manual switch) to switch between two sets of advance maps (not ideal I know) although I still wish for continuous barometric timing correction, but I've given up hope on this feature.

You are most welcome to use the custom code (which I will email to you). The code I have is based on MSEXTRA 3.2.1, but I find that I get false acceleration triggers. I believe this issue was solved with 3.3 onwards. There is nothing in the TPS or TPSdot logs that justify AE triggers and yet it triggers away regularly, more so at larger throttle openings. This can be a bother at idle when it intermittently and very briefly overfuels the engine (rpm dips). Other than that, the code works well.

[TheSilverBuick]

The code effectively does let you correct timing for barometric changes. I run %Baro on the fuel and standard speed density on the spark table, and the way my table is programmed it adds timing with lower barometric pressures and pulls timing in denser air. I live and work a bit over 2,000m and drive down to sea level several times a year and a few times driven upwards 3,400m (64Kpa atmospheric pressure!).

Since the code change to make the rate and %vacuum zero and zero, my baro correction table has become very linear where before it was a cubic curve. There are now limits on how much baro-correction is allowed on the table, and I ran into this problem and had to adjust my required fuel to move the center of the table around to maximize my ability to tune the baro table. I have zero correction at 80kPa atmospheric (normal around here). I do it simply, tune around the house, then as I drive off the mountain it leans out, I move the dot upwards until the AFR is back where it belongs. I have a dot about every 5 kPa, though now that it's pretty linear the resolution is less important.

[Zaphod]

The code effectively does let you correct timing for barometric changes. I run %Baro on the fuel and standard speed density on the spark table, and the way my table is programmed it adds timing with lower barometric pressures and pulls timing in denser air. I live and work a bit over 2,000m and drive down to sea level several times a year and a few times driven upwards 3,400m (64Kpa atmospheric pressure!).

Since the code change to make the rate and %vacuum zero and zero, my baro correction table has become very linear where before it was a cubic curve. There are now limits on how much baro-correction is allowed on the table, and I ran into this problem and had to adjust my required fuel to move the center of the table around to maximize my ability to tune the baro table. I have zero correction at 80kPa atmospheric (normal around here). I do it simply, tune around the house, then as I drive off the mountain it leans out, I move the dot upwards until the AFR is back where it belongs. I have a dot about every 5 kPa, though now that it's pretty linear the resolution is less important.

Would be interested to see how much you correct for those 5 kPa steps...?!

[TheSilverBuick]

There are dozens of factors that can change the number, engine size, exhaust size, naturally aspirated versus boosted. I imagine injector size could have an effect.

Hmm, never looked at the numbers, as the line looks linear, but the numbers slightly increase as the baro increases. Calibrated on a 250cid L6 Pontiac engine with 2.5" exhaust. MS3, sequential, COP, 32lb/hr injectors.
75kPa to 80kPa up 4%
80kPa to 85kPa up 5%
85kPa to 90kPa up 6%
90kPa to 95kPa up 7%
95kPa to 100kPa up 7.5% (I bet it could be 8%)
100kPa to 105kPa up 8.3% (Not really tuned, and that 8.3% maxes my baro correction out at 120%)

*Edit* This is a picture of the first pass of tuning I did on my Firebird. Started at 75Kp and went up from there, so haven't tuned anything below 75kPa.