[Diy_efi] CIS EFI via Fuel Pump PWM

[Kenny W]

Quoting Stephen Webb <swebb at netlab.uky.edu>:


"Since the boost pressure
is referenced to atmospheric pressure, I'm thinking the 
pressure drop across the air flap may not matter [so 
much]-- the turbo may have to work
a little harder to maintain a given pressure, but in 
doing so it
"recovers" this pressure drop."


Yes and no. The key factors in a turbo's operation and 
efficiency are the amount of air it is flowing and 
the "pressure ratio". Many people understand the 
pressure ratio to be outlet pressure+1/inlet pressure 
(in bar) which it is, but people often assume inlet 
pressure to be atmospheric, but the more restriction 
you put in front of the turbo (filters, bends in 
plumbing, flapper doors, etc) to more VACCUUM you 
actually end up wiht at the compressor inlet, so you 
are actually increasing the pressure ratio (making the 
turbo "work harder" as you say). And you can certainly 
upsize the compressor to make up for this, but at the 
expensive of overall turbo response , and the mroe you 
try to flow the worse the vaccuum gets so it's not the 
best situation. 

"For an N/A car, if the air flap *is* restricting 
airflow, would you expect
to see the vacuum gauge indicate some sort of pressure 
drop due to this
restriction?"

I certainly do on my car. it is cis and WAS a turbo, no 
turbo right now to it is essentially NA. However, my 
calibrated VDO boost gauge is still intalled, and I 
cannot get the gauge to go to 0 on WOT, it still stay's 
a few psi in vaccuum. However I also still have the 
intercooler installed. lol. So a lot of the pressure 
drop could be from that.  


-Kenny
AKA Captain Bondo

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