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MAP sensor upgrade


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#1 ucw458

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Posted 20 February 2019 - 10:27 AM

Does the 86 and older MAP sensor need to be an OEM unit?  Can a GM 3bar MAP be used in it's place?  If it can does that have an affect on boost cut?
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#2 scott87star

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Posted 20 February 2019 - 01:06 PM

Page 14-27 of the 86 service manual, http://www.starquest...fuel_system.pdf shows the range of the pressure sensor to be 65 mm Hg at 0.2 volts and 1460 mm Hg at 4.5 volts which is roughly 1 psia to 13.5 psig (13.5 pounds of boost).  So no, the GM 3 bar is reading around 40 psig at 4.5 V.  It may affect boost cut, is there any documentation the 86's have boost cut?
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#3 tux

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Posted 20 February 2019 - 04:07 PM

a GM 2 BAR map is close'ish

Manifold Pressure
(relative sea level / absolute)


2 Bar
MAP Output


14.31psi / 29.01psi


4.90V


14.00psi / 28.70psi


4.84V


13.00psi / 27.70psi


4.67V


12.00psi / 26.70psi


4.50V


11.00psi / 25.70psi


4.33V


10.00psi / 24.70psi


4.16V


9.00psi / 23.70psi


3.99V


8.00psi / 22.70psi


3.82V


6.00psi / 20.70psi


3.47V


4.00psi / 18.70psi


3.13V


2.00psi / 16.70psi


2.79V


0.00psi / 14.70psi


2.45V


5.00inHg / 12.24psi


2.03V


10.00inHg / 9.78psi


1.61V


15.00inHg / 7.33psi


1.19V


20.00inHg / 4.87psi


0.77V


25.00inHg / 2.41psi


0.35V


29.10inHg / 0.40psi


0.02V



#4 ucw458

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Posted 20 February 2019 - 04:09 PM

Well there's that fuel cut defencer mod.  Using a diode or resistor to clamp the MAP signal somehow.  I don't remember what it does exactly.  But it's only needed when you crank up the boost.

The 3bar MAP is an absolute sensor.  Meaning the first bar is vacuum to atmospheric pressure.  So it's max reading should be around 29.4 psi boost.  If the ECU is using it for fuel correction it would be lean using the GM unit.  But that could potentially be overcome by bigger injectors, higher fuel pressure or a MafT.


1460 mm hg is 28.2 psi.  I'm wondering if the use of HG in the FSM was an error.  HG isn't an absolute measurement.  And the FCD mod is needed long before 20 psi boost.  So can the stock MAP see vacuum or only pressure?  If it only sees pressure then the ECU is programmed to cut at around 2.5 volts.  If it sees vacuum as well then the ECU is programmed to cut at around 5 volts.
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#5 scott87star

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Posted 20 February 2019 - 08:48 PM

It is absolute, so 1460 mm Hg is 28.2 psiA or 13.5 psiG.  Now the odd thing about the 86 MAP is that its plugged into the vacuum/boost line AFTER a solenoid.  Since I've never had an 86 I don't know how that solenoid operates.
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#6 tux

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Posted 20 February 2019 - 09:42 PM

Solenoid probably switched from manifold pressure to atmosphere.  

At least that is what Subaru uses it for

Edited by tux, 20 February 2019 - 09:50 PM.


#7 ucw458

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Posted 20 February 2019 - 09:59 PM

So it must operate like a typical 2bar MAP then.  Switching to a 3bar MAP would work for performance setups.  A GM 2bar MAP may be a good replacement for the stock one.


I saw the solenoid thing too.  I believe it's to prevent the MAP from seeing engine pressure until the engine is warmed up.  Since that's what Mitsu used the other solenoids on SQs for.  So the ECU isn't using that signal at all when cold......... seems odd.  Not to mention the solenoid will restrict flow a bit causing a slight delay in pressure getting to the sensor.  Maybe it's only using the sensor as a safety switch?  Cutting fuel if the engine overboosts past stock specs.  I have never heard of the FCD causing fueling issues.  If the ECU needed that sensor for fuel trims then the FCD should mess that up.

Could this sensor be eliminated?
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#8 tux

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Posted 21 February 2019 - 08:16 AM

I don't see where they solenoid senses heat

#9 tux

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Posted 21 February 2019 - 09:27 AM

The FCD bleeds of voltage over 4.5 volts, as the ECU cuts fuel around there.  

Pretty sure it's used for fuel trims

#10 ucw458

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Posted 22 February 2019 - 10:22 AM

The solenoids are controlled by the ECU.  The ECU uses the solenoids to prevent vacuum advance and charcoal canister purge from activating when the engine is cold.  If the MAP sensor is on the same circuit as either of those two then it also is prevented from working when the engine is cold.
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#11 tux

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Posted 22 February 2019 - 12:27 PM

I had this idea a few months ago to use a 2bar GM MAP, but didn't look into it much further than the values being close enough.  So thanks for making me look a bit further.


"The solenoids are controlled by the ECU.  The ECU uses the solenoids to prevent vacuum advance and charcoal canister purge from activating when the engine is cold.  If the MAP sensor is on the same circuit as either of those two then it also is prevented from working when the engine is cold."

The Distributor, EGR and Purge valves are controlled not by the ECU, but by the thermo-valve that on the intake manifold.

The MAP is connected to a different vacuum port on the manifold and has a solenoid that looks to switch between manifold pressure and atmosphere pressure (much like the way Subaru documents).  I can't say for sure that it does that as I didn't test what it actually does and it isn't mentioned in the manual, but reasoning leads me to believe that's what it does.  Looks like it switches where it samples from to control the fuel trims to get a reference of boost and a reference on atmosphere to try and be accurate.  Most of the 86's run pretty ok'ish without the air can sensor so it has to be using MAP for at least part of the trims..

#12 ucw458

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Posted 22 February 2019 - 04:06 PM

It's been said that mitsu may have been looking into going speed density with their cars.  Hondas don't have mass air sensors for example.  They are speed density.

The FSM doesn't seem to describe what the MAP does.  Later FSMs have a section that explains what everything does.  If the 86 has that I haven't seen it.
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#13 tux

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Posted 22 February 2019 - 07:10 PM

Haven't seen it either to be honest.  Documentation isn't great

#14 ucw458

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Posted 26 February 2019 - 02:02 PM

I suppose the only way to find out if it's used for fuel trims is to unplug it and see how the engine runs.  Will it run with the MAP unplugged?
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#15 tux

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Posted 26 February 2019 - 07:54 PM

Good question.  Pull it and see!
Mine started doing something funky so I want to fix that before seeing if that will make it run funky

Edited by tux, 26 February 2019 - 07:55 PM.


#16 jinx

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Posted 27 February 2019 - 04:50 AM

My 1st turbo project was the older non-usa 2.0 starion motors ran that older map sensor setup
Learned an old trick from the turbo magazine mitsu colt article = put a cheap manual boost controller in line to the map sensor so it will limit boost line pressure just short of cut-psi. Hello 20psi. It worked. Ran fat up top tho

These days, ask yourself if its feasible & cheap enough to go microsquirt + fresh new sensors + add electronic timing...all tuneable

#17 tux

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Posted 27 February 2019 - 12:01 PM

Can't speak for ucw, but I'd be interested in changing the map solely for the factory one being irreplaceable.




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