CIS Injector Flow and Crack pressure

[Travis Neff]

I have a 78SC that I have pulled the engine to refresh the fuel lines (fuel leak somewhere in the engine bay. I had sent out my injectors to be cleaned and checked and the results I got back were a bit puzzling.

5 injectors cracks (open) at 3 bar (43psi) and flow around 59.4 on average
1 injector cracks (open) at 4 bar (58psi) and flows at 75 (21% more fuel)

I have attached the test results.

They all have the same Bosch part number on them, but the one with the higher output and crack pressure has an additional stamp on it (014). I ordered a new Bosch injector and the one I received also has an (014) stamp on it. I blew compressed air through it to find out where it opens and it is about 4 bar as well. I can only guess it will be a higher flow unit as well.

I don't know much about CIS - I understand you can adjust each injector on the fuel head so that each injector flows about the same amount of fuel. However, if it takes an additional 1 bar to open one injector I would imagine you can't really tune that out.

What are your thoughts?

[Travis Neff]

Bumping this up to see if anyone can share some thoughts

[boyt911sc]

Travis,

The individual fuel flow for the fuel distributor is tested and calibrated without any injector installed. Some CIS fuel distributors specially the early models are not adjustable. So you could test your fuel flow in two ways:
a). Without injectors.
b). With injectors installed.

The CIS injectors (0-437-502-004) have cracking pressures between 36 to 52 psi. (On top of my head and done this test many moons ago). There are a few distinct tests for a good fuel injectors:
a). Cracking or opening pressure.
b). Spray pattern.
c). Sealing property.

During my experiments and testing, I grouped several fuel injectors according to cracking pressures:
Group I: 36-41 psi.
Group II: 42-47 psi.
Group III: 48-53 psi.

Group X: Below 32 psi. and above 55 psi. were segregated as questionable injectors.

It would be ideal to have fuel injectors with very close cracking pressure values. Mixing them is not a problem. But your injector #6 will not PASS my test.

Tony

[Travis Neff]

Why would #6 not pass?

The reason I ask is the new one I received to replace #6 cracks at the same pressure.

Earlier this morning I ordered 4 more assuming that the new injector would have a higher spring tension and all would crack at the same/similar pressure.

[Travis Neff]

Also, I see that the fuel opening pressure on the CIS for dummies thread - for all years and models have the same opening pressure range, which is in the 3 bar range. More confusion!

[boyt911sc]

Travis,

Test the flow rates of the six (6) fuel lines in your car:
a). Without injectors. Do multi-tests and take the average.
b). Same as above but with the injectors installed.

Compare the average test results between “a” & “b”.

Tony

[Travis Neff]

I will do as you suggest and report back once I get the engine back in the car. I can do back to back tests with the old and new.

Also I noticed on the report that they are only reporting whole numbers for opening pressure - I will note what I can get them to open with on my compressor with running pressure rather than burst pressure.

Thanks for the input

[T77911S]

tony,
have you done a test with injectors ON a calibrated fuel distributor to see what the flow is.
I out test injectors that have a wide range of characteristics, like your groups 1,2 3 and X all on the same FD.

I would do 2 tests.
one that simulates idle. this one takes a long time to get any volume of fuel.
and another at WOT.

[boyt911sc]

Originally Posted by T77911S

tony,
have you done a test with injectors ON a calibrated fuel distributor to see what the flow is.
I out test injectors that have a wide range of characteristics, like your groups 1,2 3 and X all on the same FD.

I would do 2 tests.
one that simulates idle. this one takes a long time to get any volume of fuel.
and another at WOT.

Ty,

I used three (3) settings:
a). Low volume fuel flow.
b). Medium volume flow.
c). High volume flow.

I used different fixed height tools to replicate the settings but first you have to calibrate the individual flow rate. The early CIS fuel distributors are not adjustable compared to the later ones. I found out that the “medium volume flow” for 30 sec. run represents a more consistent test result and good representation of actual flow volume. This is done without fuel injector/s installed.

Installing six (6) injectors increased the complexity of the tests. I had one particular test that took me 30 trials to get the flow rates within +/- (1 mm) and later realized it was too strict. If you ever tried to calibrate a fuel distributor, make sure you have plenty of time and don’t rush it.

Lastly, unless there is significant differential flow rates among the fuel lines with injectors, the system is very forgiving and hard to see the problem.

Tony