Fuel pressure / Flow rate formula

Fran and Bud quest100 at gte.net
Wed Nov 18 01:43:44 GMT 1998


New to the group and limited in knowledge, but I believe that some GM
injectors were intentionally designed to fail to open above 45 psig.  

I picked up a set of 19#/hr injectors at the Long Beach swap meet that came
from a MAF 8? Camaro. (left over when the owner installed an aftermarket
system).  Had them flowed @ 40 psig and they were on the money, but when we
increased the pressure to 45+ they stopped dead in their tracks. Zero flow!
This was on a test bench where all eight were flowing into separate
graduated tubes to measure equal flow.  They all shut off at exactly the
same time.  These injectors were the GM fat stubby style, and these may be
the only type that act this way - I believe the Bosch style and the SVO
style will take double this pressure and still function well.

I heard later that this was intentional, in order to protect against a
failed back pressure regulator and an overachieving pump but this was all
hearsay. Anyone have similar data?

I prefer to mix and match whatever parts I can get my hands on, to try to
make something from a group of leftovers.  Has anyone out there compiled a
cross index list of parts?  A Ford 5.0 idle air in place of a GM IAC?  A MAP
sensor from a ?? will work on a ??. coolant sensors, air temps, etc???

And also has anyone posted anything about how to operate two GM TBI's
(tunnel ram) from the GM ECU.
Will one model do it ?, any model do it?, none?  I know that some of the
bigger shops make this work but I dont know if it requires circuit/component
changes or you "just do it".   

----------
>From: d houlton x0710 <tc75918 at hpr357.msc.az.boeing.com>
>To: diy_efi at efi332.eng.ohio-state.edu
>Subject: Re: Fuel pressure / Flow rate formula
>Date: Tue, Nov 17, 1998, 7:43 PM
>

>David A. Cooley wrote:
>> 
>> At 10:34 AM 11/14/98 -0500, you wrote:
>> >
>> >-> That's a pretty blatant statement isn't it?  Are you saying that if I
>> >-> go over 40 psi it will fail, period?  Everything I've read suggests
>> >-> that an injector will still work up to 120 psi, but that you should
>> >-> set as a maximum around 90 psi.  At 70, it will still work just fine.
>> 
>> Never said not to run them over 40psi even... some increase is ok, but
>> there comes a point (Found on the buick GN's for my reference) at about
>> 65-70PSI where the injector fails to open or close depending on the design.
>> This is also the pressure differential across the injector, not referenced
>> to atmospheric pressure... For a turbo'd engine with 20 psi in the
>> manifold, 80PSI fuel would not be unreasonable, as it is only 60PSI across
>> the injector.
>> Later,
>> Dave
>
>
>Yup, what you said is below.
>
>> You crank up the FP too far past the rated pressure (in your case 40psi)
>> and 1 of 2 things happen... the injector gets held open by the fuel
>
>I missed the "too far" when I read it the first time and thought you were
>saying it'd fail if I went over 40 psi at all.  Sorry about that.
>
>I'm looking at a max of about 70 - maybe 80 psi depending on how I add
>extra fuel and it seems that most everyone agrees here that that'll be 
>OK, especially for short durations.
>
>
>On a side note, I've been reading a new book I got on Nitrous Oxide.  It
>has a lot on working nitrous with fuel injection and has a section on fuel
>injectors and their capabilities.  It quotes RC Engineering and they 
>apparently swear by the Lucas disk type injectors.  They say they can run
>at 85% - 95% duty cycle with no ill effects and can handle higher pressure
>much better than the pintle type.  They can also run a much shorter pulse
>at idle than a pintle type because of the smaller lift needed for a disk
>type.  Something like a .8 - .9 ms opening time for the disk vs 2 ms or
>so for the pintle.
>
>
>--Dan
>



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