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

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).

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

I might take back what I said then. You'd have to get into a detailed discussion of the engineering to figure it out and I don't understand the manual system well enough to predict it. In general, pressure in a smaller hose will be higher, but volume lower - assuming the pump can maximize the flow to the hose in all conditions. If there is some sort of round robin going in where it applies a certain amount of fuel and expects opening, you'd be lowering the pressure point with a large hose. If, on the other hand, the pump supplies much more pressure than needed, it would reach the same pressure anyway.

Again, I lack the knowledge of that injection system to add anything but basic information at this point...

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.