Question understanding MAP sensor - Tuner-Junction

jadkar · #1 (**permalink**) March 25th, 2006, 07:30 PM

Now that I'm coming to an end of my install, and getting ready to fire up my REX tomorrow I started to ponder the MAP sensor and I confused myself

I want to understand the MAP sensor and I'm stuck trying to figure out how it work in regards to a turbocharger car and a supercharged car. It seems as though the way the stock B18C sensor is mounted something wouldn't work correctly.

I'm gonna try and explain and hopefully a turbo or SC guru can explain it.

OK....... we all know on a N/A car the Manifold Absolute Pressure sensor is going to report the manifold pressure value (either +/-) back to the ECU so that the correct fuel mixture can be determined.

Turbo car.....(during acceleration)
The entire intake pipe is going to be charged with positive pressure because the turbo is way at the front of the system. So the MAP sensor (at the throttle body is going to read positive manifold pressure. The ECU will adjust accordingly. This makes perfect sense.

The Supercharged car (during acceleration)
This is where I get confused... The unit providing the "boost" or "charge" is not at the front of the system, it's actually BEHIND the MAP sensor. So during acceleration I would think the MAP would be reading crazy vacuum. Think if it as mounting the MAP sensor in front of a turbo charger, that would be the same thing. So with that in mind how the hell does the ECU understand what the heck is going on??

This is one of those things that makes sense and then the longer you think about it you confuse yourself.

Someone please "un" confuse me

DCxMagus · #2 (**permalink**) March 25th, 2006, 08:35 PM

all the map sensor does is recongize pressure it doesn't matter WHERE the sensor is if the air in your system is pressurized it will read it.

That how the missing link works it bleeds all the pressure and has the map sensor only reconizge the 14.7 for the ambient air pressure

pressure in your system isn't relavite to the postion on where its being read if something is pressurized like an Intake manifold the entire thing is pulled up to that pressure. This is ALso why blown motors that have compression loss thru the cylinders will actually run like normal under boost. They wont have the increased power since the boost is being used as a band-aid.

Basically the map sensor has a voltage range to report back to the ecu on one side is 4.7(if you have a 5 volt map sensor) this is atomosphere pressure this is achieve on a N/A car at WOT on the other side is something like 0.5 or something voltage and thats the normal idling vaccum for the car. This relates to a 1 bar map sensor. A bar of pressure is 14.7 PSI. Now in a boosted car with a standalone fuel system you would want a higher bar sensor if you plan on running under 14.7 PSI then a 2 bar sensor if you want to run more then 14.7 but less then 29.4 then a 3 bar is for you and so on and so on up to 5 bar.

Now why doesn't puting a 2 bar map sensor on a stock car work? Simple with a 2 bar map sensor instead of atmospherical pressure being the full voltage it is now 14.7PSI OVER atmospherical pressure. Here something that makes understand this all a bit easier, you need to start realizing that a car at 5PSI of boost auctually have 19.7PSI of pressure in it's intake manifold. 14.7 for the atmosphere pressure + 5PSI being added ON top of that from your turbo/super = 19.7 your stock map sensor can't read this it maxes out at 14.7 which is why you need fuel solutions when you boost a N/A car.

in a VERY simple manner the map sensor reads the pressure in the intake manifold it reports this pressure via a voltage between 0.5 (full vaccum) - 4.7(max readiable pressure by the current sensor 1 bar, 2 bar etc.) this voltage is read by the ECU and the ECU detremines how to adjust fuel trimming accordingly. Since the intake manifold is ALWAYS pressurized or in some type of vaccum it doesn't matter where the map sensor is places since there is no why for part of the intake manifold to be pressurized and part of it not to be.

I can help explain excatly what is going on in your system if you answer the following questions.

how are you handling fuel.
what map sensor are you using
turbo/super
amount of boost you plan to run.

jadkar · #3 (**permalink**) March 25th, 2006, 10:07 PM

You explained a lot except in my particular instance it still doesn't make sense. My car is going to Jeff Evans this Thursday to get tuned and I'm just trying to figure this one piece out. Here's my current setup

B18C-r
JRSC, 9.0-10psi (not sure yet exactly where it will fall but it's somewhere in there)
LHT intercooler
Hondata S300
Stock B18C-r MAP sensor (I should be OK with my little boost)

Here's another analogy to try and explain what I'm saying......

If we had a pressure sensor in our fuel system BEFORE the fuel pump the sensor would read a vacuum or "negative" reading because it's behind the fuel pump. If we put the same sensor now in front of it the sensor would read 45,50,60psi (what ever it is).

That flash back to my situation. The MAP sensor is behind the supercharger, it's down getting pressurized like the intake manifold is. So how is it reading any boost at all??

It a turbo car the MAP is upstream, or in "front" of the turbo so it is being pressurized. It that scenario it makes sense, but for the supercharger it doesn't.

DCxMagus · #4 (**permalink**) March 25th, 2006, 10:32 PM

well with a hondata S300 you should use a 2 bar map sensor would make tuning a bit easier but the stock one will work fine.

You want the map sensor to be reading the pressure of the intake manifold. Nothing else becuase the pressure of the intake manifold is what detremeing weither or not your auctually boosting. Also the amount of pressurized air in the intake manifold is the only air that is being used for combustion so this is the amount of air you want to tune off of. As for the fuel pump thing I don't believe the gas in the tank before the pump is aucutally pressurized. The pump itself is what pressurizes the system and pumps the gas thru the lines. I just know you always want to get fuel pressure readings from the rail.

DCxMagus · #5 (**permalink**) March 25th, 2006, 10:35 PM

since your using the stock map sensor your map sensor isn't going to see any boost at all its can't measure that high your just going to get a CEL light you need to get a missing link

jadkar · #6 (**permalink**) March 25th, 2006, 10:35 PM

Quote:

Originally Posted by DCxMagus

well with a hondata S300 you should use a 2 bar map sensor would make tuning a bit easier but the stock one will work fine.

You want the map sensor to be reading the pressure of the intake manifold. Nothing else becuase the pressure of the intake manifold is what detremeing weither or not your auctually boosting. Also the amount of pressurized air in the intake manifold is the only air that is being used for combustion so this is the amount of air you want to tune off of. As for the fuel pump thing I don't believe the gas in the tank before the pump is aucutally pressurized. The pump itself is what pressurizes the system and pumps the gas thru the lines. I just know you always want to get fuel pressure readings from the rail.

Exactly .... the fuel story was just an analogy. So you see my point.....

the current placement of the stock MAP sensor doesn't seem like it would work because it's behind the supercharger, just like my story of the fuel sensor behind the fuel pump.

The thing is, all the SC guys keep the MAP sensor in it's stock location......This is why I'm confused.

DCxMagus · #7 (**permalink**) March 25th, 2006, 10:49 PM

well the fuel pressure thing is different becuase you were auctualy getting the pressure of that line not the entire fuel system.

now the map sensor on the other hand take the pressure of the entire open space of the intake manifold, and since there are no areas of the intake manifold . I think your thinking about boost as something that auctually travels like air, it's not like that. It's not like there is a pocket of vaccum before boost hits the engine. The air is either pressurized or its not. Just because the map sensor is placed near the Throttlebody that doesn't mean it's taking it's measurement there.

I understand what your trying to say, your thinking that since the charge pipe on a turbocharger is pressurized since it's a closed loop the air passes thru the map sensor pressurized and everything is fine and dandy.

IN the case of a roots blower your auctually sucking in NON compressed air that passing over where the map sensor is so you think it's never going to see the boost right?

if that what your thinking the map sensor doesn't take a measurement at the auctually throttle body the map sensor measures at the intake manifold, after the air has been compress by either turbo or super.

jadkar · #8 (**permalink**) March 25th, 2006, 11:18 PM

Quote:

Originally Posted by DCxMagus

if that what your thinking the map sensor doesn't take a measurement at the auctually throttle body the map sensor measures at the intake manifold, after the air has been compress by either turbo or super.

That's it!! Now wer're talking. So if the MAP is mounted on the throttle body but it's taking a measurement from after the supercharger how is it getting this measurement? Is there a passgage leading back to the intake??

It just seems impossible because the MAP mounts to the throttle body, then that mounts to the "S" tube which leads directly to the inlet on the supercharger. The inlet side of the supercharger is not charged (it's sucking) and the outlet part bolts to the manifold and this where the boost is.

DCxMagus · #9 (**permalink**) March 26th, 2006, 08:47 PM

I believe that where there is a port where the map sensor is to the intake manifold so thats how it gets it's measurements, it couldn't get it's measurements from at the Throttlebody becuase everytime you stepped on the gas and the butterfly opened it would instantly shot to atmospherical pressure and the sensor would be basically useless. I believe just it just sits on the TB but the passageway it is sitting in leads to the intake manifold.

jadkar · #10 (**permalink**) March 26th, 2006, 09:52 PM

Quote:

Originally Posted by DCxMagus

I believe that where there is a port where the map sensor is to the intake manifold so thats how it gets it's measurements, it couldn't get it's measurements from at the Throttlebody becuase everytime you stepped on the gas and the butterfly opened it would instantly shot to atmospherical pressure and the sensor would be basically useless. I believe just it just sits on the TB but the passageway it is sitting in leads to the intake manifold.

Thanks!! this conversation is priceless. I think you may be correct because I just looked at my setup. Remeber I said the throttle body now sits on this Jackson Racing "S" tube. Well the "S" tube has a vacuum fitting on it driectly behind the MAP sensor and I bet this vacuum line is supposed to tie back to the intake manifold AFTER the supercharger. Right now, I have the fitting connected to my by-pass valve. I'm 99% positive I have the line incorrect. I have to double check it and I'll let you know.

DCxMagus · #11 (**permalink**) March 26th, 2006, 10:46 PM

I think the way the jacksonracing kit works if they bleed off the boost from the map sensor like a missing link does so it doesn't throw a code when you hit boost.

jadkar · #12 (**permalink**) March 26th, 2006, 11:16 PM

Well, I as wrong on the hose coming off the "S" tube, it is supposed to go to the by-pass valve the way I have it.

However, I think the I made this way more complicated then it is. I'm begging to think that the way a Hondata (or whatever) fuel system is tuned on a supercharged car is they just use the MAPs vacuum reading for determining the fuel mixture. Duh, it seems like the MAP is just used backwards from how a turbo car is. On a turbo car the MAP reading + pressure to determine the boost and make adjustments to the fuel curve. On a SC car the MAP reads - pressure (or vacuum) to determine the boost and make adjustments to the fuel curve. I guess this method is the same way for a NA car since there is never "boost" in the manifold.

Let me know if this seems right, but I think it is.

DCxMagus · #13 (**permalink**) March 26th, 2006, 11:40 PM

it's not some much that as it's the fact that they needed this kit to work without throwing a CEL

if they had the map sensor hooked to the intake manifold everytime you hit the gas you would get a CEL becuase yolur map sensor is seeing boost and the ecu doesn't know what to do with that reading. So what Jackson Racing does is basically the same as a synaspe missing link, it bleeds the boost off from the map sensor so it never see it, in turn there is no CEL. Then they use a Boost based FPR or FMU to increase fuel pressure so you have added fuel. The options with the hondata they probably have it tuned without information from the map sensor so they don't have to change the system around.

You really only need the map sensor if you were using an ECU to comtrol the fuel and timing maps based off their signals. With hondata you are tuning in the fuel and timing maps yourself so you don't need to worry about that.

jadkar · #14 (**permalink**) April 1st, 2006, 10:22 AM

So anyway, I figured I'll close out this discussion because I hate when people start threads and then don't explain the outcome

I went to Jeff's on Thursday for my tuning session. Once we started to dyno the car he couldn't get it to stop pinging. I mean he kept adding fuel to the fuel map and couldn't get the car to stop. As soon as the charger boosted the engine was freaking out.

He shut it down and said this is weird I don't understand. He started to look under the hood puzzled. That when I explained my MAP sensor issue. I told him I that because I have a supercharger I believe my MAP sensor is mounted in the incorrect place now. With the supercharger installed the throttle body mounting position of the MAP doesn't make sense anymore.

He said holy crap!!...your right, I didn't even think to look at that. This is why you car is pinging. He went on to say that he couldn't figure out why the S300 was reporting all this vacuum instead of boost and now it makes sense. The MAP sensor needs to be after the charger (or what ever is doing the "boosting") He said he usually deals with turbo cars and in that case the throttle body mounting location of the MAP is fine (So he confirmed my thoughts). I asked him what do we do now?? The solution is we left the stock MAP on the car, we just unplugged it. and he gave me another MAP sensor to borrow. We hooked that MAP sensor up to a vacuum line and fed it back a fitting AFTER the charger on the manifold. We then ZIP tied (I almost had a heart attack) the sensor back to the firewall temporarily. Once we did that.....we were perfect!! Everything worked.

So here's the deal..........

The MAP needs to measure manifold pressure AFTER the unit that is doing the pressurization. This means:

Supercharged cars = MAP needs to be after the supercharger. Can't be on the throttle body. You need to block this port off and run a line from another place AFTER the charger back to a MAP sensor and mount that sensor somewhere off the engine (like the OEM CRX sensor)

Turbo cars = The stock location is already AFTER the turbo so the entire system is already pressurized. This means you can leave the MAP sensor where it is.

Now of course in both instances if you want to want more then 11psi of boost you will have to replace the OEM MAP with a "3 bar unit" but this is common knowledge.

Hope this helps anyone messing around with MAP sensor issues. I searched the net and couldn't find much.

DCxMagus · #15 (**permalink**) April 2nd, 2006, 03:42 AM

Yeah basically you only have to do with when you are going to be tuning WITH the map sensor, in the case of ROM burning or full stand-alone tuning you will need to do this. Cheeper Fuel solutions you don't need to do this modification. Fuel solutions such as (and I dont condone the use of any of these anymore

) RRFPRs or the AFC hack. So basically the Jackson Racing kit just bypasses it becuase it uses a FRP, makes sense. I wonder though if you were to NOT use a kit or not add the bypass valve if you still would need to do this?

oh well, Glad you guys got it covered and your cars runing awesome, hope you have fun with that beast. From the other thread seems like you might be needed a new tranny soon LoL.