ok stock on my car there is a flapper door style MAf (luck you guys get the wire version) but I have the chance to go to a MAP system instead...


what is the difference between the MAP and MAF wants pros and cons here if I could please




just to let you guys know this is going to be a boosted system also

 

A MAF system (the hot-wire type) directly measures the amount of air coming in; literally, counts the molecules; so it can meter fuel in proportion. A MAP, or speed-density, system attempts to infer the amount of air based on its assumptions (programming), to respond to throttle opening, vacuum, RPMs, etc. etc. The MAF system is technically far superior. However, the factory's implementation of it in these cars, is flawed in several ways, not the least of which is that its data values max out at 255 g/sec (8 bits), which is only something like 500 CFM; not a whole lot of HP. Since S/D has no such limit, it's almost a requirement if you want to exceed about 300 HP. Newer MAF systems such as the LS1 do not have such a limit.

Since a MAF counts air molecules, it does not care about boost as such, as long as the fuel pressure is boost-referenced. 42 psi in the rails into a runner at 15 psi will result in very different flow compared to 42 psi into a runner at atmospheric pressure.

 

...but, but, but....

Additionally, your stock system is technically a Vane Air Flow meter (or, VAF), which, unfortunately for you, has no compensation for air temperature, relative humidity, density altitude, and other factors that can alter the amount of air molecules in a volumetric quantity of air. Therefore, it is still an "assumptive" system like the MAP or Speed/Density type control schemes, and has the same inherent deficiencies. It uses calculations based on air volume and pressure to account for a presumed intake air mass, rather than measuring the actual mass of air molecules entering the engine.

The most accurate systems are the hybrids, which use a MAF to measure actual intake air mass, and a MAP and IAT to verify the MAF results and slightly alter the mixture based on the known effects of those variables. Still, 95%+ of the fuel calculations are based on the MAF, with only a little "trimming" left to the variables.

For a Mazda, it's probably close enough. Since you can't change your camshaft, it really doesn't matter.

I am a bit curious, though. You obviously have something in mind, or you likely wouldn't have asked. There may be a way to tailor the VAF input to squeeze a little more out of the whirring little monster. Are you finding the limits of your control system?

 

well the stock VAF sucks
nice little restriction inside the intake and also isn't the greatest piece of junk that doesn't like it when you hit bumpy bumpy roads it seems. though that could be partly due to me having it on it's side (only way to put my K&N on)

the map sensor comes with a haltech system and the haltech works with fuel management better then the stock ECU. and being boosted even more so.

a few problems with stock ECU involve fuel cut when boost goes past 8 psi to the rear rotor. fix for it is to take the boost sensor and when boost goes past 8 psi keep telling the computer you are at 7.9 psi. this way I keep fuel going to the rear rotor

larger fuel injectors only make the car run really rich on cruise and idle. you can tune it using a s-AFC but that doesn't take into account boost pressure just throttle opening.

since you can't tune the stock ECU a lot of ppl have had problems with keeping there motor running under higher boost.

then you have the VAF which puts a nice little restriction in there and prolly slows boost down just a tad

so more or less was just curious about the two being when I get the turbo in I might be switching to a haltech
and didn't know what difference between the MAF or VAF in my case vs a MAP


which would you say is the better choice between a VAF and a MAP?

 

Like you said, haltech and other aftermarket EFI systems all use MAP. It's simple and very effective. If it wasn't they wouldn't use it on few thousand horsepower race engines.
Sounds like VAF is total junk, even compared to MAF. Since it can't even accurately account for humidity or temperature you would have to end up way off a lot of the time.

 

I knew you had something cooking...

You have already discovered some of the problems of a mechanical VAF sensor (basically a spring-loaded paddle connected to a potentiometer) - really high tech, huh? One bump, and the reading gets skewed one way or the other. A little dirt on the pivots and/or wear in the pot throws it off even more. Some buildup in the vane, and it gets sluggish. Sadly, they're too lightweight to even make an effective anchor. Lots of imports used these. Ford toyed with it for a couple seasons on a few cars. And Chryslers - oh, never mind, I already covered imports.

Anything would probably be more accurate and reliable. Do you have any idea what the base control voltage of your system might be? 5 VDC maybe? If you had a common voltage, you might be able to satisfy the ECU with a MAF and translator, or MAP.

No fuel to an entire section? That can't be good.

 

can't recall what the voltage is
just remeber on www.rx7club.com someone was trying to figureout how to get the mustang MAF on the car but some problem was in getting the voltage to read the same for through different airflow values being the mustang is calabrated a little differently.


yeah the no fuel to the rear rotor is supposed to help the engine live longer by limiting boost ... and yet it has killed so many motors. don't know why they didn't just do ignition cut instead

 

RX,

I would think that cutting the ignition would send the emissions through the roof, contaminate teh oil with fuel, and wash any lubrication off the surfaces, so that isn't an option. Trimming back the fuel would send it lean, which might start to melt the rotor and seals. Retarding the timing might increase emissions and be beyond the ability of the ECU. Cutting fuel completely wouldn't hurt the engine as much as any of the other options, but I thought the more sophisticated systems used a wastegate to control boost pressure. That seems to work on a lot of other systems.

 

Yeah Merlin hit it. OEMs are not allowed to use ignition cut rev-limiters for street cars. Doing so would just blow raw fuel straight through the engine which the EPA would not let them do.

I'm aware obviously of aftermarket rev limiters. I think for exampe that the MSD 6AL might carry an EO # making it emmisions legal even though it is an ignition cut limiter. Prolly has to do with the laws being a lot stricter on a brand new car vs. one that the owner would like to personalize (within reason). Liek how the 2.73 geard LT1 cars didn't have a selectable first gear on the shifter pattern. Thats just due to a noise test at a certain speed that the gov't does and all new cars must pass. A 2.73 Lt1 would have been in first at the test speed amking it way too loud. By making first non-selectable they program the computer to be in second at that speed and it passes. But yet as soon as you buy the car you can put aloudmouth on it anyway cuz of the good folks at sema and other organizations that make sure we can have some fun and not have to break the law to do it.

 

eh emissions... like I would pass anyway being all 3 cats are gone


they do have a wastegate but the thing is so tiny you need to bore it out to do any good if you upgrade the exhuast or intake at all.

the ECU I think does retard the timing but that is only to prevent detonation not boost controll.

the fuel cut has a bad habit of blowing motors. the only thing I could think of though is the intake on the cars is tune so one intake pulse reverts back to help forece more air into the other rotor and might be getting a little fuel reversion?? donno just an idea

as for ignition cut can't you do it so it fires off every other time? wouldn't that help elim fuel wash off and oil contamination?