I bought a second MAP sensor to add to my MS3 so when I move to the mountains I'll be ready. Now I'm finding conflicting information on how exactly to install it.

Option one by Joe Perez:
Simply purchase a second 250kPa sensor ($28) and install it inside the case, upside-down and backwards across the pins of the first MAP sensor, with the signal pin bent up and wired to JS5.

Option two by richvrlimited and confirmed by Braineak:
Includes a couple extra components; this appears to be on MS1.

Obviously Joe's solution is easier, but will it work? Most of the info I find searching is about some 4bar sensor, or talks about how baro correction doesn't work well on MS1.

 

Nobody knows, huh?

 

You're asking if the solution I posted will work? Of course it works. Why wouldn't it?


What you've read about the 4bar sensor is simply an artifact resulting from the release of the MapDaddy as an alternative to this kind of hack. The MapDaddy uses 4 bar sensors of a different design, primarily because two of them are small enough to fit upright into the space consumed by a single MPX4250.

You have to use the same type of sensor for both main MAP and baro, but aside from that, you can use any two of the same sensor you want, regardless of their scaling.

 

ms3 can use different size map sensors. but you still need two caps and a resistor.

 

Joe's solution still doesn't mention caps, but he's sure it will work, and Brain specifically says to use caps and a resistor. Unless I missread Joe's solution and he also wires that extra stuff in... I wonder if I have those components laying around. It's been a long time since I have opened my megasquirt, are the caps and resistor included in Joe's solution already because they're somewhere else on the board and I'm sharing them with the MAP sensor?

 

you need them else it wont work bro. Joe's solution is "our" solution, but he left out key ingredients for success.

and i disagree with what joe said about same same baro sensor to map.

 

Yeah, we're really talking about the same thing here.

The "sensors must be the same" applied to MS1 and 2. I hadn't noticed that you had an MS3.

The .1uf cap between +5 and GND can be omitted, since there's already one on the main sensor that you're bridging across. The other cap and resistor are a noise filter on the signal line, and that must be replicated on the second sensor.

 

Serious question, as it's been a while since I've read up on any of this:

Who cares about barometric correction in a MAP-based system? Seriously, what is it accomplishing?

 

Ok, thanks guys! That clears it all up for me.

Now that you mention it, I don't know why it's necessary either I just know I'll be driving up and down big mountains a lot in the future and they say it's a good thing to have.

 

Mostly, compensating for different levels of exhaust back pressure.

 

the Baro corrections are in real time, so it'll always be referencing MAP to what the atomspheric pressure is. That can make a big change in the amount of fueling youll need during large elevation/pressure shifts.

 

Going from the 5500 ft at my house up the fun mountain road to 10,500 ft. baro correction makes a big difference. However, I found that tuning out the barometric scaling table fixed all of my issues. I think the problem roots back to the issues with the physics equations being incorrect. So MS wants to pull way more fuel than it needs to with varying altitude. By adding back some fuel % based on initial baro readings I was able to fix these problems.

 

While I haven't tuned it. I'm sure using the Baro Corrections Table in MS3 is pretty easy to tune: just add/subtract % of correction at a giving Baro reading to keep the fueling equal.

Just make sure to start with a tuned fuel map.

 

In a system where air entering the engine is measured by volume, I can see this. It's why the '90-'93 Miatæ had a baro sensor inside the ECU.

But in a MAP-based system? A manifold pressure of 80 kPa is 80 kPa whether it's the result of the throttle being partially closed or the result of WOT slightly west of Denver. When we turbocharge our cars, we don't need to give the ECU a reference as to the ambient pressure inside the intake tract just before the throttle plate.


This I can maybe see. (Instinctively, I wouldn't have though exhaust backpressure to change radically with elevation. At least, not by a significant amount as compared to the normal magnitude of exhaust backpressure. Valid point, though.)

 

Correct me if I'm wrong, but a MS running MAP alone, with a baro reading taking on startup, running on a perfect tune, then drives 5000ft. in elevation, will have leaned out AFRs until the car is reset and the thinned out air can be metered. Because that reading is part of the fuel equation.

Maybe if you turn off corrections completely you would be correct. I'm not well versed in this. But I do like that exhaust backpressure part of it. Even still, with an initial reading you'll still want to tune the non-linear table to fine tune the amount of corrections, as I'm pretty sure the default algorithum is not ideal.

 

This is exactly what I found to be true.

 

This is the exact issue I've been fighting on my NA tune. When I climb altitude the car starts to run leaner and leaner. Once I cycle the ignition and restart the car everything other than idle will be rich. Something with the default algorithm is definitely off. I never take the time when I'm at the track altitude to mess with the non-linear table as I'm usually focused on the driving.