Why is our MAP sensor in the ECU box?
 

Hi again

Keeping with my theme of asking very technical questions... I want to know why our MAP sensor is in the ECU? I mentioned in a thread a while back buying a GM 4 bar MAP sensor to strap to the intake manifold and using the built-in one for live baro correction, and was told this was a very bad thing and to use the built-in MAP sensor for running the engine, and an external sensor for baro correction. I was watching this video from Injector Dynamics, because my Subaru seems to have the common 2800rpm hesitation linked to fuel pressure pulsations, and he mentioned that long vacuum lines can lead to pulsations in MAP readings as well.

It seems to me having a short vacuum line to a sensor in the engine bay would be better than running an 8ft vacuum line to the ECU. This raises the natural frequency to beyond the operating range (as mentioned in the video) and it seems to me that your signal would react more quickly (less volume to change, less distance for the pressure wave to travel). Thoughts?

You only have 5v resolution, you shove more bars into that same 5v you loose resolution. Why do you need 50+ psi of resolution?

Otherwise whoever told you otherwise is off base. Only reason it's in the case is its easy and it has no real harmful effect on the map sensor operation.

Sorry, 4bar was a bad example. So if I want baro correction, a 2bar* map sensor near the manifold would work just fine - and since I have to pick one to use for each, I may as well use the one closer to the engine.
As I recall it was Reverent who told me not to use an external sensor.

Anyone have experience with combo sensors such as this?
http://delphi.com/manufacturers/auto...soline/map-mat

What's easier:

running vacuum line to an ecu

or

buy an external map sensor, mounting an external map sensor, running 5v power to a map sensor, running a ground to a map sensor, running the map sensor signal wire the ECU, and running vacuum line to a map sensor.



the MS Labs MS3-Basic's internal MAP sensor is probably the exact same 2.5bar sensor that almost every MS kit install uses. If you need baro, you can easily add a second identical sensor right inside the ECU with minimal effort.

Similar to what I did here:

https://cimg0.ibsrv.net/gimg/www.mia...085a949699.png

Gotta do all that if you want barometric correction anyway. I do mountain driving, so I kind of want this. The question is then would you rather run a 8ft vacuum line or 8ft run of 3 wires.

You put the baro map sensor on the back side of the main one inside the MS case. I have ran 8+ feet of line for the last 9 years without any issue what so ever. I would never bother putting the map sensor inside the engine bay and running wires, a ton more work, for zero benefit.

This sounds like splitting hairs...

Is the baro correction going to be that big of difference up in the mountains whether it's inside an ecu case + car vs being out in the engine bay? Or mounted to the roof?

http://www.onewordsubstitution.com/w...r-300x200.jpg?

Bolting an external MAP sensor to the fender and running some wire seems easier than yanking the ECU, taking the board off, soldering in a new sensor, and putting it all back together... but maybe that's just me.

It would take all of, what, 10 minutes? Not that much work. Not sure about you, but it'd take me longer than that to solder another one inside the ECU.

You're gonna have to deal with noise on those wires, probably far worse than whatever lag the vacuum tubing might generate.

Not sure what you're arguing for now, though. Do whatever makes you happy, the end result is the same.

Good point by deezums, you would need shielding on the electrical wiring to not receive false signals.

Just mount it inside the ecu. It's not air tight, your vehicle isn't air tight... The amount of atmos pressure is far greater than the physical location of where you place the sensor.

How to Read a Compressor Map - Power & Performance News - Boost related but explains the atmospheric point.

http://www.powerperformancenews.com/...-846x1200.jpg?

http://hendrix2.uoregon.edu/~imamura...-Pressure.jpg?

101 kPa = 14.7 psi @ sea level

Thanks for the physics lesson (unnecessary, I've got a master's in mechanical engineering), but I'm not concerned about the difference in baro readings. The baro reading will be fine wherever you put it.
I wanted to use a sensor mounted in the engine for MAP, and the built-in sensor for barometric correction. I was told not to do this when I bought my ECU without being told why. The noise issue makes sense, but I'm not sure why it'd be any noisier than any other signal coming from the engine bay (crank, cam, IAT, etc). Maybe because crank and cam are impulse responses (easier to filter?) and IAT is a resistive load, not an analog 0-5v. Sounds like a solution for CAN.

Not arguing for/against anything really. Just trying to understand how this all works better.

I tend to think half empty :likecat: especially this early in the morning.

Other than needing a higher bar reading, I still fail to see the point. The above comment made about resolution within the reading I think would be something to consider.

I don't think you'll have much issue with noise if you route your MAP sensor wiring appropriately (avoid coils). I do agree with you that a manifold mounted MAP sensor with electrical wiring to the ECU would be superior to a long vacuum lead. However the marginal cost (additional wiring complexity) does not outweigh the marginal benefit for most users and they use the standard pcb mounted baro sensor. It's worked fine for almost every and "why fix what ain't broke" is the mentality.

I like your idea of using the pcb built in map for baro and wiring in an external map sensor and I might give that a try. Although I don't do much elevation change driving so I'd see little to no benefit. Make sure both your PCB baro and external MAP are calibrated to agree with each other at key on engine off.

it probably had more to do with it already being built in.

doing what you want would be easy. But I'm not sure you'd find any measurable benefit by moving the map sensor to the engine bay vs. the ecu (in terms of noise and/or signal latency).

My favorite route has a 4200ft elevation change from my house to the top... I actually power cycle the car about halfway up because my AFRs start to get wonky if I don't.
For our physics-loving friends, that's a difference of 2.1psi or 14.4kpa relative to sea-level at STP ;)

Looking at that MPX family of pressure sensor it looks like the MPX4100 would be the baro sensor you want. It's a 105kpa range so scaled appropriately to take full advantage of the 0-5V Analog input range.

http://www.netzmafia.de/skripten/har...en/MPX4100.pdf
MPX4100AP Freescale Semiconductor - NXP | Sensors, Transducers | DigiKey

It should be a plug and play replacement with the 2.5 bar sensor (just a new calibration in tunerstudio) but do your own research on pinout to confirm.

Now even I, in a thread talking about marginal benefits, can admit that a higher-resolution baro sensor is probably unnecessary :party:

better is better. an oem would never put a 2.5 bar baro sensor in a ECU or vehicle. Seems like you'd want the best so just showing you your options.

MAP is a much larger part of the fueling equation, at least for speed density. So, the lag factors for MAP, MAF, RPM, TPS, and AFR are all separate. MAT is shoved in with CLT, battery and baro. They don't do as much, and don't need to update fast because they really don't change that fast either. Digital AFR over CAN is cool, but anything more is probably a bit redundant since you'll have the same voltage offset problems with whatever CAN controller you come up with.

When I finally rewire my engine bay I'll be adding a GM map sensor, and I might even try and mount it directly on the manifold if it doesn't look too hard. I don't see any need to do it sooner, though.

One more point about the standard issue Megasquirt map sensors, they are designed to be in a case and not exposed to engine heat, vibration and the elements associated to being mounted in the engine bay. That being said, you could use any number of Honda factory map sensors that are intended to be mounted in the engine bay as long as you knew the curve of the sensor and its voltages.

I'm curious how you're routing your vacuum line to need 8ft of vacuum. Are you wrapping it around the whole car?
I use 3, maybe 4 feet tops.

The NA8's have the ECU mounted behind the passenger seat, there is almost 4 feet of line needed to get from the firewall to the ECU once inside the car.

Even still, after you're at the firewall, it's like 4-5" to the back of the mani. So 4' tops

I thought the standard vacuum line to use for the MS was the one on top, near the throttle body, not the one at the back of the manifold? That's the one I'm using, I've probably got 2 feet of vacuum line inside the engine bay so that it's routed cleanly and has enough slack.

FWIW, the 4 bar MAP sensor sold by DIYautotune is a chip on a board, it's designed to go inside the ECU case, not in the engine bay. You may be thinking of the GM 3 bar MAP sensor which is a weather-sealed standalone module. I have the 3 bar GM sensor (I'm running 230 kpa, so the 250 kpa built-in sensor doesn't have enough headroom), but I have it in the glovebox with the MS3. Putting it in the engine bay would be marginally superior, but not enough that I'm inclined to go to the effort of moving it.

So the short version of the answer is that the MAP sensor is in the ECU box because mounting it externally is not enough of a win to justify the increased cost, effort, or complexity.

--Ian

Its the one that is connected to the FPR (in NAs). the one near the throttle body is supposedly noisy.

yeah, they have a tendency to fail if they get moisture in the line and then freeze.

Ok, stupid question incoming, but I have yet to come across an answer to it and I been wondering about it since I learned about speed density control.

Why does a speed debsity system care what the outside pressure is? 70kpa in the intake manifold is 70kpa. I can't wrap my head around why a second sensor is needed to compensate for altitude. Although I know that it is fairly often needed. Am I just thinking about this too simply and missing something?

because the load has also changed between those two point, and thus the fueling requirements have changes as well. Your whole fuel map would shift if you tuned at a high altitude, and then again at a low one. The baro corrections are actually correcting for the changes in VE (exhaust pressure, throttle position, crankcase pressure, humidity, etc).

Ok, so the baro is correcting for all the changes cause by things other than the pressure in the intake manifold. That was whete my hang up was. My thing was "the air is x dense in the intake manifold, meaning the engine can take in x amount of oxygen each revolution needing y fuel".

But since the exhaust pressure and other things change as well, the VE of the engine changes so it isn't taking in x amount of oxygen for x density, it is taking in some other amount (z maybe) and needs the fuel to change. Yes?

yeah, cause density is known like you said and it can calculate for that, but other variables have changed that impact VE and you're correcting for that.

And those other variables are directly related to absolute pressure outside the car, so rather than measuring and correcting for those, it is easier to just measure the pressure.

So I assume thay the baro correction curve needed will vary car by car? Or maybe engine by engine?

It will vary with the exhaust setup, particularly if there's a turbo on the car.

--Ian

From the bottom of the plenum near the FPR, routed up along the fuel rail and then back through the firewall, along the passenger frame rail to the ECU behind the passenger seat. I'm using an EUDM flat top manifold, there aren't a lot of options unfortunately.


And this is the type of MAP sensor I was talking about: https://www.summitracing.com/parts/f...2KAaAqcd8P8HAQ