Injectors and A/F Ratio

Superman · 09-02-2008, 07:26 AM

Before I remove the engine in this car and replace it with my other engine, I want to find out why it's running poorly. It seems to have good, even compression and the ignition wires are fine. That got me thinking.......

I have observed and compared CIS injector volume in the past and what I've found is that they are not entirely even. Some spray more gas, others less. But....if the Air/Fuel ratio is critical (which it is), then wouldn't this difference in volume make the cylinders fire differently and more importantly, wouldn't it mess up the setting of a proper mixture? A rich mixture in a given cylinder, or a lean mixture for that matter, might not fire. Or not fire correctly. Spewing unburned hydrocarbons which would fool the 'sniffer' machine used to set mixture.

In other words, has anybody messed with trying to match injector volumes and spray patterns, and found this to be an important impact in tuning an engine?

T77911S · 09-02-2008, 08:27 AM

good question. i have posted something similar and got no response.
i did a basic comparison of voume. i put all 3 left side injectors in a bottle and the right 3 in a bottle, lifted the AFM plate wide open for about 20 sec. i got about 900 grams on the right and 800 grams on the left. i just pulled the plugs and the plugs showed the same, a little blacker on the right and cleaner on the left.

as far as the gas analyzer (GA), i had my mixture set at 3%. the GA would read and "average" of the 2 sides together, so one side would be a tad lean and the other would be a tad rich. ( this is just based on my car and assuming each injector on the left side has the same volume and so forth on the right). if all 6 are different it would just average the 6.

whats even more interesting is that the control pressures have a fairly tight spec compared to the opening pressure of the injectors. if the injector opens at a less pressure does it dump more fuel?, i would think so.

this guy is driving himslelf crazy trying to fix fuel flow out of the FD.
erratic low flow fuel head problem
how much of a difference does it make, i dont know. i suppose you could run a cylnder a lot leaner than the others if you had just one injector bad, but that should show up on the plugs.

MatthewBrum · 09-02-2008, 08:49 AM

One point you have to keep in mind is that the injectors are not designed to meter flow. They are designed to open at a cracking pressure and give a good spray pattern. The fuel distributor is what determines how much fuel comes out of the injectors.

If you did the test that T77911S did then you want all the injectors to dispense the same amount. Now I'm sure there is also an allowable variance in this test but I could not tell you what it is.

T77911S · 09-02-2008, 09:19 AM

if one injector opens at 1 bar and another at 2 bar, fuel pressure is 2.2 bar, would the volume be different for a given amount of time?

i will have to go back and about the FD.

did you read the link i posted? is he expecting tighter tolerences than what it was designed for?

T77911S · 09-02-2008, 09:52 AM

perhaps the injector opening pressure is not that critical.
one day i may swap injectors and see if it makes a difference.

MatthewBrum · 09-02-2008, 11:03 AM

The injector opening pressure is critical in that it must be lower than the system pressure and the injectors must not leak below the cracking pressure. In the system the injectors are not the restriction. The small openings in the fuel distributor is the restriction.

It's like using a garden hose with a nozzle on it. If the water valve on the side of the house is wide open than the restriction is the nozzle. But if you just crack the water valve than the nozzle will not meter the flow coming out of the hoze. This is how the system works.

Superman · 09-02-2008, 01:13 PM

Thank you for the information, so far.

Yes, I am just as curious about the fuel distributor as I am about the injectors, and would like to know 1) Whether flow is uneven and 2) If so, what is causing the uneven-ness. But first......

I want to hear about how critical this is. For example, T77911S says that rich injectors and lean injectors even themselves out, as far as the gas analyzer goes but I am not so sure. If you get very far away from the proper mixture (13.5/1 or so) in EITHER direction, you get poor combustion. In other words you may get just as much unburned hydrocarbons from a lean cylinder as a rich one.

In addition to noticing that injectors do in fact deliver different volumes at the same air flow sensor plate placement, I also notice that most (CIS) engines have some bright white exhaust valves, and others have dark, sooty ones.

Again, has anyone tried to match injectors and/or found fuel distributors that deliver every uniform fuel volumes, and did this cause the engine to run super-smooth with maximum torque? And did it cause the exhaust valves to look the same?

911st · 09-02-2008, 03:26 PM

I am not an expert but injectors, fuel head, and intake manifold design/condition can all affect A/F's at each cylinder.

If an injector becomes dirty it can flow less than designed as it effectively makes the orifice smaller. Having your injectors cleaned or replacing them is a good thing.

Fuel heads when rebuilt are adjusted to equalize flow at each circuit. They are sensitive to dirt and improper storage. Dirt or gum in the internal orifices, slits, and or the upper or lower chambers can effect delivery to a given cylinder.

For both situations keeping a clean fuel filter in the car and running injector cleaner through the system is a good thing.

Might even be worth trying one of the industrial strength shop only fuel system cleanings.

Some cars like the 930's can have uneven air flows to different cylinders under high output. I do not know if this is an issue w the SC intake design but do not expect so. Might be worth checking for air leaks about the intake manfold.

All this is a good reason to watch your spark plugs for signs of a potential problem. If your CO or AF levels look good but one cylinder is lean and five just a bit rich it could test ok. However, it would be possible to suffer valve or piston damage from the lean cylinder condition. With 930's this happens a whole lot faster.

I suspect the cleanning is the first place to start, then the injectors. If this dose not fix it, some heads have adjustments at the head for each circut but I do not know if this is the case for SC's. Otherwise the head should be sent off for cleaning and recalibration.