Megasquirt Advice and discussion.

[TitaniumTT]

Originally Posted by Max777

Stock you got:

MAP
MAF

Oh really?

Originally Posted by Max777

Custom install, you got a few new sensors, fresh wiring, no problems.

OH REALLY?!?!?!

[antman0408]

I would rather get an older haltech, you can find e6k's for $500-600 with wiring harness and sensors.

[Whizbang]

::waits for stupid owl picture::

[djmtsu]

[Max777]

uhh... i dont get it... You have a pressure/vaccum sensor on the shock tower, and a trap door style airflow meter.... what did I say wrong?

Please educate.

[85rx-7gsl-se]

Its a pressure sensor...A MAP sensor is what is used on FDs instead of an MAF.

[TitaniumTT]

Yeah, one or the other as described in more detail in the PM I sent you.

[Max777]

Well, I know that the trap door sensor is a bit different from a "hot wire" style MAF, and that this is used for fuel on the stock ECU. I also know that the pressure sensor looks at boost/ vaccum and these parameters affect the timing on the RX-7... so what's different between a pressure sensor and a MAP sensor? I thought that these were the same thing, no? And if you cant have both, then why does this car have both systems?

I mean, a "pressure sensor" is fundamentaly the same thing as a MAP sensor, they both sense pressure, or am I totally missing something?

[RETed]

Originally Posted by Max777

Well, I know that the trap door sensor is a bit different from a "hot wire" style MAF, and that this is used for fuel on the stock ECU.

Yes, two different ways of doing this same thing.
They both measure "mass air", or the amount of air flowing into the intake / engine.

so what's different between a pressure sensor and a MAP sensor? I thought that these were the same thing, no? And if you cant have both, then why does this car have both systems?

Good question...
IN GENERAL:
A "pressure sensor" can only see pressure or positive pressure relative to "0 atmospheric (pressure).
A "MAP sensor" (remember, "MAP" stands for "manifold absolute pressure") senses both pressure *and* vacuum.
The "absolute" part means it ignores "relative to 0 atmospheric (pressure)".
What this means is that MAP sensors start at "absolute vacuum" or very near that level - this is it's zero point.
(Absolute vacuum is considered 30.0inHg or "-14.7psi" - most people don't like seeing "-" and "psi", but it's easier to explain this way.)
At sea level or "0 atmospheric pressure", a MAP sensor will actually "see" ~14.7psi.
http://wiki.xtronics.com/index.php/P...nversion_Table

Now, in the FC's case, Mazda does call the non-turbo FC's pressure sensor a...well, "pressure sensor".
The turbo FC's pressure sensor is called a "boost sensor".
That just confuses the whole issue. :P
This the learned, the non-turbo FC pressure sensor is actually a "1-bar MAP sensor" (reads vacuum up to "0" atmosphere); the turbo FC pressure is actually a "2-bar MAP sensor" (reads vacuum up to ~15psi of boost).

I mean, a "pressure sensor" is fundamentaly the same thing as a MAP sensor, they both sense pressure, or am I totally missing something?

Yes, in a sense.
I hope the above clears things up.


-Ted

[85rx-7gsl-se]

Its all formalities I think. Almost all cars either use a MAP or a MAF system exclusively. 2nd gens use a MAF system. 3rd Gens use a MAP system.

[RETed]

Originally Posted by 85rx-7gsl-se

Its all formalities I think. Almost all cars either use a MAP or a MAF system exclusively. 2nd gens use a MAF system. 3rd Gens use a MAP system.

Not true...

Although the FC uses the air flow meter as it's primary load sensor, the pressure / boost sensor also affects both fuel and ignition timing.
More specifically, we have confirmed that for a turbo FC, changes in vacuum will change fuel delivery.
Once in positive boost, the boost sensor will primarily affect ignition timing.
The RTEK guys (Henrik) will also tell you this.
So to use the word "exclusively" is not correct in this case.

The FD does use a MAP sensor based load system.
The FD MAP sensor is some funky version that goes up to like 17psi, I think.
(Most MAP sensors are in 15psi increments.)
The FD has no idea what the actual amount / mass of air going into the engine, but it does calculate that figure by looking at the engine RPM's and MAP sensor readings (primarily).
This is why it cannot tell differences in engine VE when you start modding your engine, i.e. change intake, exhaust, engine porting, bigger turbo, etc.
To the FD stock ECU, 15psi is 15psi.


-Ted

[TitaniumTT]

It should be noted that 15psi is not 15psi in all cases. Which is the main reason a MAP based system much more sensitive to changes in airflow. Taking two extremes, the stock HT-12s vs a GT45R. @ 15psi of MAP, a GT45 will flow much more air than the HT12's. So, by bolting on a GT45 you need to re-tune a MAP based system due to the drastic change in VE. When you start to mod a stock MAP based system and you increase the effiecency of the turbo's, you change the output of the compressors slightly. 15psi may still be 15psi, but the actual flow is changing and the Lambda's are now incorrect. A MAF based system is still subject to changes in VE but much less so tan a MAP based because it is measuring the airflow through the sensor, not just how much pressure is in the manifold

[RotaryProphet]

15psi is always 15psi, but 15psi at 80 degrees is not the same as 15psi at 160 degrees; this being what compressor efficiency is measuring (heat added to the intake charge). A larger compressor will generally have better efficiency numbers higher up in the airflow range, and a wider efficiency island.

You aren't changing the "flow", persay. Given a certain engine condition with a better compressor, you're flowing the exact same volume of air (assuming you have the exact same pressure), but the better compressor will be pushing a cooler, and thus denser air charge. (Which would be mass, as opposed to volume).

This is why a MAP based system also requires an intake air temp sensor to correctly calculate the air density. A MAF system measures mass directly, and is probably more accurate under a wider range fo circumstances, but the restriction to inlet flow leads me to use a MAP/IAT system 90% of the time. I would choose a MAF for things like hill-climb motors where rapid changes in altitude would necessitate the wider operating range, and self barometric correction would be more desirable.

[RETed]

Originally Posted by RotaryProphet

15psi is always 15psi, but 15psi at 80 degrees is not the same as 15psi at 160 degrees; this being what compressor efficiency is measuring (heat added to the intake charge). A larger compressor will generally have better efficiency numbers higher up in the airflow range, and a wider efficiency island.

You aren't changing the "flow", persay. Given a certain engine condition with a better compressor, you're flowing the exact same volume of air (assuming you have the exact same pressure), but the better compressor will be pushing a cooler, and thus denser air charge. (Which would be mass, as opposed to volume).

This is why a MAP based system also requires an intake air temp sensor to correctly calculate the air density. A MAF system measures mass directly, and is probably more accurate under a wider range fo circumstances, but the restriction to inlet flow leads me to use a MAP/IAT system 90% of the time. I would choose a MAF for things like hill-climb motors where rapid changes in altitude would necessitate the wider operating range, and self barometric correction would be more desirable.

Sorry, but you are dead wrong.

TTT is right.
Pressure (i.e. "psi") is only one variable when talking about turbo potential.
AIR FLOW the other big variable.
To ignore air flow is downright stupid.
15psi with a T3 is not the same as 15psi with an HKS T51R SPL...

Air temperature is tied into the whole thing (see PV=nRT), but it's a relatively minor variable.
To emphasize air temps over air flow is downright...ignorant.


-Ted

[RotaryProphet]

Originally Posted by RETed

Sorry, but you are dead wrong.

TTT is right.
Pressure (i.e. "psi") is only one variable when talking about turbo potential.
AIR FLOW the other big variable.
To ignore air flow is downright stupid.
15psi with a T3 is not the same as 15psi with an HKS T51R SPL...

Air temperature is tied into the whole thing (see PV=nRT), but it's a relatively minor variable.
To emphasize air temps over air flow is downright...ignorant.


-Ted

You're in such a hurry to contradict me, you didn't pay any attention to what I actually wrote. You need to re-read what I wrote, and what I responded to, and rethink your statement. We're not talking about total turbo potential, we're talking about two turbos on the same engine, both capable of making a given pressure at a given flow rate (CFM).

Flow and pressure are directly related; unless you have enough flow to over-supply the engine, you can't make boost. Thus, increasing flow in a system increases the pressure. With a larger turbine, you can't "flow" any more air into the engine at any given boost level than with any other turbo capable of flowing the same amount (assuming, of course, identical hot-sides, and thus equal exhaust backpressure at that boost level). So where does the power come from when you swap to a larger cold side? Increased efficiency, and thus a cooler air charge.

Being capable of flowing more air doesn't mean you actually do, it just means that it takes less work for the compressor to flow the air you do need, and less work means less heat.