[Diy_efi] How would I calculate the mass of the air w/ only IAT and MAP?

Eric D Byrd klox at juno.com
Fri Nov 1 07:29:03 GMT 2002


This raises an interesting question I have also posted on the CRX forum
at honda-perf.org.  If you swap your cam(s) for higher performance lobes,
that changes your VE map.  Therefore one wonders if you would also have
to retune the ECU.  But here is an interesting observation about my 90
CRX Si.  I read somewhere, I think from the Ecu shunt I bought to enrich
my mixture, that disconnecting the battery and reconnecting forces its
OBD0 Ecu to recalibrate or some such, and that it won't run optimally
until it finishes.  I don't know for sure, but I would speculate that it
is able to guesstimate a decent VE map just from running in closed loop
for a while, and then apply an extrapolated version to open loop
operation.  Anybody know for sure?

--Eric

On Sun, 27 Oct 2002 20:33:09 -0800 Brian Dessent <brian at dessent.net>
writes:
> 
> > I still think we are missing a variable....  flow.  Or can that be
> > assumed through the volume of the cylinder and speed of the 
> engine?  
> 
> Yep, that's it.  You can't calculate air mass with just temperature 
> and
> pressure, you need the flow rate as well.  Most speed density 
> systems do
> this with a VE table which maps RPM (and possibly pressure) to a 
> flow
> rate.  RPM is usually a known quantity to the ECU so you probably
> already have it available.  The problem is getting the VE table....
> 
> Brian
> 
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> 


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