Why are MFI injector fuel lines equal length?
 

I guess the obvious answer is related to the timing of fuel pulses at each cylinder.

BUT, if the lines are full of gasoline, a liquid, therefore non-compressible, the actual length if each pipe should not affect pulse timing, within reason of course.

Is it related to fuel volume in each pulse, or something more complex that I am missing completely??

I am asking because a non-Porsche friend, very knowledgeable in precise plumbing type matters (former machinist mate on a boomer), asked me, and I was not able to satisfy even myself with my reply.

Help!

There must be hysterisis is all I can think of--there must not always be 220 PSI injection pressure in the lines at all times, so that the length makes a difference. Either that or it's Germans being German.

I'm not sure Henry Schmidt's flex lines are the same length. Maybe he will chime in here. Also, someone look up a pic of the 917 12 cyl. Are they the same length?

Are brake lines or CIS lines the same length? If the fluid line contains no air, the contents move as a solid column.

Sherwood

maybe just cheaper to make from the same stock..... :rolleyes:

Henry's lines are not equal length from some of the photos I've seen, and I don't think that (all) 917's had equal length lines. Using the Darcy-Weisbach equation we can see that there is a linear relationship between the length of the pipe and pressure drop.

So, it appears that there will be a small difference in flow, but not enough to significantly impact fuel ratios. So John is likely right - just Germans being Germans.

I know you say this in jest or maybe it's sarcasm but it's probably the reason. The lines have a nipple on the end so it's not just a matter of cutting to length. They would have to manufacture six different versions to have them different lengths. They each need to be bent differently but at least making them all the same length simplifies one step of the production and keeps cost down.

Bobby, I really meant it that way. If there is no benefit either way they making them the same length takes one step out of the complexity of making all these parts.

Got it. :rolleyes: = sarcasm. :)

I don't think it has to do with production costs. If I were to hazard a guess, it's for when you're starting - the pump works by having a little piston push a certain amount of fuel through the lines. If the lines are full of gas, then no problem if they are not equal lengths. But if fuel drips out of the lines through the injectors, then they will not be full. If they are not full, then having unequal lengths will mean that some cylinders will get fuel before the others upon starting.

That would be my guess.

Less likely would be the friction and/or the compressible effects of the gas in the lines. MFI is pretty high pressure, and different length lines will have different pressures at the end injector, if the gas doesn't act like a 100% perfect hydraulic fluid. I hesitate to say that if Porsche was building the cars today they would have different length lines, but back then, they seemed to do things "the right way" for the sake of doing them. There are a bunch of other 911 details I can point to that fit into this category as well.

-Wayne

I'm liking Wayne's thinking here, I always wondered how the system gets filled at starting, esp. when the car has been sitting a long period. Surely at that point the lines are no longer filled with fuel.

I am also thinking about the "head pressure" effect on the in-line pump at speed. I can not find any work done in support of this idea, but it seems to me that a rapidly spinning (up to 4000 rpm or thereabouts) 220 psi six piston pump may have some harmonic or other difficulties if the loading on each piston is "unmatched" so to speak.

Any takers on that idea??:confused::confused:

Yeah, it's a German thing!
 

Probably the same reason that for the first 40 years the 911 had a fuel gauge that had widely spaced increments toward the full end and tiny lilttle increments at the empty end...where you might actually need to know more precisely how much gas is left in the tank!!

Go figure.

Just not a German thing. Also a US thing, many a Caterpillar diesel engine with mechanical fuel pump/distributor have equal length injector lines, to the extent of lenghts of pipe with U turns etc. to maintain the equal lengths. Only the other month the topic of compression of fluids was raised with some hydraulic experts, and was advised that there is minor, minor compression of liquids. The concept of equal length (and equal everything else) in the fuel lines is to eliminate any variences.

Paul

its all physics
 

Poiseuille's equation
states that flow is inversely proportion to length
and proportional to radius to the 4th power
so length is a relatively non critical factor but does matter.
a kink in your line is a disaster.
i don't know why i remember this.
you can wikipedia for the actual formula
bob

The lines are not full of gas, MFI is a metered injection system unlike CIS which is continuous. Therefore the lines need to be equal length or it will affect the fuel timing.

I think they are not the same length. If they were it would be for the simplicity of production or some other mundane reason. Of course the lines are full of fuel. The injectors open at something like 200 PSI. If there were air in there no fuel would flow. Unfortunatly I sold my MFI engine and installed a 3.0 or I'd go into the garage and check right now.

-Andy

We have built lines, both even and uneven lengths according to the customer's wishes. You will see no difference between even and uneven lines (verified with an LM-1). You also have fuel in the lines at all times if the pump is running properly. The check valves in the pump guarantee this. If air were to be in the system it would diminish both timing and volume demending on the amount of volume of air that is present in the line.

At 220 psi the lines would have to be drastically different in length for it to make a difference. It would take several hundred feet for it to make a difference if even then. The weight of the fuel in these short distances makes matching lengths inconsequential at that kind of pressure.

If you have a leaky injector then having equal length lines will not help. The pressure will be down at the time of injection.

The lines are only full of fuel during an injection event. The fuel is metered to each cylinder by the injection pump, not the injectors as in EFI. Read Dempsey's book.

FWIW,..the fuel lines on our old MFI RSR and 935 were not equal length; I used to replace them annually. :) :)

I never observed any power balance or EGT issues between cylinders that were attributed to different length injector lines.

We don't have a street MFI system here at the moment to confirm but I recollect they are not all the exact same lengths.