Oh ... FFS!!! Broken 8V Injector Hose

On Feb 24, 6:10 pm, still just me <wheeledBobNOS... RemoveThis @yahoo.com> wrote:
> On Sun, 24 Feb 2008 06:43:25 -0800 (PST), PJGH <pjg... RemoveThis @googlemail.com>
> wrote:
>
> >Okay ... I've found a chap who can make me up aftermarket PTFE lined
> >stainless overbraid hoses, but the smallest he can do is "quarter
> >bore" (which I gather is 1/4 inch).
> >In his opinion, it shouldn't matter but I can't help thinking the
> >microbore 6x2mm was used by SAAB for a reason.
>
> >Who knows for certain?
> >Has anyone commissioned their own aftermarket hoses? What bore did you
> >use?
>
> I don't know for certain, but in a high pressure system like FI, the
> diameter of the hose is likely not a limiting factor. The valve is
> doing all the fuel metering/limitation.
>
> I think it would be all right to run with it, if you can get it tight
> enough on the fittings not to spray gas. Then you can take a little
> time to look for proper replacements.

Ta for that - I'm sure on of our local SAAB Specialists will have a
suitable replacement that I can pick up tomorrow.
I want to replace the lot with good aftermarket material and have an
end to snapping these damn things.

The problem with the old Bosch CIS/K-Jet system is that the injectors
are manual - they are opened (continuously) by the pressure of the
fuel, not fired by ECU. To that end, the wider the bore, the lesser
the pressure for the same delivery of fuel? Or does the pressure
increase when the bore drops to the aperture of the injector opening
and crack the injector open. That's my dilema. I have found loads of
posts on forum boards where people have commissioned aftermarket hoses
for the CIS system, but they are very light on the technical detail.

I know years ago, I had this discussion with Grunff (now there's a
name from the past, alt.autos.saab-wise) and he commissioned a set
from a local hydraulic specialist. The guy I talked to today could not
drop down to microbore - his swaging machine would not go that small.
I've got a couple of leads to follow up tomorrow (on foot).
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PJGH wrote:

> The problem with the old Bosch CIS/K-Jet system is that the injectors
> are manual - they are opened (continuously) by the pressure of the
> fuel, not fired by ECU. To that end, the wider the bore, the lesser
> the pressure for the same delivery of fuel? Or does the pressure
> increase when the bore drops to the aperture of the injector opening
> and crack the injector open. That's my dilema.

Pressure is set by the smallest point, not the largest, and on the CIS
neither sets the pressure: the injectors are spring-loaded, not just
open jets, and it's the opening pressure that matters.


Bill
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 >> Stay informed about: Oh ... FFS!!! Broken 8V Injector Hose 

> I don't know for certain, but in a high pressure system like FI, the
> diameter of the hose is likely not a limiting factor. The valve is
> doing all the fuel metering/limitation.

The size of the hose is not very important, but since the velocity of the fuel
flow in a larger hose is smaller, and because of Bernoulli's principle, there
will be less pressure loss with the same volumetric flow rate. Consequently,
there will be a slightly higher pressure at the injector, causing it to inject
slighty more fuel. To avoid this all hoses should be the same length and same
diameter.

--
MH
 >> Stay informed about: Oh ... FFS!!! Broken 8V Injector Hose 

On Feb 25, 9:19 pm, MH <n... RemoveThis @no.no> wrote:
> > I don't know for certain, but in a high pressure system like FI, the
> > diameter of the hose is likely not a limiting factor. The valve is
> > doing all the fuel metering/limitation.
>
> The size of the hose is not very important, but since the velocity of the fuel
> flow in a larger hose is smaller, and because of Bernoulli's principle, there
> will be less pressure loss with the same volumetric flow rate. Consequently,
> there will be a slightly higher pressure at the injector, causing it to inject
> slighty more fuel. To avoid this all hoses should be the same length and same
> diameter.

That's superb - thanks for that. I did think the pressure would
increase at the injector, but wondered if the overall pressure would
drop - I'm no Physics guy ... more Humanities. As it happens,
Goodridge are going to make me up a set in 2mm and all the same length
(the 75cm length) as per the longest of the standard SAAB fuel lines.
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