[Diy_efi] Speed-density vs. MAF/MAP...

[Perry Harrington]

Adam,

I'm rarely a dick about details, but...

On Wed, Jan 29, 2003 at 11:49:00AM -0800, Adam Wade wrote:
> >From my thoughts and reading, I had always been under
> the impression that it made more sense to use
> speed-density for SMALL throttle openings and lower
> engine speeds, as MAP sensors bounce around at small

I believe you meant MAF in the above sentence?

> airflows on 4-cyl. engines (and they take time to
> respond, I understand), and a TPS would be far more
> sensitive in terms of recording changes in load... 
> And that at high engine speeds/WOT, a TPS was rather
> insensitive, but the airflow was sufficient and steady
> enough to get a good reading from a MAF/MAP.
> 
> But I was doing some reading for the book I am
> writing, and I see Kawasaki does it exactly BACKWARDS,
> on two different systems made by two different
> manufacturers (Mitsubishi and Denso).  They use
> MAP/IAT for SMALL throttle openings and low engine
> speeds, and speed-density (TPS + RPM) at HIGHER engine
> speeds and larger throttle openings.

Speed density is MAP vs RPM, Alpha N is TPS vs RPM.  I
believe that was the contrast you were making?

> 
> Am I missing something fundamental here?  Or is this
> really as ass-backwards as it looks to my eyes?

I would believe that they are using Alpha-N when they peg
the MAP sensor to atmospheric.  Bike engines have VE ranges
well over 100% for thousands of RPM of operation.  For this
reason it becomes impractical to use MAP.  Basically you'll
have a bunch of bins that are 100Kpa vs RPM.  They can then
safely assume that when they max MAP, they can go to TPS
vs RPM, as this has more resolution at that point.  If you
hit 100Kpa at say 7000 RPM and the motor redlines at 14,000,
you've got a span of 7000 RPM where the TPS goes from possibly
1/4 open to full open.  So it makes sense to use TPS instead of
MAP in that instance.

Alpha-N is used where a MAP signal is not reliable or has
too little resolution.  The primary reason people use Alpha-N
is on big cammed motors that have very little vacuum and their
vacuum signal bounces at idle.  5 inches of vacuum isn't alot
of resolution when you consider 30 inches is full scale.  Hence
here's Alpha-N again.

Now, bikes need to stay small.  But MAF would likely be the
solution to both problems.  MAF may be "slow" to respond (the
air column must accelerate after the throttles open, but with
generous accel enrichment you have less problems.

MAF would eliminate the resolution issue on bike engines, since
you have an accurate measure of air at all times.  The biggest
problems with MAF are getting the physical issues ironed out.

You need to make sure no reversion is taking place due to intake
valve pulsations.  You need an intake that is tuned to exploit
the large pulsations.

> 
> Someone tell me I'm being a dumbass.  ;)
> 
> | Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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