[s-cars] Re: Injector side of the thread

[QSHIPQ at aol.com]

RC Engineering calls 75-100psi "high pressure" fuel systems.  My exact 
questions wrt the below and some of the discussions on these high fuel pressures can 
be found on the 034 forum.  I too question why anyone would think of running 
a fuel pressure that high.  Russ Collins of RCE actually gives Javad's recount 
as an example of the wrong injector for the job at hand, almost verbatum in 
A/F terms.  But one also has to be aware that there are 2 sides to the Duty 
Cycle, the high and the low.  The high on a pintle type is about 85DC, the low on 
the otherhand, will vary by fuel injector and fuel pressure.  Just about 
every injector on the market is designed to run between 2.5bar and 3bar FP.  
Porsche (Audi) injectors run 3.8bar standard (rated, audi runs many at 4.0bar).  
Interesting to note the correlation between the RS2 injector at 3.8bar, and the 
Urs4 injector at 4.0bar.  It appears that porsche thought the extra boost on 
the RS2 dictated that a .2bar drop in RRFPR was required.  Food for thought.

SJ


In a message dated 11/11/2004 5:30:13 PM Central Standard Time, 
mihnea.cotet at easynet.be writes:
Javad,


You are totally right WRT Hap's fuel pressure, 90 psi is outright #### 
(self-censored), basically any injector out there will run out of flow 
above 65-70 psi rail pressure.

Then, 90 psi is ok at low RPMs, but at the top end the pump is going to run 
out of flow. Trevor Frank's car is running 57# injectors on 43 psi fuel 
pressure, 26 psi of boost at the redline and 12.5:1 AFR with no fuel pump 
problems so far...

Just my .02


Mihnea

www.mrc-developments.com


Hap, if you don't mind me ruminating about your dyno experience...if I 
remember correctly, by 4krpm your fuel curve was plunging below 10:1, going 
off the chart rich, then coming back lean around 6krpm and ending up 
dangerously lean at about 13.5:1 by 7krpm ish.

Due to the "Happersized" fuel pressure gauge (of which I'm now running 
;)  we could easily observe fuel pressure hitting 90psi (insanely high) and 
by 6krpm begin to rapidly drop correlating to the leaning fuel curve.

Clearly there were some mapping problems aggrivated by more fuel delivery 
than what the system was accustomed to.

I think a good solution to your fuel pressure problem would have been 
bigger injectors and a lowering of your fuel pressure.  There aren't many 
factory EFI pumps that can deliver large fuel volume at 90psi, there are 
plenty that can at 40-50psi.

Currently I'm running #75 injectors on a stock Audi CIS pump with 40psi 
base pressure.  Anyway, lets not get off topic  ;)

Javad