[Diy_efi] EFI Fuel Filters

Mike niche
Wed Nov 16 12:07:28 UTC 2011

Had a similar issue re VL commodores and skylines some decade or so back, there
was an issue with pulses causing a 'water hammer' type effect where larger pipes were
used. ie Pulses from the injectors slamming shut would go back as far as the high
pressure EFI pump to the back of the car and set up a resonance or oscillation
with the next injector opening and pump pulses causing irregular fuel delivery so that at odd revs
the ECU couldnt seem to keep the engine stable. Having some discontinuities of pipe sizes
introduced impedance mismatch which reduced the severity of the pulses and hence increased
stability. The return pulses have been known to crack steel fuel lines and I have also heard can reduce the
reliability of the crimp joint in some earlier injectors so they tend to weep fuel.  The fuel is
essentially incompressible and the energy has to go somewhere, ie its not just from
the injectors its also from the recirculating ball mechanism in the high pressure pump,
when these coincide and reinforce without a damper there can be outright failures. Most of
the factory setups I have seen have a fuel damper on the outlet of the EFi HP pump...

For the sort of power you are expecting and the duty cycle of the delivery of that power you
will very likely be fine with existing fitting sizes. If however you were running 300Kw continuously
for say several seconds up to a minute or so then you 'might' have an issue, an accumulator on
the output of the efi pump would likely get around that and make sure you have a surge tank
at input to efi pump too so it doesnt cavitate from any low pressure pump etc.

You can also check pressures at the injectors with a differential fuel pressure gauge which has
a drag pointer set to show lowest fuel pressure during a run. Or use a datalogger suitably



At 09:04 AM 11/16/2011, Dan wrote:
>I'm currently redoing the fuel lines/fittings on my '86 GM TPI system 350 chev, and something got me thinking.
>I have nice 3/8" feed line and 5/16" return line with AN8 and AN6 fittings. There is then the occational metric fitting such as the TPI, fuel pump and fuel filter.
>What has me wondering is the fuel filter as a possible flow constriction. The filter has M14 screw fittings with internal flare shape, yet the actual inlet/outlet hole inside the filter (in the flare taper is only 7/32" or 5.5mm ish). This is a OEM filter from an OZ Ford running 300+kw in factory form.
>So I guess question is, why such small holes in the filter when fuel lines are generally much bigger, or could I have run smaller fuel lines to begin with as this is all that is needed ?
>Is there a EFI filter that has larger in/out holes, I scoured through the Ryco filter catalogue and this was the best I could come up with (that had screw fittings) ?
>Thanks   Dan
>Diy_efi mailing list
>Diy_efi at diy-efi.org

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