MAFs

[Chris Conlon]

At 05:55 AM 2/8/01 +0100, Axel Rietschin wrote:

>> Yes, you missed a small point. The location of the MAF (before or after
>> the compressor), in a system that has a significant volume of plumbing
>> after the compressor *does* make a difference. The reason being the
>> "air through MAF = air into engine" relationship is only true in steady
>> state. When the pressure post-compressor is changing, up or down, the
>> airflow through the MAF is not equal to flow into the engine. Some is
>> being either pumped up into the plumbing, or delivered from the pumped
>> up volume into the engine.
>
>You mean you can get a faster response from the MAF (less lag) when it's
>located closer to the engine because the plumbing volume introduces an
>additional delay? I didn't think if it at first but I see no problem with
>that.

Hmmm I have not thought about it in terms of faster/slower. When the
MAF is pre-compressor it actually reads higher, sooner than it would
post-compressor, for a brief time when boost is rising. It's measuring
the air that's actually going into the engine plus the amount of air
needed to pump up the post-compressor plumbing to X psi of boost. The
situation is opposite as boost falls, some of engine's needs are fed
from stored boost post-compressor and the MAF does not see that air.
(It saw it earlier during boost ramp up.)

I haven't thought too hard about it yet but even with the MAF post-
compressor, location may still make a slight difference. The more
plumbing volume you have after the MAF, that is subject to wide
swings in pressure, I think the more the MAF reading will be high
under pumpup and low under pumpdown. (Could be way off base here.)

As long as your ECU knows what to expect (as far as dynamic correction)
I don't think there would be a problem either way. (This ignores the
temperature compensation issue and the turbulence issues.)


>> I've measured this effect on my supercharged MR2 and it is very real.
>> It's a very small lag in terms of watching a boost gauge but in terms
>> of throttle response it's very very noticable... if you have a
>> differently-responding setup to compare it to anyway.
>
>For the above reason, I easily believe it. It's just that the MAF-after-IC
>for temp correctness or whathever still is a no-no (or a yes-yes, I mean
>before or after the IC and/or compressor is exactly the same thing as far as
>air mass in concerned)
>
>On the other hand, faster response is very desirable and it this respect, a
>MAP sensor (2ms response time) in the inlet manifold is hard to beat.

Yeah... I'm not saying anything one way or the other about what location
is best for a MAF, since I really don't know. I would at least try to
see if turbulence and/or rapid temp swings will be a problem, though,
if I wanted to do something like that.

Personally I'm using MAP... I ran into the quirk of MAF response by
accident.


>> One thing you can do to greatly sharpen up the response of temp sensors
>> is to make a first order slope correction. Measure temp (call it C),
>
>It may be that the OEM ECUs are doing this? Dunno. For NA applications, I
>don't think they bother.

Yeah, I'm pretty sure they don't bother for NA engines. In my mind this
goes along with the slowness of typical IAT sensors. I think if they
felt there was a problem we'd see faster-responding sensors in wider use
before anybody wrote a line of code.


>> This is lots easier if your temp sensor is a bandgap or t-couple that
>> reads a voltage directly proportional to absolute temperature, eg. TMP3x
>
>Definitely. For info, I use a custom-made air temp sensor (using a
>Thermometrics FP07 NTC sensing element) for my own engine. This baby has a
>time constant of 100ms in still air (7ms in water), just to say I'm _very_
>concerned with sensor response :)

I'm with you all the way. Currently I'm using some lightweight NTC
thermistors, suspended in the middle of the airflow. (Higher flow rate
past the sensor helps a lot.) I'm still not happy with the setup though.
I'm using glass thermistors because I originally spec'ed a very high
max temp rating. At this point I'd rather have a faster responding
sensor though.


   Chris C.

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