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To: <>
Subject: Re: BSFC
From: "Lorcan Parnell" <>
Date: Fri, 19 Sep 2003 00:44:03 +0100
It's a good question, and you are correct - increasing the pressure does
increase the response time slightly, but within the range I gave it does not
present a problem on most injectors and will produce an increase in flow not
far off what you would expect (bearing in mind you have to square the
pressure to double the flow). You should try to keep below 90-95psi total
(incl boost) though as above this the injector might not open at all. I have
run 66psi + 23psi boost without a problem...there might be some scope to run
more as boost will help open the injector against fuel pressure to an
extent, although the nozzle is small. F1 cars run 150psi fuel pressure using
special injectors to get good atomization.

My point is that it's better to go big on the injector size that gives what
you want at 40psi and give yourself some room to maneuvre than to buy an
injector that is on the limit at 70psi and run out of on-time. There is a
point at around 86-88% duty cycle where injectors suffer a big drop in flow
when they can't decide to be open or shut and this should be avoided. It is
better to turn up the pressure if you need to than run into this zone where
flow can halve suddenly.

Regarding the voltage to the injectors there are two types of circuit used,
depending on the ECU. The older type, "saturated circuit" applies a constant
current, about an amp. Newer style "peak and hold" ECUs jerk the injector
open with a high current, about 2 amps, and hold it open with a low current,
about 0.5 amps. You can't really alter this without risking ECU damage or
melting the coil in the injector. Old style injectors are around 2.5ohms,
new style about 15 - 16ohms.

Obviously, you can get injectors flow mapped at various pressures or test
them yourself triggered by a low voltage battery to check for the flow you



PS Russ Collins (remember him?) is now big on EFI injection...see RC
Engineering web site.

----- Original Message -----
> Nice idea, doubling the pressure, but I do have a question.  Since the
> injector is not pilot operated (i.e., pressure balanced to lower the delta
> p) don't you increase the response time when you increase the
> pressure?

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