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Old 03-05-2008, 12:26 PM   #1
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Beating the Baro... MAP reading trickery?

Hi folks,

An interesting little nugget popped up the other day while I was looking for something else and I've been tossing it round in the noodle since. ECUs that use a MAP sensor generally take the reference barometric reading as soon as you switch on the ignition. This tells it whether the air is denser or thinner than normal and it adjusts fuel trim leaner or richer accordingly. It also will re-callibrate to atmospheric when you go wide open throttle (WOT) because plenum pressure should be atmospheric at that point.

Anyway, that got me thinking that if it sees a lower pressure at ignition on, then it's gonna run leaner until it re-callibrates (and it may do that while running according to O2 feedback, but we may as well start off with an "edge" right?) So, my usual routine at starting up would be to turn the ignition on, check the idiot lights, then start the motor. This gives it 5 seconds to see what the real pressure is... we can't have that can we? muhuhahahahaaaaa

So, I've been putting the key in and twisting it to start immediately, figuring that if the ECU is slow getting it's reading, the motor will be pulling some vacuum before it gets it, leading to a lower pressure reading, leading to a leaner initial fuel trim when it goes into closed loop.

I don't know if this is going to make THAT much of a difference, and I probably won't be able to tell at the moment how it's doing, but it kinda feels like a good thing to be trying.

Also been wondering if one could put a check valve on the MAP vacuum line, that you can work from the cabin, just before you shut off, you close it... leaving the MAP line in vacuum, then when you start up, you leave it closed until the engine fires, then open it up again... I don't know if that would be too extreme a value though and the ECU would mark MAP bad and use defaults, because it would probably be like you were flying at 30,000 feet.

I'm thinking this might be something to bear in mind for those of you that do engine off coasting, with a MAP based system, if you turn the ignition back on immediately it might retain the MAP value, also if you pop the clutch to restart the motor after you turn the ignition on, it gets a real baro reading. If you pop the clutch and then turn on the ignition, it might read the vacuum as the baro value and run leaner (Or not at all if you're pulling real high vacuum at this point) Those with vacuum gauges might be able to figure an optimum feather point for the throttle on restart such that the ECU believes the baro value and is capable of running on it.

Those who are electronically minded might figure out a resistance bias to the MAP reading that is set when you turn the ignition on for the first time, then is unset by a relay kicking it out when you engage the starter.

Anyway, thought this would be something interesting to play with.

Road Warrior
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Old 03-05-2008, 04:04 PM   #2
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Doing a quick guesstimate, I figure that the baro compensation only has a range of about 5psi, or around 10 inches of vacuum. Since there's only about 2psi of variation due to weather and probably about 3 psi allowed for altitude. This would be a reading of about 3.3V or so on a 5V 1 bar MAP sensor (non turbo MAP) I don't know how much vacuum a motor will typically pull on the first turn of the starter, will have to hook my gauge up and see.

For the engine off coast application though on a manual, with a warm engine, 10 inches might be quite a large throttle angle and it might not be worth the brief spike of fuel as it starts.

Edit: actually I notice in doing this today that on a start with the motor warmish, it does seem to have more difficulty starting, probably it's pulling more vacuum when warm.
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Old 03-05-2008, 04:37 PM   #3
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Quote:
Originally Posted by RoadWarrior View Post
Since there's only about 2psi of variation due to weather and probably about 3 psi allowed for altitude.
If this is true, then it sounds like the ECU is able to compensate for no more than about 7000-8000ft of altitude. The problem is that, while there are MANY parts of the country where you can't even drive half this high, there are MANY other places where people routinely drive this high (and higher). I would therefore think that the ECU can compensate for more than this. After all, it's not that hard to do.
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Old 03-05-2008, 05:05 PM   #4
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Ahhh my guess is off I see....


So, figuring 15,000 feet is about 8 psi and 2 psi for weather gives us about 6 psi minimum or around 18 inches of vacuum. That's quite low, some vehicles idle at about 18 inches (mine's about 22). Hmmm so that gives us about 2V on a 5V 1bar MAP.
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Old 03-06-2008, 02:43 AM   #5
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California has some high places that can be driven to as well. Here is a picture of my CRX at an altitude of about 12000ft in the White Mountains. This dirt road actually goes all the way to the 14246ft summit of White Mountain Peak (the triangular peak just to the right of center in the background). But there is a locked gate just a few miles from this point. You basically have to hoof it the rest of the way to the summit. But twice a year, you can drive two additional miles up to Barcroft Labs, which isat about 12500ft. This significantly cuts down on the length of the hike.

Anyway, the CRX felt perfectly healthy in every way possible at this altitude. This tells me that the ECU has no problems compensating for this altitude. The ONLY problem I had on this drive was a dent in the header from a rock on the road. I guess this road was never made with CRXs with B-series swaps and 4-1 headers in mind. Oh well, at least the header was not one of those expensive long-tube deals.

Anyway, could I possibly hold the world record for the highest CRX B18C5 swap?
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