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

Thu Jan 30 00:24:47 GMT 2003

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We recently did some dyno testing on a motorcycle engine with a small 
plenum and short runners.  The dyno has a MAF sensor (actually hot wire) 
built in; based on air-flow measurements, we got a VE of 150%!!  We 
think this occurs because the intake reversions are pushing air back and 
forth past the MAF, thus "fooling" it.  We put a large airbox between 
the MAF and the intake and the problem went away.  Because of space 
constraints, it's really hard to make MAF work on a motorcycle.

On the other hand, speed density can work well on engines with large 
pressure pulses in the plenum if the pressure is read at a specific 
engine position rather than at a certain sampling frequency.  I know the 
new Delphi system on the twin cam Harley works this way (don't ask me 
how I know).

Didn't the old Ducati engines use the Magnetti Marelli system with TPS 
vs engine speed lookup tables and no MAP?  They seemed to work very well 
although each lookup table only worked for a specific 
engine/cam/intake/pipe configuration.

-- 
STEWART PRINCE

PROFESSOR, MECHANICAL ENGINEERING

CALIFORNIA STATE UNIVERSITY, NORTHRIDG

Perry Harrington wrote:

>On Wed, Jan 29, 2003 at 11:43:00PM -0000, Bill SHURVINTON wrote:
>  
>
>>Um Perry VE>100% by definition says that you have inertial supercharging so
>>MAP >atmo. So you can still sense it if you are timed sampling.
>>    
>>
>
>But where in the intake tract would you find MAP > atmosphere?  I also assumed
>we were talking about standard NA MAP sensors too.
>
>Oh, another point (non-linear thought process), if you have > atmospheric
>pressure through a MAF, reversion becomes a non-issue, because the intertia
>of the air prevents reversion. This is my understanding, further discussion
>would be enlightening if you think otherwise.
>
>  
>
>>As for MAF on motorbikes I have yet to see a workable MAF solution that
>>works with IR systems and teeny throttled volumes without screwing up the
>>throttle response. Not to say it can't be done, but seems pointless. No, is
>>pointless :-)
>>    
>>
>
>I understand "if it ain't broke don't fix it".  But how would that be fun
>or challenging?
>
>Seems to me that "MAF lag" is really the inertia of the measured volume.
>This goes back to Jeff Clarke's DIY-EFI discussion on MAP lag vs TPS.
>MAP just has less lag than MAF due to where it's measured.  If you made
>4 tiny MAF sensors into each of the TBs, I'm sure you'd have a very low lag,
>but then you have a bunch of other problems.
>
>In the end, the food chain still exists as Alpha-N, MAP, MAF in terms of
>accuracy.
>
>I see the logic in MAF systems and have been working with one for a couple
>of years.
>
>  
>
>>Bill
>>    
>>
>
>--Perry
>
>  
>

E

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We recently did some dyno testing on a motorcycle engine with a small plenum
and short runners. &nbsp;The dyno has a MAF sensor (actually hot wire) built in;
based on air-flow measurements, we got a VE of 150%!! &nbsp;We think this occurs
because the intake reversions are pushing air back and forth past the MAF,
thus "fooling" it. &nbsp;We put a large airbox between the MAF and the intake
and the problem went away. &nbsp;Because of space constraints, it's really hard
to make MAF work on a motorcycle. 
 
On the other hand, speed density can work well on engines with large pressure
pulses in the plenum if the pressure is read at a specific engine position
rather than at a certain sampling frequency. &nbsp;I know the new Delphi system
on the twin cam Harley works this way (don't ask me how I know). 
 
Didn't the old Ducati engines use the Magnetti Marelli system with TPS vs
engine speed lookup tables and no MAP? &nbsp;They seemed to work very well although
each lookup table only worked for a specific engine/cam/intake/pipe configuration. 
<pre class="moz-signature" cols="$mailwrapcol">-- 
STEWART PRINCE

PROFESSOR, MECHANICAL ENGINEERING

CALIFORNIA STATE UNIVERSITY, NORTHRIDG</pre>
<br>
Perry Harrington wrote:<br>
<blockquote type="cite"
 cite="mid20030129161626.C16852 at scarab.apsoft.com">
  <pre wrap="">On Wed, Jan 29, 2003 at 11:43:00PM -0000, Bill SHURVINTON wrote:
  </pre>
  <blockquote type="cite">
    <pre wrap="">Um Perry VE&gt;100% by definition says that you have inertial supercharging so
MAP &gt;atmo. So you can still sense it if you are timed sampling.
    </pre>
  </blockquote>
  <pre wrap=""><!---->
But where in the intake tract would you find MAP &gt; atmosphere?  I also assumed
we were talking about standard NA MAP sensors too.

Oh, another point (non-linear thought process), if you have &gt; atmospheric
pressure through a MAF, reversion becomes a non-issue, because the intertia
of the air prevents reversion. This is my understanding, further discussion
would be enlightening if you think otherwise.

  </pre>
  <blockquote type="cite">
    <pre wrap="">As for MAF on motorbikes I have yet to see a workable MAF solution that
works with IR systems and teeny throttled volumes without screwing up the
throttle response. Not to say it can't be done, but seems pointless. No, is
pointless :-)
    </pre>
  </blockquote>
  <pre wrap=""><!---->
I understand "if it ain't broke don't fix it".  But how would that be fun
or challenging?

Seems to me that "MAF lag" is really the inertia of the measured volume.
This goes back to Jeff Clarke's DIY-EFI discussion on MAP lag vs TPS.
MAP just has less lag than MAF due to where it's measured.  If you made
4 tiny MAF sensors into each of the TBs, I'm sure you'd have a very low lag,
but then you have a bunch of other problems.

In the end, the food chain still exists as Alpha-N, MAP, MAF in terms of
accuracy.

I see the logic in MAF systems and have been working with one for a couple
of years.

  </pre>
  <blockquote type="cite">
    <pre wrap="">Bill
    </pre>
  </blockquote>
  <pre wrap=""><!---->
--Perry

  </pre>
</blockquote>
<br>
<pre class="moz-signature" cols="$mailwrapcol">E

</pre>
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