MAF and MAP?
 

Does the RX-8 employ just a MAF sensor, or does it have a MAP sensor as well? Which one helps the ECM adjust fuel, timing etc. better?

Okay. After checking my trusty service manual, the RX-8 doesn't employ a MAP sensor. There is a barometric pressure sensor, though. Are the two analagous?

Indeed, no MAP. As the air measurement strategy is MAFS based, it doesn't need one.
Both most probably use the same sensor technology. And that's it for the analogy.

Well, since mass air flow is temperature and pressure dependent, it shouldn't need a MAP. I think the barometric pressure sensor helps determine what the mass air flow is. Thank you for your help IKnowNot'ing.

According to my Ford EEC5 training book, BP is used in the different strategy modules for :
- crank fuel
- desired mass air flow through the idle air by-pass valve
- switching to altitude tables for EGR and spark advance
- torque converter lock-up
- shift chedules
- calculation of theoretical air charge at WOT
- self test
and I believe it also affect electronic throttle body progression.

Real-time analysis give me a headache :(.

MAP is for forced induction.

Okay, so why is a MAP for forced induction?

MAF is and Air Flow sensor....MAP is a pressure sensor

I don't think anyone is going to clarify this :confused: :(.

Just two different ways of determining the same thing: how much gas to mix in with the air going into the engine (at the most basic)

MAF measures the airflow passing through the MAF (Mass Air Flow) sensor and the baro pressure to calculate the amount of air being ingested by the engine, MAP uses the pressure inside the intake manifold (Manifold Absolute Pressure) and some assumptions/rules about the volumetric efficiency of the engine along with the RPM to calculate the same.

One advantage of MAP is that there's no (potentially) restrictive sensor that the air must travel through since all you need to do is measure the pressure in the intake. MAF is good in that if you make an adjustment to the engine which changes the VE of the engine, then it will compensate.

MAP is not solely relegated to FI cars - the 2nd gen turbo'd rx7's used a MAF system, the 3rd gens used MAP - both were forced induction.

This is somewhat a simplified description, however I believe it touches the main points.

Simon.

Manifold Absoulte Pressure.
Well you can use a MAF for FI but I think MAP is better for a few reasons. A MAP system does not have the MAF flapper restriction (FI hates restriction) and is better for FI systems that have the blow off valve venting to atmosphere. If you vent to atmosphere with a MAF system you would get un-metered air into the intake charge, or when the BOV operates you would have air that was metered by the MAF escaping and not entering the combustion chamber thereby wreaking havoc with the ECU's fuel tables.
Understand? I don't know how well I explained it?

For example, if I get a turbocharger, would I have to replace my MAF with a MAP? Can I keep both? I am aware of how both work, I was just curious if the stock RX-8 had one (a MAP). Obviously, I didn't consult my service manual before I wrote this.

Until you exceeded the flow capability of the MAF sensor you would not neccessarily need to change it if you got a turbocharger. You would likely need to put a manifold pressure sensor in the system to assist the ECU for things like advance/redard under boost and so forth, however it would play a lesser role in the engine control than a manifold pressure sensor in a MAP based system. Additionally the ECU would need fuel maps for a higher flow rate and so forth that putting a turbo on would produce.

I would be surprised if there wasn't a manifold pressure sensor on the car already - the 2nd gen rx7 (even the non-turbo version) had one in addition to the MAF sensor - it's used to help calculate engine load and stuff like that. The question would be: is it capable of measuring boost? On an NA car, having a manifold pressure sensor that could read boost and vacuum would be a bit of a waste since you'd never see much/any positive pressure in the intake manifold.

Simon.

Thank you sferrett. You are right; the RX-8 doesn't have a MAP sensor. The closest is the BARO sensor. So I would have to get a MAP sensor if I decided to go with forced induction? Electronics are a mixed blessing :confused:. Thank you everyone for answering my question :).

You'd need a MAP sensor input if you wanted to vent to atmosphere and hear that groovy cool PSSSSSSTTT when you shift :cool: or if you wanted to run boost over 6 psi reliably.
I switched to a MAP sensor and fully programmable ECU when I boosted my Miata - works like a charm. And yes my Miata will spank my 8 :eek:

Not true. There are numerous NA engines running a T-MAP sensor (Manifold Air Temp and Pressure sensor) for air charge calculation. My previous Audi V6 2.6L was one of them. I know of a lot small European cars who do too. A T-MAP is cheaper than a MAFS.

There haven't been flappers on MAFS any more for ages. They're all hot wire or hot film now with very little restriction.

Your comment about the blow-off valve vented to atmosphere and unmetered air by MAFS is very pertinent.

shelleys_man_06, another point to keep in mind when going to FI, is to prevent the triggering of diagnostic faults. For example, if I refer to the manual, re MAFS, you must not get beyond 17.2 lb/min under 2000 rpm and the MAFS signal cannot go beyong 5V. Also the difference between IAT and ECT cannot exceed 104°F (that's IAT > ECT of course).
Even if you meet these requirements to avoid a CEL, you still might enter into limp mode if the PCM strategy has authority, for example, to check actual MAFS readings versus expected, theoretical MAFS values.

This must be a good reason why the aftermarket is having such a hard time with high-boost applications for the RX-8, perhaps the 350Z, too. MAP and MAF are two different things, but they share a similar function: helping the PCM figure out fuel delivery, timing, etc. I would suppose a MAF would suffice under low-boost applications, but when the knob is turned up, the MAF can create a restriction in the intake tube, which can confuse the PCM, no? Suppose I run FI on 17+ psi (no metric units :mad: ). Is it recommended that I replace my existing MAF with a MAP, in order to help the PCM? The change-over seems quite a challenge, but I think it can be done. It will be a while before we see anything close to what the FD guys are doing, in terms of power.

Yeah, taking the MAFS out is really going to help the PCM!! It's going to help it trigger a massive 'limp' mode. Well you'll still be able to drive at 50 km/h :D to the dealer.

Stick to the MAFS, make sure signal output is within the range the PCM is expecting (see manual / diagnostic), and add a MAP and an auxiliary ECU (Canzoomer II?) to control fuelling under boost.

Yeah. Limp mode is a bad thing :o.

In short, are MAP and MAF similar to ammeters, ohm-meters, and voltmeters in circuits? They, ideally, are supposed to only take readings without restriction, no? We shall see what happens to this car in the future :).

Kind of I suppose..

A MAF could be equivalenced to an ammeter measuring the current through the circuit (airflow) directly

A MAP system could be equivalenced to using a voltmeter to measure the voltage drop across a circuit (air pressure) and therefore inferring the amperage (airflow) by having a known circuit restistance (the volumetric efficiency) and doing the math.

Simon.

Thank you sferrett for the clarification :).

Both my turbo Miata and turbo MX-3 used the stock MAF (no MAP) and open BOVs with no problems, sags, limps, idle issues or the like.

It is all in the tuning and how you handle the fuel delivery on throttle closing.

The real issue is what to do when the MAF reaches its limits. I clamp mine since the added fuel for the added air is controlled by a piggy-back system so there is no need to alarm the ECU.

On the Miata, I draw through the MAF. On the MX-3, I blew through it. It is all about logging, adjusting and loging some more.:p

To the computer!